AhID ( a RADIO REVIEW "; ;"tj (1'ti Year Publication) 4 f i 'hWU1v

No. 597. WEDNESDAY, FEBRUARY 4TH, 93r. VOL. XXVIII. No. 5.

Editor : HUGH S. POCOCK. Assistant Editor : F. H. H AYNES. Editorial Offices: 116 -117, FLEET STREET, LONDON, E.C.4 - - Editorial Telephone : City 9472 (5 lines). Advertising and Publishing Offices: DORSET HOUSE, TUDOR STREET, LONDON, E.C.4. Telephone: City 2847 (53 lines). Telegrams : " Etbaworld, Fleet, London."

COVENTRY : Hertford St. BIRMINGHAM : Guildhall Bldgs., Navigation MANCHESTER St. : 260, Deansgate. GLASGOW : WI, St. Vincent St., C.2. T legrame: "Cyclist, Coventry." Telegrams: "Autopress, Birmingham." Telegrams: "Line, Manchester." Te ms: "nitre, Glasgow." T aephone: 5219 Coventry. Telephone: 2970 Midland (3 lines). Telephone: 8970 City (4 lines). Telephegraone : Central 4857. PUBLISHED WEEKLY. ENTERED AS SECOND CLASS MATTER AT NEW YORK, N.Y. Subscription : 1 ; Rates Home, is. 8d. Canada, £I is. 8d. ; other countries abroad, 4I 3s. Iod. per annum. c s many of the circuits and apparatus described in these pages are covered by patents, readers are advised, before snaking use of them, fo satisfy themselves that they would not be infringing patents. Sources of Revenue for Broadcasting.

I J['(HE stupendous task of building up the broadcast - revenue by means which may be considered legitimate 1I ing service in this country has from the outset and proper. necessitated a continual extension of the demands In January of last year, after commenting on the out- for finds. The requirements of broadcasting, even in cry which the publication of The Listener produced, we the earliest days, were great, but the annual expenditure added : " Perhaps the recent outbreak may serve to re- then was but small by comparison with the sum involved vive the efforts of the entertai:iment industry to safe- to -day, after many years of additions to the commit - guard themselves against encroachment which they have mento of the B.B.C., many of which are self- imposed every reason to fear. In the plans for the new Broad- tasks complementary but not essential to broadcasting casting House we observe that a super studio, with as a service. accommodation for i,000 members of the public, is pro- In'.reasing expenditure on the part of the B.B.C., mised. Whether the B.B.C. derives box -office receipts nece<_sitating the finding of additional revenue, has had from such an audience or not, in either case it would the effect of making the Corporation ever on the look -out seem to us that this is equivalent to the establishment for means to an end, the end being always more funds of a B.B.C. theatre or concert hall in direct competition to m .et the growing expenditure. with those privately owned." To illustrate this point, there was a time when the B.B. 2. gladly supplied photographs of their stations' B.B.C. Applies for Entertainment Licence. apparatus, etc., kit' publication without charge. To -day In a recent issue of the Daily Telegraph we read that a Photographic Department takes the B.B.C. had applied fora music all E.B.C. photographs for publi- and danc ng licence in respect of catio i, and a reproduction fee' is In This Issue their new giant studio, which, if payable by any paper reproduc- granted, would permit the B.B.C. ing them. Special numbers of D.C. BAND -PASS FIVE. to admit the public on payment of B.B. 2. publications are produced MUSIC ON THE TAPE. an admiss.on fee. It was explained as ar excuse for charging a higher BEL -CANTO RADIO -GRAMOPHONE. by the B.B.C. that they wished rate for copies, and in every CORRECT AERIAL COUPLING. to obtain the necessary powers to direction we see the grasping at- CURRENT TOPICS. use them only if thought fit. .In titud : which the Corporation is LABORATORY TEST ON APPARATUS. other words, they wish to have at culti'ating. their disposal a new of BROADCAST BREVITIES. source Is it surprising, therefore, that revenue b be gained in competi- we should look with suspicion on THEORY OF THE H.F. TRANSFORMER. tion with the existing entertain- every fresh move which the TESTS ON DARIO VALVES. ment industry, and to create a pre- B.B. 2. takes? We have no LETTERS TO THE EDITOR. cedent to enable them to extend quarrel with the Corporation in READERS' PROBLEMS. these activities as and when they endeavouring to advance its please in the future. A 7

www.americanradiohistory.com roO Wt@Ilßgg FEBRUARY 4th, 193r. WopIlt

D. C. BAND -FASS ffIVE 11...1111.t10.... A Four stage Long Range Screen -grid Receiver. By L. E. T. BRANCH, B.Sc.

THOSE who possess direct current mains in their valves, and since this anode current is ro milliamperes, houses have at their command a very convenient the current flowing along the wire W is 0.25 ampere and cheap source of energy supply for wireless when that in the wire X is 0.24 ampere. purposes. If the voltage is sufficient to operate a fairly A red signal light G, provides an easy means of indi- generous output stage they can congratulate themselves cating whether the set is "on," and also acts as a fuse. on being able to avoid incurring the cost of a heavy Two green signal lights G, and G, are incorporated in mains transformer and rectifier. The present receiver the filament circuit, and by ganged switches show clearly is designed 'for operation on any D.C. supply from 200 whether the receiver is set for " medium waves," to 250 volts, push -pull being adopted in the output " long waves," or " gramophone." These lights were stage in order to obtain, among its other advantages, included in the first place almost as an experiment, a good undistorted output, without employing exces- and it was only during subsequent manipulation of the sively high voltages. The filaments of all the valves receiver that it was discovered that they are a real are arranged in series, which convenience and well worth has the advantage of per- SPECIFICATION. incorporating, especially as mitting the removal of any their cost is All Mail., D.C. Receiver with two Screen -grid Stases. quité small. valve from its socket while One control operates HIGH SELECTIVITY by means of ganged circuits and all the the set is working without inductively coupled pre -selector. switches. It will be noticed doing any damage to other POWER GRID DETECTION giving good sensitivity and that if the filament current valves. high quality. is followed in from the nega- The filaments of the out- PUSH -PULL OUTPUT affording adequate power output tive supply it first passes put valves are rated at 0.25 with silent background. through the push - pull ampere, and one of them is PRE -H.F. VOLUME CONTROL, which together with pre - valves and then to the first shunted with a resistance selector, minimises cross modulation. H.F. valve, next to the which by- passes the high - GANGED WAVE- CHANGE SWITCHING. second H.F. valve, and, tension current of the other PROVISION FOR GRAMOPHONE PICK -UP. finally, to the detector. one. TWO DIAL CONTROL. This arrangement is very This is more important convenient in the general than might otherwise be thought. We are so used to design of the circuit, and, among other things, has the feeding valve filaments in terms of the applied voltage, merit of confining heavy speech currents to the output that we are liable to forget that a Io per cent. increase stage. It also enables a maximum high- tension voltage in current means a considerably excessive voltage is to be applied to the output valves. An external resist- produced across the filament, and wòuld, no doubt, ance is necessary to absorb the unwanted voltage asso- damage the valve. It will be noticed that the filament ciated with the filaments. The current carried by this of the valve V, is in the negative H.T. return lead of resistance is 0.24 ampere, but since the voltage to be the valve V4, and for this reason the anode current of absorbed by it will depend upon the mains voltage, this V4 is by- passed through the resistance R,. The other latter must be taken into account when estimating the valves are shunted with suitable resistances because value of resistance required. their filament current consumptions are less than 0.25 The method of detection adopted is the well -known ampere. Here again the anode current of the detector power -grid " employing a very low impedance valve valve is passed through the filaments of both the H.F. with approximately 140 volts on the anode, and a posi- A

www.americanradiohistory.com FEBRUARY 4th, 1931. Wt@ilsoo 137 WoAcl D.0 E and -pass Five. - transformers is carefully avoide1, and in this way the tive bias of half the filament voltage of the valve en- tuning condensers gang correctly, although the step -up suring a linear characteristic. ratio of the first medium -wave transformer is approxi- It will be noticed that all the anode current passes mately I : 4, and the ratio of the second transformer is through the primary of the transformer. The AF5C I : 2. will s :and up to 3o mA. without mechanical breakdown, A high- frequency transformer having a high step -up while at 9 mA. the inductance is about 70 henrys, ratio r imparts considerable selectivity, because the whict., with a valve of 8,500 ohms impedance, gives tuning condenser causes a large capacitative load to be excellent low -note reproduction, a 5o cycle note being thrown across the primary, this load being C x r2 where repro iuced at 95 per cent. of its full value. Since the C is the capacity actually in use across the secondary. anode current flowing when no signals are being re- This large effective capacity can be considered as pro- ceived is 9 -IO mA., the efficiency of detection is high. viding a very free path for the untuned, and, therefore, Furl er, the L.F. amplification is such that, since the unwanted signals. Two condensers are exceedingly easy outp>_ t valves overload at about the same time as the to gang, and great accuracy results with the expenditure detec :or, the quality of the reception is of the highest of very little effort. order. In fact, the whole set is designed in order to The band -pass filter is of the inductively coupled type give first -class quality from the local station, good quality which provides fairly constant Feak separation over the

15 R16 EXTERNAL RESISTANCE r-- V m Re

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R1 R2 R6 560.000 POTENTIOMETER 02 w 15 164 L

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FI ;. 1. -The circuit diagram. C1, C2, C;1, C1, C,5, C,, I mfd. ; C7, C , C6, C15, 0.1 mfd. ; C11, Cr,, 0.001 med. ; C11, 0.0003 mfd. ; C11, 0.1001 mfd.; C15, 8 mfd. ; C17, 0.003 mfd. RI, R2, 0 to 50 ohms ; R3, 0 to 30 ohms ; R4, 0 to 400 ohms ; R7, R , 20,000 ohs,m centre ta wed; R7, R8, 25,000 ohms ; R9, 100,000 ohms; R10, R11, 60,000 ohms, centre tapped ; R12, 50,000 ohms ; R13, 250,000 ohms ; RI 1, R131 10,000 ohms. C16 see text. frorr distant stations coupled with good range and whole tuning scale. The amount of inductive coupling selectivity. between the band -pass coils on both wavelength ranges 01 the two H.F. stages, one is sharply tuned, while depends solely upon their distance apart and the rela- the )they is more flatly tuned, and since for local re- tionship between their axes, as described in last week's cept on the sharp one is preferably cut right out, the issue of The Wireless World. resu t is of the best when we remember that a band - The medium -wave coils of the present set consist each pass filter is included before the first H.F. valve. The of 66 turns wound upon 21n., i.e., 5 cm. diameter H.F. valves are transformer coupled, fairly efficient formers, so arranged that d can be either 8.5 cm. or coils being used in order to employ a high step -up ratio lo cm. The cross -sectional area of a 2 in. coil is 20 in one stage, thus obtaining high selectivity. It might sq. cm., hence the inductive coupling when the distance be e.rgued that good " coils are not necessary in a between the centres, d =8.=; cm., is rece ver of this kind, but in this particular case they A2N2 202 X 662 2.85 microhenrys.Y are lust as easy to wind as less efficient ones. Capacity moo d3 Imo X 8.53 5 coupling between the primary and secondary of the This is sufficient coupling to retain well Ire-

www.americanradiohistory.com ro8 Wpsiligoo FEBRUARY ¢th, 193r. WcpIlcD1 r--3Y; 5i 3 2 3z r

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E D F - L _ J J -4J J J J J J -V 35i3 3a/a" 37; ' 3 %- 3b 36 35/8- 3578---.- 30- 53/4" 33/4------1941 2%; 2-. 9tb%a r; _+J 4 0 3J. 190 tJ J +J o o AJ o- s }J -ßJ-f - J Dimensional data of the panel and terminal strip. quencies up to 5,000 cycles. When d=io cm., the is wound exactly mid -way between the coils. In this coupling is 1.75 microhenrys, and the high notes are way the mutual inductance is reduced to approximately still present to great advantage, while selectivity is loo microhenrys, and while a little signal strength is improved. The coils are arranged so that in a moment lost, the selectivity is increased. one can change from one value to the other. The medium -wave aerial coil of the band -pass filter On the long waves A =32 sq. cm., N = 200, while d has a small primary winding to which the aerial is con- 2A2N2 is fixed at 7.5 cm. Hence, the coupling is nected. This primary is connected to a tapping on the ,,00oxd3 long -wave coil in order that by a simple switching oper- 2 x 322 x 2002_ 195 microhenrys, which ensures adequate ation the ratio of the turns in the aerial to those of the I000 X 7.53 whole coil remains the same on long and medium waves, reproduction of the high audio -frequencies. Preferably, thus ensuring that the capacitative load thrown by the a single short -circuited turn of fairly heavy gauge wire aerial across the tuning condenser is the same on both

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www.americanradiohistory.com FEBRUARY 41n, 1931. Wfip®Ilmo ro9 WoAcol D.C..3and -pass Five. - denser C16 of 8 mfd. for the detector anode feed. Since

wavelength ranges. With an outdoor aerial it . is advis- the 2,000 ohm resistance 12,6 rnust be retained in the able to use an o.00i mfd. condenser in the aerial lead anode feed in order to drop the voltage correctly, it to isolate this latter from the mains. This component is is advisable to be on the safe side and use a 4 mfd. if shown as CH in dotted lines. it is not desired to incur the expense of 8 mfd. It should be noticed that for tuning the high- frequency The value of 2,000 ohms given in the present set is transformers, condensers of 0.aoo3 mfd. capacity are to be used only when the mains supply is 22o volts preferably used. The secondary inductances of the trans - or less. When it is above this amount a 4,000 ohm form rs are high, and the result is to avoid sharp tuning resistance should be used. Since the feed -resistance is and bad high -note loss. now doubled, the same decoupl ng effect is obtained by No grid -bias is used on the high -frequency valves, halving the condenser so that now 4 mfd. is ample, consequently, only the new Cossor screen -grid valves' while 2 mfd. can be used if desired. can De employed, since grid current does not flow in Two large chokes CH, and CH, are shown for smooth- these valves until the grid is one volt positive. The ing the mains supply. It is found that unless the mains 410 a eawasr om

Showing the ganged pre- selector with coils coupled by mutual inductance. volt me control is pre -H.F., and takes the form of a are particularly rough only one of these need be re- grid potentiometer on the input of the first high -fre- tained. For positively earthed mains simply omit CH, quency valve. The 50,000 ohm fixed resistance which and short the wires which go to it. For negatively is connected from the arm to the high -potential end of earthed mains we must retain CH but we cannot just the potentiometer, prevents the series grid- resistance omit the choke CH,. The voltage drop across this latter from becoming too great, and, at the same time, the is 18 volts, and contributes to the grid bias for the tuning -coil is practically not damped at all when full output valves. Hence, if CH, is omitted, a resistance volume is required on a distance station. Also when of 6o ohms must be used in ias place. When particu- the set is in its most sensitive condition the control is larly rough mains, for instance, where there mercury- more gradual than when the 50,000 ohm resistance is arc rectifiers are employed to energise this receiver, it orni tted. is suggested that both chokes be retained, and, prefer- When the push -pull valves are reasonably well ably, another 8 mfd. condenser should be connected matched it is not necessary to use a decoupling con- across the points PQ or across RS. The grid bias obtained by the valve V, when the ' See The Wireless World, Sept. 10th, 1930, p. 250. choke CH, of 6o ohms resistance, or in its place, a 6o

A 1r

www.americanradiohistory.com IIO Wfinhoo FEBRUARY 'MAC 4th, 1931.

LIST OF PARTS. 1 Bakelised board front panel, 30 x 9 x 3 /16in. 1 Condenser, 0.0001 mfd. with grid leak clip (Graham -Parish). 1 Bakelised board terminal strip, 30 x 2 x 3/16 in. 1 Condenser, 0.001 mfd. (Graham- Farish). 1 Baseboard, 30 x 121in.. 5 -ply. 1 Condenser, 0.0003 mfd. (Graham -Parish). 2 Variable condensers, 0.0005 mfd. (Polar Unirersal). 1 Condenser, 0.003 mfd. (T.C.C. Flat " S " Type). 2 Variable condensers, 0.0003 mfd. (Polar Universal). 6 Condensers, 1 mfd. 250v. D.C. working (T.C.C. Type 64). 2 Reduction gear dials and station logs (Burndept Ethorernu,r). 4 Condensers, 0.1 mfd. 250v. D.C. working (T.C.C. Type 64). 1 Mains switch (Bulgin S.56). 1 Condenser, 8 mfds. 500r. A.C. test, 240 p. working (Hydra). 1 Potentiometer, 500,000 ohms (Rothermel Centralab). 1 Condenser, 8 mfds. 500v. A.C. test, 240 v. working (Hydra) (Cis, see text). 2 Switches, 3 -pole (Utility W. 190/3). 1 H.F. choke (Climax). 1 Switch, 2 -pole (Utility W. 19012). 2 Semi- variable condensers, 0.0001 mfd. 0.000005 mfd. (Formo Type 1 Knob and rods for switches. F.). 3 Flush signal lamp holders (Bulgin D.9). 1 Push -pull transformer (Ferranti APSc). 2 Bulbs, 0.25 amp. (Bulgin Type M). 1 Push -pull output transformer, double ratio (Varley DP7). 1 Bulb. 0.3 amp. (Bulgin Type N). 1 L.F. choke (Savage 0127G). If mains are rough a double choke 1 Single closed circuit jack (Igranic P.62). assembly will be required (Savage D /27G). 1 Universal plug 2 (Igranic P.40). 2 Mains terminals, aerial and earth (Clix). Screening boxes, 9f x 51 x 8 jin. (Magnum). 1 Batten type lampholder (G.E,O. Pattern F1080) 2 Pre -set resistors, 50 ohms (Igranic 2241118). 1 Small plug and socket (Bulgin P.18). 1 Pre -set resistor, 30 ohms (Igranic 2240117). 1 Grid bias battery, 4) volts (Ever Ready UW8). 1 Potentiometer, 400 ohms (Igranic 2241/22). 2 Ebonite formers, bin. length x 2)in. dia., 8- ribbed (Becol). .5 Valve holders (Burton). Bakelised formers, 3 1-in. x Sin. dia. 2 Resistances, 60,000 ohms, centre -tapped (Coleco?). 2 Bakelised formers, 3 jin. x 2in. dia. 2 Resistances, 20,000 ohms, centre -tapped (Colrern). 1 Valve screen, 2)in. long x 2.5116in. dia.. (The "Loud Speaker" Co., 1 Resistance, 50,000 ohms and holders (Graham-Farish). Ltd., 2, Palmer Street, Westminster, London, S.W.1.) 2 Resistances, 25,000 ohms and holders (Graham -Farish). 14 Pins and sockets, for coils only (Clix). 1 Resistance; 100,000 ohms and holders (Graham -Parish). 14 Small terminals, for coils only (Clix). 2 Anode resistances. 10,000 ohms and holders (Ferranti). 1 Panel bracket. 1 Anode resistance, 2,000 or 4,000 ohms (see text) and holder (Ferranti). Wire, screws, wool, ebonite, steering, etc., etc. 1 Grid leak, 0.25 naegohm. Approximate cost, £18. .In the "List of Parts" included in the descriptions of THE WIRELESS WORLD receivers are detailed the components actually used by the designer and illustrated iu the photographs of the instruments, Where the designer considers it necessary that particular components should be used in preference to others, these components are mentioned in the article itself. in all other cases the constructor can use his discretion as to the choice of components, provided they are of equal quality to those listed and that he takes into consideration in the dimensions and layout of the set any variations in the size of alternative components he may use. ohm resistance, is used, is the voltage dropped through volts appears to make no difference whatever. There it, namely, i8 volts plus the 6 volts dropped through is no way of avoiding the use of this battery unless. the the valve V5. The bias battery B, therefore, has to be secondary of the transformer be wound in two separate inserted to increase the bias on the valve V. Although sections. However, it is a small item which requires B should, theoretically, be 6 volts, so that each valve replenishing so infrequently that it need cause no is biased to -24, actually a 4f-volt battery is used concern. because it is easy to procure, and the difference of i¡ (To be concluded.)

Finding Faults. " Touring the Tower of Babel." A talk on the discovery and remedying of The question of whether the listener can faults in receiving sets provided an interest- learn foreign languages via ether was discussed ing evening at the last meeting of the Wembley CLUB NEWS. at the last meeting of the South Croydon and Wireless Society. District Radio Society, and the conclusion Mr. Wallis dealt at some length with a reached was that the first essential was a two simple method for testing the various parts A Record Membership? screen-grid high- frequency stage set with a of the receiving set by the use of a neon tube. A membership of one hundred and sixty was frame or indoor aerial. Mr. Hon. Secretary, H. E. Comben, B.Sc., reported by the bon. secretary at the annual The Chairman said that. despite many difi) 24, Park Lane, Wembley. general meeting of the Bristol and District culties, there were no doubt many occasions 0000 Radio and Television Society, held at the Uni- when a student of languages could learn muck Preparing for a Change -over. versity on Friday, January 16th, 1931. It was by a tour of the ether's Tower of Babel. also reported that the attendance at the weekly Hon. Secretary, E. Although Muswell Still is not yet cm A.C. lectures, Mr. L. Cumbers, 14, Camp- mains it is certain that the district will demonstrations and debates was den Road, S. Croydon. steadily increasing. a. eventually be changed over from its present All D.C. supply. Thus the lecture given by Mr. local radio enthusiasts are welcome, and o000 E. B.Sc., A.C.G.I., are invited to join the society. Full particulars J. Wyborn, on Wednesday, can be obtained from the January 14th, was of no small interest to mem- hon. secretary, Mr. Precept and Practice. S. T. Jordan, 1, Myrtle Road, Cotham, Bristol. bers of the Afuswell Hill and District Radio " Good Quality Broadcast Reception " was Society, preparing then, as it were, for the 0000' the title of a lecture and demonstration at. the poiyer to come - namely, an A.C. supply! Mr. last meeting of 'the North Middlesex Radio Wyborn, who represents Messrs. E. K. Cole, Club's New Apparatus. Society, the lecturers being Messrs. Young and Ltd., gave an absorbing talk on the design and The Hackney Radio and Physical Society held Laister. its function of mains transformers. His remarks ninth Annual General Meeting on January Atr. Young first dealt with the principles in- about smoothing chokes were particularly ap- 12th. volved in delivering from the loud speaker a plicable to D.C. practice. the Society's report showed that the year faithful " copy " of the sounds radiated from 0000 1930 has been a very successful one. A notable the broadcasting stations. Re described a cir- fact was that all the lectures and talks during cuit incorporating a power -grid detector, a " Distance No Object." the year were given by members. Other activi- The lecture small power valve being used as a detector; was accompanied by a series of ties of the Society included a number of dell every precaution had been taken to eliminate lantern slides depicting in diagrammatic form days, and a dinner and dance at the Talbot sources of the operation of mains sets, distortion. transformers, and Restaurant, E.C. Mr. Laister's part in the programme was to chokes. .At the conclusion of his address Mr. The Society's apparatus at headquarters has demonstrate a receiver which he had constructed Wyborn demonstrated the Ekco three -valve been modernised during the past few weeks by screened -grid mains set, on the lines laid down by.Mr. Young. The re- which utilises only a Mr. G. V. Colle, and, thanks to Mr. Frederick shlts certainly justified the care taken. No short piece of Ilex as an aerial. Operated off Squire, the Society has the use of a very fine attempt was made to receive distant stations, the D.C. mains, this receiver showed its capa- moving -coil speaker'. but luckily there was some good test material bilities 'amazingly well. Brookmans Park, 10 The Society holds its meetings at the Hack- available from Brookmans Park. miles away, was easily eliminated, and distant ney Electricity Showrooms, Lower Clayton European stations Among the points demonstrated was the brought in at full speaker Road, E.5, .every Monday evening at 8 o'clock. effect of changing the value of the grid -leak strength. An Ekco moving -coil speaker was An excellent syllabus has been prepared by the in the detector circuit. When the., grid-leak used. Tise set showed itself to be equally Technical Committee for the coming year, and in the detector was replaced by the once almost capable of handling enormous volume from the those desirous of becoming members will receive universal 2 megohm leak, a marked deterioration local station. a cordial welcome. of quality was apparent. Hon. Secretary, Mr. C. 39, J. Witt, Coniston Hon. Secretary, Mr. G. E. Sandy, 4, Meadway, Ilon. Secretary. Mr. E. H. Laister, " Wind - Road, N.10 Raynes Park, S.W. flowers," Church Hill, 5.21 A I2

www.americanradiohistory.com FEBRUARY 4th, 7J37. Wllpds@o MpIld IIT IMUSOC''"

The tie illustration shows a sprsimen of the " Sel - enop°one" record taps, actuel size. ÖTl TI-lE TAPE

The " Selenophone" : a Light Cell Gramophone. By A CORRESPONDENT. EVERAL disturbing threats have been levelled at ' ` transverse " system of recording, as opposed to the the orthodox disc method of recording in the last probably more popular " intensity " system, in which year or two, but possibly a direct challenge the sound strip is occupied by a streak of unvarying to its supremacy in the realm of sound reproduction width, but varying blackness, corresponding with the comes from a new device manufactured in Vienna and changes in frequency. In the transverse system known the as the "Selenophone. " For the " Selenophone " sound record is represented on the photographic film apps ars to overcome the primary drawback of the disc, as a streak of uniform blackness, " jagged " at one edge, viz., brevity of content. There is literally no limit to each " jag " representing one cycle of the sound wave ; the ength of performance which can be given by the the closeness with which one jag follows its neighbour cont: nuous strip of paper which comprises the " record " gives the frequency of sound, while its loudness is in this new instrument. determined by the size of the jags. About a year ago the Austrian Selenophone Com- pany, whose " talkie " system, developed by Thirring Galvanometer Recording and Selenium Reproducer. and Richtera, is well known on the Continent, demon- A simple and effective device provides the jagged strated to a select circle a small recording and repro- edge in the Selenophone system. The amplified micro- ducing instrument embodying the principles employed phone currents pass along the :,tretched metallic thread in film work, but omitting the picture side of the enter- of a string galvanometer. This thread, about i / io mm. tainment. The results were then most promising, and in diameter and 20 mm. long, is stretched so tightly that the :'ew difficulties which presented themselves are now its natural frequency is of the order of 15,000 cycles reported to have been overcome, rendering it now pos- per second. It lies in a very powerful magnetic field, sible to market a home- reproducing instrument compar- in an air gap of only about o.f mm. The poles of the able in performance with that of the gramophone, and electromagnet are drilled through with a hole in which using cheap paper strip records capable of providing a system of lenses is fitted which throws the image of a abot.t 7o minutes' performance per Soo -metre drum. long narrow slot on to the film. The thread is so The Selenophone method employs the well -known arranged in relation to the slct that when the former is at rest it prevents half the image of the latter from reaching the film. Owing to the small angle between the directions of thread and image, a very small move- ment of the thread will cut off or uncover quite a lot of the image. In this way the " jagged edge streak " is prcduced along the sound strip. We now come to the re- production of the sound, the special method of doing \vh ch gives the system its name. Here we find, for once, no sign of the ubiqui- tous photo -electric cell ; its place is taken by the special 711 rring selenium cell. As is «e l known, the selenium DEMONSTRATING THE " SELENOPHONE." The tl e presence new light cell gramophone under test in cell has so of officials of the Austrian State Broadcasting Company. On the left is much inertia or M. Oskar Cseika, the Director- Generel. " lag " that at the higher A 13

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112 - "Wflpdagg FEBRUARY 4th, 1931. WHIZ. Music on the Tape. - of immense value to the blind. Last, but not least, the frequencies the amplitude of the reproduced sound falls transport of paper rolls requires none of the anxious off seriously. This difficulty is said to be effectively care necessary in the despatch of gramophone records.

compensated for by the customaryy method of intro - _ 0 O O ducinr, a resistance -capacity input filter. The back- ground noise, usually troublesome with selenium cells and due to imperfect contact between the selenium and Books Received. V,f ì)(i1 y. its elecfrodes, is not noticeable with his cell, and the The Physical Lab°,at r collected Researches, Vol. 15II. 1930.-A collection of authoritative articles on vari- light -sensitive surface is made so small that a very ous wireless subjects which have occupied the attention of the concentrated beam of light can be focused on it. In this N.P.L. during the past year. The writers include F. M. Cole- way a high sensitivity is obtained, no very bright source brook, R. M. Wilmotte, H. A. Thomas, D. W. Dye, E. H. M. and of light is and a few stages of Amplification Rayner, S. W. Melsom, A. H. Arnold, L. Hartshorn, required, J. E. P. L. Vigoureux. Published by H. M. Stationery Office, are needed. price £1 net. The use of actual film for home -reproduction purposes The Radio Amateur's Handbook (7th Edition). A manual of would tend to increase costs ; the Selenophone Corn- amateur short -wave radiotelegraphic communication, including pany has, therefore, adopted " records" reproduced on narrow paper tape by means of the offset printing method in which an inked photographic impression of the original on a metal plate is transferred to a rubber " blanket " and thence to the paper. Since the paper and the printing process are cheap, such re- cords fulfil the requirements of a mass -produced article. It must be assumed that, in the case of paper records, the light beam is reflected from the paper, not passed through it as is the manner with a film. The tape, which is A close-up" of the " Selenophone Junior," specially designed for home use. 12 mm. wide, carries four separate sound tracks, which are automatically switched general advice to the amateur, practical articles on receivers, in, one after the other, without interruption. Each roll transmitters, aerials, .power supply, operating a station, the " Q " code, miscellaneous abbreviations, and a quantity of other is 30o metres in length ; hence we have no fewer than useful information. Pp. 218, with numerous illustrations 1,2oo metres of record, representing a playing time of and diagrams. Published by the American Radio Relay about 35 minutes. And it must be remembered that League, Inc., Hartford, Conn., U.S.A., price $1. the reverse side of the paper can also be printed on. Radio- Kurzaellen und ihre Eigenschaften, by Franz Anderle. When we recollect that the ordinary gramophone disc A textbook on the nature and characteristics of short waves, plays for only about three minutes, the extraordinary with chapters on reflectors, aerials, crystal control, and other kindred subjects. Pp. 122, 160 diagrams and illustrations, of become clear, even with possibilities the new instrument and 3 maps. Published by Franz Deuticke, Vienna and Leipzig. ignoring the fact that the paper tape is not subjected to Price M.6 in paper cover, or M.8.40 in cloth. the wear caused by a needle, and, also, that a complete Der Bau Von Uberlagerungsempfängern (Building the Super- roll playing for seventy minutes can be produced at a heterodyne Receiver), by Dr. Walter Daudt. The theory of cost not exceeding that of one gramophone disc. the superheterodyne and practical directions for building a typi- The reproducer itself is not unlike the ordinary gramo- cal receiver. Pp. 48, with 40 illustrations and diagrams, and a phone, though it possesses two turntables, since the tape sheet of the symbols used in wireless diagrams. Published by Rothgiesser und Diesing A.G., Berlin, price Rm.1.50. must be rolled on to an empty drum from a full one. The " Selenophone Junior," as it is called, passes its Die Elektrische Schallplatten- Wiedergabe (Electrical Record it need hardly be said Reproduction), by Manfred von Ardenne, with descriptions and output to a loud speaker, and illustrations of various gramophone pick -ups and other apparatus that the whole apparatus can be operated from the connected with sound reproduction. Pp. 80, with 69 illustra- mains. tions and diagrams.. Published by Rothgiesser und Diesing, If it can perform all that is claimed for it, the new A.G., Berlin. Price Rdi.1.70. instrument opens up a new field of possibilities. The Télérision et Phototélégranbiie, Principes Fondamentaux, by recording of complete operas, symphony concerts, E. Aisberg, with a preface by E. Belin. A popular explanation of the principles and practice of phototelegraphy and television, speeches, and broadcast dramas becomes a simple with an appendix describing the Belin system. Pp. 172, with matter. A weekly spoken news bulletin or magazine 82 illustrations and diagrams. Published by Etienne Chiron, also becomes possible, a development which would be Paris, price frs. 10. A I4

www.americanradiohistory.com FEBRUARY 4th, 1931. WIpg@g0 rr3 WOpl1d.

An A.C. Mains Receiver - Electrically Driven with - Turntable.

HERE is no real reason why the changing. A differential condenser, ever-popular H.F.- det. -L.F. with its moving vanes at earth poten- three -valve circuit, which has tial, acts as a reaction control. proved so successful in receivers in- Power -grid re:tification is a feature tended purely for broadcast reception, of the receiver. The detector works shoulc not be adopted as the basis of a with over 120 volts on its plate, and :adio- ,gramophone. Its range is ,wide passes a current of about 6 milliam- enouga for the average listener's needs, and, as a gramo- peres. Thanks to the fact that tie grid leak has natur- phone reproducer, sufficient magnification for domestic ally a low value, it becomes possible to insert the pick- purpo ¡es is obtainable from the detector, converted into up at the low -potential end of the circuit, so that impulses an L.:1 . amplifier, in conjunction with the output valve. are fed to the grid through the leak resistance. In this Even if a comparatively insensitive pick-up be chosen position there is less risk of intrcducing parasitic noises there will seldom be complaints on the score of insuffi- through the pick -up leads, etc., and switching is sim- cient volume, particularly if efficient modern valves are plified. used. The detector is coupled to the output stage by means The Bel -Canto Radio -Gramophone, with which we are of an auto -transformer (actually a double -wound R.I. here concerned, is an example of this circuit arrange- " Hypermite " component, with primary and secondary ment. Starting at the input end, it will be seen that joined in series), which is paralle'. -fed through the usual an unusual method of resistance-capacity filter. An volume control, in the form H.F. stopping resistance is of a p )tentiometer, actually SPECIFICATION. inserted in series with the with a resistance of 25,000 CIRCUIT : Screen grid H.F. valve, coupled by choke -fed output valve grid, and the ohms, is adopted. This tuned -grid circuit to a power grid detector. Single L.F. loud speaker is directly con- system has the advantage stage, coupled by resistance fed auto -transformer. Full - wave power rectifying valve. nected in its anode circuit. that r: does not introduce CONTROLS : (1) Independent tuning of input and inter -valve A Bel -Canto loud speaker is any serious disturbance of circuits. (2) Combined wave -range and radio -gramophone fitted ; this instrument em- change-over switch. (3) Reaction. tuning ; although it makes (4) H.F. input volume bodies a large, lightly sup- no cor tribution towards the control. (5) Gramophone volume control. (6) Mains on -off switch. ported cone driven by a inherent selectivity of the GENERAL : A.C. mains operation with inside or outside balan :ed -armature move- set, it (foes allow interference aerial. H.F. input potentiometer. Direct loud speaker ment. to be avoided by limitation connection. Anode current supply is of input. PRICE: £37 108. Makers: Bel -Canto Radio, Ltd., 34 -36, Oxford Street, London, W.1. through a Mullard D.W.r Indi -ectly heated A.C. full -wave rectifying valve ; valves are used throughout, the srnóothing and feed cir- and nogative bias is applied to the grids by inserting cuits are straightforward and simple. Tappings from a resistance (with the necessary shunt condenser) in the power transformer primary are led to pins mounted each cathode lead. radially round a centre contact, and the appropriate A tuned-grid coupling, with parallel choke feed, is connection to suit the user's supply voltage is easily employed for the H.F. stage. Medium- and long -wave made by means of a special dot.ble- contact socket. coils are connected in series, and the latter are short- A universal motor of the commitator type is fitted fox circuited by a switch in the usual way for waveband driving the turntable, and an automatic stop is provided ; A 17

www.americanradiohistory.com I Ti MED®11 FEBRUARY 4th, 1931. W©u3llci.

AERIAL TUNING CONDENSER

11F TUNING CONDENSE2 //YPUT VOLUME CONTROL. poTENflomETER

AERIAL -GRID COIL H F COUPLING CO/L HFVALVE 54k'

POWER TRANSFORMER PR/MARY TAPP/NGS

OUTPUT VALVE AC/PI

DETECTOR VALVE 3541/

COIL SCREEN RECTIFYING VALVE DW/

WAVE-CHANGE SWITCH

REACT/ON CONDENSER

INPUT VOLUME CONTROL

AERIAL-GRID CO/L HFCOUPLING CO/LS CONNECTIONS CONNECTIONS

SMOOTHING CHOKE

TO LOUD SPEAKER

POWER TRANSFORMER PegfARY TAPP/NOS

L F TRANSFORMER POWER TRANSFORMER

METAL CHASSIS

SMOOTHING CONDENSERS

Upper and lower sides of the Bel-Canto receiver chassis. One of the coil screens is removed. A 18

www.americanradiohistory.com FEBRUARY 4th, r();r. Wap@hoo r/5 Woa.c1 Bel- '7anto Model R.G.4 Radio- Gramophone.- of the tuning range. There is lardly any reaction over- this must be in the " off " position when the set is used lap on the medium -wave side, and very little on the long for radio reception, as the main on -off switch controls waves. the current supply to both motor and power transformer Sensitivity is ample to ensure the reception of most prir cary. transmissions with a real programme value, and is about C instructional details of the chassis are clearly shown up to the usual standard of a set of its class with A.C. in the accompanying illustration. The receiver com- valves. Similarly, selectivity is as good as can be ex- ponHnts, with the exception of tuning and reaction con - pected from two tuned circuits, and does not seem to den ;ers, switch and volume -control resistance, are be adversely affected by the potentiometer volume con- mounted on either side of a large aluminium sheet, which trol. A short indoor aerial is quite adequate except occupies the full width and depth of the cabinet, and where consistent long- distance reception is desired. can be withdrawn after removing a few wires. With regard to the capabilities of the set as a repro- Edgewise thumb -operated tuning drums seem to be ducer of gramophone records, it was found that there is hardly the most convenient form of condenser controls a good response over a frequency range from slightly when they are mounted on the panel of an upright over ioo cycles to 3,000 cycles per second. After this cab net receiver of this type, but one soon gets accus- point a gradual falling off is observable, but there is

Schematic circuit diagram. A volume -control potentiometer (not shown) is embodied in the )ick -up carrier. toned to them. In any case, the receiver is not intended still a detectable output at 6,000 cycles. No resonances primarily for long- distance reception, and so the need are sufficiently marked to be unpleasant, or even to for operating the dials continuously is unlikely often force themselves to one's notice. to r rise. When radio signals are being received the perform- The aerial potentiometer makes a very satisfactory ance is substantially that described in the preceding para- volume control, although most of the signal strength graph ; the output at the higher audio -frequencies is well var ation is introduced as the knob is rotated through maintained, provided that reaction is not used to excess. first degrees the few from the " full-on " position. The In spite of the simplicity o f the smoothing arrange- pre: ;ence of the potentiometer resistance across a part of ment there is a commendable freedom from hum, and no the tuned coil has an entirely negligible effect in increas- complaint whatever can be registered on this score. ing circuit damping. The set handles very well, tuning A D.C. model, with indirectly heated valves con- anc reaction controls operating in a smooth and logical nected in series, is available at the same price. The manner. As the condenser drums are mounted side by general circuit arrangement a::rd layout is similar, but side , they may be rotated simultaneously over a section an 11VI.L.4 output valve is fitted. A tg

www.americanradiohistory.com iih Wfladato FEBRUARY 4th, 7931. NVopIldl

07)-RREC R At IIIIIIiINIIIIIIIil1111111111

uuillu!u1iuluil

COUPUN G

IIIIIIgI111111IÎ By W. T. COCKING. 1111111111111 1 The Efficiencies of the Band =pass Filter arid the Single Circuit Compared.

firHE aerial -tuning system is not only of the is shown in Fig. 4, and it is reasonable to expect that, greatest importance from the point of view of since there are two tuned circuits, each with certain selectivity, but it also determines to a large extent losses, the efficiency will be lower than that of the single the efficiency of a receiver. Owing to the unique pro - perties of a series- resonant circuit, it is one of the few places where a definite magnification can be obtained without the use of a valve. There is little doubt but that the usual single -tuned circuit, with tapped or " aperiodin " aerial coupling, is the most efficient of the many possible methods ; and it would be universally adopted were efficiency the only point to be taken into consideration. Under modern broadcasting conditions, however, the Fig. 2. -The equivalent electrical circuit of Fig. 1. L,, R., conflicting problems of selectivity and quality become Co represent the aerial inductance, resistance and capacity of almost greater importance. With the single -tuned respectively. Lt is the primary of the coupling transformer, while L, R and C are the inductance, resistance and capacity circuits of Fig. 1, (a) and (b), it is well known that of the tuned circuit. M is the mutual inductance between the windings, and E represents the voltage set up in the aerial by the lower the H.F. resistance of the tuning coil the a signal. greater is the efficiency and selectivity, but the poorer the quality. Since quality is of the utmost importance, circuit. It becomes of importance, therefore, to deter- a very low- resistance coil cannot be used, and it is then mine the relative efficiency of the two methods of coup - found that the selectivity becomes too low to prevent pling, in order that an estimate can be made of the trouble from " cross -modulation " and " beat- inter- extra amplification desirable when using a filter. ference." The Single Circuit. The remedy for these troubles lies in the use of the band -pass filter, with which sideband cutting can be There is little data available as to the efficiency of non -existent. The circuit of a capacity- coupled filter the single circuit, and so it is necessary to investigate this well -known circuit before proceeding to discuss the filter. In Fig. 2 is shown the equivalent electrical cir- cuit to that of Fig. z, in which the aerial inductance, capacity and resistance are represented by La, Ca, and Re, respectively. The voltage set up in the aerial by an incoming signal is represented by the generator E. Now, the complete expression giving the ratio of the voltage e, applied to the grid of the valve, to the volt- age E, set up in the aerial, is complex. It is well known, however, that the effect of the aerial resistance with the "aperiodic " aerial coupling is very small, and may vary within wide limits without affecting the results to an appreciable extent. It becomes permissible, there- fore, to assume that this resistance is zero, for the error so is in Fig. 1. -One of the most efficient aerial tuning circuits, the introduced considerably less than that found commonly used " aperiodic " coupling (a) and tapped aerial practice when an attempt is made to build the circuit coupling (b). A magnification of some 10 times is readily obtainable, with medium -efficiency coils. to conform with the calculated constants.

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www.americanradiohistory.com vv FEBRUARY 4th, 1931. p®Il®@0, 117 WopIld. Corre 7t Aerial Coupling.- It is worthy of note that the figures for magnification Ti- e tern magnification will be used to denote the calculated from the formula in the Appendix are directly ratio e /E, and this must not beconfused with the coil applicable to receiver design. It is merely necessary magnification, which is equal to 0,14R. Under the con - to multiply the magnification by the product of the ditio z of zero aerial resist- effective aerial height in ance, the formula (1) in the metres and the field strength App ;radix enables the mag- TO what extent must signal strength be sacrificed when replacing a single tuned circuit by a capacity band-pass in volts per metre in order nification at any frequency filter ? In this article calculations of aerial circuit efficiency to obtain the actual voltage to be readily calculated ; are made under different coupling conditions and the conflict- applied to the grid of the whil the magnification at ing requirements of selectivity, quality and signal strength first valve by a signal, and, are carefully examined. resonance is more easily therefore, to estimate the calculated from formula amount of H.F. amplifica- ti (2). The curves of Fig. 3 show the results to be ex- tion required in any given case. pect d in a typical case ; it is, of course, necessary to assume certain.- values for the coil and aerial constants, The Capacity Còupled Filter. and :hese will be found in Table I, together with details The circuit of a simple capacitatively- coupled band - of tl,e type of coil to which they may reasonably be pass filter is shown in Fig. 4, and formula (4) in the expe:ted to apply. Appendix allows its magnification to be calculated at TF e effect of varying the aerial coupling is well any frequency, and brought out by these two curves. Curve A is for a the simpler formula mutt al inductance to the aerial of 8.9 µH., and it will (5) at resonance only. he seen that, while the magnification at 1,500 kc. (200 It can easily be seen metr ?s) is a times, at 55o kc. (55o metres) it falls to that the efficiency of only 3 times. An increase in the coupling to 19.35 such a circuit depends µH. (curve B) reduces the magnification at 1,50o kc. not only upon the to 1c times, but increases it at 55o kc. to 5.4 times, and aerial coupling, but the efficiency over the whole broadcast band varies in also upon the filter nearly a 2 -I ratio instead of about 4 -1. coupling. This latter A further increase in the aerial coupling leads to in- is determined by con- creased efficiency at the lower frequencies, but only at siderations of the band the expense of both reduced efficiency and selectivity at width required, but the 1 igher frequencies. There is no point in giving curves with such a simple cir- Fig. 4. -The simple capaci- tatiiely coupled band -pass for greater degrees of coupling, therefore ; and, in prac- cuit this remains by filter. Although the efficiency tice, one usually compromises no is Icwer than with the single with a degree of coupling means uniform as circuit it offers very decided simil>.r to that of curve B. The magnification obtain- the tuning is varied advantages as regards both over the waveband. selectivity and quality. As a result, we should expect to find that the efficiency 14 varies more than that of a single circuit. That this is indeed the case can be seen frcm the curves of Fig. 5, 12 in which curve A is for a mutual inductance to the A aerial of 8.9 µH., and curve B for 19.35 µII. lo A comparison of these curves with those of Fig. 3 is B extremely interesting, and shows up several important 8 facts which have been noted in practice. In each case consider curve B, which represents the degree of coup- 6 ling, perhaps the most generally satisfactory and the most commonly used. At 1,00e kc. the filter magnifica- 4 TABLE 1. A 2 Aerial constants. Inductance .. 20 µH. Capacity .. .. 0.0002 mfd. 1300 1100 900 700 500 Resistance .. 0 FREQUENCY kC. Coil constants. Inductance 200 µH. Fig. 3. -The magnification obtainable with either circuit of Resistance at 1550 kc., 5:3 ohms. Fig. 1 can be seen from these curves. Curve A Is for a mutual Inductance to the aerial of 8.9 ,H., and curve B for 19.351 H., 750 kc., '7.5 ohms. a more usual value. Note the large magnification at the high 1,000 kc., 12 ohms. frequencies. 1,200 kc., 16 ohms.. 1,500 kc., 27.5 ohms. able with single circuit aerial coupling is too little appre- ciatel, for it is not usually understood that the ordinary Coil construction. 80 turns of No. 26 D.C.C. close wound on 2in. diameter aerial circuit gives a voltage step -up of some io times former. , without employing a valve.

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www.americanradiohistory.com I r, WApSIlSOO FEBRUARY 4th, 1931 WoEIlCI Correct Aerial Coupling.- frequencies, since the falling off is due to the filter lion is 6.5, which is about 70 per cent. of that of a coupling. In no way can the efficiency of the simple single circuit; at 55o kc., however, the filter magnifi- filter be improved without at the same time entirely cation is only 1.6, and the relative efficiency about 29 altering the band-pass characteristics. Of the two per cent. ; while at 1,500 kc. it is only 2.56 as com- methods of coupling, inductive is the better on the score pared with io for the single circuit, or a relative effi- of efficiency, since it does not suffer from low efficiency ciency of 25.6 per cent. That matters cannot be greatly at the high frequencies. Unfortunately, however, it is improved by an alteration in the aerial coupling can not very satisfactory, for at this part of the waveband be seen from an inspection of the curves ; for an altera- it is liable to become tion in the mutual inductance only results in increasing unselective. Progress the magnification at the high frequencies and reducing in filter design will 16 1111 it at the low, or vice versa. undoubtedly lie in 11 combining 14 Capacity Filter Less Efficient than Single Circuit. the good 1111r points to be found in t; RI 1111 The difference in shape of the curves of Figs. 3 and both of Y 12 types coup- z ER 1111 5 is of great interest, since it contains the clue to the ling -by the use of a 0 10 1111 reason for the low efficiency of the capacity filter at mixed capacity and á U/ cc NRIMMINNININI high frequencies. For all frequencies lower than about inductance coupler. a 1111 1,000 kc. the filter efficiency falls away at about the Before concluding w 8 a 11 same rate as that of the single circuit ; but, instead of this article it would be 1111 increasing for the higher frequencies, it again falls off. well to make some á 6 MEI á Now, the curve of Fig. 6 shows the peak separation mention of the com- 11Pi111111 with this filter, which has a coupling capacity of o.ó1:5 monly used circuit in A111111 mfd., and it will be seen that the two peaks coalesce at which the aerial is ri11111 95o kc. This, then, is the reason for the low efficiency connected through a I111 of the capacity filter at high frequencies ; the greatest small fixed condenser E11 0 =11111 efficiency is always obtained when the coupling is such directly to the upper 1000 900 800 700 600 500 is that the reactance of the coupling component equal end of the tuning FREQUENCY kc. to the H.F. resistance of the tuning coil. This is only coil. possible at one frequency, which, in this case, is about This method has Fig. 6. -The vacation of band- 1,000 kc. For frequencies lower than this the coupling the great width with frequency In a advantage capacity filter, with 200 pH., is greater than the optimum, and the double -peaked of simplicity of me- coils and a 0.015 mfd. coupling condenser. The two peaks turning curve is obtained ; the efficiency, however, does chanical construction, coalesce at 950 kc., and for not fall off rapidly. For frequencies higher frequencies higher than this than since the use of tapped the efficiency is reduced. (See 1,000 kc., however, the coupling is less than the opti- windings is obviated Fig. 5.) and the switching for the two wavebands is simplified to a great extent. It can be shown mathematically that this circuit is fundamentally the same as the " aperiodic " type ; for it can be redrawn as the circuit of Fig. 1(a) with the addi- tion of a fixed condenser in the aerial lead. The differ- ence between the two circuits, however, lies in the values of the different components. With the directly coupled circuit, the inductance of the hypothetical aerial winding is the same as that of the secondary, which is also equal to the mutual inductance between them. Series Aerial Condenser. Now it has been shown that the best value for the mutual inductance depends upon the aerial reactance ; in this directly coupled circuit the mutual inductance is fixed, and so to obtain the best results we alter the aerial reactance by means of the series condenser. In other words, with " aperiodic " coupling we have a more or Fig. 5. -The magnification obtainable with a capacity coupled less fixed value for the aerial reactance, and we adjust band -pass filter is shown by the above curves. Curve A Is the mutual inductance for the optimum results ; but for a mutual inductance to the aerial of 8.9 ,H., and curve B for 19.35 H. Note the falling off in efficiency below 1,000 kc., with the directly coupled circuit we have a fixed mutual which is due to the filter coupling. A capacity filter is con- siderably less efficient than a single circuit. inductance, and so we adjust the aerial reactance in order to obtain the best relation between them. mum, and the efficiency falls off very rapidly indeed, There are, of course, other considerations involved, while the double -peaked tuning curve disappears. for with the directly coupled circuit it is doubtful No alteration to the aerial circuit itself will improve whether it is justifiable to assume that the aerial resist- the magnification characteristics appreciably at the high ance is zero. Further considerations as to selectivity,

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www.americanradiohistory.com FEBRUARY 4th, 1931 WAITe®MT, Ir9 7A\ 0wild Correct Aerial Coupling. - and at resonance this becomes: and -.he apparent change of secondary reactance due to 11X, /C - .. (6) the aerial, will be different in the two cases, so that it 1/1w'M4R2 + A2 (R2 + X.n,2)21 is irr possible to decide which is the better circuit for and the optimum value of aerial courting is given by : general use without a detailed investigation into both. w11 = / rA (R` + Xm')1 . . .. (6) APPENDIX. V L

Wh an the aerial resistance is zero, the magnification of the where 111 = mutual inductance in henrys. ordins ry single tuned circuit with "aperiodic" aerial coupling, C = capacity of tuning condenser in farads. is giv m by : 11/C L = Inductance of tuning ccil in henrys. /E _ .. (I) L1 = inductance of primary winding of aerial coil .n -/L {wall'` - AB)' + A2R2] henrys. and a; resonance this becomes : M/C La = aerial inductance in henrys. a .. (2) Ca = aerial capacity in farads. A/[w'M' RZASJ -i- R H.F. resistance of tuning coil in ohms. and the optimum value of the aerial coupling is given by: = coM= VILA (3) Xrn = reactance in ohms of the filter coupling. 1 /wOe) Under the same conditions, the magnification of a two -stage and A = (cuLa (Li - l /wC) band -pass filter is given by : B = (wL - MX,n/C = 0, at resonance. .. (4) - e grid of the valve. V[{A (1t2 -} X,n' - B2) + Bw=1121= = voltage applied to the { 112(o02112 - 2AB)2] E = voltage induced in series with the aerial. r CDIII Events of the Week in Brief Review. NEWS OF OLYMPIA. GRID SCORES QUARTER- CENTURY. CHESS 3Y AMATEUR RADIO. A louble -size show is the promise for Dr. Lee de Forest has received con- Six inembe:s of the Sydney Chess Club the 1'131 Radio Exhibition. An official of gratulations on the twenty -fifth anni- have arranged to play a match by the I adio 1)lanufacturers' Association in- versary of his invention of the three - amateur wire.ess with six chess experts in formed The Tireless World that the ex- electrode valve. The first valve containing Otago, New Zealand. hibition in September will occupy the a grid, or third electrode, saw the light 0000 New Hall, as previously, and also the of day in January, 1906. GERMAN SETS IN BRITAIN. grourd, first, and second floors of the Em- 0000 According to a Berlin message, Great pire Hall. PRISON FOR " PIRATES." Britain was Germany's best " wireless Th: Association has, in addition, signed The Austrian postal authorities an- customer during 1930, taking radio, tele- a contract under which the Exhibitions of nounce that wireless listeners who fail to graphic, and telephonic equipment to the value 1932 and the four subsequent years will take out or renew licences are liable to of neatly £570,000.0000 be held in the Olympia Main Hall a month a fine of £14 or one month's imprison- ment. WIRELESS SCHOOL FOR BOYS. earlit r than has been the practice in the A boys' wireless school has been past. 0000 founded in Middlesbrough by the Rev. P.O. PROSECUTION FAILS. L. H. B. Staveley, vicar of St. Peter's An interesting case under the Wireless Church, and Sir. V. Leggett, emigration Act Nas heard by the Bridgwater magis- officer. Invitations to attend the first trates last week, when they dismissed a meeting, which was held on January 23rd, summons against Mr. Joseph Lowe for were extended to two boys of fourteen having worked a wireless receiver without years or more from each juvenile organ- a licsnce. Mr. Lowe proved to the satis- faction of the magistrates that his valve amplification equipment was used solely for the reproduction of gramophone recor ds. 0000 ITALIAN CONCERTS FOR AMERICA. Italy is to provide American listeners with a weekly musical programme, accord- ing to an arrangement entered into by the American National Broadcasting Company and the Italian broadcasting organisation. The concerts will be transmitted every Sum ay evening from the Rome station, now operating on 25.4 metres. 000 0 INTERNATIONAL RADIO SHOW. Bcrdeaux will hold an international wireless exhibition during October next. Manufacturers all over the world are in- vited to apply for particulars to the Syn- dica . des Radio- Electriciens de Bordeaux THE KITE SET. An officiai 48, - at the Radio Research Board's experimental s-aaon at et du Sud -Ouest, Cours d'Alsace Slough with one of the transmitter -equipped kites used for direction-finding tests. Lorraine, Bordeaux (Gironde). The padded valve is in the container at the end of the stick carrying the battery cells. 1 23

www.americanradiohistory.com 120 Wilp@Il@OO FEBRUARY 41h, 1931. Vvifild. isatiou in the town. A complete radio WIRELESS GOODS BY AIR. wave wireless transmitter and receiver course has been prepared lasting twelve Lnperial Airways, Ltd., report that which was specially installed for the months. nearly 2,000,000 wireless valves have now Prince's use on the Pacific Stearn Naviga- 0000 been carried in their machines between tion Company's liner, Oropesa, on which WIRELESS AND WINE. London and the Continent, in addition to he is travelling, will enable him still to To relieve the slump in the wine in- approximately £80,000 worth of loud maintain constant and direct communica- speakers. dustry. the Italian broadcasting authori- 0000 tion with this country through the short- ties are granting free advertising period-. wave wireless coast station at Portishead. THE LATE SIR WILLIAM BULL, in the programmes to wine merchants. BT., M.P. No ship fitted with short -wave wireless who will sing the praises of Bacchus in In few of the obituary notices which apparatus for communication with Eng- five languages, says a Twin corre- have appeared concerning the late Sir land has yet made the voyage along the spondent. William Bull, who died of January 23rd, South American coast which will be fol- 00430 lias any mention been made of lits close lowed by the Prince of \Vales. MUST WE CORK OUR AERIALS? association with broadcasting and the 0000 A resolution in favour of the compulsory wireless industry. Yet Sir William was "PIONEERS OF RADIO." Mayors, school- teachers, and parish " corking " of all wireless to one of the original directors of the British aerials pro- priests are among the enthusiasts in tect pigeons was passed unanimously at Broadcasting Company at its formation in France who are petitioning for the estab- a meeting of the General Council the 1922, and at the time of his death was of lishment of a body be National Union of President of the Radio Manufacturers' to called " The Shunt -distance Flyers, Pioneers of Radio. Its function held in Manchester on Association, a position he had ably held would January 24th. The be to supply resolution ran for many years. each village community with Speaking last week on behalf of the an experienced representative capable of " That the woe reiterate its demand for doctoring it to be made compulsory for all wireless 11. 1\1..\.. Capt. J. W. Barber, C.M.E., wireless sets and giving much - needed aerials to be corked, as the mortality of said : " During his service on the Board advice. sporting pigeons is heavily increasing owing 0000 to the large number of aerials that have LISTEN FOR WORLD -WIDE been erected and are likely to be erected TELEPHONY TESTS. on account of the opening of the Regional Throughout stations, which would bring back into favour the present month short- the crystal receiver." wave listeners all over the world are urged O000 to make a special effort to pick up a SPAIN'S NEW BROADCASTING special series of 31.35-metre telephony SYSTEM. transmissions from WIXAZ, the new ex- Within a month Spain should be eiju perimental station of the Westinghouse ing the benefits of a reorganised broad Electric and Manufacturing Company at casting system under the supervision of Boston, Mass. Reports should be ad- General Beretguer's Government. The dressed to the organisers, the Inter- provisional schiente comprises a national Short -wave Radio League, " National " station of 60 kW. at Mad Jamaica Plain, Boston, Mass., who will rid, with " Regional " stations at Madrid appreciate observations as to signal (30 k'IV.), Catalona (30 kW.). Audalusi:. strength, fading, and other phenomena. (20 kW.), Valencia (20 kW.). Galatia The daily transmission schedule is as (20 k\\'.), and Vascongadas (10 kW.). follows : 1200 to 1400, 1700 to 1800, 1900 Half an hour each day will be reserve,' to 2100, and 2300 to 0200 (G.M.T.). for Government announcements, and the 0000 stipulation is made that commercial pub- SURVEY OF WORLD'S licity shall not occupy more than fir.. BROADCASTING. There are a mere 24,000,000 broadcast minutes in each hour. 0000 receivers in use in the world, according to Mr. Lawrence D. Batson, of the elec- THE BOHEMIAN TASTE. trical equipment division of the U.S. De- The Radio Club of Bohemia (Czech. A nROADCASTING PIONEER. 'l he late partment of Commerce, who has just pre- Slovakia) has Sir William Bull, whose death occurred on just held a plebiscite January 23rd, was one of the original pared a remarkable survey of the world's amongst its members to discover the rela- directors of the British Broadcasting radio markets. One of Mr. Batson's cal- tive popularity of foreign broadcasting Company. culations, writes our Washington corre- stations. Preferences were expressed by of the original 11.11.C., Sir William Bull spondent, is that it would require 380 totes as follow : Künigswusterhausen, 80; revealed a remarkable capacity for states- million sets to equip all the homes in the Vienna, 70; Langenberg, 45 ; Graz, 75; world that are within constant listening Gleiwitz, manship; although a director of that com- 50; home- Milan, 40: Toulouse, pany. he never allowed the interests of range of broadcasting stations, a statement 30; Frankfurt. 25; Hamburg, tlii 24; Daven- public and of the manufacturers to con- which should encourage the radio trade. try, 22; and 22. Stockholm, flict so far as he was able to determine. Though the book abounds in statistics It was discovered that the percentage " \Vheu the was appointed President of and is intended primarily for exporters, of members who listened consistently to the R.M.A. we began to discover exactly it contains a good deal of interest for the the national station at Prague was only how fortunate we were in having as guide layman. For example, Mr. Batson ob- 20! serves that in some foreign countries 0 0 0 3 and philosopher a man with both the legal and political mind. was " there is a definite indication of a trend U.S. AMATEURS ASSIST ARMY. Sir William Bull thoroughly interested in the radio indus- toward the adoption of the American Thousands of American amateurs acted try. and yet was able to be entirely un- sponsored -programme system." as observers for the U.S. Army Signal biased, and whenever, inside the Associa- The report gives very full data concern- Corps on January 23rd and 24th, when tion, we differed in domestic policies we ing the status of foreign broadcasting. an Army Fokker plane equipped with a all had confidence that Sir William could Licence fees or brcadcasting subscriptions, special short-wave transmitter carried out assess the relative merits Alf our diverse it shows, vary from Is. 8d. per set per a round trip by way of Nashville and annum in France to r8 16s. in Turkey. arguments and produce a thoroughly 0000 Louisville to Montgomery, Alabama. judicial and scrupulously fair judgment." Throughout the flight transmission from 0000 INDUCTIVE BAND -PASS FILTERS. the plane was maintained on a wavelength SHORT WAVES FOR THE PRINCE. In the article tinder the above title in of 85.7 metres, and it was the duty of Now that the Prince of \Vales has our issue of January 28th the formula amateurs appointed by the American passed Bermuda, the range of the normal after line 14 on p. 79, col. 2, should be Radio Relay League to compile a log of ship wireless installation will be insuffi- A= H= microhenrys, where A is cross - all messages received en route with par- cient to keep him in direct touch with 1000N d' ticulars of signal strength and fading. Great Britain, but the Marconi short- sectional area and N number of turns. A 24

www.americanradiohistory.com FEBRUARY 4th, 1931. Ff@k0s 12I WopIlCl WIR.ELESS wORLD

LABORATORY TESTS A Review of Manufacturers' Recent Products.

CELESTION D.20 CHASSIS. the reproduction of speech, and, taking Terminals are fitted for the attachment Hi -berlo Celestion loud speakers have the results as a whole, we can state with of aerial earth and reaction leads. In been produced only in complete cabinet confidence that only a trained ear could addition, two flex leads are provided form but a chassis unit for incorporation differentiate between the D.220 unit and a which should be attached to one in transportables of the and radio gramophones moving -coil loud speaker. terminals on each of the tapped aerial has now made its appearance. The dia- The windings have a nominal resistance coils. phragm, which is of the well -known Celes- of 750 ohms, and the impedance varies The make's are the tion London Electric reinforced type, has a wide angle, with frequency as follows :- Wire Co. and Smith's, Ltd., Church and is supported in a particularly rigid Frequency (cycles). Road, Ley ton, London, E.10, and the cast-aluminium frame finished in grey Impedance ( olimos). 50 .. 1,590 price is 12s. 6d. enamel. The movement is of the non-di f- 0000 100 1.930 ferential single- acting type, and is mas - APPLEBY COMPONENTS: sivel' constructed throughout. The air 200 2,920 400 ... 3,550 A new range of components has recently gap is adjustable and a high degree of been introduced by E. sensi ivity can be obtained. 800 5,650 Hetherington 1,600 9,370 Appleby, Chapel Street, Marylebone, Lon- 3,200 12,600 don. These include a small binocular H.F. 6,400 6,800 choke, sectional wound, priced at 3s. 9d., a standard type, also The unit is made by Celestion, Ltd.. sectional wound on a London Road, Kingston -on- Thames, and grooved ebonite for- the price is three guineas. mer, listed at 6s. 6d., 0000 and a decoupling re- LEWCOS sistance of E00 ohms. TWIN TWO -PIN BASE. The price This of this com- is fitted with four sets of plugs ponent is Is. 6d. and sockets to accommodate the standard pattern Lewcos two -pin plug -in coils ; one set being mounted at right angles to the other set. The base of the unit is moulded in bakelite, and in the hollow recess underneath is a three -pole change- over switch consisting of cam -operated leaves with silver- tipped contacts. This is actuated by an extension rod, thus

The App'eby components referred Cel.,stion type D.20 loud speaker chassis. to above. The diaphragm is 14in. in diameter, " VEE " EARTH PLATE. and, as be night expected, the low -fre- Where facilities are available a metal quen y response is above the average. At plate buried ip close juxtaposition to the 50 and 100 cycles the output is only point where the earth lead enters sligh .ly less than the that of the average mov- building, amw for preference ing -ciil immediately loud speaker. A resonance, prob- below the aerial, forms one of the best ably due to the reed, occurs at about 300 earth counec:ions obtainable. cycles. From 300 to 5,000 cycles the out- The " Wee " earth plate made by J. B. put is sensibly uniform, apart from a dia- Trueman, 141, Ince Avenue, phra,;mn resonance Walton, at about 2,500 cycles, Liverpool, should prove admirable for and Wove 5,000 cycles there is actually a this purpose. rise in It consists of a square the characteristic up to 6,000 copper plate of 24 cycles, the highest frequency Lewcos dual -range coil unit for two -pin S.W.G. measuring tested. coils with wave -change switch in base. 16in. x 16in., to- which is firmly attached Thum the general effect has a crispness and brilliance a length of No. 14 S.W.G. copper wire. which is seldom achieved enabling the unit to be mounted on a Copper_is ideal for with a this purpose, since it diaphragm of such large diameter. convenient part of the baseboard, and does not corrode. The 300 -cycle resonance The price complete does not affect well back from the panel. with lead -in cleats is 4s. 9d. . 25

www.americanradiohistory.com l '2 Wilp®Il®N FEBRUARY 4th, 1931. Warta G.E.C. MAGNET H.T. BATTERY. it we accept this as being the useful To obtain the best all -round results, it we, to precede the trans- The battery e. 1fb which this test report life of the battery some 290 kury would be advisable is concerned is officially known as a triple hours can be expected when the initial runner by a valve of medium A.C. resist- capacity size and listed as type L4903 by current taken from the battery is of the ance, and for preference one not exceed- order of 20 mA. From the discharge ing some 12,000 ohms A.C. resistance. curve it can be seen that this represents Even so, should the steady anode current about 4,200 m i 1 l T- rise above 3 mA., it would be best to ampere hours actual adopt a resistance capacity feed arrange- capacity. ment, thus deflecting the steady D.C. The overall size of from the primary winding. the battery is 131¡in. This transformer does not include a x51¡in.x3in. h i g h. condenser across the primary winding as The nominal voltage is usual with most Ferranti models. is 60, and the battery When the transformer follows the de- is tapped in steps of tector valve and a differential reaction 6 volts throughout. The price is 13s. 6d. oo:o G.E.C. Magnet 60 -volt triple capacity dry -cell H.T. battery. FERRANTI A.F.8 L.F. the makers, the General Electric Co., TRANSFORMER. Ltd., Magnet House, Kingsway, London, The type A.F.8 L.F. transformer is a W.C.2. In view of its size it was decided new model just introduced by Ferranti, to commence the discharge at 20 mA., Ltt1., to meet the demand for an inex- which should be well within its capacity. pensive component of sound quality. The The discharge was not continuous, since overall dimensions are quite small in spite dry -cell batteries must be allowed periods of the fact that it embodies a nominal of rest during which they partially re- type of core, and not one constructed cuperate, and unless they are given these from high- permeability material. rest periods the full value cannot be de- The internal construction follows the rived the usual Ferranti practice, the winding being New model Ferranti A.F.8 L.F. trans- from battery. former and uncased sample, showing the Consequently the discharge was re- sectionalised and carried on a special sectionalised method of winding. stricted to periods of four hours, with spider -type former. This method of similar periods for recuperation. In the winding .assures a low self- capacity. The condenser is employed, no further capa- graph showing the performance of the casing is a neat bakelite moulding with city is desirable, but in other cases a battery the rest periods have been the terminals placed in an accessible fixed condenser of 0.0003 mfd. in shunt omitted and only the actual working time position. with the primary winding is recom- mended. The step up ratio of the A.F.8 is 3.5 6 20 to 1, and the makers are Femmanti, Ltd., G.E.C. MAGNET TRIPLE CAPACITY 18 11ollinwood, Lancashire. The price is DRY - CELL H.T BATTERY - 5 Ils. 6d. ) NOMINAL VOLTAGE 60 . -18 -14 -12 E IGRANIC LARGE CONDENSERS. z Included in the new components re- I- 3C 10 o cently introduced by the Igranic Electric > -8 Co., Ltd., 149, Queen Victoria Street, 2 - 6 London, E.C.4; is a range of large capacity fixed condensers of the paper 4 These are made in 1 I dielectric type. 2 capacities ranging from 0.1 mfd. to

I o 50 100 150 200 250 300 350 400 450 500

TIME (WORKING HOURS)

Discharge curve of the Magnet 60 -volt H.T. battery.

included. Theme was a fairly rapid fall Some measurements were made of the in voltage during the first twenty -five primary inductance at 50 cycles with hours, after which the decline was more various values of D.C. flowing and with gradual. This rate of fall was maintained a small maintained A.C. voltage across until the battery had been, in use for 290 the coil. The values found are given in hours, when a well- defined cut -off was the table below :- noticed Pius hat pens to be a distinctive feature IQranle 2 mfds. condenser tested to of all G.E.C. batteries; they maintain D.C. A.C. co.nponent Indu.-tance 200 volts D.C.; the working voltage in ntA. in mA. in henrys. 250. their output at a reasonably high level is for a certain time, governed by the type 0 0.213 02.3 46 of battery, and then show a rapid fall. 2 0.314 4 mfds. The price of the first- mentioned 4 0.433 30.7 size at 700 volts is Is. 6d., a 2 mfds. In the present case the cut -off point was 6 0.54 24.3 tested found to coincide with a drop in voltage 8 0.64 21 size of the same test voltage costs 3s., per cell to approximately 0.75 volt, which and a 4 mfds. 55. The 2 mfds. and tested is generally accepted as being the lowest 4 mfds types are available to useful limit to which dry cells can be The measured D.C. resistance of the 1,650 volts, the price of these being miischamged. primary winding is 1,040 ohms. 4s. 2d. and 7s. 7d. respectively. A 26

www.americanradiohistory.com FEBRUARY 4th, 1931. VflirmIl@go 123 WopIlcl

The Erince's South American Speech. tions to the consequently impoverished On good authority I learn that the members of the entertainment profession Princi of Wales's speech at the British 0000 Emplie Trade Exhibition in Buenos Aires The Boat Race. on March 14th will be broadcast. If it The Oxford and Cambridge boat race is at all possible, the B.B.C. will relay will, as usual, be described in a running the speech to British listeners, either commentary broadcast on Saturday, picking up one of the American short- March 21st. wave retransmissions, or employing the Probably mere. people listen to the Boat transatlantic Telephone Service. Race relay than to any other sporting 0000 event broadcast by the B.B.Ç. When Accent Tells. 0000 This will be one of the first occasions, Lady Balloonist -and Others. I bel eve, that it has been possible to " The Pursuit of. Pleasure," which is hear ti pure English accent across the Mr. Lance Sieveking's programme for Atlanuic. It is well known that His By Our Special Corresponden!. February 12th and 14th, will give Royal Highness possesses an ideal micro- listeners the atmosphere of night life phone delivery, so it would not be sur - "Music and Dancing." during the past two centuries. prising if the speech reveals that the The characters include Mozart, Surely it is not surprising that the Grimaldi, Handel, Jenny Lind, Henry indistinctness of previous transatlantic entertainment industry sees speeches has not something Irving, Madame Bonzo, the lady balloon- been entirely due to ominous in the application of the B.B.C. ! ist, Marie Lloyd, Sir William Sterndale atmospherics for a music 0000 and dancing licence in Bennett. and t host of others who helped respect of the new " Broadcasting Combating : to drive away chill care in the days of Interference An Inter - House." Armed with such a concession, Inational ? our grandfathers. Move the B.B.C. could legitimately admit pay- 0000 Semmering ! Another obscure locality ing audiences -incidentally, at competi- which may will renown through broad- tive priées. Saturday Afternoon. casting. For it is at Semmering (Aus- 0000 Two o'clock on a Saturday afternoon tria) that the Union Internationale de is the time, I inderstand, when a million Radicfusion is this week broaching a An Official View. toiling sons .Y Britain, having just re- quest-on of the utmost importance to In a chat with a Savoy Hill official turned from their work, sit down to a listen ais in all parts of the world. last week I was assured that the B.B.C. " mid -day " repast. At present the The Union is about to agitate for an hopes to secure a licence in the interests B.B.C. chooses this moment to conclude International Conference enjoying the of the staff. Several staff dances are held the lunch -titre concert of restaurant official status of the famous -Washington throughout the winter season in a public music, and the workers are left to munch Wireless Conference, but devoted en- hall rented for the occasion, and it is the more or less a silence. I -am glad. to tirely to broadcasting questions. custom to make a nominal charge. With bear that the " O.B." department is 0000 the large studio at Broadcasting House thinking of postponing the opening of by Official Lax ty. available, the renting of a public hall is the Saturday lunch -hour concert half held be unnecessary. " It is possible," (as they, call Hitherto broadcasting has occupied a to an hour, so that the session comparatively unimportant place in the added my informant'', " that on certain it in America) will last from 1.30 to 2.30. agenca of world wireless conferences, with the result that there is still an aston ailing official laxity in matters con- cerning broadcast wavelengths and power. The feeling of the Broadcasting Union is that if the Governments of the world can be persuaded to take an in- telligent interest in the subject, much of the existing confusion in the broadcast- ing Cher would disappear. 0000 RaprEsenting 100,000,000 Listeners. Thy delegates at Semmering and. a little later in the year, at Copenhagen. will prepare a scheme for submission at the great wireless conference to be held in M tdrid in 1932. MUSSOLINI BROADCASTS. The Duce is an enthusiastic believer in the powerful It is estimated that the delegates at influence of broadcasting. This recent photograph shows him in a characteristic Semmering this week represent more than attitude at the microphone. a hundred million listeners. 0000 occasions when members of the public Hunting for a Voice. would be admitted to broadcast perform- Objections to Italian Proposal. At the tints of writing, Mr. Philip ances a contribution to a charity would Th+ greatest enthusiasm for a broad - Ridgeway is still searching a be useful." for female castii g conference is being shown by 0000 " personality " voice which shall make Italy and a proposal is already afoot our loud spet.kers ring with freshness, that the first meeting should be held in In the Name of Charity. charm, and freedom from accent -Cock- Rome, which was recently the venue for It is interesting to note that listeners ney, " refaned," or any other kind. He the International Cinematograph Con- are now admitted to the new Edinburgh is collecting material for " The Ridge- gress studio for a nominal sum, the proceeds way Parade," which comes back to the I understand, however, that the B.B.C. being devoted to charity. microphone on March 2nd (National) and is offaring objections to this proposal on I wonder whether the entertainment March 3rd (Regional). the grounds that the Italian capital industry will always condone competi- " I can only describe what I want as woulel give the gathering too national a tion of this kind, provided that what it the pure, young English voice," he says. flavour; Savoy Hill prefers an inter- loses goes to charity ? And, further, He has already given auditions to more national centre like Berne or Geneva. whether " charity " will cover contribu- than it haudrt d girls ! ., 27

www.americanradiohistory.com WIp®Uog FEBRUARY 4tlt, Woad. !r2o[ ,., 1111-firmsformer

The Reaction Between By S. O. PEARSON, B.Sc., A.M.I.E.E. Primary and (Continued from page 90 Secondary Windints. of previous issue.)

alternating- current trans- deal chiefly with the latter arrange- former consists essentially ment as regards the theory of the of two coils so arranged tuned transformer. (If both that a certain degree of mutual primary and secondary circuits are inductance exists between them. tuned the circuit as a whole An alternating current flowing in either one of them becomes a filter, whose theory is not so simple as that will cause an electro- motive force of the same frequency of a transformer). to bé generated in the other. In the case of power Phase Differences between Primary Current and transformers for low- frequency work the two coils are Induced E.M.F.s. wound on a single iron core, for the purpose of obtain- us ing the highest possible degree of magnetic coupling Let assume that a high- frequency transformer between them. With such an arrangement practically consists of two coils whose self inductances are L, the whole of the lines of magnetic force produced by a and L0 henrys respectively, disposed relatively to each current in one of the windings are linked up with the other in such a way that the mutual inductance between turns of the other, the coefficient of coupling being them is M henrys. Suppose, further, that an alternat- very nearly unity. ing current whose R.M.S. value is I, amperes is driven As a general rule, however, it is not permissible to through the coil L, by means of an alternating current employ iron cores for high- frequency transformers be- generator A, as shown in Fig. z (a). Then, as ex- cause the power losses incurred in the iron rise very plained in the previous article dealing with mutual rapidly as the frequency is increased, and in many induction,' the alternating voltage generated or induced in instances it is essential to have a more or less loose the second coil L2 will be coupling or low value of coupling coefficient between E, = (1MI, volts (I.) the windings. For these reasons a high- frequency where (O = 2r, x frequency, and this electromotive force, transformer usually consists of two inductive coils whose 'produced in the secondary coil, lags by just a quarter of relative positions are so fixed that the required amount a cycle behind the primary current. of magnetic coupling exists between them. There are Similarly; the self induced E.M.F. or back E.M.F. several ways in which the in the primary coil itself is primary and secondary given by .,LI volts, and windings can be disposed also lags by a quarter of a relatively to each other, cycle behind the current but all arrangements in- and magnetic field produc- volve the same principles. ing it. Thus, if a simple The turns are wound on vector diagram is drawn in non- magnetic formers and which the current is the coefficient of coupling represented by a vertical Fig. 1.- Theoretical circuit diagrams of a high- frequency is always less than unity. transformer (a) with secondary circuit open, (b) with straight line OI,, as shown The primary winding of secondary winding tuned to resonance by means of a con- in Fig. denser C2. M is the mutual inductance between the windings 2, the primary back a transformer is the one whose self -inductances are L1 and L,. The coupling co- E.M.F. and the secondary which is directly connected efficient is k = induced E.M.F. can be to the source of current, A/L1L2 represented respectively by that is to say, the one to which electrical power is two lines 0E, and 0E, drawn horizontally to the right. supplied. The secondary winding is the one in which For the present it is assumed that the secondary coil L2 the voltages and currents are entirely due to the is on open circuit, as shown at (a) in Fig. Z, and that magnetic influence of the current in the other. no self -capacity effects exist in either coil or between In nearly every instance either the secondary winding the coils, so that the coupling is purely magnetic. or the primary winding of a high- frequency transformer Since the induced E.M.F. in each coil is given by is tuned by means of a condenser to resonance at the the product of the primary current and the reactance operating frequency. Since it is nearly always the secondary winding which is tuned in this way we shall ' See The Wireless World, January 28th, 1931. A 2 4

www.americanradiohistory.com FEBRUARY 4th, 1931. WlilmIl®..po WopM 125 Theor;, of the H.F. Transformer. - suppose that this capacity tunes the secondary circuit (wL, or wM, as the case may be), it follows that if the to complete resonance with the requency of the voltage R.M. S. value of the primary current is one ampere the induced in L, by the current in L1. Under these con- induced E.M.F. becomes numerically equal to the reactance, and from this fact we can evolve a simplified ditions the condensive reactance I completely definition reactance, W C2 of particularly suited to the explana- neutralises the inductive reactance of the secondary tions to follow. Namely, the reactance of a circuit is coil, and the impedance of the closed circuit becomes numerically equal to the back E.M.F. produced by a cur- numerically equal to its effective resistance R2 ohms. rent of I ampere, the phase difference between the voltage and i.urrent being just go° or a quarter of a cycle. The secondary current is therefore I2 = R2 amps., and Similarly, the resistance of a circuit is numerically is exactly equal to the opposing in phase with E2, volta. a set up by a current Reaction of the Secondary Current on the Primary of 1 impere in the circuit, Coil. this voltage being exactly The main object in view at the present juncture is to 180° out of phase with re- find out the precise effects of the tuned secondary spect to the current. The circuit on the apparent impedance of the primary wind- quest on of phase difference ing of the transformer. Fortunately, the theory is is a very important one greatly simplified by the fact that the electro- motive indeed, especially where -1-E2 force and current in the secondary O E circuit are in phase interaction occurs between with each other. But, in any case, the reactions two which :ircuits each carrying occur in the circuits will be rr.ost easily alternating currents. explained by For Fig. 2.- Vector diagram first treating an actual transformer of known constants insta ice, if a certain cir- showing the phase differ- ence between the primary numerically, deriving theoretical expressions subse- cuit is carrying a current current I1, and the voltages quently. and if an alternating Et and E2 generated by this I current in the coils L1 and Let us then suppose that the primary coil inductance E.M.F. is induced into this L2 respectively of Fig. 1 (a). is L, = ioo microhenrys and tl- e secondary circuit by action inductance the of a is L2 =200 microhenrys and, further, that the mutual neigi.bouring one, the exact effect on first the circuit inductance between the coils is M =70.7 microhenrys. mentioned depends almost entirely the phase relation- on With these values the coefficient of coupling between ship between the current I, and the induced voltage caused by the presence of the second circuit. For the coils is k = M 0.5 or 3o per cent. Assume that instance, if the induced voltage is 180° out of phase with respect to the current, that is to say, always in the secondary circuit is to be tuned to .a frequency of direct opposition to it, the effect is equivalent to the 1,000 kilocycles or 106 cycles per second (corresponding addition of resistance. Or if the induced E.M.F. lags to a wavelength of 30o metres) by means of a suitable by coo the effect is the same as adding inductive condenser. Then at this frequency the quantity reactance to the circuit. In general, when the voltage u, = 2s x frequency will be 6.283 x roe radians per introduced into the circuit is somewhere between go° second. To - complete our list of constants, let the and 180° out of phase with respect to the current, the high -frequency resistance of the primary coil and effec: is the same as adding both resistance and reactance in series with the circuit. These are the conditions which arise when the secondary winding of a trans- former is allowed to carry a current, that is to say, :11 the r pparent impedance of the primary winding depends on the nature of the load across 1 the secondary coil as ' 2 '3 well as on the actual inductance and resistance " of the primary winding. Conditions when Secondary Circuit is Tuned. (a) (b) On connecting a circuit or " load " of any kind across the terminals of the coil L. in Fig. i (a) a current will, of course, flow round the closed circuit thus formed. If Z, is the impedance of the complete closed secondary Fig. 3.- Vector diagrams showing the phase differences between the primary and secondary currents when the circuit the current will be secondary circuit is tuned to resonance. E1' is the voltage I,= E amps., where E2 is induced back into the primary winding by the current I2 in the secondary coil. the I.M.F.3 set up in L. by the action of the current I, in t;ie primary coil. The phase position of this of the secondary circuit each be ro ohms, so that secoidary current will depend on the ratio of reactance R, = R2 = ro ohms. to resistance in the circuit. The primary coil alone, with the secondary winding N )w suppose that the secondary coil L, of Fig. r (a) either on open circuit or removed altogether, will be has a condenser of capacity C, farads connected across Z, = ti /Ri2+(wLi)2 ohms. This equals wL, approxi- its terminals, as shown at (b) in Fig. r, and further mately, since R1 is small compared with wL1, the

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126 Mp@hog FEBRUARY 41h, 1931. WoadoO acol Theory or the H.F. Trausïormer.- bined diagram is shown in Fig. 4, where E2 has been numerical value being 628.3 ohms. But knowing omitted for the sake of simplicity. We see at a glance beforehand that the current in the tuned secondary that OI, and OE,' are in exact anti -phase, that is to circuit is bound to influence the primary circuit, we do say, El' is just 180° out of phase with respect to the not know the relationship between the primary applied primary current. This means that the voltage induced voltage and the primary current. We must first find in the primary coil by the current in the tuned secondary the exact effect of the secondary current, and perhaps coil is directly opposing the flow of primary current. the easiest method of procedure is to assume some definite value of current in the primary coil L,. Apparent Increase of Primary Resistance. Accordingly let us assume that the current in the coil Now, it was explained above that it is only in the L, is I, =5o microamps. , the frequency being io6 cycles case of pure resistance or its equivalent that a counter per second. This current will induce an E.M.F. whose voltage is set up in exact opposition to the current at value is E2 = w,MI, volts in the secondary coil, from all times. It follows, then, that the effect of the equation (1) above. Now in this instance ,,,M works secondary current in the tuned circuit on the primary out to 444 ohms, so that E, = 44.4 x 5o x 10-6 = 0.0222 coil is virtually to increase its resistance. The extra volt in the secondary coil, and this voltage lags just or apparent resistance introduced in this way is given 90° behind the primary current. Thus, in Fig. 3 (a), in magnitude by Ohm's law by dividing the voltage E,' if OI, represents the primary current, the voltage E E' will be in the direction OE_ at right angles to OI, and by the current I its value being therefore R,'= Ii to the right. ohms. In the present case E,' =o.985 volt and Now when the secondary circuit is tuned to complete resonance the resulting current is given by I, =50p so that R,' - °'98o x 106= 19,70o ohms. En 0.0222 0 I, = 0.00222 amp. or 2.22 Compared with this large figure the actual R2 ZO - 2 or ohmic resistance (I o ohms) of the milliamps. and is in phase with E_. Thus, primary winding fades into insignificance, in 3 (a) representing Fig. the vector this and thus there is no point in designing the as current will lie in the same direction primary winding to have a low actual 0E,, being indicated by Oh in the o I2 resistance -the thinnest wire compatible diagram. with mechanical strength will suffice. the tuned The voltage developed across A further important conclusion result- secondary circuit will be ß0L2 x 13=-1.257 x ing from our calculation is that the 2.22=2.79 volts. reactance of the primary coil is not in Effects on the Primary Coil. any way affected, because the secondary We now come to the main point in the E current does not induce any component of calculation, namely, the determination of voltage at right -angles to the primary cur- the effects of the tuned secondary circuit Fig. 4. -Vec tor diagram rent, or 9o° out of phase. The reactance obtained by c ombining the on the impedance of the primary coil. diagrams (a) and (b) of of the primary coil itself is (.,L, =628 coil Fig. 3, the transformer Just as the current in the primary secondary be ing tuned to ohms, and therefore the apparent im- induces an E.M.F. in the secondary, so reson ance. pedance of the primary coil when will a current in the latter generate a the secondary circuit is tuned to voltage in the primary additional to the normal back resonance at 1,000 kilocycles per second is Z,'= E.M.F. of self induction ; and the phase relationship of V19,7102+ 6252 = 19,720 ohms, so that even the this voltage - to the primary current determines the reactance is negligible compared with the apparent nature of the reaction, that is to say, whether it will increase of resistance. This apparent high resistance of produce an apparent change in the primary resistance the primary winding constitutes the dynamic resistance or reactance, or both simultaneously. of the tuned transformer. Let E,' denote this extra E.M.F. induced in the The voltage which must be applied to the terminals primary coil. Its value will be E,' =0,102= 444 x of the primary winding to drive 5o microamps. through 0.00222=0.985 volt. As in the previous case, this it with the secondary tuned to resonance is equal to voltage will lag behind the current producing it by the product of current and dynamic resistance, namely, 90°. The vector OI9 has been selected from Fig. 3 (a) 5o x Io x 19,720= 0.986 volt. The voltage developed and redrawn in Fig. 3 (b) in the same direction as across the secondary circuit was seen to be 2.79, and before, and the voltage E1' is here represented in its therefore the actual step -up effect or voltage magnification correct phase position by the line OE,' drawn vertically 2.7 Q given by the transformer is ni = 2.8 times. downwards, that is, lagging by 90° behind Oh, because 0.986 - the vectors are always assumed to rotate in a counter In the next part of this article simple theoretical clockwise direction about the point O. formula will be developed for calculating the dynamic We are immediately concerned with the phase rela- resistance of tuned transformers in terms of the constants tionship between the primary current I, and the voltage of the windings, and the effects of the primary circuit E,' in the primary circuit arising from the secondary reactance on the tuning of the secondary circuit will current, so that by superimposing the two diagrams of be considered. Fig. 3 we obtain the required information. The corn- (To be continued.) . A 30

www.americanradiohistory.com 1931. Wfirnhoo FEBRUARY 4th, 7 WOpIld VES WE ITAVE E

Four Representative Specimens Chosen from the as 0.005 micro- mfds., so that a i : 1 ratio transformer would appear to Dario Range of Valves. be the optimum coupling. With ARIO valves are made in SCREENODION (BIVOLT). this arrangement selectivity is likely France and handled in this This is a screen -grid H.F. ampli- to be rather poor unless a very good country by the Impex Elec- fier fitted with a 2 -volt filament. The band -pass i iput circuit is adopted. With normal trica , Ltd., 538, High Road, Ley - anode terminal is mounted on the tuning arrangements a tonstone, London, E.u. The com- top of the bulb and the other con- 3: Z ratio step-up is advised, and, plete range of receiving valves corn - tacts located on a 4 -pin base cup in assuming a dynamic resistance of prises some eighteen different types, the usual manner. Its nominal 250,000 ohms for the. tuned second- the majority of which are fitted with characteristics are :- ary circuit, a stage gain of 53 can two- and four -volt filaments, the six - A.C. resistance, 250,000 ohms. be assured. With a secondary coil volt type being apparently un- Amplification factor, 25o. of solid wire and giving some favoared in France since there are Mutual conductance, i mA / volt. 150,000 ohms for the dynamic re- no specimens of this particular class Maximum anode potential, 200. sistance of the circuit, the stage gain available. Included in the samples Normal screen voltage, 80. will fall to 32, approximately. sent in for test were some indirectly A set of curves were prepared con- heat -xd valves designed for operating necting anode current and grid volt- HYPE,L POWER (BIVOLT). from a step -down transformer de- age with various screen potentials. This is a 2 -volt super-power out- liveiing between 3.5 and 4 volts Grid current was found to start at put valve having the following rated A.C. zero grid volts, consequently it will characteristics : - be satisfactory in practice to operate A.C. resistance, 2,70o ohms. DARIO SCREENODION Amplification factor, 5. (BIVOLT) Ea7200 the valve with a grid bias of 12 3 A8pÇ volts when the maximum anode Mutual conductance, r.8 mA. / _....FILAMENT 0 2 AMR po volt. 4T2V Ea°150 2 voltage is applied. Measurements ii were then Maximum anode voltage, 200. 2-4 }}` i. s made of the A.C. resist- W numum Ca=200I.. Although the largest G ance, amplification factor and power 20 ; C'.Ilrray i1 handling valve in this class, so far O ... a 150..M Z mutual conductance under these a as we are aware, the maximum 6 ... .'/Il!r.P.11 operating conditions with 6o, 70 g and 8o volts on the screen grid, the undistorted power, allowing 5 per Q2: t . /. 15 111/Ea=200 . results being tabulated below : O - C8 10 . 7 DARIO I4VPER POWER =i/c Ea. 50 H . (31VOLT) .III CA AmDlill- =i- C Screen A.C. cation Mutual Ef .. .. E volts. Resistance tactor Conductance 35 .E -0..... -7 -6 -5 -4 -3 -2 -1 0 +1 (lie) (li) .. mm.oi. (K) dm Eg-15 GRID (VOLTS LIMA 30 MN.. E 20.Ism IImIyNN'jI`ly Grad volts -anode current curves of the 60 800,000 ohm 400 25 Eg2 Bholt Screenodion with various values 0.5 1uA /volt. II of screen potential. 70 390,000 ohm 210 0.6 mA/volt. 20 E ` -30 .UI Eg .. . -35 lll,.,4 80 310,000 ohm 210 0.68 mA /volt. , Rather than deal very briefly with 15 . each of the eighteen specimens, I HU 10 . "f l LOAD LINE many of which show quite familiar There is very little change in these ' 5 441.1"iii" characteristics, it was decided to values with higher screen potentials, - dev )te the space available to a more the drain on the H.T. battery in- 0 iüiiiíiiiilii comprehensive test 40 60 020 160 200 240 280 and report on creases, but to no good purpose. r ANODE VOLTAGE those of greater interest, such as the Between 7o and 8o volts would ap- screen -grid battery valve, its A.C. pear to be the most Anode voltsanode current curves of the satisfactory Bivolt Hyper -power valve. From these counterpart, an A.C. detector, and screen potential. can be ascertained the most suitable loud speaker impedance, also the maxi- the two -volt hyper -power valve The anode-grid capacity is given mum power output. A 31

www.americanradiohistory.com 128 MpS11SM FEBRUARY 4th, 19 31. NVopIld. cent. second harmonic, that it will I.2 amps. Its rated characteristics grid valves these constants vary deliver was found to be 32o milli - are: - widely with change in screen poten- watts from the specimen tested. A.C. resistance, i,000,000 ohms. tial and grid bias. Curves connect- This figure was obtained by prepar- Amplification factor, i,000. ing the three principal constants for ing a set of curves connecting anode Mutual conductance i mA. / volt. various values of screen voltage at volts and anode current for equal Maximum anode potential, 25o. a fixed anode potential of 200 volts increments of grid bias. The opti- Maximum screen volts, 150. and a grid bias of - 1.25 volts have mum loud speaker impedance was The anode and screen voltages are been prepared and are reproduced then determined and the power somewhat higher than usual for herewith. triangle sketched in. The optimum valves of this type, and where these The rated characteristics apply bud speaker impedance to obtain potentials are readily available, it apparently to working conditions maximum power output is of the would be well to apply them. With using the maximum voltages all order of 6,48o ohms at mean speech a view to ascertaining the performance round, as an examination of these frequency. of the valve with voltages of the curves show a definite improvement order more generally used, a set of with higher screen potentials. In DARIO HYPER POWER curves were prepared connecting (BIVOLT) r 36 moomm grid volts and anode current for O> a_MWIM DARIO SCREENODION 32 !LAMENT 036AMPMNEmg various screen potentials from 6o to (INDIRECTLY HEATED) ¢ 13o volts, the anode voltage being 4000 ° 28 EW /I% maintained at 200 volts. X I , 24 IIIm g . _ .. Grid current was found to com- A° \\_ < 121 b2 ' i 3000 - 20 _V mence at - 0.75 volt bias ; conse- á o A - N Cm O 'a_itflilm f quently under normal amplifying /' 16 mo i 20 ó 10w° Ì%iiï/iñ conditions at least - 1.25 volts grid M_\M_ á 8 Z 12 iii I,. 11'II1'Iq 15 I, 2000,-_ i bias should be used. Under work- < w F '-A: ,.,m_a\`© 8 ÌÌÌÚ o ing conditions only a very small J M. _ 621 IIIfi/B ulm maa\'- rip grid swing is permissible if distor- 1000 44 4 z 4 %IImmH "i48 tion in the waveform is to be 2 .2 mmoLB _\',,_a_a á -40- -35 -30 -25 -20 -15 -10 -5 0 avoided in the anode circuit, especi- aN.-- _ ¢ GRID VOLTS 0 07 ally when the valve is coupled to a 60 70 80 90 100 1 0 120 130 140 f aSCREEN GRID VOLTS Grid volts -anode current curves of the tuned circuit of comparatively high Dario Bivolt Hyper -Power valve. Average dynamic resistance. Therefore, a Relationship between A.C. resistance values under working conditions. A.C. amplification factor µ and mutual con- resistance 3,250 ohms, amplification screen potential of the order of ioo ductance g with 200 volts on the anode factor 5.2 and mutual conductance volts may be used in practice with and various screen potentials. The grid 1.6 mA /v bias is 1.5 volts throughout. an anode voltage of double this Good results should be forth- value and the grid bias stated above. many cases Continental valve rating coming with reasonably sensitive has been found to apply to working cone -type moving armature loud conditions. DARIO SCREENODION speakers, but two valves in parallel (INDIRECTLY HEATED) The anode -grid capacity is stated 32 40 w - G 1130 will be required to operate a moving w w to be 0.0045 micro -mfds., and with OaO Es coil model at its best. With the 1- 28 - 35 < !' 120 the constants found by measurement Es a 1 maximum anode voltage, grid _ 24 amoom 30 O¢ using the voltages chosen for this 1 °. O bias of about 22 volts should be E 20 ii1oo 25 test, the maximum 'stage gain with Z /i Es- ? applied, the average anode current _ U6/17@ unneutralised circuit calculated H E°. an then being of the order of 22.5 Z 16 IR1Jlai 20 1- e ti 60 2 at 200 metres is 205. Since this ¢ a__ee ti w Es ° milliamps. The sample tested was 12 15 a transformer of j maÌSR postulates about O 0/Q1 P= dead hard and grid current started 60 O 08 II/IIp 10 unity ratio, the selectivity may not am/ O at zero grid volts. O be all that could be desired, and, loud 04 Ì/% 5 Since the average reed -type I //Im on substituting this for one with a speaker has an impedance of about :ii%iii o -3.5 -30 -25 -20 -1.5 -1.0 -05 0 step -up ratio of 3 : 1, the tuned cir- 5,000 ohms at 400 cycles the r GRID VOLTS cuit of which shows a dynamic re- speaker may b fed through a sistance of 250,000 ohms, a stage Grid volts -anode current curves of the straight - forward choke - capacity Screenodion for various screen potentials. gain of 8o may be reasonably ex- filter circuit, the need for this being The anode voltage is maintained at 200 throughout. pected. obvious in view of the comparatively Through the range of measure- high anode current. These being the minimum safe ments made, the screen current was SUPER- SCREENODION (INDIRECTLY operating voltages, further measure- considerably less than i mA., and HEATED). ments were made of the electrical in many cases below 0.5 mA. Screen Designed for A.C. circuits, this constants shown by the valve. The potential should, thefefore, be taken valve operates from a step -down amplification factor was found to be from a potentiometer, which might filament transformer giving between 1,70o, the A.C. resistance 1.75 be designed to pass about 3 mA. to 3.5 and 4 volts, the current taken megohms, and the mutual conduct- give good voltage regulation. A by the heater being approximately ance 0.97 mA. /volt. In all screen- series resistance is definitely not ad- A 32

www.americanradiohistory.com F.s13RUARY 4th, 1931 WinIlwo 129 Wont]. vised. The anode is brought out amps. Grid current was found to cathode. Care must be taken to keep to a terminal on the top of the valve start at - 0.7 volt grid bias, so that the anode current within reasonable in the usual manner, and a five -pin the valve would seem to be suited bounds, and an actual apode base cap is fitted, the centre pin for use as a power -grid detector by voltage of between 8o and ioo connecting with the cathode and the returning the grid circuit to the would be about the best value. heater connections brought out to Even so, the anode current will rise the normal filament pins. 18 DARIO SUPER DETECTOR to approximately 12 mA., but may (INDIRECTLY HEATED) 40 vary with different specimens. SI PER DETECTOR (INDIRECTLY w HEATED). c, As an intermediate L.F. amplifier 1' MI 35 A though designated a detector < FILAMENT 1.2 AMP fI1,l î with 15o anode volts and a grid bias AT 3 8V. A.C. vale e, it would be more in keeping 12 30 of -6 volts, the' steady anode cur- , I/I5I/51 O with its characteristics to place it in rent will be of the order of 7.5 mA. 10 the category of small -power L.F. r .. z_ Under amplifying conditions, the z s since will All measured A.C. resistance was 7,150 vals es, it serve equally cc crr z well as a first -stage low- frequency IlI 15 s ohms, the amplification factor 14.3, O I.III.III11IIAI, amFlifier. The rated characteristics 10 O and the mutual conductance 2 mA. o ° I,Ì%1Ì% o volt. A five is fitted are :- 2 2 per -pin base a %S,21 A.C. resistance, 7,50o ohms. Ì0%/01iW111 with the cathode connection in the o Irls I Amplification factor, 15. °-14 centre, the remaining pins being dis- Mutual conductance, 2 mA. / volt. GRID VOLTS posed in the usual manner, the fila- Maximum anode voltage, 150. Grid voltsiI -anode current curves of the ment pins being, in this case, the A.C. Super Detector (indirectly heated) heater connections. The arrange- The heater is designed to operate valve. Average values under amplifying at l: etween 3.5 and 4 volts, the cur- conditions, A.C. resistance 7,150 ohms, ment conforms with the accepted amplification factor 14.3 and mutual rent taken being of the order of 1.2 conductance 2.0 mA /v. practice in this country. Letters to the Editoro The Editor does not hold himself responsible for the opinions of his correspondents. Cor espoudence should be addressed to the Editor, "The Wireless World," Dorset House, Tudor Street, E.C.4, and must be accompanied by the writer's name and address. MU.,TIPLE BROADCAST. circuits whose response falls off suddenly and completely tc

zero. - Si', -Your article on multiple broadcast is especially interest- square -topped response curve, Dr. Robinson then to the Leaving the ing to me as for some years I have been trying get proceeds to discuss the effect of modulated oscillations on a ordinary Press to publish my scheme for a way out of the comes . conclusions which he find in my tuned circuit, and to various present chaos in the ether. (The earliest I can apparently wishes us to regard as new, but which can, in fact, is one 29, 1929.) returned M.S.S. dated October be deduced much more easily and rigorously by the usual T ris is simply the scheme of von Ardenne reversed. methods. For it is well known that reducing the decrement I suggest that : of a circuit does not diminish its response to frequencies well (a) Each country is allocated a fundamental wave which away from resonance, but increases its response to frequencies is radiated, perfectly synchronised, by all its stations. in the immediate neighbourhood of resonance. The sidebands (b) Each station then imposes on this its local HF wave - wave is increased ; and as the fundamental (or are not reduced, but the carrier tl ese local Waves avoiding heterodyne with amplitude of the sidebands is a measure of what Dr. Robinson it ; harmonics). calls the absolute modulation, it follows at once that the latter (c) This local frequency is then modulated at L.F. is unaffected, although the percentage modulation is reduced. T le receiver, as von Ardenne states, would need two stages When he comes to the question of an interfering carrier of cetection. wave, however, Dr. Robinson merely states that its effect is of trouble? S-irely this is the way out our present "small," but from what has just been said it is obvious that I know that my knowledge is lacking in the scientific side, it is not small ; like the sidebands (or, if you prefer it, the but von Ardenne's plan bears out once more the possibility is as great in the case of a circuit on absolute modulation) it just of r scheme which I have grown tired of inflicting patient of very low decrement as in any other. The alleged explana- edit3rs. WILLIAM B. WEST. tion, then, turns out to be no explanation at all, and on top Deal. of this comes the admission that the Stenode does not, in fact, eliminate the interference, and that " other means " have to

be adopted to do so ! STENODE. This would seem to indicate that some of the claims pre- S:r, -The article in which Dr. Robinson seeks to explain the viously made were misleading. Would it not be more satisfac- prirciples of his Stenode receiver can hardly be allowed to tory if Dr. Robinson were to give us, instead of theories of past without comment. Not having access to the original doubtful value, a precise description of the experimental facts paper, I must perforce take the abstract as it stands. which those theories are intended to explain? The author begins by throwing doubt on the application The audible effects of an interfering station may be divided of Fourier analysis on the ground that " there can be no into two parts :- discontinuity in the nature of the physical response " of a (1) The actual speechh or music. circiit with a square- topped resonance curve, as the signalling (2) Heterodyne tones (including the heterodyning of the spe. d is varied. I cannot see the force of this argument. A interfering sidebands by the carrier of the wanted squ ire -topped resonance curve implies a discontinuity in the station). response to a varying carrier frequency ; why, then, should A theoretical consideration of the Stenode receiver as hitherto we deny the possibility of a discontinuity in the response to a described (making reasonable assumptions as to the rectifier varying modulation frequency ? And if we do not get such an characteristic) indicates that it would eliminate (1) without effect in practice, it 'is because in practice we do not get affecting (2). if the interfering station is more than about A 33

www.americanradiohistory.com 130 Wfirnhoo FEBRUARY 4th, 1931. íWpIl. 10 kc /s. away, only (1) occurs, and in such cases, therefore, the falls, due to diaphragm loss and interference, a point which I Stenode circuit may prove to be a useful alternative to the have discussed several times in this journal. baud -pass filter. With smaller separations the second type of The results for the M.C. speaker are somewhat different. From interference predominates, and the arrangement would appear 300 to 1,000 cycles the actual diaphragm gives apparently four to confer no great advantage. - times the output from a rigid disc, whilst at 3,000 cycles it gives If the results of this analysis give a fair indication of the over forty times that from a rigid disc. The reason is simply that behaviour of the receiver, it would not appear to entail any great the coil and diaphragm form a complex resonating structure revolution either in theory or in practice. In view of the which " breaks -up " throughout the audible range. The drop fact that the device has already been advertised to the public, in efficiency at 2,000 cycles should be noted. It extends over a it-is surely not too much to ask for a straightforward scientific fairly wide frequency band, and is characteristic of M.C. statement of what it will actually accomplish? speakers, although it does not always occur at this frequency. Oxford. L. YATES -FISH. Owing to the inclusion of diaphragm losses i!'e above efficiencies are somewhat optimistic, but in the aforementioned M.C. loud RECEPTION OF AMERICA. speaker one might expect efficiencies from 5 to 10 per cent. Sir, -I wish to bring to the notice of fellow readers the fact over a certain frequency range. These results prove that conditions are very favourable for long- distance reception. very definitely that rigid diaphragms are not desirable I have myself received several American stations on the medium for loud speakers. In a forthcoming article,* the M.C. waveband at headphone strength on an S.G. three -valve set. speaker is reviewed in the light of our recent research. Amongst those I have heard announced are WTG, Atlantic It is shown that the upper register is due in part to the dia- City, WGY, New York, and WJG, New York. The strength phragm per .se, but mainly to the flexibility and resonances of and clearness of these stations are good enough to repay any the coil itself, i.e., its ability to take up an oval shape -thereby enthusiast who cares to stay up till the early hours to listen to distorting the diaphragm-as when pressed between one's fin- them. IL W. CAPEWELL. gers. If experimenters will use a well -resined violin bow on Stoke -on- Trent. the edge of the coil my meaning will be clear. Jan. 12th, 1931. When the valve resistance is added, the overall efficiency of valve and speaker is reduced considerably. For example, with EFFICIENCY OF LOUD SPEAKERS. a valve resistance of 3,000 ohms the efficiency of the above M.C. Sir, -The efficiency of loud speakers is raised in your issue of speaker falls from 16.1 per cent. to 6.6 per cent., whereas with a December 31st. As Mr. Barclay remarks, the efficiency is now pentode it is about 1 per cent. The efficiency r) is, of course, greater than it was for the short horn and reed -type speakers only a criterion of the power output when the current is con- used in the early days. Provided the diaphragm moves as a stant at all frequencies, e.g., the pentode case. With a low - whole the efficiency is obtained from measurements of resistance resistance triode the increase in resistance and reactance of the Rm- 1'R 4 speaker reduce the current, and, therefore, the output at high and inductance in air and in vacuo. The efficiency q_ Re frequencies. where R , is the motional resistance in air, i.e., the difference Again, by plotting an [efficiency /frequency] curve for the range between free and fixed resistances : Rm is area the motional resist- 30 to 10,000 cycles and taking the main valuer ance in vacua, k is the ratio of the motional impedance in L base line air to that in vacuo, and Re is the resistance when the rl can be reduced to a comparatively low value according to the system moves in air. Another method is to measure Re in extent of the response curve of the speaker. A discussion on air with the driving agent (coil or reed) free and then this would be too protracted for inclusion here. with it fixed. R is the difference between the two, So far as I am aware, the most efficient loud speaker is the excepting that it includes diaphragm losses, since the moving coil-15ft. horn variety of the W.E. Co. used for cinemas. diaphragm moves in one case but not in the other. It Over a wide range it is 30 per cent. or more efficient. This will be convenient to refer to the value thus obtained as the high value of efficiency is due to the resistive air load on the apparent radiation resistance. A series of measurements have diaphragm being greater than in the ordinary speaker. been made by Mr. G. A. V. Sowter and myself, from which Some freak values of qa may be of interest to show what can be achieved at resonance frequencies. The diaphragm of ,la can be calculated. We are, unfortunately, not the proud the possessors of the necessary vacuum chamber and pump, so that above M.C. speaker resonates on its surround at 18.5 cycles, and the efficiency 90 the results are apparent values, i.e., , . The diaphragm loss exceeds per cent. The efficiency of a thin should not be serious below 1,000 cycles, but it increases at the aluminium disc driven by a M.C. and resonating at 120.5 cycles higher frequencies. In fact, in reed -driven apparatus, removal was 97 per cent., whereas at 250 cycles it was only 2 per cent. if the diaphragm does not affect the output above about 8,000 The A.C. resistance of the coil was about 1 ohm, whilst the radi- cycles, although this is due in part to interference. ation resistance was 37.5 ohms -a truly efficient arrangement. Data are given in the accompanying table showing the apparent Those interested will find some of the above points treated in efficiency of (1) a reed- driven conical diaphragm 18 inches in detail in the " Philosophical Magazine," pp. 1 -54, January, 1931. diameter with a baffle 3. feet square,. (2) the writer's M.C. loud London, S.W.1. N. W. McLACHLAN. speaker with a baffle 6 feet square, (3) both speakers if their Jan. 2nd, 1931. diaphragms behaved as rigid discs in infinite baffles. WIRELESS RELAY SERVICES. Apparent Efficiency with Apparent Efficiency with Sir, -As a regular reader of your paper I have read with Frequency (cycles efficiency Rigid Disc Efficiency Rigid Disc interest your remarks and the replies on the subject of wire- per second). % % °ó less relay services. I would like to point out that this method of Reed. Reed. Moving Coil. Moving Coil. wireless distribution has been in use nearly three years in this district, and in some instances three and four programmes are given; the subscribers choose whichever they desire. A 300 .. .. 6 40 (resonance 16.1 4.3 at 305 N.) few advantages of the service are as follows : (1) Low initial 8.4 13.3 15.9 4.3 outlay. (2) Low rental. (3) Twelve -hour service. (4) Freedom 1,000 .. .. 7.1 2.88 18.3 3.5 from electrical interference (tramways, etc.). (5) No upkeep 1,000 .. .. 7.7 0.43 7.9 0.00 3,000 .. 1.4 0.12 12.0 0.26 cost, such as batteries, valves, etc. With regard to tramway interference, we have taken a number of good receivers from people resident on the in With a rigid diaphragm the reed -driven type resonates at 305 car routes exchange for our relay is cycles, and its efficiency is 40 per cent. If the iron loss and the service. A further interesting point that some of our sub- winding resistance were not so high, the efficiency would have scribers get first. hand knowledge of good wireless reception and been much greater. As the frequency rises, the efficiency gradu- buy receivers, a step which would not otherwise be taken in ally fades into insignificance -in fact, the [efficiency /frequency] numerous cases. J. PRESTON. curve is just like the selectivity curve of a tuned radio circuit. Briercliffe Wireless Relay Station, Due to diaphragm "break -up," the apparent efficiency of the Harle Syke, near Burnley. actual speaker is uniform from 300 to 2,000 cycles, after which it * In the issues of January 21st and 28th, 1931. A 34

www.americanradiohistory.com FEBRUARY 4th, 193r. WApdmg 137 WOpEEI READERS' PROBLEMS

ii!1lil11 ' !!!!Pii!!i!!!!!i!ï'ts !?! ;I'" Îis ;!c!!1!!! ;;;ï ""rlirimr.l:itlq?!í!!i!l9iniell;o::r,.-::::aitMlEiá!'°ilR:113°emil:t::ln:olpl:llll:l''f; Replies to Readers' Questions of General Interest. Technical enquiries. addressed to our Information Department are used as the basis of the replies which we publish in these pages, a selection being made from amongst those questions which are of general interest.

Failing Emission. Two -station Reception. I am ..sing an " L " class valve as a first - Will you please tell me what is the stcge L.F. amplifier, and hare just simplest form of circuit for the recep- noticed that quality is greatly im- tion of two alternative programmes, proved when Trirl bias for this valve with a switch change -over from one is reduced from 4q volts to 3 volts. station to the other? If it is possible, taken to L.T. negative, through a bias is no pos- Iy H.T. and L.7'. accumulators are I should like Co use ready -made com- battery where necessary, there in good condition, and the usual tests ponents. It is proposed to adopt a sibility of trouble through the combina- fcél to reveal any fault. Can you tell detector -L.F. circuit with reaction. tion of valves with different filament ar- m.?, why this change should have taken We cannot think of any simpler voltages. 0000 place? rangement than the modified Hartley cir- in Fig. 1. Ordinary centre - If we can assume that no abnormally cuit shown Frame Aerial Switch. resistance has developed in the anode tapped coils may be used, and entirely high tell nie what was the circuit, it is logical to conclude that the separate tuned circuits are provided for Will you please each wavelength. actual type of switch used in the emission of your valve has fallen off con- Range siderably. Input from the aerial, and consequently construction of the " Dual 0000 the selectivity of the set, may be con- France Aerial" described in your trolled by cperation of the aerial series issue of January 7th? Increased Output : Shorter Life. condenser U, which may have a value of This was a " UtiIlty " switch, made by I haie found that the current obtainable about 0.00025 mfd. If it is necessary to Wilkins and Wright, Ltd., and described f -one my eliminator can be increased depend to any very great extent on re- as the " anti- capacity change- over " type. by fitting a larger condenser across t'ce valve rectifier output. Is it H.F.C. l l,elq that any harm will be done by R.C. 600-00rC J TÓ retaining this extra capacity as a per - nuauency? L.F It is risky to attempt to increase the output of a rectifying valve by operating it in a manner other than that intende by the manufacturers ; its working life is likel- to be seriously reduced by doing so. o 000.. 0000 mfd Condenser Working Voltage. When designing an eliminator, I take it 'hat the large condenser joined across he 11.7'. rectifier output will he galled upon to withstand a higher voltage than the remainder of the condensers used in the unit. Is this correct? Yes, the unsmoothed rectifier output, to- getl-er with a ripple voltage component, is a-.rplied to this condenser. It is gener- ally agreed that it should be capable of Fig. 1. -An easily assembled circuit for reception of alternative programmes. dearing with a voltage equal to twice that of the unsmoothed rectifier output. action, it would be convenient to insert a No Volts to Spare. 0000 semi -variable condenser, Cr (shown in (With reference to previous correspond- lead of Pentode Detection. dotted lines) in the appropriate ence). . You suggest that in- I should like to obtain a larger output whichever circuit is found to oscillate cipient L.F. oscillation might be pre- from my anode bend detector (at pre- most freely. A maximum capacity of vented by using decoupling resist- sent a 20,000-ohm triode), so that the from 0.0003 to 0.0005 mfd. would be right ances of higher value, if this course succeeding resistance- coupled output for this component. is possible. Unfortunately it is not, valve may be more nearly fully loaded ooco as there is no surplus voltage; indeed, on signals from the local station. Grid Voltages Undisturbed. the detector and first -stage L.F. am. Would it be worth while to use a pen- I am thinking of using 2 -volt screen -grid 7difier are being run at considerably tode as a detector? battery valves in combination with, a below their rated anode voltages. Your detector is obviously called upon to 6 -volt detector and output valve. Is Will it be necessary for me to reduce deal with quite large H.F. inputs, and in it possible that the insertion of the magnification, or is there any less these circumstances a pentode would give necessary L.T. voltage -reducing re- drastic remedy that can be tried? good results. For a given input, it should sistance will upset the grid bias? As you cannot very well use larger de- provide an output of about twice that No ; provided that the voltage -reducing coupling resistances, the alternative plan obtainable from a triode of normal charac- resistance is inserted in the L.T. positive is to increase the capacity of the associated teristics. lead and that the grid return leads are by -pass condensers.

A 35

www.americanradiohistory.com NWpcIlsoo FEBRUARY 41h, 1931. W"o0rr'Ilcl A Long -wave Filter. have overlooked the fact that L.F. im- Tunable Interference. (Pill you please give me a circuit diagram pulses may be passed back to the detec- Since anode current from of a capacity -coupled my II.F. -det.- aerial input tor grid via the H.F. coupling coil, which 2-L.F. receiver has been derived from filter for long traces only? This is we note is actually a tuned anode coil D.C. through to precede mains an eliminator I a screen -grid H.F. valve, connected as an auto -transformer. To have been troubled by intermittent and I should to use con- like ganged prevent the possibility of L.F. reaction, interference, which is mainly confined densers with electrically connected the decoupling components in this circuit to the rotors lower end of the long -wave tun- for tuning both the filter and should have values that will deal effec- ing scale, although it is faintly the H.F. stage. tively with. low- frequency currents. In- audible A on other wavelengths. I sup- circuit diagram of a suitable arrange- stead of a resistance and condenser of 600 pose that this is due to H.F. energy ment to fulfil your requirements is given ohms and 0.1 mfd. as shown in your dia- generated in Fig. by electrical machinery con- 2. We have not assigned a value gram, we recommend 20,000 ohms and nected to the same supply mains, and Co the coupling condenser Cm, as this will 2 mfds. respectively. 0000 should be glad if you would suggest depend on -the design' of your tuning coils. how it might be prevented. An Improvised Potentiometer. We expect that your assumption is cor- I have a simple H.T. battery eliminator rect, and you are advised to connect an with two positive terminals, one of H.F. choke -which must, of course, be of which gives maximum output, while adequate current -carrying capacity-in the other is connected through an in- each lead from the mains to your ternal- series resistance, variable in eliminator. steps. This eliminator proves to -lie 0 0 0 0 suitable for operating a 1 -v -1 set, but it is impossible to obtain the correct Tapped Coupling Coil. operating voltage for the screening grid of the H.F. valve. Do you ad- With regard to the " Flexifl, Two " re- vise me to fit a high -resistance poten- ceiver, described in your iNsue dated tiometer, or is there some simpler December 31st, 1930, I ant not quite way? clear as to how connection is picked No doubt the present series resistance up with the contact brush of the has too low a value for reducing screen- selector switch. Should No. 10 ing grid voltage, and in any case a poten- switch stud be earthed? tiometer is best for purposes This stud is joined to the extreme end of regula- of the long -wave tion. As an easy way out of your diffi- coupling coil, and should culty, we suggest a not be earthed. Connection can most that fixed resistance, conveniently of some 40,000 or 50,000 ohms, should be be made to the brush by join- joined between the ing all the back studs of the switch to- screening grid and gether and negative filament terminals of your H.F. also to earth. The back studs valve holder. are those mounted on the plate which is Fig. 2.- Circuit arrangement of a capacity.. 0000 nearest to the panel. An insulated spindle coupled filter for long -wave reception. is supplied with this type of switch. Alternative aerial couplings are provided. Separating H.F. and L.F. Components. .fly 2 -v -1 receiver is quite stable on the Testing an L.F. Transformer. medium waveband, but self- oscilla- tion is produced when all its circuits FOREIGN BROADCAST GUIDE, Suspecting that my L.F. transformer was are tuned to a long -wave signal. I faulty, I recently made a test with believe this is due to the action of 'phones and a dry battery. There H.F. currents in the L.F. amplifier. B N appears to be continuity through Will you please tell me how to deter- R ® both windings, but the loudest click mine whether this assumption is (Czechoslovakia). is produced -when the testing appara- correct? Geographical : tus is joined across the secondary ter- position 49° 12' N. ; l6° 57' E. A test may be made by connecting, as a Approximate minals. Surely this is wrong. Is not air line from London : 752 temporary measure, an abnormally large miles._ the secondary winding usually of condenser between detector anode and much higher resistance than the earth. A capacity of at least 0.002 mfd. Wavelength : 342 m. Frequency : 878 kc. primary? should be used, and, if instability is cured Power : 3 kW. (temporarily). Your transformer would appear to be by this addition, it may be assumed that Time : Central European (one hour in faulty. There is doubtless a break in the your present H.F. filtering devices are advance of G.M.T.). primary winding, and the click heard in inadequate. the phones is due to the flow of charging 0 0 0 c Standard Daily Transmissions. current into the capacity existing between 08.00 G.M.T. (Sun.), relay of promenade sections of the Increased Range. coil. There is the addi- concert from Carlsbad ; 09.00, concert tional possibility Interference from the local station is so that there is a built -in (Sun.) ; 10.15, gramophone records ; strong that distant reception is im- 11.00, primary condenser. time signal, carillon (from Prague); 17.15, Normally, the primary winding of a possible over a large part of the tun- ing scale, Germany transmission (concert or talks); transformer has a much lower resistance and I am considering the 19.00, main fitting of an input evening programme ; 21.20, than the secondary. filter to my H.F.- relay of concert, dance music from 0000 det.-L.F. three -valve set. Do you consider Morayska- Ostrava, or organ recital from that this addition will reduce Beranék Cinema (Prague). H.F. and L.F. Decoupling. its range to a serious extent? Will you please examine the circuit dia- In your locality it will no doubt be Frequently exchanges programmes with gram of my proposed H.F.- det. -L./". necessary to operate any receiver having Prague, Morayska -Ostrava and Bratislava. receiver (on which values are a single -tuned input circuit with an aerial Man announcers. Call : Allo Radio Journal marked), and say if it is in order ?" coupling much below the value giving Brno (phon. Broono). Announcements Tuned anode coupling is to be used loudest signals. But when a filter is fitted are sometimes made in German and for the H.F. stage, on the score of it should be possible to adopt a much French. simplicity and economy. I ant not closer aerial coupling ; in consequence, aiming at extremely high magnifica- although Closes down with good -night greetings the addition in question will in (Dobrou Nor), followed tion. itself introduce some by Czech National loss, there will prob- Anthem (Kde Domou Muj). A receiver constructed on these lines ably be a ne, gain in sensitivity rather should give satisfactory results, but you than a loss.

A 36

www.americanradiohistory.com C- e AN r RADIO VIEW 18/y Year of Publication)

WEDNESDAY, FEBRUARY IITH, 1931. VOL. XXVIII. No. 6.

Editor : HUGH S. POCOCK. Assistant Editor : F. H. H AYNES. Editorial Offices : 116 -117, FLEET STREET, LONDON, E.C.4. - - Editorial Telephone : City 9472 (5 lines). Advertising and Publishing Offices :. DORSET HOUSE, TUDOR STREET, LONDON, E.C.4. Telephone: City 2847 (13 lines). Telegrams: "Ethaworld, Fleet, London."

C)VENTRY : Hertford St. BIRMINGHAM: Guildhall Bldgs., Navigation St. MANCHESTER : 260, Deansgate. GLASGOW: WI, St. Vincent St., C.2. Te egrams: "Cycliet,Corentry." Telegrams: " Autopress, Birmingham." Telegrams: "IliRe. Manchester." Telegrams: "IIiRe. Glasgow." Te ephune: 5210 Coventry. Telephone: 2075 Midland (3 lines). Telephone:' 8870 City. (4 lines). Telephone: Central 4857. PUBLISHED WEEKLY. ENTERED AS SECOND CLASS MATTER AT NEW YORK, N.Y. Subscription Rates : Home, L1 Is. 8d. ; Canada, £s is. 8d. ; other.countriers abroa d, L,I 3s. sod. per annum.

A e many of the circuits and apparatus described in these pages are covered by patents, readers are advised, before making use of them, to satisfy themselves that they would not be infringing, patents. Avoidable Interference.

ril HE attention of listeners is being concentrated practicable, and the only effective remedy lies in curing more and more on the problem of avoidable inter- the trouble at the source. Unfortunately, it is not ference, which is produced by local electrical illegal to radiate electrical interference until a listener appal atus or machinery, such as motors, tramways, and is able to show that such action is interfering with the electric signs. We have seen it suggested that undue quiet enjoyment of his rights. It would be generally concern is being shown over this problem, and it is conceded, we believe, that reception of broadcasting is statec that up till now listeners have managed very well to -day a part of the amenities to which an individual without seriòus interference with reception, so that there is entitled, but it normally rests with him to lodge the shoul I be no reason why they should all at once wake complaint and take action where his enjoyment of up with the idea that interference of this kind has sud- broadcasting is being interfered with from such causes. denly become intolerable. The only other remedy is for those responsible for Bu : if we consider the facts, it will be obvious that causing interference to voluntarily take what steps they interf .rence that can quite well be regarded as intolerable can to effect a cure. A good deal of research work has to-day, a year or two ago could have been endured. been done recently, notably by the General Electric The amount of electrical machinery in use has increased Company, to devise means for reducing the causes of very substantially in the past few years, thereby aug- electrical interference. The B.B.C., too, has taken a menting the sources of interference, and at the same keen interest in the matter on behalf of listeners, but time the average sensitivity of wireless receivers has there is room for much more active efforts yet before increased by leaps and bounds, the cure can be general ; more until to -day interference which especially is it desirable that the of users of was normally almost inaudible In his Issue ever -increasing body provides at the best a seriously T electrical apparatus should be increl sing background noise to TRACING HUM IN MAINS SETS. educated to appreciate the cir- programmes of stations at any RECENT DEVELOPMENTS IN AMERICA. cumstances in which radia- distar ce. It must be remem- CORRECTING PICK -UP CHARACTERISTICS. tion can take place and the bered too, that public interest in CURRENT TOPICS. methods of cure which can be distant reception has increased; UNBIASED OPINIONS. resorted to. are responsible are thus providing another reason LOTUS MAINS RECEIVER REVIEWED. Those who why Local electrical interference mostly unaware themselves of to-day appears as a more serious D.C. BAND -PASS FIVE. the trouble which they are caus- matte: than formerly. LABORATORY TEST ON APPARATUS. ing and would, we believe, in The: idea of being able to lessen BROADCAST BREVITIES. nearly all cases most readily co- interference of this nature at the LETTERS TO THE EDITOR. operate when once they were receiver has long since been READERS' PROBLEMS. shown how a remedy could be abandoned as theoretically im- effected. 's 'I

www.americanradiohistory.com WAp@IliagO FEBRUARY nth, 193f. W3pllcl

How to Obtain a Silent Background.

Systematic Stage=by =stage Tests. By W. T. COCKING.

VERY reader who has experimented with mains fairly easy. In order to illustrate the principles in- receivers must d at some time or other have experi- volved, the commonly used circuit of Fig. i will be enced the annoyance of hum.' The locating of taken as a basis for discussion, and the modifications the source of trouble is by far the most difficult part necessary for other circuits will become apparent. It of the problem, but once it is found its cure is usually will be assumed that the complete apparatus is built into one .unit, and that it performs satisfactorily with the exception that there is excessive hum. It will be realised that a systematic investigation of l the set is essential, for the hum may be introduced in many places at once, ut and the elimination of one source alone often makes -VVVv- little audible difference. 04 1 The obvious starting point i a is the loud speaker ; it is L.S. very unlikely, although not o ó impossible, for there to be 0 hum in a reed -drive -type 1 .. Y Y X x speaker, but it is a very 9 C probable source of trouble if it be of the mains -ener- gised moving -coil type. The procedure to be adopted, therefore, is to connect the moving -coil to the loud speaker terminals on the set in the usual manner, and to switch off ti the set, leaving only the field energised. Any hum must now be due to the field supply, and must be eliminated in one of the many well-known ways. If Y the field be of the low - voltage type supplied by a Fig. 1. -A typical three -valve receiver. The elimination of hum is carried out in stages; by metal rectifier, the trouble short- circuiting the points a, b, the output stage is isolated, and can be dealt with by itself. can usually be cured Similarly, by short- circuiting the points e, f, the detector is isolated from the H.F. stage. The by condenser CD is for the purpose of eliminating a particular type of modulation hum. connecting in parallel with B I2

www.americanradiohistory.com FE3RU.1RY 1rth, r93r. NWEnIl@oo WaFIl r3.5

Traein; Hum in Mains Sets. - due to the filament supply ; the mains transformer tap- the winding an electrolytic condenser of high capacity. ping is out of balance, and the potentiometer should be With a high -voltage field winding supplied by a valve retained permanently.. rectifier, or D.C. mains, it will usually be necessary to If no better results are obtained by the use of a add a smoothing choke in potentiometer, however, the series with the winding, in trouble must lie in the H.T. additon to a 2 mfd. or 4 fundamental principles of mains- operated receivers or grid bias supply ; unless mfd. condenser in parallel THEare now so well known tint it is readily possible to design the valve used is one with with it. a set with the assurance that. when completed, it will give no a thin filament and itself in- trouble from hum. " In general, however, this presupposes troduces hum, or there is an The Output Stage. a " clean" layout with considerable distances between the components, which makes for a large and unwieldy set. excessive capacity between Having made certain that Where a compact receiver is desired such assurance is hardly the different windings on the loud speaker introduces possible, for the necessary proximity of components is one the mains transformer.' The no h im one can tackle the of the greatest sources of hum. In this article it is shown is most that hum can be traced methodically by examining the H.T. supply easily set in earnest. The first receiver stage by stage. tested by increasing the step is to make sure that capacity, say, by 4 mfd. , there is no electro- magnetic of the condenser C1 ; if this inter iction between the output choke, ór transformer, results in an improvement it is the smoothing circuit and the mains transformer. To do this disconnect the which is at fault. The choke inductance must be in- choke from the valve and H.T. supply, and connect creased or the capacity of the condenser C1, or both, in across it a resistance equal in value to the anode A.C. accordance with well -known principles ;2 a further pos- resis ance of the power valve, as shown in Fig. 2a for sibility, however, lies in electro- magnetic induction be- choke and Fig. 2b for transformer coupling. Under tween the smoothing choke and the mains transformer, thesu conditions there should be no audible hum what- and the effect of rotating the former should be tried. ever and if this be the case, the connections may he The grid bias supply may be tested by connecting an put back to normal. If hum be found, however, the additional 4 mfd. capacity across the condenser C2, or chok e or transformer must be rotated, or moved to a across the bias resistance R1. If any improvement different position in the set, until a position is found results, a condenser of the correct capacity should be which gives silent operation. Trouble in this portion connected permanently in these places, or in extreme of the receiver is rather unlikely, for no amplification cases an additional choke can be used. follcws the output circuit, and any pick -up is not The Detector. like'y to be at all serious. We next come to the T ie next step is to isolate detector and its intervalve the )utput stage, and to do coupling, and the short this we . short -circuit the circuit to the L.F. trans- sect ndary of the intervalve former secondary should transformer by connecting be removed. Now this a length of wire between transformer is, perhaps, the points a, b, Fig. i. We the most likely source of have proved the loud hum in a compact set, speaker and output circuit and very careful attention to be free from hum, and should be given to its posi- an hum with thé inter - tion. The test is the same val re transformer second- as that for an output trans- ary short -circuited must former ; the primary must obx iously be due to the be disconnected, and then H.T. supply, the grid bias connected to a resistance sur ply, or the filament of whose value is equal to the power valve. the normal working resist- he latter is the most ance. That is to say, in easily tested by heating it the case of the circuit of Fig. 2. -The method of testing for eiectro- magnetic pick -up teniporarily from an accu- in the output circuit is shown in the above figures; (a) is for Fig. i, a resistance equal mulator, when the possi- the case of an output choke, and (b) for a transformer. The H.T. supply and the valve should be disconnected as shown, to the internal valve re- bil ty of trouble in the and a resistance connected across the choke or transformer. sistance, some io,000 filament supply ceases. An ohms for an AC /HL- accumulator is not always available, however, and in type valve ; but where a resistance -fed transformer is this case the return lead c to the centre tap on the mains used, the resistance should be equal to the valve resist- transformer should be disconnected and connected in- ance and the coupling resistance in parallel, some 6,000- stead to the slider of an adjustable potentiometer, which 7,000 ohms for an AC/ HL and the circuit of Fig. 4. in turn is connected across the valve filament yy, as shown in Fig. 3. If position can be found a on the ' Sec The Wireless World, page 652, December 10th, 1930. potentiometer at which the hum is absent, then it is s Seo " Smoothing," November 19th, 1930.

13 tt

www.americanradiohistory.com ,r16 VIpsIlmo FEBRUARY 11th, 1931. WOpIlCd Tracing Hum in Nains Sets.- voltage source. The positions of the wiring in the H.T. The necessity for this primary shunt resistance during rectifier and smoothing systems should be examined, but testing is to simulate the working conditions as far as the most likely source of trouble is the mains trans- slum pick -up is concerned ; it will be found that without former. Fortunately, screening is often of great use in this resistance there is a very large amount of hum, eliminating electro -static pick -up, and the effect of en- which is not normally audible, while if the transformer closing the detector together with its grid leak and con- primary be short-circuited, all hum vanishes. Having denser in an earthed metal box .should certainly be connected the correct value of resistance across the tried. In this connection it must not be forgotten that transformer primary, it should be rotated to the position interaction between the valve electrodes themselves can occur, and the writer once traced a persistent hum to interaction between the electrodes of the H.T. rectifier and the detector valves, which were rather close to- gether ; the trouble was cured by screening. The H.F. Stage. Having cured hum as far as the grid circuit of the detector, the short-circuit between the points e, f, should be removed. There should still be no hum, and in the very rare case when it is found that this alteration introduces it, it can be cured by rotating the tuning coil itself. Next reconnect the condenser C, to the tuned circuit, and test for hum. During this test the set must be so tuned that no signal is audible, for this may give rise to modulation hum, which will mask other effects. Any hum which can now be heard must be due to the H.T. supply to the H.F. valve, and appro- priate additions should be made to its smoothing cir- cuit, and also to the grid-circuit bias resistance by -pass condenser C,. In the absence of a signal, the set should Fig. 3. -The alternative connections to the filament are shown now be perfectly quiet, and with no trace of hum, be- in this illustration ; the potentiometer should be of low resis- yond, perhaps, one foot from the loud speaker cone. tance and adjustable. This form of connection is likely to give less hum than the centre -tapped mains transformer of Fig. 1. Modulation Hum. of minimum hum ; and in order to do this, it is, of Now it may quite possibly be found when course, necessary to replace the normal connections to that a station is tuned in, a fairly strong hum it by sufficiently long flex leads. be becomes evi- It may mentioned dent. This is known simple rotation as modulation hum, since the hum that will not always cure hum, and the actually modulates carrier transformer must sometimes be its in the of the incoming signal, turned on side, or and it is often difficult cure. some other curious position. to There are several causes of modulation hum, and each has its When a suitable position has been found for the own remedy. In the first place, it may be caused by insufficient transformer, its primary should be reconnected, and the grid of the detector valve short- circuited to the cathode by connecting a length of wire between the points d, e, in Fig. i. Any hum is now due to the H.T. supply to the detector, and more smoothing is indicated. The effect of adding capacity to the condenser C, should be tried, and if there be an improvement, a larger capacity should be connected permanently in that position. In a few cases, of course, an excessively large capacity may be needed, and it is then more economical to add an additional choke and condenser at the point z in the H.T. supply. The next step is to check the detector grid circuit, which can be done by removing the short- circuit between the points, d, e, and applying it to the other side of the grid condenser ; that is, to the points e, f. At the same time, the connection between the condenser C4 and the tuned circuit should be broken. The detector grid is now connected directly to the cathode through the grid leak and condenser, and any hum will usually be Fig. 4. -The parallel feed transformer -coupled circuit ; when due to electro -static pick -up on the grid. This will occur testing for electro- magnetic pick -up in the transformer, the if the heater wiring is placed close to the grid of the primary shunt resistance must be less than the valve resistance. It should be equal to the valve resistance and the coupling valve, but more usually it is due to pick -up from a higher resistance in parallel in order to simulate working conditions. B 14

www.americanradiohistory.com FELRUARY 11th, 1931. ®Il,sm 137 WopIlcl Tracing Hum in Mains Sets.- leads and earth. The best capacity must be found smoothing of the grid bias and H.T. supplies to the by experiment, but a value of 0.0005 mfd. is often H.F. valve. The former is the more prevalent source sufficient; in some cases, however, a capacity of 0.001 of trouble, and it may be necessary to use a total capa- mfd. or 0.002 mfd. may be found necessary. The full city for C, of Fig. i of about 4 mfd. before the hum mains voltage is connected across this condenser, and can be cured. Extra capacity across the other H.F. so it is very important that it should be rated for by -pa;s condensers should also be tried. continuous working on not less than 25o volts A.C. Another source of trouble lies in the H.T. rectifier It will usually be found possible to eliminate all valve , this may generate traces of hum by a sys- H.F. pscillations which are tematic search conducted modu ated by the A.C. on the above lines, and mains. This state of affairs the whole principle lies can Le cured by adopting -' - in working backwards the rectifier circuit shown from the loud speaker, in Fig. 5, where the valve and checking each point is prevented from oscillat- in turn. During this pro- ing by the two condensers cess a watch should be C, v hich should have a kept for high- resistance capacity of o.1 mfd. and connections, since these be tested to at least 1,000 are a prolific , source of volts - hum from electro- static In the writer's experi- pick -up ; apart from badly ence, however, the most soldered joints, they will troublesome source of be found most frequently mod elation hum is the in valve holders and plug - supply mains; and this pe in coils. There is always part cular form can occur the chance of a faulty ever when no H.F. stage valve, and this is the is wed. The cause of this most likely source if the FIA. 5. -The addition of the two condensers C to the H.T. type of hum is rather rectifier prevents the valve from generating H.F. oscillations, strength of the hum is obscure, but it is apparent- which would be modulated by the mains and cause hum. variable. In obstinate cases ly due to the presence of the effect of using a H.F. currents in the mains leads. The cure is simple potentiometer for th e connection between the cath- and satisfactory ; a condenser C, (shown dotted in odes and heaters o f the indirectly heated valves Fig 1) should be connected between one of the mains should be tried, a s recommended for an output a Correspondence, page 26, January 7th, 1931. valve.

R.S.G.B. Tests and Competition. G2XY H. T. Littlewood, 82, Stainburn Crescent, Chapel Merton, Leeds. (Change of '.1' to annual tests on the 28 mC. wave- address.) band have not proved very satisfactory up Transmitters' G5MI I. F. lleiklejohn, Royal Signals Mess, Alder- to the time of writing. Conditions have shot. and GSYH C. H. Chorley, 78, Nightingale Lane, S.W.12. beet: against long -distance reception, Notes. 06XP (ex 2BPU), A. J Perkins, 127, St. Asaph Rd. we mderstand that very few transmitters Brockley, S.E.4. (Transmitting on 1.5, 7 wer3 able to establish connection with dis- and 14 mC. and welcomes reports.) GBPA H. C. Page, Phunford Farm, Ospringe, Nr tal'', stations; in fact, the only one of Faversham, Kent. (Change of address.) wham we have yet heard was G5BY, Mr. An Eltham Enthusiast. O6RH R. Holmes, 446, London Road, Isleworth, H. L. O'Heffernon, who managed to get Mr. K. .1. Freeman (2ADL), 189, Well Middlesex. (Change of address.) COSR S. Rowden, Rosebank, Pilrig Street, Edin. across to the 8th District of U.S.A. on Hull Road, Eltham, S.E.9, asks us to burgh. Jaruary 11th. state that he is willing to co- operate with 2ABW E. Gankrodger, 4, Montrose Villas, Chewton Rd., Keynsham, 0 0 0 0 any amateur transmitter in his experi- Somerset. ments. He stands by for telephony at 2ADC F. S. Mizen, 28, Brunel Rd., Bridgwater Rd. Lo-y Power Tests. Nr. Bristol. the following times : Saturday, 14.00- 2ADL K. J. Freeman, 189, Well Hall Road, Eltham, Preparations are being made for the 20.00 G.M.T. on the 40 -metre waveband ; S.E.9. " One Watt Week " in April, in which 09.00 -18.00 on 40 2ADM A. J. Peck, 21, Greere Rd., E. 15. Sunday, metres, and ZAIN H. A. White, 39, Royston Ave., Wallington, cor ipetitors will be limited to an input of 18.00 -20.00 on 80 or 150 -175 metres, and Surrey. (Change of address.) 1 watt of pure D.C. supplied by accu- will also be ready to listen throughout the 2AMZ F. R. Drew, Frampton Cotterell, Nr. Bristol. mulators or dry batteries. 2AWV R. W. Wright, 42, Cedar Road, Aintree, The tests will night on 40 metres for long- distance tele- Liverpool. begin on Saturday, April 11th, and finish phony by arrangement. 2BAJ J. F. Stanley, The Frith, Aldington, Mersham, on Saturday, April 18th, and the times on 0000 Kent. each day will be from 21.00 to 08.00 NEW CALL -SIGNS AND CHANGES OF ADDRESS. 2BVL A. E. Brookes, 19, Alexandra Rd., Uplands, G2DC J. Drudge Coates, Burleigh, Manor Rd., Bedminster Down, Bristol. (Change of G. M.T. We hope soon to be able to pub - Farnborough Park, Hants. address.) lisa further particulars of this interesting 0200 (ex 2BIC), L. R. Seal, 90, Wollatent Rd., 2BXP A. N. Le Cheminant, 21, Raymond Rd., co upetition, which is being organised by Beeston, Notts. Victoria Park, Bristol. A. S. 100, 2BXZ W. A. Clark, 89, Laburnham Ave., Garden Mr. M. W. - G2XH Williamson, Rushdale Rd., Meen- Pilpel, G6PP. It is under brook, Sheffield. Village, Hull. st,iod that the points scored will vary G2XT (ex 2BZT), J. R. Wilson, 23, Salter's Rd., INDIA. from 1 to 5 in proportion to the distance, Gosforth, Newcastle -on -Tyne. VII2FZ Capt. de Lisle Carey, R.A.M.C., c/o Messrs. ai d that no pre -arranged schedule will be G2XII C. W. Shillam, :36, Bayswater Ave., Redland, Grindlay & Co., Ltd., Bombay (transmitting Bristol. C.W. on 7 and 14 mC, and will welcome allowed. 62XX F. Wilson, 85, Risca Rd., Newport, Mon. reports). R 1î

www.americanradiohistory.com Wflitd@ag FEBRUARY rrth, x931.

...... The R.C.A. Home Recording Superheterodyne. and Micro =Synchronous Filmoradio. Radio. By A. D 1NS DA LE.

'110 readers of The Wireless Two views of the R.C.A. World the many merit- superheterodyne are shown in orious advantages of the composite illustration. This the superheterodyne are too receiver has nine selective cir- well known to require reitera- cuits, three at radio frequency tion here, and the same applies and six at intermediate fre- to its disadvantages as present quency, which in this case is in the older models. In 175 kc. In order to eliminate America the superheterodyne as much interference as pos- has been out of fashion for a sible prior to signal amplifica- number of years for two reasons. From the buying tion, " pre- selection " at the incoming broadcast fre- public's point of view it was a case of " give a dog a quency is adopted. It is also desirable in a superhetero- bad name and hang him." From the manufacturers' dyne to have a relatively high signal level at the grid point of view the patent position was so obscure that no- of the first detector, or frequency changer. In the re- body knew for certain who had a cast -iron right to ceiver under review, therefore, there are two tuned cir- issue a manufacturing licence ; consequently, and very cuits ahead of the first valve, and these circuits are naturally, nobody was willing to risk his money on so arranged that they reject unwanted interfering signals, the manufacture of a set which, when all was said and being coupled so that great attenuation results in the done, was not so very superior to other types of set case of frequencies lying outside the desired band. which could be manufactured The coupling between the and sold without fear of H.F. and first detector valves complications. NOW that the furore over the Atlantic City, New York is a capacity -coupled H.F. The position to -day is that and Chicago radio shows has died down, and the transformer which, together has been winter radio season is in full swing, it is possible to with the valve, gives a uni- the patent situation review the radio-set position in the United States with definitely cleared up, and some degree of calmness, and endeavour to pick out the form amplification of about the R.C.A. has the mono- most outstanding developments of the 1930 -31 season. thirty over the broadcast poly of superheterodyne There are three such major developments, for all of which waveband. The oscillator the Radio Corporation of America, as the virtual mono- circuit is a conventional one patents. The development poly patent holders, may be said to be responsible: and improvements w hic h (1) the return of the superheterodyne ; (2) the still fur- for use with a standard 3- have been effected in screen - ther development and perfection of T.R.F. (tuned radio electrode valve, and consists grid valves and associated frequency) sets ; and (3) the introduction of home record- of a tuned grid circuit with an equipment have ing into all electric gramo -radio sets. A fourth inno- anode reaction coil coupled to enabled vation is just now beginning to loom up over the many of the old difficulties to horizon, viz. : the incorporation in gramo-radio sets of it. The grid is connected to be overcome, and the super- a moving -picture film projector, making use of the Bell - the mid -tap of the tuned grid heterodyne is once more a Howell home cinema equipment. circuit to minimise changes desirable set. The R.C.A. has in the oscillator frequency recommenced the manufac- which might result from a ture of an improved model, and licensed a number of change of valves. The tuning elements of the oscillator competitors to do likewise. Very few competitors can circuit are so arranged that the oscillator frequency is afford the licence, however, for it is of a very dictatorial always approximately 175 kc. higher than the fre- character and requires a terrific cash deposit or advance quency to which the H.F. system is tuned. on account of royalties- something in the neighbour- The intermediate frequency amplifier consists of three hood of £20,000. transformers and two valves. The first I.F. transformer B i6

www.americanradiohistory.com FEI,RUARY rrth, 193r. WipsIl®@e, 139 W0r 114. Recent Developments in America. - In the power supply a full -wave rectifier valve is is connected to the " local- distance " switch so that used, and the filter system is specially designed to elimi- when it is in the " local " position the selectivity of the nate hum from the loud speaker. The entire set is, of circuit is slightly impaired by the introduction of resist- course, designed to operate off the almost standard ance. This results in a wide acceptance band which American no volt 6o cycle A.C. mains. The loud prevents any possibility of side -band cutting, and en- speaker field is used as one of the chokes of the filter, sures the highest -quality reception on local stations. so that the total rectified current flows through. its When the switch is thrown to the " distant " position winding, thus providing a powerful magnetic field. The the timing of the transformer is made very sharp, so filaments of all valves are of the indirectly heated that, tt the expense of quality, interference is minimised cathode variety. All grid returns are earthed, and most when listening to distant stations. of the valves are self- biased by their plate current flow- Thy; transformer consists of a tuned primary con - ing through resistances in their cathode circuits, which resist- necte 1 in the plate circuit of the first detector, and a reduces any chance of coupling through common tunee secondary connected in the grid circuit of the ance. first [.F. valve. The secondary is partially shielded As regards the performance of the receiver, the degree from the primary in order of selectivity is so high, in to loosen the coupling and both positions, that a con- improve the selectivity. siderable amount of , side - The entire transformer is band cutting takes place. mounted in a copper can Performance. to keep the losses at a mini - murr and at the same time Maximum response takes shield the transformer place between 30o and windings from other parts í,00o cycles, after which of the circuit. When the the response falls off local- distance switch is rapidly. But to an Ameri- thro .vn to the local position can any receiver which a re sistance is connected does not reproduce with a across the primary and an- most decided boom or other resistance is placed in drummy effect in the lower series with the secondary register is definitely bad ; winding. he likes his bass (pro- Aroidance of High -note nounced " base," please, Loss. to avoid any misunder- The second and third standing ! ), and plenty of I.F, transformers are both it, so the R.C.A. super- alike, and consist of tuned heterodyne meets his prilnary and secondary requirements. In some coupled tightly enough to models there is included give a broad -top resonance with this superheterodyne chz racteristic with high an electric gramophone and att muation to frequencies home recording set. The outside the desired band. T.R.F. models provide for These transformers are three screen -grid H.F. me unted in iron cans to valves, power detector, shield them from other and two L.F. stages, the parts of the circuit, and to second of which mounts FILMOPHONE- RADIO. Home talkies, home movies, gramo- add sufficient loss to pre - phone and radio in one instrument. Side view showing movie two power valves in push - projector, gramophone turntable and pick -up. A front view pull. vent double peaks in the of the complete equipment in Its containing cabinet is shown resonance curve. For ac- in our title illustration. It is in the tuned circuits curate tuning, all three I.F. of this receiver that we transformers have adjustable condensers across both encounter the so- called micro -synchronous principle of primary and secondary windings. tuning. In the first place, tuning is effected by means Anode -bend rectification and a single L.F. stage com- of a single knob on the front of the cabinet. This knob plete the circuit, two power valves connected in push - is only turned for fine tuning ; for rough tuning it is pull feeding the dynamic speaker through an output swung laterally across a long horizontal slot in the transformer. Great care has been taken in the design panel, carrying with it a celluloid indicator which sweeps o1 the cabinet to prevent that bane of the compact, all - across a scale calibrated in kilocycles. The knob itself ir -one receiver, cabtriet resonance. Volume control is is attached to one end of a lever, the other end of which effected by varying the control grid bias on the H.F. is attached to a round plate, or cam wheel, around the a id first I.F. amplifier valves. This gives a balanced circumference of which are arranged the five variable reduction in amplification at both radio and intermediate tuning condensers. Each condenser is connected to the f: equency when the volume control is turned down. edge of the cam wheel by means of a lever, and is pivoted B 17

www.americanradiohistory.com I4 FEBRUARY 11th, 1931.

Recent Developments in America. - twisting action which, with the ordinary tone arm, would or supported at one point only. The side of the con- be imparted to the tone arm from the needle. denser opposite the pivot point is free to move within Perhaps no other feature of the combination instru- certain limits. A roller on top of the chassis is attached ments is quite so fascinating as home recording, which to this moving side of the tuning condenser. opens up a new and unlimited field of home entertain- The driving action, which produces a scissors -like ment. The small hand microphone is operated in con- motion in the condensers when the tuning lever is junction with the L.F. amplifier of the wireless set, the operated, is not from the rollers, but is through the output of which is fed back through the electric pick -up, micarta connector links which are attached to the five causing the latter to vibrate in such a manner as to metal rods fastened to the underside of the cam wheel. record on a special disc. This home recording is Thus, the condenser moves in two ways -a compara- available for either microphone or wireless by simply tively large amount due to the action of turning the turning the control switch in the gramophone compart- cam wheel, and a very small amount on one set of ment. Thus, either a home -made or a wireless pro- plates due to the roller on top of the chassis following gramme can be recorded. the irregular track around the outer edge of the cam As soon as the record is made, the control switch can wheel. This last-mentioned movement is the compen- immediately be turned back to " Record Reproduc- (Below) R.C.A. superhetero- dyne. The three I.F. transformers are housed in separate metal screening boxes.

(Above) Top view of the superheterodyne chassis. The partial screening of the valves will be noted.

sating motion which is the secret of micro-synchronous tion," and, using the same special recording needle, the . tuning, for, by means of adjusting screws, each con- record can be reproduced right away without any pre- denser can be individually adjusted before the receiver vious processing. A small weight is placed on top of leaves the works, so that all five circuits are exactly in the pick -up when recording, so as to provide the addi- tune at any setting of the dial, thus ensuring maximum tional mass necessary to ensure correct engraving of efficiency at all times. the record grooves. This weight is always removed when reproducing. The records themselves are of the un- Gramophone Refinements. breakable variety, and can thus be sent by post to The gramophone turntable is driven by an induction- friends. They measure six inches in diameter and play type motor, and micarta gears replace the metal gears for one minute twenty seconds. formerly used. A heavy felt pad, permanently impreg- The recording of wireless programmes is accomplished nated with vaseline, replaces the governor friction leather in the same manner as microphone recording, except formerly employed. These two items, it is claimed, that the output of the detector valve is substituted in the assure absolute quietness of operation and perfect speed circuit for the microphone. In both cases monitor repro- regulation. The motor is started simply by lifting the duction of comparatively low volume is obtained in the pick -up arm and moving it to the right beyond the outer loud speaker during the recording. edge of the record. A starting switch lever is thus The quality of reproduction from these records does eliminated. not, of course, in any way approximate that of com- The pick -up is of a new flexible impedance type, with mercial gramophone records, but they are, neverthe- oil damping, and the pick -up arm itself has several dis- less, surprisingly clear and free from distortion. Their tinctive features. In appearance it can best be likened playing time might, however, be increased with to a cobra's head. It is counter -weighted at the back advantage. to prevent excessive record wear, and it is weighted at What may be termed the very last word in home the two sides of the wide front portion to prevent the entertainment is an instrument which incorporates in one B Ió

www.americanradiohistory.com FEi3RUARY rith, r93r. Wp@ll®-N 141 W01i-110 Recent Developments in America. - list of silent and talking films for home entertainment con.ole : home movies, silent or talking, radio, and xvliich can now be obtained in the United States from graricphone. This departure has just been announced photographic dealers. Radio and gramophone combi- by Messrs. Bell and Howell, of Chicago, well -known nations have already been worked out satisfactorily, camera manufacturers. A Bell and Howell film pro- but the additional problems involved in balancing the jector, utilising standard i6 mm. home -movie size film, other units with the film projector have not previously is used for the pictures, and a Howard chassis is the been so successfully solved. bass of the radio portion of the instrument. A gramo- These developments provide ample evidence that the phcan f turntable is so arranged that it can be operated wireless receiver, once the hobby of a few enthusiastic at either the standard speed for ordinary gramo- and highly technical amateurs, is rapidly developing and phone records, or 331 r.p.m. when the records for being associated with other inventions to the end that talkirg pictures are played. Thus, either silent or talking it shall provide the basis of a single instrument capable pictures can be shown, or the pictures may be shown of delivering, at will, every form of home entertainment. to the accompaniment of either the wireless programme In operation, these compendiums of entertainment are or a r ordinary gramophone record not synchronised practically one hundred per cent. dependable; in out- with the film. In addition, of course, either the wireless ward form they are pleasing to the most a:sthetic taste ; set o: the gramophone is available independently. and their application is rapidly becoming universal -at This development has been inspired by the growing least, in America.

DESIGNING AN EFFICIENT ALL -WAVE RECEIVER. Overcoming Tuning Condenser Difficulties. LTHOUGH it is possible to use a short -wave re- condenser in parallel a itli each existing tuning condenser ceiver with tolerably good results on the broad- of the short-wave receiver. A cast band by means of suitable plug -in induc- It is quite a simple matter to mount terminals on tam es, the converse is by no means true. The average the panel which are connected internally to the fixed broadcast receiver usually contains switching arrange- and moving vanes of the tuning condensers. if care is ments for changing over to the long waveband when taken in choosing the position of these terminals so that des red, and various other arrangements, including the the internal connecting wires are short and direct, there layout, designed to give maximum convenience on the will be not the slightest detrimental effect on the per- s a ^elengths upon which programmes are usually sent formance of the receiver on short waves. In this manner cut. The result of this is the existence of various stray it is possible to construct an all -wave receiver which is c:içacity effects which, while having negligible effect really a practicable and worth -while proposition. By on broadcasting wavelengths, have such serious results winding suitable coils it will be possible to use it for. on the short waveband as to preclude all possibility of listening -in to aircraft telephony transmission on goo sat sfactory reception. Apart from this the fact that metres, which is a wavelength just missed by a large the tuning condensers in an ordinary broadcast receiver number of broadcast receivers with the dual -wavelength usually have a maximum capacity of 0.0005 mfd. pre- type of tuning. set is further difficulties, since even if an attempt is made to use them only from minimum up to a capacity of 0.00015 mfd., tuning becomes exceedingly critical, as it s all crowded into a few divisions of the condenser scale, whereas in the case of a receiver specially designed fo short waves the use of tuning condensers having a maximum capacity of 0.00015 mfd. enables the full seile of the dial to be employed. Fitting a Parallel Condenser. The only difficulty likely to arise when using a short - w we receiver on the broadcast band is also due to the tuning condensers, which, as already mentioned, usually have a low maximum capacity. No difficulty will be e::perienced, of course, in winding suitable plug -in in- d lctances to cover the normal broadcast band, but the h gh L/C ratio will not be very conducive to good selectivity. In order to overcome this, and also in order to enable the receiver to be used on the long-wave broad - c fisting band without the necessity of an unduly large "NOISES OFF." A glimpse in the effects" studio at Munich number of plug -in coils, it is advisable to make pro - during the broadcasting of a play in which the aids to realism ision for the external connection of included a drum, Klaxon horn. air pistol, megaphone, steel a 0.0003 mfd. chain, and a dinner plate. n in

www.americanradiohistory.com 'p Wpshoo FEBRUARY rrth, r93r. íi.`ï'ila CORRECTING. ,PICK-UP

CHARACTERISTICS How to Make Use of a Simple Filter to Remove Resonances. By H. E. WATSON, D.Sc. pN inspection of the characteristic curves of with the Parlophone gliding -tone record, by recording gramophone pick-ups, published in The Wireless galvanometer readings every five turns. With a little World for March 26th and April 2nd, 1930, practice, no difficulty was experienced in counting turns reveals the fact that the majority of them exhibit a and writing down readings simultaneously, the only resonance peak in the neighbourhood of 3,000 cycles. essential being a piece of paper attached to the turn- Although a rising characteristic is desirable to counteract table, which gave an audible click at each revolution. high -note loss in the amplifier, a pronounced peak not This method is not very reliable in the neighbourhood only introduces unpleasant of resonance peaks owing to quality owing to exag- for those who do not possess the means for measur- the lag in the movement of gerated overtones, but also EVENing output voUages, attempts at modifying the quality the galvanometer needle cause overloading of from a gramophone pick -up form an interesting and instruc- when the displacement is may tive study using the ear alone as a guide, and there are few the amplifier on the high pick -ups on the market which will not respond to the treatment. rapid, and it may miss notes. The practical data given in this article will assist in removing minor resonances, but as It occurred to the author the resonance peak which commonly occurs in the upper these variations cannot be that a defect of this type register. detected by ear, strict could be cured very easily accuracy is unnecessary. by the use of a simple band -stop filter, and a few The record is also unsuitable for frequencies much experiments showed that an excellent characteristic could below 500, but as the present experiments were con- be produced at small expense. A small choke was fined to resonances in the high notes, the wide scale in made by winding 1,800 turns of No. 33 S.W.G. enamelled wire on part of the core of a burnt -out L.F. B.T.H. PICK-UPREVOLUTIONS transformer with a cross 12820 200 180 160 140 120 100 60 60 40 20 0 section of tin. x ¡in. The choke and a condenser were 16

connected in series across the I 14 pick -up terminals, a 50,000 - l ohm potentiometer volume control forming the output co 1.2 load. The arrangement is 0'0024 mfd+5J00011_ 0 1.0 shown in Fig. I. > ,ri O'0024mfd In order to test the effec- C'003mf. ' 41;050,0000 Fig. 1. -A band -stop filter tiveness of the system, 0e consisting of a choke and á r Eau condenser in series, shunted measurements were made 06 across the pick -up. using a B.T.H. pick -up. A W characteristic for the pick -up alone was first plotted, 0'4, with the help of H.M.V. standard records, the output being measured on a Moullin voltmeter. The curve 0'2 agreed very closely with the one given in The Wireless World, but the observations required a considerable o 200 5Ò0 1000 2,d00 3,000 4,000 5,000 6,000 time, and correction of the values to a constant ampli- FREQUENCY tude was rather tedious. It was found that almost Fig 2.- Curves showing the effect of different compensating identical results could be obtained in a few minutes filters on the characteristics of a B.T.H. pick -up. Is 20

www.americanradiohistory.com FEBRUARY 11th, 1931. Wfp®Ilgoo 143 WmIlcl Correcting Pick -up Characteristics. - identical in construction with the B.T.H. instrument, this region, as shown on the diagrams, was a distinct taken from a commercial radio- gramophone (pick-up B advantage. in the diagram). The bass output in this case is larger Fig. 2 (i) shows the output characteristic of the than that from the resonance peak which occurs just newest type B.T.H. pick -up with a medium " chromic " below 4,000 cycles, so that overloading would take place needle. The voltage at the resonance peak is nearly first on the low notes. The effect of the filter is less double that at any other part of the curve, and conse- marked, but is still detectable by a softening of the quently overloading may easily occur on the high notes. quality. The experiments so far described were made with the REVOLUTIONS object of deliberately 220 200 180 160 140' 120 100 80 60 40 20 0 altering the characteristics in 2-2 order to obtain improved results. However, it may not be out of place to point out that unintentional 2'° \ \ í variations of equal or greater magnitude may be easily ,6 produced by using a pick -up in an unsuitable circuit. This ',M 2 was strikingly brought home to the author on substi- : tuting a Marconiphone 1 6 pick-up for those previously tested, using the same potentiometer, but no filter. The ¡h1 \\ output in 1 4,i +\ the upper register was very small, as shown \ ,` iI!' 1' by the lowest curve (M,) in Fig. 3. On disconnecting u) V : \ ®I Ell the potentiometer, the upper dotted -line curve (L) O 12 Ili I > :" II was obtained, showing an output three to five times \ i Y 1'0 ` 01 as much as before. \, Important Effect of Connecting Cord. PICK UP B -+0'003mfd+5, ` - 0 I \ In the case of the two other pick -ups, the 50,000 -ohm 0'6 \ output load introduced a small difference, but it was <. f-\ evident ` 11 that the present effect was of an entirely different oa order. As the curve with no load still -'M1 differed con- ', _-.,___.-__i / \ , siderably from the one given by The Wireless 02 World, attention was next directed to the cord connecting the o, pick-up to the amplifier. This was a flat twin braided 200 500 1 d00 2,000 3,000 4,000 5,000 6.000 telephone cord of a common type, r2ft. long, belonging KREQUENCY to another pick -up. Subsequent measurement showed Fig. 3.- Curves M1 and M2 show the effect of working a Mar - the capacity between the leads to be about 0.0005 mfd. coniphone pick -up into an unsuitable load. The characteristic of pick -up B is considerably modified by introducing a filter. and the power factor to be poor, the D.C. insulation resistance being 24 megohms. The introduction of the choke and a o.003 -mfd. con- On disconnecting the cord and using two short inde- denser reduces the curve to the shape shown in (2), pendent leads to the voltmeter, the remarkable peak while with a 0.0024 -mfd. condenser (3) the resonance peak disappears altogether. A 5,00o -ohm resistance in MARCONIPHONE (GOLD TUNGSTYLE) REVOLUTIONS 220 200 180 160 140 series diminishes the effect of the filter and gives curves to 120 100 80 69 40 20 u (4) and (5). with a distinct peak. Intermediate values can be obtained with smaller resistances. It will be s seen that the value of the condenser is by no means critical. 8 It must be remembered that although the curves (2) NO LOAD to (5) appear irregular, the variation is scarcely detect- 7 able even by a very critical ear. The effect of the filter N 6 as a whole is, however, quite appreciable in the form J p5M of an improvement in quality. O > 5 When the pick -up was shunted by a 0.125 -mfd. con - denser ¢ there was a pronounced resonance peak at 500 á 4 cycles, this being an electrical resonance, whereas the 3,50o -cycle peak is due to mechanical resonance. It 3 might be expected, therefore, that the introduction of CRD CORD a o.00z -mfd. condenser in parallel, as indicated in 2 Fig. I, would produce resonance at about 5,60o ' cycles - or rather less, allowing for lead capacity, and thus 1 -.. _r/.,---- , improve the ---_. output at this frequency. Measurements ..... Ç_. DM _. _ _OR'+005----'i2 showed that a condenser of this size did increase the o, ----' \ 200 500 1000 2,000 3,000 4,000 5,000 6000 output at very high frequencies, but the increase was FREQUENCY so small that it would make no appreciable difference. 3 Fig. 4. -The capacity and power factor loss of a long connect- Fig. shows characteristic curves for a pick -up almost ing cord are strikingly brought ont by these curves. 13 2I

www.americanradiohistory.com 144 WApaS00 FEBRUARY 11th, 1937. WOpIlCI Correcting Pick-up Characteristics.- These curves, as well as others which cannot be voltage of 9.& was obtained at 5,000 cycles, as shown discussed in the present article, show that the pick -up in Fig. 4. A 0.5- megohm load, as recommended by under consideration depends to a large extent upon the makers, reduced this to 5.7, but even so it is evi- electrical resonances for its successful operation, and dently inadvisable to connect the pick -up to a valve great care should be exercised when connecting it to with only 2 or 3 volts grid bias. Another curve shows an amplifier. It must also not be forgotten that the that connection of the cord, as well as the resistance, characteristic rises very considerably below a frequency almost causes the disappearance of the peak. The of 200, and due allowance must be made for the high vertical scale in Fig. 4 is one -fifth that of Figs. 2 and 3, voltages which may be developed on law notes. The and the two broken -line curves of Fig. 3 have been question of low -note output has not been dealt with reproduced in Fig. 4 to show the relative magnitudes. here as it has been discussed several times previously.

How to Manage a Set. On the following Tuesday, the 20th January, Valuable hints on the management of wire- the popular taste was catered for by a series less sets were given by Mr. Scott Sessions in of tests of loud speakers, using sound test re- a recent talk before the Golders Green and CLUB NEWS. cords. The club arrived at some interesting Hendon Radio Society. In the course of his and also some surprising conclusions, one of professional work of testing sets, Mr. Sessions which was that the club speaker was had gained useful experience concerning the still Mr. capable of giving a good account of itself! most likely sources of breakdown and the Garside demonstrated a large four -valve The beginners' lectures mistakes instrument in an oak cabinet. were continued from most often made by novices. He Men- Utilising S.G. the point reached last session, with two lec- tioned case a amplification operated off the of large and costly set in it the D.C. mains by tures on the 16th and 23rd January ou Induction which the grid -bias battery was accidentally means of four of the new type indirectly heated and Self Induction. disconnected. The result was that the heavy valves. A large variable resistance and pro- vision for Hon. Secretary, Mr. C. J. Townsend, 14, current drawn from the H.T. battery ruined the use of either a moving coil or Hamilton Street, N.W.1. the - valves, transformers, chokes and con- an ordinary speaker were only two o4 the novel densers, and involved a heavy repair bill. points about this receiver, the power- handling 0000 In the subsequent discussion Col. Ashley qualities of which were particularly noteworthy. Transformer Hints. Scarlett emphasised the neoessity of a milli - Hon. Secretary: Mr. C. J. Witt, 39, Coniston ammeter with every set. Road, N.10. A lecture on " Transformers " by Mr. J. Hon. Secretary: Lt.-Col. II. Ashley Scarlett, 0 0 0 o Baggs, of Messrs. Ferranti, Ltd., proved a great 60, Pattüson Road. N.W.2. attraction at a recent meeting of blade oono For Short -wave Beginners. Radio (Birmingham). Radio in Bermondsey. A paper intended as an aid to the newcomer After dealing with the earliest commercial to short -wave work and entitled " Listening on use of transformers, the lecturer passed on to The classes held at the Bermondsey Men's Short Institute continue to flourish. Sections for in- Waves," was-read by Mr. Harold Hodgens, describe the firm's latest product, the A.F.B. termediate and advanced work did very well Chief Radio Instructor, City of Dublin Tech- Constructional details and characteristics were during last term, and it has now been found ideal Institute, before members of the Trans- given, and the effects of back coupling and of expedient to start a third class for beginners, mitters' Section of the Wireless Society of two transformers in a receiver were described. due largely to the number of new members Ireland on Monday, -January 26th. Valuable information was given concerning im- who have applied for membership since re- Having dealt with the allocation of the pedance. inductance, output chokes, resistances, opening after the Christmas vacation. short wavelengths, Mr. Hodgens went into de- D.C. resistances of speakers, and different types An attractive series of lectures and demon- tails of day and night range and skip- distance of output. ' strations has been planned for the rest of the effect, mentioning particular cases of the Details of the Society may he obtained on session, and all interested are invited to apply Chelmsford B.B.C. station and the Continental application to the Hon. Secretary, 110, llillarire at once to Mr. G. Hall, Headmaster, Bermondsey stations at Eindhoven and Zeesen. The varia- Road, Gravelly Hill, Birmingham. Men's Institute, The Alma L.C.C. School, tion of signal strength with the seasons was 0000 Southwark Park Road, Bermondsey, S.E.1. illustrated by graphs giving the received signal o00o intensity for two New York stations. The Stenode and Television. A Ferranti Demonstration. Hon. Secretary : Mr. H. Hodgens, 12, Trinity " The Stenode Radiostat and its Application That Messrs. Ferranti, Ltd.. are friends of Street, Dublin. to Television " is the title of the lecture and the amateur radio societies was amply borne o000 demonstration to be given this evening (Wed- out at the meeting of the Muswell Hill and Catering for Everybody. nesday) before the Television Society by Mr. District Radio Society on Wednesday, January The Kentish Town and District Radio Society E. L. Gardiner, B.Sc. The meeting, which opens 28th, when Mr. Garside, representing that firm, began its new session in good style on Tuesday, at 7 p.m., will be held at University College, made a journey especially from Lancashire to January 13th, when a lecture of especial inter- Gower Street, London, W.C.1. Non -members of give a talk. Furthermore, the company sent est to the transmitting members was delivered .. the Society may obtain cards of admission ou a large packing case full of apparatus for the on the subject of " The Theory and Practice application to the head office of the Society at demonstration. of Crystal Control." 4, Duke Street, Adelphi, London, W.C.2.

TE STING THE LIFE OF VALVES. Equipment installed at the Wembley laboratories of the General Electric Company for maintaining close control of the life performance of valves under working conditions. B 22

www.americanradiohistory.com FEBRUARY rrtlz, ro3r. Mp@ISOO MpIld 145

Events of the Week in Brief Review.

CHANTICLEER CALLING. 100 KW STATION FOR RADIO AT LILLE FAIR. Radio -Beziers (France) has adopted the LUXEMBURG. A wireless section will be included in cockcrow as an identification signal. A French syndicate has obtained a con- the famous Lille Commercial Fair, which 0000 tract from the Grand Duchy of Luxem- will be held from burg for April 4th to 19th. NEW POLISH STATION. the construction and operation Contrary to the previous custom, radio of a 100 kW. broadcasting station. The new 20 kW. broadcasting station 0000 demonstrations will be permitted. at Vilna (Poland) will be opened' in 0000 SHORT WAVES FOR LIGHT WEIGHTS. March. The wavelength will be 312 COHERER INVENTOR metres. John J. Long, chief engineer of WHAM "HONOURED." 0000 broadcasting station, Rochester, U.S., M. Edouard Branly, who invented the coheres, and is BILINGUAL PROGRAMMES IN has applied to the Federal Radio Com- generally regarded as BELGIUM. mission for a test permit for the " useless France's " pioneer of wireless," was Belgium's new State- controlled broad- frequencies " between 50,000 and 100,000 recently fêted at a demonstration in his casting scheme came into force on Feb- kc. (6 to 3 metres). His object is to honour at the Sorbonne. Unfortunately, ruary 1st, when the National Broadcasting develop lightweight transmitting and the meeting was poorly attended, and the Institute made its first dual transmission receiving equipment for aircraft. Paris Soir remarks bitterly that many from the Velthem station. French pro- professed admirers of the savant were grammes are sent out on 508.8 metres and conspicuous by their absence. FORTHCOMING Flemish on 333.5. - EVENTS. o o o o. O O O O WEDNESDAY, FEBRUARY 11th. R.S.A. LECTURE ON TELEVISION. VATICAN CITY CALLING. Golders Green and Hendon Radio Soriety. Mr. W. G. W. Mitchell, B.Sc., joint -Annual 7 To -day (Wednesday) will feature in dinner. At p.m. At the Brent hon. secretary of the Television Society, Italian radio history as the occasion of Bridge Hotel, Hendon. !Haswell Hill and District Radio Society. - will deliver a lecture entitled " Develop- the official At 8 opening of the Vatican City p.m. At To(lington School, Tether - ments in Television " at a meeting of the wireless station. Special significance down, N.10. Lantern lecture: "Simple Facts about Radio," arranged by the Royal Society of Arts at John Street, attaches to the event owing to the pre- Mallard Wireless Service Co., Ltd. Adelphi, London, W.C.2, on Wednesday, sence of the Pope at the inaugural cere- North Middlesex Radio Soriety. -At 8 p.m. At St. Paul's Institute, Winchmore Hill, February 25th, at 8 p.m. Apparatus mony. We understand that the pre - N.21. Load Speaker night. illustrating the principles of television liminary transmissions will be made on Television Society. -At 7 p.m. At Univer- will be exhibited. The chairman will 50.26 and 19.84 metres. sity College, Gower Street, London, be 0000 W.C.1. Lecture and demonstration: Sir Ambrose Fleming, M.A., D.Sc., "The Stenode Radiostat and its Appli- F.R.S. RADIO TELEGRAMS FROM ration to Television," by Mr. E. L. Gar- 0000 AEROPLANES. diner, B.Sc. The Lufthansa Air Service has in- BEYOND A JOKE. THURSDAY, FEBRUARY 12th. A wireless augurated a radio telegram service in prank carried out by students Radio Society of Great Britain. -Joint of Yale University came to an conjunction with the German postal informal meeting with Lensbury Radio abrupt authorities. Passengers on the regular Society. At 6.15 p.m. At the Lerture end a few days ago when the ringleaders Theatre, 16, were routes can now hand telegrams to the Finsbury Circus,- E.C.2. warned by the Federal Radio Conn - Slade Radio (Birmingham). -At 8 - -p.m. mission wireless operator on the 'plane for de- At the Parochial Hall, Broomfield Ri'ad, that they were liable to fines of spatch to a ground wireless station and Erdington. Lecture and demonstration: £1,000 and imprisonment for five years. " Dynamic Inductor Speakers." by Messrs. The students had transformed a thence to the nearest telegraph office for M. B, Simmonds and G. T. Peck. radio delivery to any address in Germany. receiver into a low- power, transmitter, 0000 MONDAY, FEBRUARY 16th. and although their original intention was Hackney Radio and Physical Soriety: At merely to startle the district with strange RADIO SMUGGLING IN CZECHO- the Electricity Showrooms 18-24, Lester SLOVAKIA. ('laptop Road, E.5. Disrussinn, "T'he calls, they grew more enterprising and A wireless receiver; believed to be one Design of L.F. Amplifiers." attempted to sell " time on the air " to of a number smuggled into the country local merchants. from Germany, has been captured by the SHORTER WAVES FOR THE SOS1 0000 Czecho -Slovakian police at Leipa, Bohe- An international regulation reserving a SHORT WAVES FOR GENTLEMEN. mia. The police suspect that a regular band of short waves exclusively for SOS " Short waves have donned evening traffic exists in radio sets which escape calls is urged by Capt. S. C. Hooper, clothes. They have gone ` class,' " runs import duty. - director of U.S. Naval Communications, a circular which we have received from 0000 who points out that the 600 -metre wave- an Altoona, Pa., U.S.A. radio manufac- NEW ITALIAN RESEARCH CENTRE. length is often inadequate over vast ex- turer who is marketing a new super- The erection and equipment of Italy's panses of water like the Pacific, Indian heterodyne short -wave adaptor for the new radio research Station is the main task and South Atlantic Oceans. He contends ordinary radio receiver. " Short-wave to be undertaken in 1931 by the Italian (writes our Washington correspondent) reception has definitely left the category National Council of Research, of which that the maximum daylight range on this of merely a pastime for nuts, cranks, the Marchese Marconi is president, writes wavelength rarely exceeds 500 miles, and and wild -eyed experimenters," continues our Turin correspondent. The council that an alternative short wavelength the refined writer, who explains that will also investigate precision methods of would guarantee reception over much " one can now listen to short wavelength -wave signals measurement. greater distances. without mussing one's hair or being on B 2 5

www.americanradiohistory.com 146 Wpemo FEBRUARY firth, 1931. V7aellcll speaking ternis with kilocycles, audio ings in the studio while listening to the wire has the same average distance from frequencies, capacities, impedance, or performance ou loud speakers. the centre of the main. As the frequency other obstructions to normal enjoyment." The Funkhaus occupies a dominating of the supply during the life of this patent The news that short waves are now position in the western suburbs of Berl' rarely exceeded 100, this idea had no " classy " will come as a great relief to adjoining the famous Radio Exhibil4 great development, but, with high fre- our readers, many of whom must have grounds. quency wireless work, it is now in com- been leading a Jekyll and Hyde existence. 0000 mon use, and sometimes called Litz wire. 0000 SETS FOR BLIND LISTENERS. " Another patent of the year 1888 re- REGI)NAL SCHEME FOR NORWAY. More than 7,500 wireless receivers, lated to the first and master effort to Ninety per cent. of Norway's popula- mostly crystal sets, have been distributed provide a perfect means of smoothing tion will be vi ithin " single -valve range " to. blind listeners in various parts of the and filtering any ragged edge or uneven- of a broadcasting station if a new scheme country in connection with the " Wireless ness in the flow of a uni- directional cur- of reorganisation prepared by the Nor- for the Blind " Fund, the total sum col- rent. Incidentally, it is a clear wegian Chamber of Commerce and the lected by the end of January being anticipation of the audio transformer, Telegraph Department is passed by £30,000. and users of wireless apparatus ought Parliament. The scheme provides for a Work has now begun on the delivery of to feel grateful that here at any rate is total of forty -one main and relay trans- 5,000 single-valve sets to blind listeners something free. There is only one claim, mitters to replace the existing ten. in districts which are beyond the crystal and, put briefly, it is ' to equalise 0000 service area of a broadcasting station. strength of current by means of a self - PERPLEXED. To serve areas still more remote an order induction coil or coils or condenser,' and Front the Editor's Postbag, Croydon Paper :- AERIAL LOOPS. A resident, I note, has attached several wire loops to his radio aerial and a number of little black condensers along it. I wonder if any of your readers can tell me whether there is any advantage gained by these. Do they give greater selectivity or stronger volume? AMATEUR. Carshaltoi lull. Our office boy suggests that the object is to condense the signals to enable them to pass through the loops. 0000 LECTURING OVER THE ATLANTIC. Sitting in an armchair in his home at Oxhey, Herts, at 1.45 a.m. on Friday January 30th, Mr. C. C. Paterson, president of the Institution of Elec- trical Engineers, delivered a " lantern " lecture' to members of the Engineering Institute of Canada, who were assembled in their lecture theatre in Montreal, 3,000 miles away. The lecture was transmitted by direct radio- telephony from this country to Canada. The circuit was from the Post Office Rugby station and the Bodmin beam station of the Imperial and Inter- national Communications Company to the Canadian Marconi Company's station at Yamachiche. Landline communication was established between Mr. Paterson's home and Bodmin and Rugby, as well as between Yamachiche and Montreal. A NEW RELAY.- Salzburg, Austria, the birth -place of Mozart, was recently honoured with a new broadcasting station relaying the Vienna programmes. The 0000 photograph shows the castle and gives a good idea of the picturesque surroundings. GERMANY'S " BROADCASTING HOUSE." has just been placed for 1,000 two -valve there are three drawings which prove the A Radio Museum will be one of the in- sets. genius of the patentee. Not content to teresting features of the Berlin " Funk - The committee of the Fund states that place one coil in series, lie provides for haus " -the headquarters of German the amount still needed for sets alone, another in parallel, and then suggests a broadcasting. The new building is already combination of the two, which is the other apart from any question of maintenance, in use, but the museum and several is about £15,000. Donations modern audio transformer." (writes estimated at departments are not yet completed should be sent to the offices of the Fund, 000a a correspondent). London, of 226, Great Portland Street, FREE LICENCES FOR THE The main studio occupies the centre W.1. the building and measures 130ft. by 70ft. 0000 BEDRIDDEN? could be per- It is 40ft. high and is provided with a RADIO'S DEBT TO DR. DE Whether the Government gallery running round three sides. In- FERRANTI. suaded to grant free wireless receiving geniously housed in a compartment on the A point of special interest to wireless licences to permanent invalids was dis- fourth side is a large organ. enthusiasts was mentioned by Mr. Frank cussed at the recent annual meeting of There are two smaller studios fitted Bailey, managing director of the City of the Manchester and Salford Society for with movable walls, permitting the size London Electric Lighting Company, in the Provision of Wireless for the Bed- of the studios to be regulated according his recent lecture on the " Life and ridden Poor. It was stated that for the to the type of performance for which they Work of Dr. de Ferranti, F.R.S.," at the past year the fees paid for licences are required. All the studios are fitted Manchester College of Technology. " As amounted to nearly a third of the with special soundproof windows of a far back as 1888," said Mr. Bailey, "Fer- Society's expenditure. type designed at the Heinrich Hertz In- ranti obtained a patent for the idea of The Committee is anxious to hear cf stitute in Berlin. Through these the an electrical main composed of thinly anyone willing to " adopt" a case in control engineers can observe the proceed- insulated wires so put together that each which a set has already been provided. B 26

www.americanradiohistory.com FEBRUARY 11th, 1931. VrpssIlsoo,s Upl ! to date. Naturally, he could not expect the makers of the set to un- dertake this work, and as the local /D dealer ' FREEGR seemed shy of tackling the bthse job I agreed to do it. A cursory Caution. actually necessary to remove the examination of the set, which was I notice that certain Continental back in order to connect up the pick- very compactly built, showed me receivers which are quite unscreened, up, and this almost makes me think that I should save myself a lot of although having several stages that certain radio manufacturers time if I had the theoretical circuit of are H.F., have lately been advertised fearful lest the electric gramo- diagram of the instrument before phone, me. in the columns of certain daily pure and simple, should be- I accordingly wrote , to the come more makers papers. The price is certainly at- popular than the radio of the instrument explaining tractive, but I should advise intend- receiver itself. the circumstances, and asked them ing purchasers to be wary, as a if they would be good enough to multi -stage H.F. set which requires American Memories. supply me with the necessary no screening probably requires no The picture of the Cortlandt printed diagram. Their reply was volume control either. Street Radio Market in New York somewhat ambiguous, it merely City, which was recently published stating that they were unable to give A Pick -up Plea. in this journal, brought back me a circuit diagram. They prob- memories of the first triode which I ably meant that they were unwilling The position of the jack usually to do so, although, since I found out supplied for the connection ever purchased. This was actually of the in Fulton on disembowelling the set that most gramophone pick -up is, I find, very Street, one of the many drab thoroughfares at the of its components were of foreign awkwardly placed in most sets. It back of the Woolworth building, of which origin, I might have been justified in is usually situated somewhere at the assuming that their knowledge of the back of the set in Cortlandt Street is another. I still an un-get -at -able have set did not extend beyond the impor- position, and, since in most it in my possession, and, curi- cases it ously enough, tation of the parts and the assembly is so connected that it is not possible the filament is still intact, although it of them from a blue print. If this is to leave it in circuit when the re- has long been out of action. so, I shall have much pleasure in ceiver is being used for radio work, It cost me ten dollars, I recollect, but it never supplying them gratis with a copy of the whole business of connecting or did very much for . its living. The principal the diagram, which I eventually disconnecting the pick -up is very memory worked out after much tedious wire - temper- trying, especially in which I associate with it is the case tracing ; indeed, the disposition of of a heavy set. I realise, of course, the parts was so peculiar, and the that in most cases its position is de- wiring so unnecessarily compli- termined by the necessity of the cated, that it almost seemed that it avoidance of a long lead hanging on had been deliberately " scrambled " to the grid of the first valve of the in order to prevent the ordinary gramophone amplifier, but some amateur from finding out the theo- makers seem to have deliberately retical circuit diagram. The circuit, exercised Machiavellian ingenuity in however, proved to be a very ordin- making the use of the pick -up as ary affair, and I could only con- difficult as possible, although clude, therefore, that after all they whether this is by accident or with simply did not want me to know the malice aforethought I cannot say. circuit. Surely in these enlightened days I have referred previously in these there is not the slightest reason why notes to this reluctance on the part the gramophone pick -up connection "You mean 'toob'?" of certain manufacturers to disclose should not be made a permanent fix- what they called their mystery cir- ture, the actual pick -up itself being the look of bewilderment which I cuits, and I thought that the practice thrown in and out of circuit by a received from the shop- assistant- had quite died out. It seems very simple switch mounted on the front sorry, sales clerk -when in a foolish, as any rival manufacturer of the panel. It is no use arguing moment of mental aberration I called who desires to know the circuit has that serious loss of efficiency would it a wireless valve instead of a radio only to buy a set and put one of his result owing to the fact that one ter- " toob. " technical men on to the tedious but minal of the switch would always be straightforward task of tracing it permanently A out. connected to the grid Bad Sign. When information of this kind is re- of the valve. The days of low -loss The other week I was asked by a fused I think that one is fully justi- circuits in commercial sets have gone friend to make a major modification fied in concluding that the manufac- for ever-if, indeed, they ever to an all- electric receiver which he turer has got something unsound in existed -and the slight extra " loss " purchased somewhat over a year the design of the set which he caused by does this wire would be neglig- ago ; he required the alteration in not want his more enlightened cus- ible. In the case of certain sets it is order to bring the set completely up tomers to get wind of. B 27

www.americanradiohistory.com 148 Wilp®Il®00 FE13R1 _ittY 11th, 1931. WoirM LQTLIS

Natural Quality of Reproduce Range and Selectivity of a tion of Both Speech and Music. High Order on Short Waves.

HE mains transportable is a range key switch, and knobs for type of receiver which is controlling reaction and L.F. volume steadily gaining in popu- respectively. The metal panel is larity. With its self -contained screwed to . the face of the receiver loud speaker and frame aerial it chassis, and is finished, in brown presents a neat and compact crystalline enamel. Hand capacity appearance which makes an im- effects are entirely absent. mediate appeal to those with an The receiver circuit employs three instinct for orderliness. While valves (H.F., det. and L.F.). The retaining all the best qualities of its illustrious ancestor, first two have indirectly heated filaments, while the third the battery -driven portable, it is able to give a far better valve -a pentode --is directly heated from a separate output from the point of view both of quality and filament winding on the mains transformer. volume ; the H.T. voltage is always up to concert pitch and the cost of running for, say, 200 hours -the average Aerial Circuit Connections. life of a portable H.T. battery -is about 4d. compared The aerial circuit consists of a short-wave frame with 18s. 6d. for H.T. replacement and L.T. charging aerial wound round the interior framework of the set. in the case of the portable. It is loaded for long waves by a separate inductance, The cabinet design in the Lotus All- Electric Trans- and provision is made for the attachment of an external portable is of a type which has previously met with con- aerial and earth. There are two aerial sockets, and siderable success in battery portables. Standing when changing from short to long waves with an ex- vertically on a ball -bearing turntable, it has a sloping ternal aerial, the lead -in must be moved to the appro- control panel above the lotid speaker fret. The cabinet is priate socket, as both long- and short -wave windings cut away to frame the controls so that the whole of the are provided with tapping points to reduce aerial load- interior of the set (with the exception of the loud ing. Wave -range switching is accomplished by a system speaker) can be withdrawn without disturbing con- of switches which are ingenious adaptations of the well - denser dials or control knobs. known Lotus jacks. They are operated by a horizontal The receiver chassis and power unit are mounted brass push -rod running parallel to the control panel and inside the wooden framework supporting the short- controlled by the wave-range key. wave aerial winding. Both are very completely The screen -grid H.F. valve is followed by transformer screened, and the receiver coupling. A tin. diameter chassis, which is sub - SPECIFICATION. ribbed. and slotted ebonite . divided into two compart- CIRCUIT : Three valves. Screen -grid H.F. (tuned trans- former carries the trans- ments, is provided with a , former coupling), leaky grid detector with reaction trans- former windings and also a former coupled to pentode output valve. Direct loud which i3 readil3i detachable clip -on speaker feed. Full -wave valve rectifier. reaction winding, back, giving easy access to CONTROLS: (1) Tuning controls, independent side -by -side capacity -controlled from the the valves. The screening - drum dials. (2) Wave -range switch. (3) Reaction. detector anode. box of the power unit is (4) Post -detector volume control. The detector functions as GENERAL : Provision for gramophone pick -i p. Self - a grid rectifier provided with ventilating contained frame aerial. Turntable in base of cabinet. with zero louvres, and the cover is PRICE: £25 10s. Makers: Garnett, Whiteley & Co., bias. The bias resistance removable for adjustment Ltd., Lotus Works, Mill Lane, Liverpool. shown in the detector of the mains transformer cathode lead in the circuit primary and insertion of the valve rectifier. The power diagram is for the gramophone pick -up, and comes into unit is mounted on a shelf immediately below the re- play only when a plug is introduced into the jack switch ceiver chassis, and to facilitate removal of the cover on at the back of the receiver chassis. Bias for the H.F. the power unit the shelf can be drawn forward after and L.F. valves is also obtained from separate cathode extracting two fixing screws. The length of the multiple resistances, a separate centre -tapped filament winding cable connecting the two units is just sufficient to permit being incorporated for the directly heated pentode. this operation, and does not leave an undue amount óf The detector is followed by L.F. transformer coupling slack with the power unit fixed in position. to the pentode. Post -detector volume control is pro- The control panel carries side -by -side drum dials fòr vided by a variable resistance in parallel with the L.F. tuning the aerial and H.F. amplifier circuits, a wave- transformer primary. A resistance is connected in B =8

www.americanradiohistory.com FEBRUARY rrth, r93r. VIIp4sag %EDE!

Nlllllllltllll!+"- ECE/VEE CHASSIS WITH DETACHABLE BACK' Illaol° -_ PICK-UP JACK CO/YE LOUD SPEAKER

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REVERSIBLE MA /NS PLUG

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Layout of components in the Lotus Transportable. The power unit cm be pulled forward for removal of the aluminium screen. B 29

www.americanradiohistory.com WApAlmo FEBRUA1tY izth, 1931. r:0 MpN Lotus Mains Transportable A.C.- - were tuned in -two at full programme strength. The series with the grid terminal of the secondary winding total number received on short waves (excluding 5GB) in order that stray H.F. currents may be prevented from was 22, and of these at least nine could be enjoyed, for reaching the loud speaker leads. In a compact receiver long periods without, fading or interference. of this type, in which the frame aerial and loud speaker The instruction book gives an approximate calibra- are in close proximity, an H.F. stopping device is tion of the tuning dials, and in the receiver tested this essential if back coupling between the output and input calibration was found to hold within two metres over circuits is to be avoided. The stopping resistance also the whole of the short -wave range. The long -wave assists in keeping down the higher audio- frequencies, range, in which the frame aerial is loaded by a small which are generally over- emphasised by the pentode. coil seen at the right -hand side of the screening in the A compensating condenser and resistance are also con- pictorial diagram, is not so sensitive as the short. nected in parallel with the loud speaker winding with Daventry (5XX) is quite satisfactory, but an outdoor the same end in view. aerial would be necessary in order to enjoy the long - wave foreign transmissions at reasonably good strength. H.T. Smoothing. A happy compromise has been effected in the match- The power unit contains the mains transformer, a f ull- ing of impedance values in the output circuit of the wave valve rectifier, the usual 4 mfd. rectifier output pentode, with the result that the quality of reproduction load, and a smoothing choke. It is surprising to find is very pleasing indeed. \Ve award the designers full that no by -pass condenser is connected on the receiver marks on this score, for quality is difficult to achieve

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Schematic circuit diagram of the Lotus receiver Chassis and power unit. side of the smoothing choke, which is connected in the with a boxed -in loud speaker movement of small dimen- H.T. lead. Further, only the detector anode circuit is sions. There are no objectionable resonances, and the decoupled. Yet there is no trace of motor-boating reproduction of both speech and music has a natural under working conditions. and unforced quality. In spite of, or perhaps because of, the rather uncon- ventional arrangement of the smoothing circuits there Outstanding Features. - is literally no trace of mains hum. When first switched The exceptionally good performance on short waves, on there was, however, a distinct background hiss. the pleasing quality of reproduction, and the neat lay- Reversal of the mains plug at the back of the set con- out are the three outstanding features of the set. The siderably reduced this interference, which is of a high - frame -aerial reception on long waves might be improved frequency character and enters along the supply mains. with advantage, but those who have had an oppor- The selectivity on the short -wave range is of a high tunity of hearing this set will agree that the number of order, and at a distance of only five miles from Brook - stations available on the short -wave range more than mans Park there was a clear space of 33 degrees between compensates for this deficiency. the two stations without any trace of background. The makers are Messrs. Garnett, Whiteley and Co., Actually, under these conditions the National transmitter Ltd., Lotus Works, Mill Lane, Liverpool, and the price (261 metres) extended from 236' to 272 metres and the of the receiver in oak, mahogany, or walnut is X25 los. Regional (356 metres) from 333 to 369 metres. In the This includes valves, royalties, and accessories such as free space between the two stations nine foreign stations gramophone pick -up, jack and reversible mains plugs. B 30

www.americanradiohistory.com rF.hRr'_1RY rIlh, ro;r. \VIpA Ig59 G`'IoEIlco1 tom-,U.C.,Qonc%/'oss i/ve -wr- Coil Details. Hints on Construction. By L. E. T. BRANCH, B.Sc. (Concluded from page 110 of previous issue.) ALTHOUGH the baseboard is rather large, being 2o. Another i8 is lost if one choke is employed for 3oin. x r24in., the weight is kept down by using the smoothing. Hence E =M 38 where M is the mains ¡in. ply with two tin. thick battens underneath voltage. When two smoothing chokes have to be at each end, thereby leaving a space under the rest of employed E = M 56. Table I shows suitable values the baseboard to take some of the wiring. The panel for the external resistance for different mains voltages. is -in. thick, and is 3oin. x gin. A terminal strip The simplest way to test the suitability of the resistance tin. wide of the same thickness is used right along the is to measure the voltage obtained across the filament of back of the baseboard and carries the terminals as well the valve V,, which should be not more than 6 volts. as the mains supply socket, and also the socket for con- Only two screening boxes are employed, the coils Old necting the external resistance. Since the current flow- valve for each stage being in their appropriate boxes.

Rear view of receiver. The two valves in push -pull can be .seen on the left. Note the method of mounting the smoothing chokes when two are used. ing through this resistance is approximately 0.24 ampere, Screening covers are used to shield the part of each valve below the internal screen and the grid wire for its value should be-E ohms where E is the voltage 0.24 each high- frequency valve enters its box at the point to be absorbed. With a 4 -volt detector the voltage where the valve screen touches the side of the box. dropped through all the valve filaments is approximately These screens must be in metallic contact with the boxes. Three holes must be drilled through the bottom of TABLE T. the front box and one through the bottom of the back box to accommodate the wires carried under the base- One Choke. Two Chokes. Mains Voltage. board. A small metal plate washer of about 14 gauge Ohms. Ohms. aluminium should be placed on the tuning condenser 200 680 600 spindles between the two boxes to prevent the two sides 210 720 640 becoming clamped tightly together. Another washer, 220 760 680 230 800 720 which need only be i8 gauge, should be placed between 240 840 760 the panel and front box. When fastening down the 250 880 800 switches see that they are correctly in line. If the B 3T

www.americanradiohistory.com NWp@ Ilmo FEBRUARY rrth, 1931. rj? Wald D.C. Band -pass Five. - points are not used. The same lettering is used through- interconnecting rods do not fit tightly any play will out. cause the back switch to lag in its action. Pieces The S.G. filament shunting resistances are mounted of wood ¡in. thick and about tin. smaller each way on the sides of the boxes. The trimming condenser in than the inside dimensions of the boxes, are screwed the front box is mounted on a piece of wood near the down at the bottom. The switch which is behind the top so that it is adjustable by means of a screwdriver back box must be supported on a small piece of wood inserted through a hole in the lid. of the same thickness. The connecting rods of the The tuning condensers which are associated with the ganged switches are all in metallic connection with one band -pass filter coils are mounted on a small platform another, and should also be connected at a suitable formed of one upright piece of tin. five -ply, Min. high point of the screening boxes in order to keep the rods by 2tin. wide, and one horizontal piece, Elkin. long by at earth potential. 2*in. wide. When the switches are turned full over to the right When commencing the wiring it is advisable not to the long -wave coils are shorted and one green signal have any of the coils in position. The long -wave coils is illuminated. With the switches in their central posi- are inserted when as- much wiring as possible has been tion two green lights appear and the long-wave coils done without them. When inserting in the boxes the are all brought into operation. By turning the switches small strip bases of the coils see that the distances from full over to the left both green signals are extinguished the sides of the boxes are the same in each case. If this as..vell as the filaments of the H.F. valves, and the set is not done trouble in ganging may be experienced. is now suitable for gramophone reproduction purposes. After connecting up the terminals on the long -wave When wiring the switches Table II will be found to be coils to their appropriate points the short -wave coils are useful in checking the connections. The unnumbered inserted and the last few connections are made. The

S11 68 TURNS SECONDARY N° 22 D.S.C. 70 TURNS N° 22 D.S.C.

WOUND REVERSE PRIMARY DIRE TION 3/2 32 TURNS 19 N° 40 D.S.C.

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Dimensional data of the pre -selector coils and the H.F. intervalve couplings for both wavebands. The secondaries of the H.F. transformers are identical, thus ensuring satisfactory ganging. 11 32

www.americanradiohistory.com FEBRUARY rrth, 193r. 'Mp®Rem WEIl! 153 D.C. Band -pass Five. - with the two ends near the centre of the former, both coils are of a semi -plug -in type, and are quick and easy joined to the pin P5 as these are the low- potential ends. to insert or extract. The tapping is connected to the socket S2 and the The medium -wave band -pass coils are wound on two terminal T2. The high -potential end of the tapped coil bakelised formers, ein. diameter and Ain. long, and con- is connected to the socket S, and the terminal T,. The sist of 66 turns of No. 25 D.S.C., which occupies rein. high -potential end of the other long -wave coil is con- winding space. These coils must be wound in opposite nected to both the two sockets S6 and S'6, and also to directions, otherwise capacity effects are introduced the terminal T6. The pin P is only used to help sup- which considerably modify the filter. The grid coil is port the coils. Corresponding lettering is used in the tapped at the twenty -fifth turn from the top, and the other diagrams, so that no mistake ' is likely to be tapping brought to the made in wiring up. If terminal T8. In both coils increased selectivity is de- the low- potential end is at sired a short - circuited the bottom, and simply turn of heavy -gauge wire, connected to the plugs P, e.g., No. 20, must be in- and P6 respectively. The cluded in the manner indi- high -potential end of the cated in the first part of aerial band -pass coil is this article. connected to the terminal The medium -wave H.F1 Ta while that of the grid coil transformers are both connects to the terminal wound on bakelised formers T,. At the bottom of the of Sin. diameter and 31-in. aerial coil of the band -pass long, and have 7o turns filter there is wound a of No. 22 D.S.C. wire. small earial winding of 17 The one for use in the front turns of fine covered wire. screening box - that is, The gauge is not critical, immediately before the and No. 36 D.S.C. is suit- detector -is tapped at the able for this purpose. A 25th turn from the top, space of din. is left be- and the tapping brought tween this coil and the to a terminal T15. Since main coil. The 17 -turn the tuning condensers are winding should be wound ganged, it is impwative in the opposite direction to see that the number of to the 66 -turn winding on turns, and also the lengths the same former. The top of the windings, are iden- end of the fine winding is tical. The low- potential connected to the pin P2, ends are connected to the while the lower end goes pins P1, and P20. The to the terminal T1. high -potential ends, which The long -wave coils of are at the tops of the coils, the band -pass filter are are joined to terminals both wound on the same Plan view showing ganged intervalve couplings. The cylindrical T1., and T18 on the coils. ribbed ebonite former, portions of the valve screens are removed. Ten spacers Ain. thick are which is 21in, diameter then placed round the and 6in. long. The dimensions and positions of the bottom half of the coils, and in the case of the first slots are clearly shown in the illustration, each slot H.F. transformer 19 turns of No. 40 D.S.C. wire are containing 40 turns of No. 34 D.S.C. wire, the aerial wound on, while in the second H.F. transformer 32 one being tapped at the fiftieth turn from the low - turns of the same wire are used. Not less than ten potential errd. The coils thus consist of a total of 200 spacers should be employed, otherwise the capacity in- turns each, and must be wound in opposite directions troduced between the primary and secondary flattens the TABLE II. tuning considerably when the long -wave coils are in operation. These fine windings must be wound in the Switch Coil I Switch Coil same direction as the secondaries. The bottoms of these Point. Connections. Point. Connections. windings are connected to the pins P and P16, while the top ends to 1 S,, 10 810 are brought the sockets S and S at 2 SS the top ends of the coil formers. 11 Sol When the coils are in 3 S, the set these sockets receive plugs which are connected 4 T, 12 T20 to the anodes of 5 T, 13 Ti, the screen-grid valves. The long-wave 6 T2 14 Sl, H.F. transformers are wound on ribbed ebonite formers 7 S14 15 21in. diameter and Sin. long, the windings being clearly 8 C17 16 See Fig. 1. shown in the coil diagram. The primaries and second- 9 Ta 17 J aries are, as usual, wound in the same directions. The n 33

www.americanradiohistory.com Wt@li@oo FEBRUARY 11th, 1931. 751 MTh' _ o o '. O O .1l, o o o o

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The wiring plan above and below the baseboard. The drilling data for tige panel and terminal strip illustrated in last week's issue ; are as follows: A, I in. dia. ; B, } in. dia. ; C, ¢ in. dia.; in. dia. ; E, in. dia. F, } in. dia.; G, >Pa in. dia.; H, drilled and tapped 5 B.A.; J, I in. dia. e 3 q

www.americanradiohistory.com FEBRUARY rrth, WRp®Il@oZ 193. 155 WOirIlcI D.C. Band -pass Five. - that the positive end is nearest the grid. In this way low- potential ends of the secondaries are joined to the the valves are biased to approximately 19 volts. The pins P,,, P21 respectively, while the high-potential ends corresponding Cossor P.625 output valves can be used go to the sockets S13 and S_0 and the terminals in push -pull at this bias. T and T20. The low-potential ends of the primaries The set is not really intended for use with valves of are joined to the pins P1, and P15, and the high -potential the P.625A. type, which require a large grid bias, but

The completed receiver without valves, valve screens and lids to screening boxes. The layout is quite straightforward and the spacing of components generous. ends to the sockets S9 and S, and also to the terminals they can be used if a resistance of 6o ohms is inserted T, and T10. in series with the choke CH,. This resistance increases If the greatest selectivity is required, the medium - the bias up to 40 volts when the 44-volt grid battery is wave band -pass grid coil is plugged into the socket S'0 used in the grid of the valve V, with its negative end -that is, in the farthest position from the aerial coil - nearest the grid. Alternatively, a grid bias battery in which case the coupling is 1.85 microhenrys. If of 18 volts may be inserted in series with the decoupling your mains are earthed on the positive side, you may resistance R9-either side of R, is satisfactory. have decided only to wire in the choke CH, and omit Since it is a simple matter to operate the set on one CH2. If the negative side is earthed you will have H.F. stage by connecting the anode of the first S.G. CH2 ; but, unless your supply is rough, you will have valve to the socket S of the coil in the second stage, it omitted CH,, and in its place have inserted a fixed 6o- is suggested that the set be started up in this way. In ohm resistance. In either case the next thing is to insert fact, when the highest possible quality of reception is two Bulgin fuse lamps, type M, in the green signals, desired from the local station only one H.F. stage should and a type N in the red signal. Ordinary battery be used. valves are used, and for this reason the set is easily In case the inductances of the coils are not quite adaptable for battery operation. Two Cossor S.G. 220 identical it is advisable to adjust the condensers in the valves are required for the H.F. stages, and by means following way. With both H.F. stages operating, and of plugs connect their anodes to the sockets provided the trimming condensers set to minimum, turn the on the coils. An L410 type valve is required as the volume control practically to the bottom and set both detector. The shunt resistances of the S.G. valves dials for the London National Station. Now adjust the should be set at 35-40 ohms, while that across the de- trimming condensers if it is found that an improvement tector should be 24 -27 ohms. The shunt across the out- can be effected. If the aerial capacity is slightly lower put valve V5 should be set at 300 ohms to pass the than the average (as with a short aerial) the condenser anode current of the valve V4. tuning the aerial coil will require trimming, whereas if The bias values, obtained by employing the set in the aerial capacity is a little high the condenser tuning the standard way, are suitable for the Marconi Osram the grid coil will require trimming. Next, turn the P.625 or Mullard P.M.256 valves in the output when dials for a station near the top of the dial, e.g., the the mains are 230 volts or above. Below this mains Midland Regional, and adjust the condenser vanes on value it is advisable to take the 4I-volt grid battery the spindles. from the grid of the valve V, and insert it in the cor- Now return to the lower end of the dials and repeat responding position in the grid of V,, but reversed so the operation upon more distant stations, such as Cork B 35

www.americanradiohistory.com z56 WIlp@Ilmo FEBRUARY rzth, 1931. WopIlca D.C. Band -pass Five. - when tuning in to a powerful station is to set the volume and Budapest. The tuning over the whole range should control very low and, when attempting to increase it then be found satisfactory. Any tendency towards oscil- after tuning full in, just check first that the volume is lation should be quenched by turning down the volume not increased by decreasing the volume control. control. If the adjustment of the vanes on their If this latter is found to be the case it shows that spindles is found to be more than about two degrees the detector was overloaded and the volume control had it is best to check the inductances of the coils. The not been turned low enough when tuning in. There is. easiest way of doing this is to change over the tuning another corollary to this. If the volume control is too condensers. high when tuning in, the station will appear to have When operating the volume control a peculiar effect two strong peaks one or two degrees. apart. The middle \rill be noticed. For instance, imagine the set is tuned of this is the correct setting, and the volume control in fully to a powerful station, and the volume control should be turned down, when the volume should increase. is very low so that only very quiet signals can be When the gramophone is operated, turn the wave - received. Now, on increasing the volume control, change switch so that both the green lights are ex- signal strength will increase at first and then, after tinguished. The H.F. valve filaments are then shorted reaching a maximum value, it will actually decrease. so that no filament current flows through them, and The decrease is due to overloading of the detector also a negative bias of 3 volts is automatically given to valve, and is peculiar to any set which is designed so the detector valve so that it functions correctly as an that the output stages overload at about the same time amplifier. In the list of parts in last week's issue two as the detector. The method which should be adopted valve screens should be included.

KEEPING THE H.F. WITHIN BOUNDS. The Use of a High= frequency Choke in the Output Circuit. iT is sometimes found that a receiver, especially A cure may generally be effected, without detriment when tuned to stations on the upper broadcasting either to the volume or the quality of reproduction, by band from i,000 to 2,000 metres, is inclined to a combination of a high- frequency choke (which must depend for its stability on the position of the loud be wound with a generous amount of wire if it is to be speaker. One may observe, for example, that the effective on the long waves) with either one or two receiver tends to oscillate if the loud speaker leads condensers, making a filter circuit which is interposed are brought anywhere near the aerial lead -in, or perhaps between the " live " speaker lead and the set. Often the tuning or reaction adjustments are upset if the one condenser only (C,) will be needed ; the complete speaker is handled or moved. filter, with both condensers, is an almost infallible The inference to be drawn from misbehaviour of this cure. The greater the capacities of C, and Cî the more kind is that high- frequency currents are present in the effective the filter will be, but if the capacities chosen loud speaker leads, giving rise to an amount of back - are too large a loss of the higher audio- frequencies will coupling that depends on their position. The trouble result. A total capacity of o.00i mfd. is generally is more likely to occur on the upper waveband, because very suitable from both points of view, but this figure may be increased considerably if the speaker impedance is not too high. A. L. M. S. H t BOOKS RECEIVED. H.F. CHOKE The Practical Ailectriciau's Pocket Boot., 1931. Edited by 111111111111 F. H. Robinson. The 23rd annual edition of this well -known pocket book, containing a mass of useful information, tables, data, etc., relating to all branches of electrical engineering, including a short section devoted to wireless and a tabular OUTPUT LOUD SPEAKER list VALVE Of particulars of electrical supply throughout Great Britain and Ireland, giving the supply voltages in all the principal towns. Pp. 560+lxxii., with numerous diagrams and illustrations. Pub- lished by Electrical Trading and Electricity, London, price 2s. 6d. net. Rurlfunl, Jahrbuch, 1931. The year -book of the Reichs -Rund- LT - funk Gesellschaft, Berlin, relating the progress and performance of the various broadcasting organisations in Germany, with articles on technical matters, events of the past year, and numer- ous other features. Pp. 417, with 248 illustrations. Published by Union Deutsche Verlagsgesellschaft, Berlin. By interposing a radio- frequency choke filter arrangement as shown, 11.F. impulses can be prevented from passing to the Données Numériques (le Radioélectricité, compiled by R. Mesny, loud speaker. and extracted from Vol. VII of Tables Annuelles de Constantes et Données Numériques. Comprising 18 pages of useful fornnthe the usual precautions adopted for by- passing stray H.F. and data taken from standard publications during the years 1925 -1926. Published by Gauthier- Villars et Paris, and impulses away from the L.F. amplifier are very much Cie, by McGraw Hill Book Co., Inc, New York. Price : Paper Fes. more effective on the short waves than on the long. 35, cloth Fes. 40. B 36

www.americanradiohistory.com FEBRUARY zzth, 193r. Wpcdgoo Wald 157

LABORATORY TESTS

A Review of Manufacturers' Recent Products. RIPAULT'S 99 -VOLT H.T. BATTERY. If we accept this as being the useful life Ltd., Morley House, 320, Regent Street. This is a standard -capacity dry -cell type of the battery, it can be shown that a London, W-.1. The resistance element battery having a nominal voltage of 99, capacity of some 1,750 mA. hours can be consists of tr synthetic material, and and measuring 9 in. x5 *in. x3áin. These obtained with an initial discharge as high moulded into the two ends are long sol- dimensions will permit the battery to be as 10 mA. dering tags. used in portable sets, since the space Sprague resistances are available in allotted is usually of this order. Tap- values' ranging from 5,000 ohms to pings are provided at 9 -volt intervals RIPAULT'S DRY -CELT 5 megohms, and they are rated to dissi- throughout the battery, and in addition 120 H T. BATTERY pate 1 watt. A sample 20,000 -ohm size the first 9 STANDARD CAPACITY -volt portion adjacent to the NORMAL VOLTAGE .99 was measured, its resistance being 19,000 negative end is sub -divided by further 100 10 ohms. A further measurement was made tappings of 12 volts each. Grid bias up 9 < after a B E current of 2 mA. had been passing to 9 volts can be derived from this sec- 7 80 for a a considerable time. There was no tion, but, of course, at the expense of re- change in the resistance value. ducing the ó80 available anode voltage by the 5 The price of these resistances is ls. amount required > i for this purpose. 40 4 rc each. With a view to ascertaining the work- 3 0 0 0 0 ing life of the battery, it was subjected 20 2 O to a discharge test extending over a pro- FRANKLIN CONDENSERS. longed period. Intervals o for recuperation 50 100 150 200 250 300 350 400 We have recently received were allowed, but in the discharge curve some samples TIME (WORKING HOURS) of the new large- capacity fixed condenser Marketed by the Franklin Electric Co., Discharge curve of Ripault's 99 -volt Ltd., 187, 189, Ilford Lane, Ilford, dry-cell H.T. battery. The well- maintained voltage after the early decline, coupled with the rapid fall after a prolonged life, are two very com- mendable features. This battery is a British product, the makers being Ripault's, Ltd., 1, King's Road, St. Pancras, London, N.W.1, and the price is 15s. o 000 SPRAGUE RESISTANCES. Ripault's British -made 99 -volt dry cell Made by the Sprague Specialities Co., H.T. battery of standard capacity. U.S.A., these resistances are marketed in this country by Howard Thomas and Co., reproduced here only the actual working hours have been included. The initial current was set at 10 mA., and the Franklin 500 -volt and 1,500 -volt D.C. test ter- I mfd. condensers. Both are the same minal voltage at the commencement of size externally. the test was 104. This fell rather rapidly during the first 100 hours, but during the Essex. these are encased in moulded following 150 hours there was very little bakelite containers change arranged for upright in the voltage. During this mounting, and are supplied in sizes rang- period it was maintained at 60 volts. ing from 0.1 mfd. 4 At to mfds. Two types the end of 250 hours' work a shat'p are available, the one tested at 500 volts decline set in, which was continued -with- D.C. out and the other at 1,500 volts D.C. break until the battery was exhausted. Prices are according to size and test volt- This natural cut -off point coincides with age; for example, a 0.1 mfd. 500 -volt test a drop in voltage 0.9 to per cell, a value type costs ls. 6d., its 1,500 -volt counter- which is generally accepted as being the lowest Sprague 20,000 -ohm part costing 2s. 3d. A 4 -mfd. 500 -volt useful limit of discharge for dry resis- type is priced at 5s., cells. tance with long soldering and the same size tags. rested to 1,500 volts D.C. is listed at 108. B 37

www.americanradiohistory.com '7flif@Ilgoo Wald FEBRUARY 11th, 1931. " POPULAR " STATION INDICATOR. 3,500 cycles with a major resonance be- our own preference is for the full high - The purpose of this device is to assist tween 150 and 200, and a sharply defined frequency response up to and above 6,000 in the reception of distant statiens when subsidiary resonance at 2,200 cycles. Be- cycles. In some circumstances, however, a frame aerial typo receiver is used. The tween 300 and 2,000 cycles, however, the the filter is indispensable in eliminating opened indicator is placed on the table output is uniform. The sensitivity is high- frequency interference and mush. somewhat below the Undistorted power inputs up to 5 watts average, but the can be satisfactorily handled by the unit. non -adjustable move- The workmanship throughout is of a high ment has been set standard and the performance places the to handle a consider- type 2063 unit in the highest class of mov- able power input. ing coil loud speakers. The price com- The price is £3 10s. plete with the filter and switch is £12 lOs., Type 2063. -De- and supplies are distributed by Messrs. signed for dperation Philips Lamps, Ltd., 145, Charing Cross from A.C. mains, Road, London, W.C.2. this moving -coil unit 000u is suppled with field "ANDY" ACCUMULATOR CARRIER. current from a type Made of lead- coated metal, the 506K full -wave valve " Andy " carrier can be adjusted to fit rectifier. The dia- any shaped accumulator up to 6in. square. phragm is 8in. in A rubber- covered carrying handle is fitted. diameter, and is The makers are Beaufoy and Co., 87, driven by a low -im- South Lambeth Road, " Popular" station indicator for use with pedance moving coil. Full of the portable and frame aerial type receivers. details London, S.W.8, and the output transformer ratio required for price is ls. 11d. valves of various A.C. resistances are and orientated until the compass needle given in the instruction leaflet. points to the name of the station required. Our tests revealed that the sensitivity The portable set is then rotated so that is above the average, while the reproduc- the plane of the frame is parallel with the tion is characterised by exceptional bril- indicator. In this position it is correctly set for reception of that particular station. Loose cards are provided for use in the principal cities in this country, and in- structions are given as to the method of calibrating the indicator for places not so provided for. The makers are the Dampad Rubber Co., Ltd., 5 -7, Market Street, Finsbury, London, E.C.2, and the price is 5s. o 0 0 0 TWO NEW PHILIPS LOUD SPEAKERS. Type 2032. -The non -adjustable moving iron movement and cone unit are enclosed in a moulded " Philite " case, the back of which is well ventilated to reduce box The " Andy resonances. The average impedance of carrier. the winding is 5,000 ohms, but the im- pedance rises considerably at very high ,md very low frequencies (14,500 ohms at CATALOGUES RECEIVED. 50 cycles, 32,900 ohms at 6,400 cycles). Igranic Electric Co., Ltd., 149, Queen The frequency response is from 100 to Victoria Street, London, Philips type 2032 loud speaker in moulded hated loose-leafloose -leaf catalogue of new sea cabinet. son's components and accessories. Also a 20-page booklet dealing with the Igranic- Philips type 2063 liance in the upper Elkon rectifiers and their uses. moving coil unit 0000 for A.C. mains. register from 3,000 to 6,000. From 3,000 Varley (Oliver Pell Control, Ltd.), 103, down to 50 cycles the Kingsway, London, V.C. -1930 catalogue CTAPPEe MAINS output, although at a of receivers, components and accessories. TRANS FORM ER? lower level, is uni- 0000 form, apart from a Dayzite, Ltd., 17, Lisle Street, Leices- slight resonance be- ter Square, London, W.C.2.-104-page tween 100 and 200 catalogue describing the proprietary re- cycles. To meet the ceivers, components and accessories demand of those who handled by this firm. object to the effect 0000 of the high fre- Belling and Lee, Ltd., Queensway quency response, a Works, Ponders End, Middlesex.-Illus- filter and switch trated booklet describing the new range of have been included connectors recently introduced. in the equipment. 0000 The effect of this Radio Instruments, Ltd., Purley Way, filter is to introduce Croydon, Surrey. -New season's catalogue a well- defined cut -off of receivers, mains units, components and at 3,500 cycles, but other accessories. B 38

www.americanradiohistory.com FEBRUARY Iltk, 1931. l59

More Regional Rumours. Now that the un April 1st. Scottish station is under Once again the B.B.C. are displaying construction thoughts are turning to their wonted fondness Western Regional, and for " one over the already the public eight," by deciding to replace the can take its choice from a fine crop of Northern Regional orchestra freshly gathered rumours, mostly with a com- contra- bination of nine players on April 1st. dictory. They are inspired, for the most Remembering, however, part, by the news (authentic) the gibes thrown that B.B.C. at them when they followed the same engineers have been investigating sources plan at Birmingham, the of water supply in Somerset. B.B.C. are 0000 rigorously excluding any mention of the By Our Special Correspondent. term " Nonet." Search for Water. 0000 The question of water supply is always that The Ideal Combination? the first to be several listeners wished to question tackled, and during the past him on B.B.C. I learn from the Music Department at week the broadcasting officials policy. He proceeded to Savoy Hill nine have been the agreed rendezvous, expecting to meet that players are con- prosecuting enquiries in the Bridgwater sidered to provide an ideal studio com- a small deputation, and was not a neighbourhood. This fact has been taken little bination just suited to the needs of the to mean that Bridgwater dismayed to 'find himself confronted by is the B.B.C.'s 200 gesticulating Welshmen. microphone. Usually the instruments choice for the Regional station. comprise two violins, viola, 'cello, bass, 0000 0000 and pianoforte, together with woodwind A Football An Unwanted Neighbour. Broadcast. of the flute, clarinet, and oboe types. That Bridgwater Fortunately the gathering was presided There is no brass. will not be chosen is over by a chairman 0000 as certain as the fact that the Marconi who blended firmness with tact, but it was Mr. Cock's verbal Crumbs of Comfort. beam station already occupies the best agility site in the district. Incidentally, and presence of mind that turned The only word of consolation which the a hostile demonstration into a gathering B.B.C. is not anxious to have such a can be offered to Northern listeners on neighbour! of gentlemen unanimous in their admira- the loss of their orchestra is that this oouo tion for the B.B.C. nine -piece assemblage will contain the hear Testing in the I that the latest outside broadcast cream of Northern orchestral talent and " Wee, Sma' Hours." to be arranged is Not for several years have the broad- a running commentary that it will be augmented from time to on one of the semi -finals of the Football time casting engineers of different nations co- for performances on a larger scale. Association Cup matches on March 14th. Performances by the B.B.C.'s operated in tests of the kind now being 0000 main conducted in the early hours of the orchestras will be available through the morn- new Northern ing by London Regional and Mühlacker. Is the B.B.C. "Running" the Census? Regional0000 transmitter. Most of the experimentation is going on If the 1931 Census omits any man, at the German end, and it is noteworthy woman or child in the United Kingdom, Birmingham University Broadcast. that the engineers at Mühlacker are tak- it will not be the fault of the B.B.C., for Sir Oliver Lodge's speech proposing ing great pains so to adjust the modula- the Corporation is taking quite a sur- " Tlie Memory of the Founder, Sir tion system that the minimum interfer- prising interest in this decennial event. Josiah Mason," and Sir Austin Chamber- ence is caused to other stations. Not content with organising a series of lain's speech proposing " The University Both in Germany and England the tests six weekly talks on the subject -the first of Birmingham," will be relayed from the are being listened to on special receivers. is by the Registrar- General, Mr. S. P. Jubilee Celebration Dinner of the Bir- 0000 Vivian on Tuesday next, February 17th- - mingham Guild of Graduates (London the B.B.C. has Branch) at the Trocadero Four Years prepared an historical Restaurant on Ago. leaflet which contains a specimen census February 20th to Midland Regional. The last international test of import- form and an invitation 0000 ance in Europe to listeners to occurred four years ago correspond with Savoy Hill if they foresee with the inauguration of the then revolu- Hair -greying Course (continrei). any difficulties in rendering the returns. From the Savoy Hill Postbag tionary Geneva plan, when the whole It is hoped :- of Europe's broadcasting that questions can thus be Question : Are you using an outside, stations changed answered by those taking part in their wavelengths in a single the inside, or frame aerial ? week -end. talks. .4asier: Inside The whistling and shrieking on that -lead -in outside. auspicious Sunday night conjured up an oscillator's paradise, but I doubt whether it was so very much worse than a normal night in 1931. 0000 The Boxing Broadcast. Numerous friends who disclaim any interest in boxing have confessed to me that they sat through every round of the Brown -Kirby fight as it was broadcast by the B.B.C. One man who usually retires promptly at 10 p.m., nearly froze at a cold grate waiting for the knock -out that never came. The relay was an entire success and a triumph for the " O.B." department, which is surely one of the hardest- worked organisations within Savoy Hill. 0000 A "Deputation" of Two Hundred. Mr. Gerald Cock, who is " O.C." of the department, can tell queer tales of his experiences when in search of fresh material. Sometimes PROGRAMME when he is not the EXCHANGES WITH ANTIPODES ? Newly erected short wave sleuth he is the ambassador. transmitters at Hilimorton, Rugby, for When in United the Post Office telephony service to the -Wales a short time ago he was informed States, Australia and South Africa. The possibility of relays to aid from Australia by this new link has been discussed at Savoy Hill. R

www.americanradiohistory.com t 6o Wtmhog FEBRUARY 11th, 1931. WopIl2 C©RR IFSP I) ND1ENCIFo The Editor does not hold himself responsible for the opinions of his correspondents. Correspondence should be addressed to the Editor, "The Wireless World," Dorset House, Tudor Street, E.C.4, and must be accompanied by the writer's name and address. included, and the arrangement would, in the opinion of the STANDARD TERMINOLOGY. to all parties concerned. in stating, in writer, be advantageous Sir, -Mr. W. H. O. Tweeny was quite correct Perhaps other readers of your journal may 'wish to express his article on " Talking Films," in your issue of January 28, which the writer considers many people views on this particular subject that the TU or transmission unit is " unfamiliar to to be of considerable importance from every point of view. in this country," the reason being that it was superseded some Manchester. J. BAGGS. time ago by the more appropriate term decibel (abbreviation db.). Apart from the general desirability of adopting standard CENTRING OF RECORDS. terminology, the need is particularly pressing in this case, as Sir,-As an ardent gramophonist since I was seven years of TU now represents the traffic unit (the unit of traffic flow), age, and as a contributor to the gramophone Press, I feel that I which is defined by the British Engineering Standards Asso- must drop you a line in reaction to one or two references I have ciation as " a unit employed in estimating the amount of switch- seen in your publication recently regarding gramophone repro- ing equipment required in automatic exchanges to carry the duction. traffic." I am quite convinced that the gramophone disc- producing com- Sydenham, S.E.26. BERNARD C. HOLDING. panies have not yet paid sufficiently concentrated attention to what we call the " centring " of a record. Take any dozen records, place them on a standard gramophone turntable one at EXCEPTIONAL RECEPTION. a time, noting the lateral sway of the tone -arm (or pick -up arm). them will preserve a steady spiral. tracking Sir, -I wonder if other readers of The Wireless World Perhaps three of this evening (January 27) towards the turntable spindle, and the remainder will show a noticed very exceptional conditions its creep inwards. in reception on the wavelengths 950 -1,400 metres. decided oscillatory motion during 8.30 -9.30 p.m. I have heard many people exclaim that the hole was not in My is S.G. det. L.1". mains- driven, outside aerial, L -type, set is erroneous. The trouble lies about 60ft. by 30ft. high. I normally receive Oslo and Motala the centre of the disc; but this in the pressing of the recorded matter out of centre with the at good loud speaker strength; Moscow T.U. and Leningrad maddening To -night all have been spindle hole. Only the quickest ear can detect this at weak loud speaker strength. in when a piece of Moscow and being quite as strong lack of attention to detail manufacturing, abnormally strong, Leningrad is when sustained notes is no of wrong identification, music having a lively tempo played, but as Daventry. There possibility is more stately character, both being announced frequently; Moscow was giving announce- are being rendered, or if the music of listeners to write to the station the veriest tyro in musical appreciation cannot help but place ments in English and requesting his and shriek with horror. This is the with comments and suggestions. his hands over ears seemed normal; there were most important point in the otherwise phenomenally fine pitch Reception on medium waveband in the recording of to -day. I do not know whether atmospherics on the long wavelengths. of perfection very considerable the turntable spindle of the amplifier at the B.B.C. is of less : and windy. I should be very Weather Cloudy, cold, raining size not, but I do know that the awful interested to know whether these were local conditions at Cam- than the standard or Perhaps wailing of some records leads me to suppose that this is the case. bridge, or whether these conditions were widespread. FREDERICK JACKSON. some of your readers can suggest an explanation. Hampstead, N.W.6. There was no fading ; Leningrad overloaded the push -pull large Blue Spot THE STENODE. output stage, two Mazda A.C. /P. driving a Dr. Robinson's hypothesis that " there volume control was used, and then could S'ir, -I disagree with cone speaker, unless can be no discontinuity in the nature of the physical response still be heard some 50 yards outside the house. to one SCIENTIST. as we go from a signalling speed of one per second Cambridge. of 5,000 per second." Of course, there is no sharp discon- tinuity ; but there is a progressive change in the response RADIO DEALERS AND RADIO SOCIETIES. throughout the frequency range. An adequate reason for this Sir, During the last few years the membership of the many is the fact that the response of tuned circuits is essentially radio societies which used to exist throughout the country has a phenomenon concerned with E.M.F.s, which have a perfect fallen to a very great extent, so that in many cases they are sinoidal form and are therefore uniform and continuous. An .supported only by the really hard- boiled enthusiast. E.M.F. of limited duration can be an approximation only, and This change is undoubtedly due not to any diminution in as the duration is decreased by increasing the signalling speed the interest in radio, but to the fact that its technical de- the approximation becomes less close. The only method of velopment has now reached a stage where a certain amount investigating theoretically the response to a non- uniform of knowledge is necessary if progress is to be made by any E.M.F. is to analyse it into a number of sinoidal components amateur, and, in consequence, it tends to become rather beyond (e.g., carrier wave and sidebaids); if the legitimacy of this the understanding of the man who used to be the backbone of process is denied, it should follow that the converse process the average radio society. of combining two frequencies to form a heterodyne is also The time, however, seems to the writer to be ripe for a inadmissible. general resuscitation -of radio societies, and, if necessary, the I think Dr. Robinson would agree that if an aperiodic instru- creation of entirely new societies in large numbers throughout ment were used to measure the electro- magnetic field at a point the country, and I would suggest that such societies should within range of several C.W. transmitters, it would indicate obtain their membership mainly from the radio retailer and a single resultant field of complex wave -form ; this must be others associated directly with the radio industry, and possibly interpreted by analysis into components corresponding to the with the music trade. different transmitters, with frequency differences of perhaps If this proposal could be put into force and the membership 20 kc. each. Can he show any reason why there should be of individual societies made appreciable (say an average member- a discontinuity which invalidates this process when the analysis ship of not less than 50) such societies would, I feel sure, be indicates components with a frequency separation - between warmly welcomed by manufacturers as enabling them, by 50 cycles and 5,000 cycles (i.e., a carrier and sidebands)? means of lectures and demonstrations, to get in direct touch In conclusion, I would like to point out that although a in a very simple manner with those who have a really live decrease in damping may cause an increased absolute response interest in the sale of radio apparatus, and (of equal importance) to a certain range of modulation frequencies it will cause a knowledge of that apparatus and its after -sales service. decreased absolute response to any frequencies which suffered Whilst a largo proportion of the membership might consist much proportional loss in the original circuit. of radio traders, the serious radio enthusiast would also l'e Berkhamsted. D. A. BELL. B 40

www.americanradiohistory.com FEBRUARY rrth, 193r. WApA®m rnr ' MpN you to construct an eliminator with a generous voltage output ; by doing so it lt Jli' will- become possible for you to make the deooupling arrangements of your receiver all the more effective. We cannot hazard an opinion as to the future, but doubt if appreciably higher voltages than at present will ever be used-in domestic re- ceivers, at any rate.o:oo Double -wound Aerial -grid Transformer. In the published description of the t " Flexible Two" receiver it was ott., i, stated that under certain pp conditions it is advantageous to use separate ,L aerial windings for the input coil. il'ill you please gire U\\\íltitit \y \IUC% /®U Ifll[l\\"l// r' \\f lam' lit me a sketch showing horn these windings should Replies to Readers' Questions be placed in relation to the tuned of General Interest. coils? Technical enquiries addressed to our Information Department are used as the basis of It will be found convenient to place the the replies which we publish in these pages, a selection being made from amongst those extra aerial windings in the positions questions which are of general interest. shown in Fig. 1. The short -wave wind- ing may be supported on thin strips of Stroboscopic Speed Indicators. insulating material laid over each of the Series Filaments. coil former The periodicity of my A.C. supply is non- 7'he usual advice, that a receiver in which ribs, while the corresponding standard, and consequently the ordin- a fault long -wave coil may best be wound in a has developed should be narrow groove ary stroboscopic device will not func- tested by the stage-by-stage method, formed by cutting slots in tion as an indicator of the speed of seems hardly each rib. No. 38 D.C.C. and No. 40 to apply to the " All D.S.C. wire rotation of a gramophone turntable. D.C. Three," in which the valve fila- would be suitable for the Will you please tell me how to calcu- short- and long -wave windings respec- ments are connected in series. In tively. late the number of " spokes," so that consequence, it is impossible to I may make one for myself? eliminate a valve without putting the The number of black bars on the disc whole set out of commission. I am can be determined by multiplying the fre- having a certain amount of trouble quency of the supply mains by 120, and with this receiver. Do you think it dividing the product by the required would be worth while to make resist- speed (in revolutions per minute) of the ances (of the saine value as that of turntable. the valve filaments) for temporary 0000 insertion in each valve holder? Bias Resistance Calculations. Although it is true that individual Although i believe the necessary instruc- valves cannot be removed, it is quite un- AERIAL tions have been published st veral necessary to do as you propose. Provided times, I am unable to trace a refer- that the grid and anode circuit inpedances ence to the method of determining of any valve that is to be eliminated are the correct value of resistance for use temporarily short- circuited, it is possible in an automatic grid -bias circuit. to make stage -by -stage tests in the ordin- SHORT I, trill you please WAVE give me the formula? ary way. AERIAL WINDINGS} This information can be obtained by 0 0 0 0 LONG applying Ohm's law in the form of R= Increasing Undistorted. Output. WAVE . Is there EARTH E . In this formula R is the resistance any ?ray of increasing the power e output obtainable from the Band -pass required, E the bias voltage to be de- Superheterodyne without impairing veloped across the resistance, I the total its characteristics, and without intro- current (in amperes or fractions of an ducing any serious modification into ampere) to be passed through the design? the resist- SWITCH ance under operating conditions. We suggest the use of a P.T.625 pen- 0000 tode output valve, which will provide some Hum and Loud Speaker Characteristics. 1,500 milliwatts, as opposed to the 500 Fig. 1.- Construction of double -wound elfter rebuilding my A.C. receiver and milliwatt output of the specified valve. It aerial -grid transformer. changing over from will be best half-!cave to full - to retain two -volt valves in It will be leave rectification, I am disappointed the remaining positions. necessary to obtain a change to find that over switch with an extra pole so that the A.C. hum, though still 0000 long -wave aerial not excessive, is distinctly worse than winding may be short - The Prophet's Mantle. circuited in the manner shown. before; this in spite of the fact that I am about to buy a new power trans- 0000 the smoothing arrangements are un- former, and if you think that the Power Transformer changed. Can you suggest an ex- tendency Modifications. in the future will be to use The power transformer described in " 7'he planation? higher anode voltages I propose to Wireless By fitting full -wave rectification World " for January 22nd, you obtain an instrument capable of de- 1930, is suitable for my requirements, have doubled the hung frequency, and no livering considerably greater pres- doubt have brought except that a voltage of 300 is needed it up to a periodicity sures than are actually required at across each to which the loud speaker is H.T. secondary section. capable of present. i assume that there will b% responding to Will you please tell me how to modify a greater extent than harm in having an excess voltage? the winding? formerly. Further, it is likely that the Care must be taken not to present ripple apply a You should add 300 turns to each frequency is magnified to a higher voltage to your rectifier -of what- secondary bobbin, greater extent by the L.F. amplifier and as the bulk of the of ever type -than it is intended to handle. winding will naturally be increased by the . receiver. The remedy is to include \Vitlh this proviso, it is all to the good more effective smoothing. to this addition, it will be necessary to use choose a transformer which will enable No. 38 enamelled wire. B 41

www.americanradiohistory.com 162 Wtp@ho@ FEBRUARY iith, 1931. WouIlcll

Ganged Switches. _feeding an A.C. pentode connected as a Instabilitll in any "2- II.b'." receiver has combined detector and output valve. A been traced to the use of a metal con - circuit diagram (omitting the H.F. input FOREIGN BROADCAST GUiDE. necting rod between the ganged wracc- circuit) is given in Fig. 2. changing switches: the set is coni- c 0 0 o pletelg stable when this rod is tem- Heat Dissipation. VELTHEM- poranly removed. Will you suggest I am constructing a compact D.C. Re- how this source of interaction may be ceiver, and uni finding considerable L®üVAIIl1 removed? difficulty in dedising a voltage -redne- (Belgium). The transference of H.F. potentials ing resistance which will absorb the from one circuit to another can generally necessary power without getting un- Geographical position : 50° 52' N. ; 4° 43' E. be avoided by earthing the connecting rod duly hot. 4 large resistance wound Approximate air line from London : 218 -at more than one point, if found neces- with thick wire tales up too much miles. - sary-through a direct math of low resist- room,- while a small one raises the Wavelength : 338.2 m. Frequency : 887 kc. ance. But it is probably better to use temperature inside the container to Power : 12 kW. (eventually 15 kW). links of insulating material between the an extent that is greater than can be Time : *Greenwich Mean Time. various sections of the control rod. tolerated. Will you make a sugges- *Belgium adopts B.S.T. 0000 tion? Standard Transmissions. Half -wave Rectification. This is a fundamental problem, and the For local station reception I intend to dissipation of even 30 or 40 watts calls 18.00, C.M.T. outside broadcast (Sun.) ; build the " Regional One " receiver for adequate ventilation. You might 20.00, talk ; 21.15, gramophone records or as described in your issue of mount your resistance in a sub -base, mak- concert (Sun., Tues., Thurs., Fri.). Special August 13th, 1930. 1 already have a ing good provision for the circulation of test transmissions are made on Sundays poker transformer capable of giving air, or, alternately, it might be mounted between 11.0 and Mid -day. a large current output, but ii-itli a external to the receiver, with a protective Man and woman announcers. Call : (for the centre -tapped 11.7'. winding provid- cover of perforated metal. K.V.R.O.), Hier Velthem, de Katholieke ing only 150 volts between each end 0000 Vlaamsche Radio Omroep ; (for S.A.R.O.V.) and the tapping: it occurs to inc that Designing a Reaction Coil. Flier Socialistische Arbeiders Radio Omroep this component might be used if 1 I wish to add a rem -lion roil to my II.F. 000r Vianderai, abbreviated to Hier Sarov. adopt half -wave rectification, which transformer, which was originally in- Announcements are made in both Flemish should be adequate for a receiver that tended to precede an anode -bend de- and French. is -obviously largely immune from tector. Medium- and long -wave wind- Opening signal (for SAROV broadcasts) : " hum." Of course, an extra 4 -colt ings are connected in series, and are imitation of carillon from Malines Cathedral. transformer Will be obtained for feed- supported concentrically on the same ing the valve heater, as. my present ribbed former. how many turns Closes down with Goede Nacht, followed by transformer dors not include a wind- would be necessary for a single Belgian National Anthem (La Brabançonne). ing for this purpose. Will you please reaction coil to be effective on both give me your views, and, if possible, wavebands? a circuit diagram showing the neces- It is hardly possible to design a reaction Push -pull Output. sary modifications to the " Regional coil by other than empirical methods, and, Will it be necessary to have a separate One." in any case, we have hardly sufficient data 4 -volt L.T. winding for feeding the If your transformer is capable of giving on whieh.to make a definite recommenda- heaters of 4.C. valves used in a push - a sufficiently large output, there is no tion. It is suggested, however, that you pull circuit? reason why it should not work satisfac- should start with 40 turns of fine wire, It is not essential that push -pull output valves should be heated from a separate transformer output, but it is generally pre- ferred that this should be done, particu- larly when automatic0000 bias is provided. Mains Feed for a Superheterodyne. I have recently tried an eliminator for supplying anode current to my super - heterodyne receiver, which, incident - ally, is of rather out -of -date design. It is noticed that rather more hum is produced than is pleasant, and that it is particularly noticeable when the receiver is tuned to a carrier u-ave. What can 1 do to improve matters? We recommend that you should add extra smoothing in the oscillator anode feed circuit, and if this does not have the desired result, similar precautions should be taken with regard to the supply to the intermediate- frequency amplifier. 000o NOTE. " All -wave Reception." With reference to ft published reply under the above heading in 7'he Wireless World of January 21st, we are reminded rig. 2.- Obtaining a higher voltage by the use of a half -wave rectifier in the Regional by Messrs. Burne -Jones and Company, One" receiver. The transformer H.T. secondary tapping is ignored. Ltd., that they also manufacture a set - the " Universal Three " -for the recep- toril- in conjunction with a half -wave and that this winding should be placed tion of all broadcasting wavelengths be- rectifying valve; the output from such a considerably closer to the long -wave tween 15 and 2,000 metres. This receiver rectifier, after a reasonable amount of secondary winding than to the correspond- was reviewed in our issue for August 14th, smoothing, should be quite adequate for ing medium -wave coil. 1929. B 42

www.americanradiohistory.com C G he U1= - le AND r ¡; RADIO REVIEW ( 18lh Year off Publication)

No. 599. WEDNESDAY, FEBRUARY 18TH, 1931. VOL. XXVIII. No. 7.

Editor : HUGH S. POCOCK. Assistant Editor : F. H. H AYNES. Editorial Offices : 116 -117, FLEET STREET, LONDON, E.C.4. - - Editorial Telephone : City 9472 (5 lines). Advertising and Publishing Offices : DORSET HOUSE, TUDOR STREET, LONDON, E.C.4. Telephone: City 2847 (i3 lines). Telegrams: "Ethaleorld, Fleet, Lo::don.,. COVENTRY : Hertford St. BIRMINGHAM: Guildhall Bldgs., Navigation St. MANCHESTER: 260, Deansgate . GLASGOW: 101, St. Vincent St., C.2. Telegrams: ry-e,li,t. " Coventry." Telegrams: " Autopress, Birmingham." Telegrams: "Ilitfe. Ma nehester." Telegrams: Telephone: 5210 Coventry. Telephone: "Ilitfe, filasgosv. "- 2:170 Midland 13 lines). Telephone: 8070 City (4 li oex). Telephone: Central 4807. PUBLISHED WEEKLY. ENTERED AS SECOND CLASS MATTER AT NEW YORK, N.Y. Subscription Rates : Home, LI Is. 8d. ; Canada, LI is. 8d. ; other countries abroad, LI 3s. rod. per annum. As many of the circuits and apparatus described in these pages are Covered by patents, readers are advised, before making use of then, to satisfy then:ce're., that they would not be infringing patents. Radn® Societies. N our issue of Iast week a contributor to our Corre- they had a new set to demonstrate, they could take it spondence columns, writing on the subject of the down to a local society and talk about it to an audience amateur radio societies, put forward the suggestion largely composed of those who would subsequently be that a revival of interest in societies could be brought called upon to sell and to service it. about by appealing to the radio retailers and service One of the difficulties which radio societies have to men throughout the country and adapting the activities contend with to -day is obtaining lecturers for their of the societies more particularly to their requirements. meetings. As it is, the manufacturers do supply lec- Even though, at the time of writing, that letter has turers in fair numbers, but they would be much more only recently been published, we have had sufficient ready to do so and to go to pains in the preparation evidence to show that it has provoked a good deal of of the subject matter and the choice of representatives interest and some criticism. A number of readers have if they felt that they were by that means getting into asked that we should give our support to the proposal direct touch with those individuals whose interest in and encourage other readers to express their views on their products might subsequently be of the utmost im- how best the proposal might be carried out. portance to themselves. Our correspondent remarked in his letter last week There are many points to be considered in connec- that the membership of the many formerly active radio tion with this proposal, and for the time being it is societies which throughout the country had fallen to a sufficient to invite our readers, and particularly those very great extent. This statement is undoubtedly true, who are actively concerned in the management of radio but there are outstanding excep- societies at present, to contribute ...... tions ; some of the radio societies their views on the suggestion. If are to -day carried on with greater a success is to be made of con- enthusiasm than at any time in In This Issue ducting radio societies on the new their history, but everything lines NEW BAND -PASS FILTER. suggested, then it is un- appears to depend upon whether doubtedly from those who have UNBIASED OPINIONS. a society has a definite policy already had wide experience in OUTPUT to pursue and an aim over which VALVES IN PARALLEL. connection with societies that we the members can enthuse. We REGENTONE A.C. FOUR. must look for suggestions. We therefore think `that to encourage CURRENT TOPICS. have no doubt that it will be said all those who are engaged in the TESTS ON NEW APPARATUS. that already many of those en- wireless industry in any capacity BROADCAST BREVITIES. gaged in the wireless industry are where technical knowledge is re- SOUTHEND'S RECORD RADIO SHOW. members of local societies, but quired to join local radio societies THEORY OF THE H.F. TRANSFORMER. we believe that they are mostly would be an excellent move, and READERS' PROBLEMS. members by reason of their it would be very gratifying to the amateur status rather than as manufacturers to feel that, when members of the wireless industry. A9

www.americanradiohistory.com rol WAp@Il@Og FEBRUARY r8th, 193r. MAO New

The " Mixed Filter."

Constant Selectivity over Constant the Waveband. Peak Separation.

By W. I. G. PAGE, B.Sc.

rirHE reprisal method of combating interference London National station, although the curve is between European broadcasting stations gains sufficiently flat -topped to retain sidebands. The new adherents every month. Power is answered capacity filter at this wavelength, on the other hand, by still more power, resulting in the creation of a shows only a single peak which is less flat, but the vicious circle, and the European ether may soon begin selectivity is good. On London Regional's transmission to resemble an arena for rival town criers. Meanwhile the two filters give an almost similar response, whilst the humble listener finds, to his dismay, that with on the Midland Regional the peak separation of the simple tuning devices certain stations cannot be capacity filter is sufficiently great to give double tuning. separated, and he is somewhat bewildered in attempting The inductive filter on the upper wavelengths tends to be- to choose a method of obtaining better selectivity. come more selective, and the peaks to get closer together. Of the various schemes available none seems to have For the sake of comparison, the response of the well - so many advantages as the coupled circuit, band -pass known single tuned circuit is given by the dotted curve filter, or pre -selector-call it what you may. Not only in Fig. 2. Here there is an inordinate lack of selec- does it confer the benefit of far better selectivity, but tivity with the London National station, but as the the whole range of audible frequencies in the trans- ratio of tuning capacity to inductance increases as we - mission can be retained, go up the wavelength scale, due to the flat top of the the selectivity improves and response curve. While the of HETHER the coupling in a band -pass filler is effected the single peak becomes capacity and inductive by fixed capacity or inductance, change of selectivity sharper. band -pass filters developed and peak separation is likely to occur as one tunes across Next to the response in this country by The the wavelength range of the set. A new method of filter curve of the single circuit is Wireless World represent an coupling is described in this article, resulting in the achieve- that of the mixed capacity - important advance in re- ment of constant peak separation and constant selectivity over inductance band -pass filter ceiver technique, in certain the tuning range. Constructional details of a practical filter (full- line), which it is the circuits their excellent are given, and should provide the reader with the necessary purpose of this article to characteristics can be made details for introducing the arrangement into his set as a describe. Before giving full to hold only over a limited pre -selector. details of this new tuning waveband. In Fig. r. re- unit it would be as well to sponse curves for induc- discuss one other charac- tive (dotted line) and capacity (full line) filters have been teristic of the four tuners, namely- sensitivity or signal plotted for three important wavelengths -those of the volts developed, as measured by the height of each London National, London Regional, and Midland curve against the vertical scale. So much attention has Regional stations. The coupling components Lm and been paid to the selectivity- cum-quality problem that Cm of fixed value have been chosen to give the best the question of selectivity- cum-quantity has been rather peak separation at the middle of the wave- range. neglected, the word " quantity " referring to signal It will be seen that with the inductive filter the strength. None of the three filters gives as great a selectivity is distinctly poor at the wavelength of the signal strength as the single circuit, although the mixed A IO

www.americanradiohistory.com FEBRUARY i8t11, 1931 WbsIlmo OpDI 165

Fig. 1.- Response curves for typical capacity and inductive filters. The coupling components C and I,,,, have been given values so that the peak separation is correct at the middle of the wave- band. Note the lack of selectivity with the in- ductive filter at the lower wavelengths.

CAPACITY FILTER INDUCTIVE FILTER

10

5

/ o LONDON NATIONAL LONDON REGIONAL MIDLAND REGIONAL filter falls only a very little short. The capacity filter above the given wavelength double- humped tuning may on the London National wavelength gib -poor signal be too pronounced. Both these conditions cause strength, a.,; does also the inductive filter at the top end uneven selectivity and inconstant peak separation over of the tuning scale. At the middle of the scale the the waveband. As already explained, the inductive two filters of Fig. i share equal efficiency, but their filter suffers from poor selectivity at the lower wave- curves do not rise vertically to the same height as those lengths, and although peak separation tends to decrease either of the single circuit or the mixed filter. at the upper end of the range, the variation is not very Summarising, it can be said that a fixed capacity - great, and the filter certainly does not offend in this coupled filter can give a definite peak separation only respect as much as the capacity type. It will be shown at one wavelength ; below this figure the coupling that the mixed filter not only gives constant selectivity becomes deficient or too loose, and signal strength drops ; and constant peak separation from about 24o to 600

Fig. 2.- Response curves of the single circuit and the mixed capacity- inductance filter. It will be seen that with the mixed filter the peak separa- tion and selectivity are almost constant over the waveband and, furthermore, that the efficiency is not much below that of the single circuit.

MIXED FILTER SINGLE CIRCUIT

10

o LONDON NATIONAL LONDON REGIONAL MIDLAND REGIONAL

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www.americanradiohistory.com nive'r y:

¡66 Witea.M0 FEBRUARY 28th, 1931. WOpIld New -pass Band Filter. - 1,100 metres, but that the signal strength obtained is greater filters. The twelve 1,000 than that from either of the simple a response curves in Figs. i and 2 have been made assum- ing the same aerial coupling and valve-damping con- 900,1?o",,,'"""" ditions in each case. 800 In tackling the question of constant peak separation ( in a filter, various coupling arrangements suggest them- selves. Five different methods are given in Fig. 3, 00 1,0",,,","'wLm+ soo °i4 where the coupling component is shown in a dotted '' \ ° P.© U enclosure in each case. The first (a) includes a variable G O 500 '.\ a coupling condenser, the rotor of which is controlled by , \../'/.s OFII the spindle of the ganged tuning condensers. A O . :JM., , O OF-J special condenser with a maximum value of about 400P Jp ., 300 Miá NÌII WI o.oz mfd. is required, and, unfortunately, the price X..;al C!...11 G becomes prohibitive. An inductive coupling with a 200Ma'` o; ,NCiA., baby variometer actuated by the ganged tuning control x is shown at (b). For the total movement of the tuning - G. Mom nm :_ condenser control the inductance of the variometer 0 \ would need to change through only about one micro - 5 0 700 900 I 1,100 1.3.0 ,5QC to a KILOCYCLES I henry. While not being impossible make, such 6 . 5 0 4 0 360 20 200 small component would need almost the skill of a iMETRES watchmaker. Fig. 4. -By plotting a curve of the high -frequency resistance of a tuned circuit and comparing it with curves for the impedance of The Importance of Negative Inductance Coupling. various couplings, it is possible to choose the best form of coupling to give constant peak separation. Greater promise would seem to come from a coupler not requiring variable control. Fixed -capacity coupling which give as constant peak separation as that due to suffers from reduction of peak separation in the opposite capacity or inductive coupling taken alone, and we direction to that of fixed -inductive coupling ; why not, know, from Fig. I, that neither of them is entirely therefore, mix these two forms of coupling, as in satisfactory. Perhaps capacity and inductance in Fig. 3 (c), and smooth out their individual deficiencies parallel, as in Fig. 3 (a), will help. Here again, even to form the ideal constant filter? It is found that there at resonance of the coupler or either side of resonance, are no values of inductance and capacity in series the peak separation over the waveband is less constant than with either com- ponent part - whatever values be chosen. It will now be as well to study the formula for peak separation, which is N/x r cycles, where x is 2irL the impedance of the coupling device, r the high -frequency resistance of one of the tuned cir- (a) (b) (e) cuits (the tuning coils and condensers are assumed to be matched), and L the inductance in henrys of one coil. It is quite evi- dent that change in wave- length (i.e., frequency) does not affect 2s-L, and we are left to consider x2 r2. How can we make the difference between these two remain the same (d ) (e) between zoo and 600 metres? Let us plot the square of the high -fre- Fig. 3. -Five suggested methods of obtaining constant peak separation. The first two, (a) and (b), quency resistance against have variable couplers but are difficult to construct. In (c) and (d) the desired results cannot be obtained, while the filter shown at (e) gives nearly ideal characteristics. frequency of a typical A i2

www.americanradiohistory.com FEBRUARY 18th, 1931. WAp@Il@&T 167 WOpIld. New Band -pass Filter. - are farther from our objective than ever ; but if we tuned circuit with a tin. coil. This is -quite easy, as were to find a coupling circuit represented by the simple we have only to square the figures already given by adding together of inductive and capacitative impedance A. L. M. Sowerby' in an article on small coils. The in a series circuit we should obtain a curve likely to be curve obtained is shown as of the right shape. a heavy line in Fig. 4. For further details of the There is nothing we can do mathematics of this type of to alter its shape except by mixed coupling where the the use of Litz, and then the coupling impedances are change is only slight. If additive, the reader is re- we can obtain another ferred to an article by curve parallel to it repre- E. A. Uehling, in the Sep- senting the square of the tember, 1930, number of impedance with frequency Electronics. Suffice it (x2) of some coupling de- to say that the coupling vice, we shall have achieved impedance x becomes :- our object, for when the -(0,L, implyingthe vertical distance between -)o,C,n, the two curves representing use of negative induct- x2 and r2 respectively is ance, which can be looked constant across the wave- upon as a capacity follow- band, obviously x= - r2, and ing an inductive law ; x', hence the formula for peak therefore, separation will give the I,,, (i)Ln, + same value over the wave - 2. range concerned. The first The square of the im- coupling device to be taken pedance with frequency is a fixed condenser of o.oi for a mixed coupling of 2.6 mfd. -the value commonly microhenrys negative in- employed in capacity fil- ductance and 0.05 Plan view of the unit. The lids of the screening boxes mfd. ters. The impedance of have been removed. common capacity is given this capacity with frequency as a curve in Fig. 4, -and is can be found very rapidly by reference to The Wireless seen to be practically parallel to the r2 curve from 24o World Radio Data Charts, and x2 is got by squaring to 600 metres. This solves our problem. the figures obtained. The curve is seen to be anything A filter made up to the circuit of Fig. 3 (e) but parallel to the r' curve, thus explaining the mis- gives the desirable constant peak separation and con- behaviour of the capacity filter. Where x2 is less than stant selectivity referred to earlier. The winding of a r there is no peak separation at all, and at the other coupling coil to give a small negative mutual induct- end of the x- curve it can be seen that the difference between x2 and r2 is too great to give normal OTO 5 -100 PLATE separation of peaks. 5-100 mm d The next coupling to mm d TO o GANGED GRID he examined is the fixed po inductive type having a typical value of 2.5 micro - po 0'00035 mfd henrys. When the im- 22 P 18 82 TURNS pedance of this is squared 15 18 FIg. 5.- Winding the nega- (x2) and plotted, it will 12 b TURNS tive mutual inductance for be seen that the curve 9 0 the mixed coupler. In all 6 o 26 turns of No. 22 D.C.C. (dotted line), although not are wound on a 1 -inch ribbed former. parallel to r2, is much nearer our ideal than the capacity curve, but still is not good enough. A clue to the solution of the problem is given by Fig. 3 (c) where the impedance of the series coupling circuit o is got by subtracting the impedance of the condenser TO from that of the inductance. As these two vary oppo- FILAMENT sitely, their difference causes the combined impedance to change very considerably with frequency, and we Fig. 6.- Circuit diagram of the filter unit with mixed coupling. is the low -potential 1 See article The reaction winding placed 5116. from entitled " Size tersas Efficiency of Small Coils." end of the secondary tuning coil. Note that the earthed Fig. 2. January 29th, 1930. screening boxes have been omitted in this diagram. A Y3

www.americanradiohistory.com r68 WiVigIliggIO FEBRUARY 28th, 1931. WOpIlcI

LIST OF PARTS. 2 Screening boxes, 6Iins. X 4dins. x bins. (Magnum). 1 Ebonite former, 21ins.dia. X 41ins. long, eight- ribbed (Redfern). 1 Variable condenser, 0.00035 mfd. twin -gang (Cyldon). 1 Ebonite choke former, 1in. dia. x Sins. long, six -ribbed 1 Variable condenser, 0.0003 mid. (Utility "Mite "). (Becol No. 7). 2 Semi -variable condensers, 0.9001 -0.000005 mfd. (Formo 4 Terminals. Type F). 1 Fixed condenser, 0.05 mfd., mica (Dubilier B.775). 2 Terminal mounts. 1 Bakelised board, 12ins. X lins. x in. 1 Ebonite former, tins. dia. x 32 ins. long, eight- ribbed 1 Baseboard, 12ins. X 10ins. (Redfern). Sleeving, wire, spacers and screws. ance requires description. Use is made of a one -inch with a few turns of wire on a one -inch former costing (overall) ribbed ebonite former around which is arranged but a few pence. No mathematical knowledge is re- a two -start winding. Two separate wires are laid o_ n quired to construct a filter of this type to give ro kc. parallel to each other, as shown in Fig. 5. The end c peak separation on the medium band with tuning coils is connected to the low-potential end of the primary other than those of 2:12-in. discussed in this article. If, tuning coil (see Fig. 6) and b to the end of the second- for instance, ein. coils of zoo microhenrys inductance ary coil ; d and a are joined together and taken to the were used, the number of turns on the one -inch former common coupling condenser. To obtain z.6 micro - would be about II plus ii. It is quite easy to start with henrys for the medium broadcast band when using 2jin. 14+ 14 turns, and remove them gradually until a milli - tuning coils, 13 turns are required for each winding ammeter in the anode circuit of the detector valve shows on the one -inch former -that is, 26 turns in all. The that the desired response has been obtained. There wire used is No. 22 D.C.C., and the turns are wound will be no need to deviate from the value of 0.05 mfd. touching. coupling capacity -all fine adjustment being carried out Shorn of all theoretical considerations, the mixed in the negative inductance windings. coupler consists of a condenser of rather larger value Little work has yet been done on the long waves, but than is usually employed in a capacity filter, together for those wishing to experiment with 2,000 -microhenry

. . .. . rnirriir TO . 000 3 : . PLATE - -ó; ° 82 TURNS : NO 22 D.C.C. i; i1I1ji TO 1f 9 GRID (eff 11 a.,o 18 TURNS 11!-84 SCREEN -a,q 0'05 mfd I e 34 D.S.0 N9 fa'1M ., o rI - m- 82 TURNS, óc.... 7ui - h el i I NO 22 D.C.C. ,--.- e III IOW ;. Vjøiw ..... :. .II 0p 5-100mmfd _ ^..-

-...... lit,:i___ cWgg r 22 TURNS NO 34 D.S.C. EARTH & 1 AERIAL FILAMENT SPACEC 32 TURNS PER INCH TAPPEC AT 6TH 9TH 12TH 15TH 4 5 6 7 B 9 10 11 12 18121 22NNNii TURN FROM EARTH END INCHES

Fig. 7.- General layout and wiring plan of the unit. The separate aerial winding is wound on 3;32in. spacers -matches will do.

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www.americanradiohistory.com FEBRUARY 18th, 1931. Wflpdwo W©pIlcl New Band -pass Filter. - tuning coils, the following coupling values are sug- gested : 6o + 6o turns No. 36 D.S.C. on a one -inch ribbed former for the negative mutual inductance, and 0.02 mfd. for the common capacity. Constructional Details. Details for the construction of a mixed filter are given in Figs. 6 and 7 and in the photographs. The unit pro- vides a highly satisfactory input circuit to a detector L.F. set, and can also form the basis of design for a pre -selector for H.F. sets, as the suitably shaped re- sponse curves should assist in preventing cross -modu- lation with the screen -grid valve. There is a separate input winding, as it has not been found advisable to couple the aerial through the negative inductance. Total screening of both tuning coils is essential, for even one microhenry direct coupling between them upsets the selectivity. The inductance of each coil when screened 15 10 0 5 10 15 RESONANCE is about 230 microhenrys, which, in association with a KILOCYCLES OFF Fig. S. -Using an oscillator, these two response curves of the 0.00035 mfd. tuning condenser, covers roughly the 200- unit were plotted. The readings were taken with a valve metre band. Ganging is found to be simplified by voltmeter. Similar curves to this are given at any wave- 550 length between 240 and 600 metres. For comparison the the use of a 5 to ioo micro -microfarad trimming con- curves are plotted to the same height. denser across each circuit. Two response curves of the unit using an oscillator and measuring the volts fully in line, having turned both trimmers to zero. developed by a valve voltmeter are given in Fig. 8 ; Tune in a station of low wavelength and increase reac- similar curves are obtained for any wavelength between tion to lower detector damping ; alternately increase 24o and 600 metres. each trimming capacity until the signal is at a maxi- Having completed the filter, tap the aerial to, say, mum. The ganging is correct when reducing or increas- the I2th turn and set the tuning condenser rotors care- ing either trimmer, reduces signal strength.

CONVERTING A BATTERY SET FOR A.C. MAINS OPERATION. i not infrequently happens that a set is primarily If the tuned anode system is employed it may be neces- designed for use with battery valves, and that later sary in the case of certain sets to consider the advisability on opportunity arrives for the use of A.C. mains, of changing over to H.F. transformer coupling, not so and it is consequently desired to convert the receiver much in the interests of stability as of complete absence to an all -mains instrument at a minimum of expense from mains hum, for it must not be forgotten that the and trouble. The first thing which must be done is properly designed H.F. transformer will not pass on to rewire the filament circuits. The new filament wiring the low- frequency impulses corresponding to mains hum must be carried out in such a manner that there is no owing to the low capacity between primary and second - external field, for it must be remembered that the wires ary, whereas the grid condenser; which is a feature of will be carrying A.C., and serious interference would the other system, will obviously do so. This must not, result if this precaution were not observed. This end however, be taken to mean that the tuned -anode system may be most conveniently achieved by merely twisting is not advised in an " all -mains " receiver, for in a the filament wires together in the manner of ordinary properly designed receiver it may give no more trouble electric lighting "flex," and if desired electric lighting than an H.F. transformer. The point which it is desired - " flex " may itself be employed. Alternatively, the pair ' to emphasise is that they may be troublesome in a con of wires going to each valve- holder can be laid side by verted set which was not, of course, primarily designed side in close proximity to each other, and yet a third for mains operation. . method is to use lead -covered wire, not forgetting to earth the lead sheathing. Dispensing with Intermediate L.F. Stage. This having been done, it will possibly be found that It may be possible to dispense with one stage of low - there is a tendency to oscillate due to insufficient screen- frequency amplification and to couple the detector direct ing, for it must not be forgotten that owing to their to the output valve or valves. greater efficiency the amount of screening which is suffi- Finally, the question of grid bias has to be considered cient to prevent oscillation with battery valves may not and the necessary provision made for obtaining this be nearly enough when " mains valves " are employed. from the mains. This may be done either by the so- If H.F. transformers are employed in the receiver it called " automatic " method or by the use of a separate may be necessary in order to achieve complete stability grid bias eliminator. Whichever arrangement is em- to remove one or two turns from the primary, even ployed, however, it is always advisable to get the H.T. though theory would dictate an addition owing to the and L.T. portions of the main equipment going first, higher A.C. resistance of the valve. a battery being used temporarily for grid bias supply. A Ij

www.americanradiohistory.com 170 VpoIl000 FEBRUARY 28th, 1931. Would. A Note of Hcpe. I have been doing a great deal of set testing lately-commercial sets, I mean -and as a result of my UNbIA3D labours I have been able to form BY FREE GRID some very definite opinions con- cerning them. The sets which I The Obsolescent Aerial. "The Wireless." have tested have included most of As time passes and receivers be- Certain combinations of letters, the leading makes of both battery come more and more sensitive, out- such as M.C. for moving coil, B.P. and mains sets. The sensitivity I door aerials will no doubt entirely for band -pass, and W.W. for have found is, in nearly all cases, vanish, with considerable advantage Wireless World, have literally to the appearance of the landscape ; passed into the language, and the and, indeed, in many of this CAREY year's same thing is true of certain words, ìTREET, i eceivers a frame aerial is incor- such as triode and pentode. Unless porated or provision is made for something is done towards the evo- using the mains as an aerial. The lution of a proper system of nomen- chief disadvantage of the frame clature by a responsible body, such lies in the fact that it is neces- as the R.M.A., or a special com- sary to rotate it at intervals, in mittee of etymologists, utter chaos order to obtain maximum signal will eventually arise. As has al- strength, and, in addition, it is ready been pointed out by a corre- rather clumsy in appearance. In spondent in The Wireless World, the certain transportable receivers the word radiogram, which has for many question of appearance is got over years been a recognised abbrevia- by incorporating it in the cabinet of tion of the post -office word radio - the set, together with the loud telegram, is already being applied speaker ; but even though the set is to radio -gramophones. riling my ebonite rather than my This will petition. sufficiently sensitive to receive a eventually lead to confusion, and large number of stations without the should be checked at once. I am of a very high order. Two or three necessity of pointing the frame afraid that the horrible expression years ago there were comparatively directionally, it often happens that, " the wireless," meaning a wireless few sets available which had any owing to the general cussedness of receiver, has taken root too firmly pronounced range -getting proper- things, the distant station which one for it to be killed now. In my ties unless the reaction coil was skil- particularly desires to receive ne- opinion, the word " radiophone " is fully used. Manufacturers have cessitates the turning of the set far more euphonious -pardon the completely overcome this difficulty, -and, therefore, the front of the pun -and, although containing more and the great problem which they loud speaker -away from the fire- syllables, is easier to say and is more must now face is getting rid of un- side. comprehensive in its meaning than wanted stations. Almost without any of the expressions now in use. exception, all the sets which I have Yet Another Knob ? tested have had very poor selec- In the case of some of the larger tivity, although quality is on the console - type receivers the frame whole good. At any rate, their is incorporated and provision is sharpness of tuning does not war- made for rotating it by a small rant keeping a first -aid outfit handy knob. This is a perfectly satisfac- when manipulating these receivers. tory solution so far as I am con- Hitherto it has, of course, been ex- cerned, but for the completely non- tremely difficult to make a set selec- technical man it does mean, of tive without marring quality, but course, that there is still one more with the coming of band -pass tuning knob to twiddle. Some manufac- and other methods of obtaining high turers have attempted to solve the selectivity without sacrificing quality difficulty by making provision for this should be a thing of the past, using the mains as an aerial. It we and may look forward this year is my experience that while this Certain combinations of letters ... to receivers which are first class in method works quite well on the nor- every respect. I hope, however, the mal broadcasting band it gives rela- " Pick -up " is another cumbrous ex- prices will be a little more attract- tively poor sensitivity on the long - pression ; one might as well speak of ive. At present I find that the cost wave band. I suppose the problem a loud speaker as a " put- down," of complete sets makes it more pro- will be eventually solved by making although the expression loud speaker fitable for me to continue filing my the set so sensitive that an inch or two itself is ugly enough in all conscience, ebonite than to run the risk of filing of aerial wire concealed in the lid and badly needs the attention of a my petition. will be all that is necessary. competent etymologist.

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www.americanradiohistory.com FEBRUARY 28th, 1931. NVATesgoo 'VDT cl

Charts to Simplify Calculation of Output. By W. A. BARCLAY, M.A. HE. practice of combining two or more output suppose in what follows that the load in the anode cir- valves in parallel to secure a large power con- cuit is arranged as in Fig. r, in which the loud speaker sumption in the loud speaker is one which is offers a resistance of r ohms to currents of speech fre- steadily increasing in vogue. A general review of modern quency, all direct current through its windings being receivers demonstrates -if demonstration were necessary eliminated by the large condenser C, while the H.T. -that the multiplication of L.F. stages is now a thing is supplied to the anode system of the bank through a of the past, and that from the standpoint of quality the resistanceless choke, L. When C and L are both very paralleling of power valves in quite small receivers has large, this is equivalent to the well -known " choke -feed " everything to commend it. For the energising of the arrangement, the resistance r representing the dynamic larger phonic mechanisms of commercial use, banks of resistance of the loud speaker windings. The modifica- paralleled valves are, of course, standard practice. tions for other methods of connecting up the loud speaker The mathematical theory of parallel output valves has will readily suggest themselves. been dealt with by Baggally in Experimental Wireless.' It will be apparent that the conditions under which The subject is, however, fairly complicated, and the each individual valve of the system works without intro- writer believes that a simplified treatment on general ducing distortion differ in no respect from those which lines will be useful to many readers who may wish to must be fulfilled were the same valves working entirely ascertain without much troublesome arithmetic the " on their own." If, for simplicity, we exclude from practical effect of parallel- our discussion all mention ing their output valves. of the so- called " har- Here, as so often elsewhere monic " forms of distortion in wireless theory, the prin- arising from the unequal ciple of alignment will spacing of the resistance prove a useful aid, and the characteristics, we may set writer has accordingly de- down briefly three main signed an Alignment Chart " limits " which must be from which the approxi- set to the operating condi- mate performance of such tions of power valves :- combinations may be read (A) The anode dissipa- off with the minimum of tion, or product of the calculation. supply voltage and mean The Limits of value of anode current, Distortionless Output. must never exceed the safe Before describing wattage which is usually this Fig. 1.- Schematic arrangement of N output valves in parallel, Chart we shall first recall working with a dynamic resistance of r ohms. quoted by the makers. the general conditions for (B) The anode current the distortionless working of output valves. The mode must never exceed a certain value, viz., the " satura- of connecting such valves in parallel is illustrated in tion " point, in the region of which the resistance Fig. r, from which it is seen that all the grids, as well characteristics become curved. Nor should the anode as all the anodes, have a common connection. While current fall below a certain minimum, fixed by the the same signal e.m.f. is impressed on the grid of each position of the " lower- bend " curvature of the charac- valve, the anode current passing through the load will teristics. have N times the value of each individual valve current, (C) The instantaneous potential of the grid should N being the number of valves in the bank. We shall never be allowed to become positive, owing to the unde- sirability of the resulting grid current. I " On Banks of Paralleled Valves Feeding Resistance Loads With- The nature of the limitations imposed by these three out Distorting the Wave Form." Experimental il'ireleas, June, 1928. considerations upon the path of the working point under A I9

www.americanradiohistory.com 172 VIpWs00 FEBRUARY z8th, 1931. v` oTRZ Output Valves in Parallel.-. AA with the 30o volts line. Since P2 is the centre of the given operating conditions may be illustrated by refer- " run," the top half, a2P2, must equal the lower half, ence to Fig. 2a, which shows a set of resistance charac- P2b2, and hence the total extent a,b2 representing the teristics for a certain supposedly ideal power valve. The signal input may be set out. It will be seen that a2b2 is characteristics shown are equally " spaced, and are larger than albs, the corresponding " run " for a supply assumed to be without lower- bend " curvature. For of 35o volts ; nevertheless the 300 -volt supply is un- most power valves the permissible minimum of anode economical, for if a still lower value of H.T. were used, the working path could be still further extended. For 90 250 volts H.T. the total permissible working path is a3b2. Here, however, a fresh limit is imposed, namely, 80 that set by the line BB, so that if the supply voltage be yet further reduced, say, to 200 volts, the resulting run" will still be of the same length as before, e.g., WZi a4ó4. It will, of course, 70 be most economical to obtain WalrairdENrcarr this maximum length of working path with the least 5 oo 4 3,ó possible value of H.T. supply. On reducing the H.T. RMI below 150 volts, however, the third limiting condition ? 80 \ I, I comes into play, and the permissible " run " is I i,I rI r,r, I curtailed r ,NI OI,ENI % by the line CC. Thus, for a supply of ioo volts, the total extent of the working path ir 40 available with this load EEEME i'I is restricted to a6b6. It will now be evident that the Ir1IIlgI1®1III optimum position of the working line is that at a5b;, for 30 MI which the greatest value of " run " (otherwise signal o II,/E E/AFI e.m.f.) is obtained with the minimum necessary voltage supply. Thus our object in designing a bank of parallel 2 o IIIII1IIIIIb output valves will C be to ensure as far as possible this I ideal condition for each individual valve. 10 . The above relative disposition of the limiting factors A, B, C, illustrated in Fig. 2a, is, however, very often tN% modified by the fact that the maximum permissible M1.iÌ50 700 75. 0. 2 0 0a I 00 ANODE VOLTS E value of anode current fixing the position of the line BB Fig. 2a. -Ideal characteristics of a power valve, illustrating the three limiting conditions which restrict output. 110. . current is small relatively to the average value of this current, and can usually be neglected altogether in power 100 calculations to the great simplification of the work. The anode dissipation is rated at 7.5 watts, and the curve 90 WAWA corresponding to is OdII.g, this value shown at AA. The maxi- mum permissible anode current is 6o mA., this value 00 Cairrf being shown by the horizontal line BB, while CC repre- r,I sents the straight portion of the characteristic for zero grid volts. Now, it is well known that for a given value 7D 121F21/11111 of resistance load the working path on the diagram of < Fig. 2a is a straight line of constant slope whose actual :flp:rii r position depends on the values of grid 60 ,m bias and supply I: voltage in use. As an example, we shall assume a IiII/m ,` value of 6,667 ohms for our load resistance, and draw 50 %raIa2 ro° on our diagram a succession of lines of corresponding slope. VA \W REN ; MU; o 40 I,TNrl oo W M.EI The High- tension Voltage. .. It will be found that if a, b a.,b2,...a,;b5 be several IAIr a 30 ^ , N w such working lines, the total extent upon each which ; 5r,r IA is available for the excursions of the working point is -. rAa,5Mu ° 1 limited by one or other of the three considerations A, 20 , Na?1 A B or C previously noticed. For large values A of the supply MrrIIraN voltage, the total extent of the permissible " run " will 10 _MEr,IAEMM be limited to such small lengths as alb, or a2b2, the iíMM governing consideration being that the centre point of bi, o L'kl \b2 the " run " must lie on the curve AA of maximum anode 50 00 150 200 250 300 350 400 dissipation. Thus, when the supply H.T. is 30o volts, ANODE VOLTS ! the necessary grid bias will be - 52 volts, since this is Fig. 2b. -Ideal characteristics of power valve. In this case, the value of the characteristic at P2, the intersection only two of the limiting conditions are operative, the value of of I,,,,,r being- too great to have any restrictive effect on output. ,. A 20

www.americanradiohistory.com FEBRUARY 18th, 1931. VAp,ging 173 Woplld. Output Valves in Parallel. - the curve of anode dissipation being 7.5 watts, as in the is so large that condition B does not come into operation previous case. The maximum extent and ideal position at all. For most output valves, indeed, the characteristic - of the working path is here shown at a41)4, for which the limiting scheme is similar to that shown in Fig. 2b. Here supply voltage is 200, and the negative grid bias 24 the maximum value of anode current is shown as 90 mA., volts. This path is situated at the junction of the

100- -100 -90 -80 50- -70 -1,000 70- -900 -60 80- 90- -800 100- - 700 -51 -600 150- -500 -40

200- -400 - 35 -30 300- - 300

200- 400- 250 - 25 500- -200 600 - 20 700- 800- 150 900= 250- 1, 000=

- 100 1,500 - -90 - 300- 2,000- 70 ,n - N E f -60 O - > =O .3,000- 350- -50 4 000- -9 -40 5000 - - 7 400-4 6.000 - 7,000- -30 -0 8,000- 9,000= - 25 10,000= -20

500- 150007- -40 - 15 20,000-

-10 30,000 - -9 40000 - -8 - 2'5 -7 50,000- -6 700- 60,000= -2'0 70, 000- -5 80,000- 90,000- 100,000- -4 - 15 Fig. 3.- Alignment chart for output of power valves in parallel. It is possible, given the number of parallel valves, to find the amount of distortlonless A.C. power output supplied and also the optimum 900- value of the supply voltage to effect this. If the number of parallel valves and the supply of H.T. voltage is fixed, it is also possible to find the A.C. power delivered to the load. A third use of this chart is to find the best number of parallel valves to supply a definite A.C. power output to a given load when the value of the H.T. voltage is fixed. 1,000-

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www.americanradiohistory.com 174 WIp@hoo FEBRUARY ,8th, 1931. - WoaIl& Output Valves in Parallel. - r conditioning limits A and C, as shown graphically on the power scale. Lastly, the point will indicate by 2 the curve AA and line CC, the limiting condition shown ! by BB not being utilised at all. It is easy to see that the position of the value of upon the resistance all other positions 2 of the working line for the same load scale. are necessarily curtailed either by AA or by CC. Three general problems will now be discussed with The Problem of the Bank of Parallel Valves. the aid of this Chart. In all cases the value of the load Having considered, generally, the nature of the resistance which is to be worked will be supposed limitations imposed upon the working of the individual known. power valve, we come now to the questions arising from I. Given the number of parallel valves : to find the amount of the performance of a number of such valves connected distortionless a.c. power supplied by the bank, and also the in parallel. We shall assume that all the valves con- optimum value of the supply voltage to effect this. cerned are of similar type, so that the characteristics Our object here is to find the points E and P on the of each are approximately the same. It will further be Chart. Join the known points A and C. There are supposed that the resistance, r ohms, of the load is in now two possible cases, depending on whether the all cases known. The points which arise are varied. point B lies above or below AC. If the supply H.T. voltage and the number of valves Case (1). -If B lies below AC, the required point E in the bank are both fixed to start with, it may be re- is found in alignment with B and C. The power P is quired to ascertain the amount of a.c. power which r found in alignment with and B (see Fig. 4). can be delivered to the load without distortion, together 2 with the value of grid bias required to secure this. Case (2). -If B lies above AC, let B' be the point Conversely, if the amount of a.c. power required is in which AC meets the current axis. Then E is in specified, we may wish to determine either the number alignment with A and C, while P is in alignment with of valves in the bank or the optimum value of supply Y voltage. To and B' (sec Fig. 5). such questions it is hoped that the Align- 2 ment Chart now to be described will supply a ready Example. -We shall illustrate answer. this procedure by con- sidering a bank of five parallel valves How to Use the Chart. each having the following constants : D = io watts ; Imaz= ioo mA. ; The Chart itself (Fig. 3) Ra =2,000 ohms. Suppose consists of four parallel ¢ it required to find the W Fig. 4. (Left) Method of axes, or scales, bearing re- U using Chart of Fig. 3 when approximate a.c. power t- w 1- p B lies below AC. spectively C.) Z The numerical w dotted which could be delivered ó á A lines need not actu- values of voltage, resist- > 1- ally be drawn; their posi- tions only need be indicated by this bank to a load of ance, current and power. o by a ruler or 500 ohms, n other and further to From the data supplied straight -edge. ascertain the H.T. supply for the individual valve, is necessary. From equa- take the value k e of maxi- C í , tions (I) above, the fol- mum permissible current, á lowing values are easily I,aax, the anode dissipa- Er a ó z o obtained : A=25 watts ; tion D, and the anode > B =25o mA.; C =65o u, a.c. resistance, Ra. We ohms. An inspection of then find values of the the Chart shows that B following quantities :- lies ; A above AC hence D Case (2) applies. Using x watts C 2 the diagram as in Fig. 5 we find the necessary H.T. B =N x ' "' milliamps. -(r) Fig. 5.- (Right) Method of Et to be 255 volts, -2 using the chart the a.c. when B lies above AC. An auxiliary power in the load being point B' is found, which in C=-a+ ohms. turn is used to deter- 9.7 watts. The necessary mine P. grid bias with which to r being, as usual, the load operate will, of course, be resistance and N the num- read from the valve ber of parallel valves. characteristics as usual. Further, let E represent the value of the supply H.T. II. Given the number voltage. of parallel valves and a fixed supply of In using the Chart, the points A, B, C, and H.T. voltage : to find the maximum distortionless a.c. power E will always refer to the values of these quantities when delivered to the load. taken on the appropriate scales of the Chart ; thus, the We here suppose that, owing to exigencies of supply, D point A refers to the value of N x taken on the the actual H.T. voltage on the anodes is not the opti- 2 mum value ascertained immediately above, but is of power axis. In addition, if P denote the value of a.c. some definite fixed amount. We have already seen power delivered to the load r by the bank of N valves, (Figs. 2a and 2b) that a variation from the optimum the point P on the Chart will refer to this value taken value of H.T. supply will usually necessitate a restric- A 22

www.americanradiohistory.com FEBRUARY 78th, 193r. WRrIl®/og,s 175

Output Valves in Parallel. - Joining 450 volts to B, we obtain A and C. Thé values tion of the extent of working " run " if distortion is are, A= 31.8 watts ; B =142 mA. ; C =1,580 ohms. to be avoided ; consequently, the a.c. power now to be But from equations (i), delivered will be the same or less (but never more) than with the optimum H.T. voltage. The procedure is A= NxD =Nx5 now as follows: - Join E to A, B and C, and note the point B' in which B = N x 2""x =N x 50 the lowest of these lines meets the current axis. (If Eli is the lowest line, B' will be the point B.) Then C= ,x24 -- =2 0-{-1000 P is found in alignment with and B' (see Fig. 6). It We thus have three equations for N, yielding respec- 2 tively the results is to be remarked that the limiting condition governing 5N = 31.8, or N = 6.4 the extent of the working " run " will be (A), (B) 50N = 142, or N = 2.8 or (C), referred to at the commencement of this 2000 moo = 1580, or N= 3.4 article, according as EA, EB or EC is the lowest on N-{- the Chart. We therefore select the t h e required Example.-With 2 value 6.4, i.e., the W w Fig. 6.- (Left) In this case same five valves as were 0 EC is shown as the lowest number of parallel valves of the three lines through used above, let the fixed J aZ ¢W á will be 7, taking the next > E. It therefore fixes the H.T. supply be 200 volts. Ñ position of the auxiliary greatest integer to be on Y, U NA w point B', which in turn is The point E now referring cr used to find the point P. the safe side. to this value, the points % A, B, C being as before, ' Anode Dissipation Limit. the line EC is found to be the lowest of the three. a The reason why the --AB 3 Having found the position 2 largest indicated number value of P is í of valves will be that re- of B', the r E?-- ______B read off as in Fig. 6 to be --C.-----4.-----...... --- quired will appear if a 6 watts, showing a con- ---ep lesser number be chosen, siderable reduction on the r A and the a.c. power com- former case. The limiting puted under the same con- condition is here (C) that ditions. Should we decide due to the zero grid volts to use only 5 valves instead Fig. 7.- (Right) Illustrating p line, which precludes sig- the inverse method of using E of the 7 found necessary, nals of any greater magni- the Chart. When 2, P and we could find the a.c. tude being obtained. The E are given, A, B and C, power supplied by apply- remedy is, pf course, to and hence N, may be de- ing the procedure of II rived. increase the H.T. value to above. It would now be that found in the first found that example, viz., to 255 volts. III. Given the value of the available H.T. supply : to find the A =5X5 =25 watts best number of parallel output valves in order to supply a definite B= 5X5o= 25omA. a.c. power output to a given load. C = 400+1000 = 1400 ohms In this case the value of N itself is unknown, and so none of the points, A, B, C, is available directly. We Joining E =45o volts to each of these, we find that must, therefore, proceed to work inversely. EA is the lowest of the three lines, and note the position The point B will be found on the current axis in of W. The resulting a.c. power is easily read off as r only 12.5 watts, whereas it was stated that 20 watts alignment with the known points and P. (See Fig. 7.) 2 were required. Moreover, each valve is working at its The points A and C will be found in alignment with extreme limit of anode dissipation. This is otherwise E and B, A being on the power axis and C on the shown by the fact that EA was the lowest line of the resistance axis. three, showing, as we should expect, that anode dissipa- We have now obtained values for A, B and C, and tion is here the limiting factor. The load imposed on can substitute these values in the equations (1) above, the five valves is thus seen to be excessive. which thus give us three equations for N. Solving for In conclusion, it should be said that the results N in each case, the greatest value obtained is the one obtained by this diagram are approximate only, this required. being necessarily due to the simplifying assumptions Example. -How many valves of the same constants which were referred to in the text. It is believed, how- as before would be necessary to supply an a.c. power ever, that the loss of accuracy thus entailed is sufficiently of 20 watts to a load of dynamic resistance 2,000 ohms, small to justify a fair measure of confidence in results using 450 volts H.T.? Here we join 1,000 ohms to only otherwise obtainable with a disproportionate 20 watts, and note the point B on the current axis. amount of arithmetical labour. A 23

www.americanradiohistory.com 17(5 WAp@Il@OO FEBRUARY 18th, 1931. WopIld 000114011,0 .A.C. FOUR A Compact, Effective, and Easily Operated Receiver with Two H.F. Stages. Tuned anode couplings are used in the H.F. ampli- fier, and matters are so arranged that the variable con - denser rotors may be earthed, thus avoiding construc- ONE could wish that there was a wider choice of tional complications. Waveband switching is effected commercial broadcast receivers with multi -stage by the simple expedient of short -circuiting the long - high- frequency amplifiers. The better H.F. - wave sections of the tuning coils. det.-L.F. sets are capable of making a wonderfully fine The detector grid circuit constants have obviously showing, even for long- distance work ; they fully deserve been chosen to ensure the avoidance of high -note loss, the wide popularity that has been . accorded to them, and to give sensibly linear rectification. Transformer but an extra H.F. valve is well worth having. A re- coupling is adopted as the link between the rectifier and ceiver with two H.F. amplifiers should be capable of the L.F. valve, which is of the directly heated variety, doing its work without effort and with certainty, even with a heavy filament consuming one ampere at four in unskilled hands ; further, it almost follows that there volts. A choke filter output for the loud speaker is will be sufficient reserve of amplification to mask the provided. ill- effects of irregular valve charac- There is no reaction in the usual teristics or operating voltages that sense, but the pre- detection volume differ widely from those intended by SPECIFICATION. control takes the form of a grid the designer. CIRCUIT : Two screen-grid H.F. valves potentiometer for the H.F. valves. With regard to selectivity, there is with tuned anode couplings ; grid Matters are so arranged that, by also an important advantage. When detector, transformer- coupled to triode making the grids operate at zero or a superabundance of H.F.'amplifica- output valve. Fitter feed to loud at a slightly negative potential with speaker. tion is available, it becomes possible Westinghouse metal rectifier respect to their cathodes, self - to work with an extremely loose in voltage- doubling circuit. oscillation can actually be brought aerial coupling, thus largely avoid- CONTROLS: (I) Single -dial control about over the majority of the tun- ing the particular type of inter- of three tuning condensers. (2) Ganged ing range. ference to which the screen grid_ wave -range switch (3) pre -detection Decoupling resistances and con- valve is liable, and at the same time volume control. densers are fitted where necessary retaining sufficient overall sensi- in grid, GENERAL : Provision screening grid, and anode tivity to satisfy all reasonable needs for gramophone circuits. pick -up. For operation on A.C. Automatic grid bias for as to range of reception. the output valve is provided in the This principle has been put mains and with inside sor outside into open aerial. conventional manner. practice in the Regentone A.C. Constructionally, a somewhat un- PRICE £31 10 0 mains- operated four -valve receiver, : complete with valves. usual plan has been followed, in which made its first appearance last MAKERS:.Regentone, Ltd., Regentone . that the receiver proper and the autumn at Olympia, where its com- House, 21, Bartlett's Buildings, power supply units are entirely pact and workmanlike design London, E.C.4. separate though housed in the same attracted considerable attention. cabinet. The latter unit includes a As shown in the accompanying power transformer with outputs for diagram, the basic circuit arrangement embodies two H.T. supply and filament heating, a Westinghouse H.F. stages, a grid detector and a triode output valve. rectifier, and also the usual smoothing equipment. The The aerial may be connected at will to the input coil, receiver proper is built as a chassis assembly ; valves, either through a single fixed condenser or through two screened tuning coils, and a triple " Polar " tuning con- condensers in series. As each of these capacities is denser, are mounted on the top of a shallow aluminium rated at 40 mmfds. only, it will be appreciated that aerial box, in which are mounted the remaining components. coupling will be extremely loose in either case, and, Although there is no overcrowding- indeed, there is further, that the capacity existing across the input cir- space to spare everywhere -the receiver is exception- cuit will not be affected by changes of aerial capacity. ally compact, the chassis measuring a bare 12in. wide A 24

www.americanradiohistory.com FEBRUARY 18th, 1931. WIpgigoo 177

POP/ER un/T DIAL LIGHT GANGED NFVALVES. MULLARO 54VB CONDENSERS

OUTR7T VALVE. SCREENED MOLLARD AC/064 TUN/NG CO /LS

7V VGLUME CONTROL POTENTIOMETER ON SIDE OF CAB /NET

TRANSFORMER PR/MARY TAPPINGS li

._: I . IIII I I ;I II,p I -í II p . gg .It :QlÌmlc piI III IUG '' II Ulullílìo \ AER/AL./. I IIIIh II !`)IIIII EARTH AER/AL.T. I : ' IIIIIIIb \ DETECTOR. MULLARD:\ 35r V 91ldlb; HFVALVE SCREEN P/CK-UP LOUD SPEAKER

METAL CHASSIS

CATHODE GRID CONDENSER AND LEAK POTEIYT/OMETEP AERIAL SEP/ES CONDENSERS

DE-COUPLING RESISTANCES FILAMENT POTENTIOMETER

GAA/CED WAVE -RANGE SWITCHES' INTERVALVE TRANSFORMER

3/AS RESISTANCE

SkAGHETT/ FEED RES /STANCES

OUTPIT CHOKE

HtNO Receiver and power supply chassis. Below is shown the underside of the base compartment.

A 2 i

www.americanradiohistory.com y; fi:_ .: : ï . : -.4. 4 ;44_1143._., _

178 WAynDc/oo} FEBRUARY 18th, 1931. VVl7L`f1lll Regentone A.C. Four. - in transmissions intermediate between the local twin by about rosin. in depth. Both receiver and power stations when testing the set in North London. supply units are readily removable from the cabinet. As. far as quality òf reproduction is concerned, the About ioo volts is applied to the detector anode, and capabilities of a good modern receiver are largely deter- this valve passes a current of nearly 5 milliamps. mined by the characteristics of the loud speaker with Although overall II. F. magnification is not excep- which it is operated. The Regentone receiver leaves tionally high, it is much greater than would be obtain- no grounds for unfavourable criticism on this score ; able from a single stage, even of the " laboratory " although its bass cut -off is perhaps a shade higher than is very well maintained. type ; it is no exaggeration to say that, under average in some cases, the upper register conditions, any transini -,ion with the slightest " pro- The special directly heated output triode, as used in one gramme" value is xvi thin rain(' of the set. On the this receiver, is entirely satisfactory, and rather

* o ourruT

A1 A2 1 mid 61 ----

OFT I .r 0 000 1rd

0 1 M u

c

O

BIAS RESISTANCE

'theoretical circuit diagram. gramophone pick -up sockets (not indicated) are Joined across the detector grid circuit medium waves its behaviour is particularly pleasing, wonders why this particular type of valve is not more and an almost unlimited choice of stations was found widely ((sect. It is operated under optimum conditions to be available when a practical test was made in cir- and gives an output of ego milliwatts- adequate for all cumstances that were by no means ideal. The long- ordinary needs. wave performance was perhaps slightly less striking, Volume regulation is effected entirely by operation of but almost all the high -power stations operating in this the Soo -ohm. potentiometer which controls the grid band were well received. potentials of the H.F. valves. This was proved to be Selectivity is surprisingly good. The adjective is a very satisfactory device ; by its use even a strong used advisedly as it is rather difficult to account for such signal can be reduced to inaudibility. a satisfactory showing in this respect in the case of a There is an almost complete absence of hum, and, receiver with but three single -tuned circuits. Without indeed, the receiver is exceptionally free from back- any special operating artifices, it was possible to tune- ground noises of every kind. A 25

www.americanradiohistory.com _ FEBRUARY 18th, 1931. NRTAmboo WaiIl4 179

N FiNV EA FRS' NUM ) ER Next week's issue will he, by " Special request," devoted principally Parts used in the A.C. Three: to the interests of the new reader. Introduce The Wireless World 1 11riple variable condenser, 0.0005 mfd., with drum drive (Polar Tub) to your friends with this issue. The following are some of the I Variable condenser, 0.0003 mfd., with knob (Utility, W.218) special features to be included I Variable condenser, 0.0002 mfd., with knob (Utility, W.219) :- 1 Variable condenser, 40 mmfds. (J.B. Midget) 1 Differential condenser, 0.0003 mfd. (Utility, W.211) PRE -SELECTION 1 Condenser coupler (Ormond) A.C. THREE 3 Condensers, 1 mid. (200 volts working) ... (T.C C., No. 50) 3 Condensers, 4 mid. (400 volts working) ... (T.C C., No. 80) How to build the New Band-pass Set 4 Condensers, 2 mfd. (200 volts working) ... (T.C C., No. 50) 1 Condenser, 2 mid. (400 volts working) . (T.C C., No. 80) 3 Condensers, 0.0003 mfd. (Dubilier, No. 610) 1 Condenser, 0.0001 mfd (Dubilier, No. 610 1 Condenser, 0.05 mfd (Dubilier, No. B.775) 1 Semi -variable condenser, 0.00002-0.0002 mfd. .... ( Lewcos) 1 Semi -variable condenser, 0.00015-0.002 mfd. (Lewcos) 1 Resistance, wire -wound, 30,000 ohms, and holder (Varley) 1 Resistance, wire -wound, 50,000 ohms, and holder (Varley) 2 Resistances, wire-wound, 10,000 ohms, and holder (Varley) 1 Resistance, 400 ohms (Clarostat, Type F.W., Claude Lyons) 1 Resistance, 100 ohms (Clarostat, Type F.W., Claude Lyons) 1 Resistance, 100,000 ohms (Ediswan) 2 Resistances, 250,000 ohms (Ediswan) 2 Grid leak holders ( Bulgin, Porcelain) 2 1..F. chokes, 40 henrys (Bayliss) 1 L.F. choke, 300 henrys (R.l.) 1 L.F. choke (R.I., Pentomite) 1 Power transformer (Bayliss, Type W.W.8) 1 Valve screen (Colvern) 3 Coil screens (H. & B.) 1 Metal rectifier (Westinghouse, H.T.7) 1 Flash lamp holder 1 Fuse bulb, 100 mA. (Bulgin, Type B) 1 H.F. choke (Lewcos) 1 H.F. choke (McMichael, Binocular Junior) 2 Terminal mounts (Belling -Lee) 4 Indicating terminals, aerial, earth, 2 output A compact receiver (Belling -Lee) 2 Panel brackets, 4in (Magnum) J embodying the 3 Tuning coils (Type WWTG, Tangent) 3 Valve holders, 5 -pin (Lotus) most recent Wire, screws, sleeving, sheet tin, sheet improvements aluminium for brackets, etc., etc. in modern set design.

THE EVERYMAN TWO

An easily con- structed, self.- contained,trans- Large portable re- Coloured ceiver for use Supplement with either in- door or outdoor aerial. STATION

IDENTIFICATION Complete constructional details CHART and full explanatory diagrams. Components used in the Everyman Two: 1 Variable condenser, 0.0005 mfd. (Polar, No. 3) 2 Valve holders (W.B., " Universal ") 1 Piece of ebonite, 2 x 1 x ', in. - 1 Variable condenser, 0.0003 mfd. 1 " Spaghetti " resistance, 30,000 ohms .. (Magnum) 1 H.T. battery, 120 -volt (Pertrix) (Ready Radio, " Brookmans ") 1 Grid leak, 2 megohms (Sovereign) 2 Grid bias batteries, 9 -volt (Pertrix) 2 Vernier dials (Formo) 1 Switch, three -pole double -throw (Eureka) 1 L.T. accumulator, 2 -volt (Ex doe D.F.G.) 1 Differential reaction condenser, 0.0003 mfd... (J.B.) 1 L.F. transformer (Ferranti, A.F.8) 1 Loud speaker unit (Ormond, R.4mO) 1 Condenser, 0.0003 mid., and clips (Sovereign) 3 Solid plugs (Clix, No. 25) 1 Small chassis and cone ... (Ormond, R.451) I. Condenser, 2 tnfd. (Dubilier) 4 Sockets, insulated with bush heads (Clix, No. 24) 1 Cabinet (Cameo) 1 Tuning coil, base mounting, tapped 20% 2 Hook terminals (Clix, No. 4) 2 Valves (Nullard, PMIHL, and PM2A) and 00% (Tunewell, Dual X) 6 Wander plus, 2 H.T. +, 2 H.T. -, G.B. +, t Pkt. insulated links.... (Ready Radio, " Jiffilinx ") 1 H.F. choke (Ready Radio, " Hilo ") G.B.- (Belling-Lee, " Midget ") Wood screws, flex, etc.,

A 27

www.americanradiohistory.com r"t 18o WIpA®go FEBRUARY i8th, 1931. WodIE MORE POWER FROM PARIS. The Pope's address, in Latin, took about The new Radio Paris will shortly begin a quarter of an hour to deliver and was experimenting on 80 kilowatts. Tests will heard in practically all countries of the follow with Petit Parisien on 60 kilo- world. The relay from the B.B.C. watts. transmitters was entirely successful, but 0000 it was found that better results were A 500 -KILOWATTER. obtained by picking up the transmission Europe's most powerful broadcasting C URREST from W2XAI) on 19.5 metres than re- station, now under construction at Nog - ceiving direct from the Vatican Station. hinsk, a Moscow suburb, will have a power The transmitter, which in its main fea- of 500 kilowatts. tures follows the design of the high- In accordance with the famous Five speed British beam transmitters, consists Years' Plan, the Soviet Republic hopes ToPicS of four main panels, and is designed for to commission eleven new broadcasting telephony and high -speed telegraphy on stations of 100 kilowatts each and twenty - smwi^.,t,rd1o!rwp11iivaa»imi^dii,. either 19.84 or 50.26 metres. eight of 10 kilowatts before the end of On telephony the transmitter is rated 1933. to deliver from 8 to 10 kW. of un- The new Kolpino station, which will News of the Week in Brief Review. modulated carrier wave energy to the relay the Leningrad programmes, will aerial feeder system, the output depend - shortly begin transmission with a power TELEGRAMS FROM TRAINS. ing slightly on the wavelength used. The of 75 kilowatts. Radio enterprise on the French State normal degree of modulation is 80 per 0000 Railways has not stopped short at the cent. The rating on continuous wave tele- RADIO AMATEURS IN N.Z. installations of broadcast receivers ou graphy is 13 -15 kW. to the aerial feeder. EARTHQUAKE. trains. Preparations are now being made The energy from the transmitter build- For the story of how New Zealand for the inauguration of a radio -telegraph ing is conveyed to they two aerials by wireless amateurs rendered invaluable service for the convenience of passengers two separate concentric copper tube public service during the earthquake we to and from Paris on the Brest, Havre, feeders. The transmitting building itself are indebted to Mr. F. A, Mayer Dieppe and Strassburg lines. is situated near the railway terminus. '(G2LZ), of Wickford, Essex, who has 0000 Every effort has been made to harmonise, been constantly in touch by wireless with THE VATICAN WIRELESS STATION. as far as possible, the transmitting build- Station ZL -2BE, operated by Mr. Mills, The short -wave broadcasting and wire- ings and aerial towers with the graceful of Hastings, in the devastated area. less telephone and telegraph duplex surro',ndings of the Vatican City. 'l'he by As soon as the main shock subsided SI r. station which has been supplied transmitting building is of sober but Mills collected the remains of his trans- Marconi's Wireless Telegraph Company to pleasing architectural design. which was formally mitter and re- erected it on packing cases the Vatican City, and A special receiver, partly made of in the open. He was working under inaugurated by His Holiness the Pope on standard of normal range. parts the telephone difficult conditions with fires raging all Thursday last, has a world -wide arid high -speed Marconi receiver and tele- around him. His station proved the Illustrations will be found on pages 183 phone- terminal four -wire two -wire equip- only means of communication between and 184 in this issue. ment, will secure good telephone and tele- Hastings and the outer world for several graph duplex communication between the days until the land lines were restored. Vatican City and any part of the world. Communication with the stricken dis- ooco Zealand trict was organised by the New TELEVISING MARKET PRICES. Amateur Radio Transmitters' League, W9XAP, Chicago, transmits the New ZL2GK at Wellington being appointed York and Chicago Stock Exchange quota- official station under the control of tions by television. As it emerges from Messrs. Taylor and Perkins. In Napier, the machine the tape is carried past the where conditions were exceedingly bad, scanning apparatus. The reproduction on Station ZL2GE did good work. In television receivers is stated to be quite spite of difficulties the little chain of reachable. stations handled large nùmbers of mes- 0000 sages at an average speed of 35 words DUTCH AMATEURS WARNED. per minute. Ultimately they were taken The Supervisory Committee of Radio over by the Government for official Transmission in Holland has issued a traffic. 0000 special notice to Dutch amateurs forbid- ding them to transmit anything in the WHAT WAC STANDS FOR. The letters WAC mean " WPorked All nature of a criticism of the Government's Continents. " -South African Wireless policy in broadcasting matters. Weekly. 0000 This explains the.term " ether hog." BIGGER PROFITS FOR N.B.C.

00 00 .. The gross revenue of the American BROADCAST JOKE SERVICE. National Broadcasting Company in 1930 A startling report comes from a Madrid was in excess of £4,500,000, according to correspondent to the effect that the the annual report of its President, Mr. famous Spanish humorist, Ramon Gomez, Merlin Aylesworth. The gross revenue has been permitted to install a permanent in 1929 was £2,862,000. microphone at his home linked up by land With the addition of six new trans- line to the nearest broadcasting station. mitters to the N.B.C. system in 1930 the The innovation has been arranged " so permanent wire network now totals that he can intervene at pleasure during 34,500 miles, linking seventy -four broad- the transmissions for the purpose of throw- casting stations, two of which are in ing in a joke." Canada. There is no truth in the rumour that The synchronisation of stations. i.e., -the B.B.C. is installing a line to the RAILWAY SIGNALS. A German loco- linking them together by short-wave residence of Tommy Handley with the motive fitted with a frame aerial used for channels or by wire so that a group can reception of shunting Signals - instructions. wave, object of brightening the Sunday pro are transmitted from an 'serial" along- operate simultaneously on a single gragimes. side the track. is soon to be put to practical use. A 23

www.americanradiohistory.com

FEBRUARY 18th, 1931. Nrpg@go 181 Wait].

AUDION L.F. TRANSFORMER. cabinet. The makers are Messrs. Lan - Made chester Laboratories, Ltd., Spring Road, by Graham Parish, Ltd., Masons Tyseley, Birmingham. Hill, Bromley, Kent, the Audion L.F. ocoo transformer is available in two ratios - 3:1 and 5: 1, the price being 12s. 6d. NEW W.B. SWITCHES. The new push -pull switches introduced and 15s. 6d. respectively. The 3 : 1 model has been tested and measurement recently .by the Whiteley Electric Radio made of its pritnary inductance at 50 Co., Ltd., Nottingham Road, Mansfield, cycles Notts, make use of a method of contact when passing various values of a high order and quite equal to the aver- D.C. With no D.C. flowing, an induct- not hitherto met with in switches of this age moving coil loud with - ance of speaker mains type. Two samples have been sent in 24.5 henrys was obtained; this energised field. falls to 23.5 henrys with 2 tuA of D.C. for review, the one a simple make -and- The moving coil is of the low impedance break switch, and the other a three -point type and a 25 : 1 transformer is required switch. Stout nickel- silver springs are for normal low- impedance power valves. mounted on a bakelite base, and in the Models fitted with internal transformers suitable for either medium impedance triode valves or pentodes are available. The response is characterised by crisp- ness and brilliance in the upper register, the reproduction of speech being quite exceptionally good. From 50 to 150, cycles the output rises rapidly to a value slightly below the average for the whole characteristic. This output is maintained without any sign of resonance up to 750 cycles, after which there is a rise to the average value at.1,000 cycles. The curve New W.B. three -point switch and, inset, continues at this new level up to 2,500 enlarged view of double -cone contact cycles and rises above the average between " on " position the of these 3,000 tips springs and 6,000 cycles, the highest output snap into a re- entrant cone carried on the being located between 4,000 and 5,000 end of the plunger. This affords a larger cycles. Below 100 cycles the amplitude area of contact than possible with the developed by the diaphragm is adequate, Orthodox arrangement, since the springs but the acoustic output appears to be are gripped by the double cone making limited by the small diameter of the dia- contact on both sides of the leaves. All phragm (5 in.). Nevertheless, this slight contacts are insulated from the plunger deficiency is more than Graham Farish Audion L.F. transformer compensated for so that the switches may be mounted on in moulded bakelite case. by the performance in the upper register, metal panels without the aid of insulating with the result that the general and to 19 henrys with 4 mA. On passing effect is surprisingly good for so 5 mA of D.C. through the primary, the small an instrument. inductance recorded was 16 henrys, and with 8 mA 14.5 henrys. A practical test using a valve of some 15.000 ohms. A.C. resistance and passing a steady anode current of approximately 2.5 mA revealed a noticeable falling off in amplification below 300 cycles and above 6,000 cycles. Between these two limits the amplification was good and, ,judged aurally, appeared to be reasonably constant. There was no improvement in the lower register by changing to a value of 8,000 ohms. impedance, since the inevitable increase in anode current reduced the primary inductance to a level where the ratio of primary impedance to A.C. re- sistance of the valve was of about the sane order as in the previous case. o ^_ o o Lanchester moving coil loud speaker LANCHESTER MOVING COIL with permanent magnet field. LOUD SPEAKER. bushes. Single -hole fixing bushes are This neat little instrument is contained fitted, and all metal parts are nickel- in a rexine covered cabinet measuring only plated. Soldering tags, in addition to 13in. x 10 in. x 54in. In spite of the small The price is 4 guineas, and models now terminals, are provided, and the prices dimensions of the 35 per cent, cobalt steel in production are fitted with fret of simple are : Is. 6d. for the three -point model, permanent magnet, the sensitivity is of design in keeping with the lines of the and Is. 3d. for the two -spring type. A 29

www.americanradiohistory.com FEBRUARY 18th, 1931.

CLIX NEW " ALL -IN " TERMINAL. The makers are H. Clarke & Co. (Man- POLAR "DISC" DRIVE. Since the introduction of the resilient chester), Ltd., Atlas Works, Old Traf- Below is illustrated the Polar " Disc " socket to replace the split pin ami plain ford, Manchester. drive fitted to their " Tub " three -gang socket for plug -in type connectors, Clix 0000 have remodelled many of their standard EXIDE INTRODUCE DRY lines to enable the new arrangement to BATTERIES. Reliable figures relating to the per- centage of broadcast listeners who use dry -cell H.T. batteries are exceedingly difficult to compile, but the evidence col- lected by the makers of Exide batteries is apparently sufficiently weighty to in- duce this firm to enter the dry -cell market. The batteries are officially known as Clix new "All -In " insulated terminal Drydex batteries, and include ILT., grid with solid plug and terminal socket. bias and sundry sizes of torch and pocket lamp batteries. be adopted. The insulated "All -In " ter- The H.T. range, with which we are minal is now fitted with a solid pin particularly concerned, consists of four mounted in the fixed position of the con- types -the Red Triangle, Green Triangle, nector and the removable part, to which Orange Triangle, and Blue Triangle. The the flex lead is attached, has a helically first two mentioned are standard capacity slotted resilient socket. size, but the Green Triangle class is of All metal parts are fully insulated and superior quality. The prices of the Red The " Disc " drive. the terminals are designed to safeguard Triangle series range from 7s. for a 60- Polar the valves, since the various flex portions volt size to 14s. for the 120 -volt unit, are not interchangeable. condenser which was inadvertently The complete terminal costs 8d., en- omitted from the illustration accompany- graved or plain. The flex portion and the ing, our test report on this condenser, panel fitment can he purchased separately which we published on page 711 in our at 4d. each. issue of December 24th last. The price The makers are Lecho Linx, Ltd., 254, of the drive is 5s. and the makers are Vauxhall Bridge lid., London, S.W.1. Wingrove and Rogers, Ltd., Arundel Chambers, 188 -189, Strand, London, oouc w'. C.2. 0 0 0 0 ATLAS SHORT -WAVE COILS. These coils are made in four sizes, and enable a wave range of from 15 to 100 Catalogues Received. metres to be covered a with 0.0003 mfd. Mallon Radio, 14, Golden Square, Lon- tuning condenser. The coils are wound with don, W.1.- Illustrated folder dealing with stout gauge tinned copper wire and their range portables, transportables, mounted on porcelain, the plugs, sockets, of clamping screws, and other metal parts, and radio- gramophones.0000 being nickel- plated. The set consists of The M.L. Ltd.. Vic- a two -turn coil, a four -turn coil, New Drydex battery just introduced Magneto Syndicate, a six -turn Coventry. The Book of coil and a nine -turn coil, by Exide. toria Works, -" the price being the M.L. Rotary Transformer," describing 10s. the complete set. The prices of the while the corresponding types in tine and explaining the uses of the various ma- separate coils are, No. 2, 2s. 6d.; No. 4, superior class cost 9s. and 18s. 6d. respec- chines made by this firm. 2s. 7d.; No. 6, 2s. 8d. ; and No. 9, 2s. 9d. tively. 0000 The Orange Triangle series are triple Celestion, Lid., London Road, Kingston - capacity designed to meet the needs of on- Thames. -Illustrated folder in colour those listeners using power pentodes or dealing with the new season's range of power triodes in the output stage of the loud speakers, the Celestion -" Woodi offe " set. A heavy duty 60-volt unit in this pick -up and the 'l'iltatone. class costs 14s., a 105 -volt 24s. 6d., ococ and a 120 -volt 27s. Asea Electrical, Ltd., 5, Chancery Lane, The Blue Triangle London, W.C.2.- Illustrated price list, series have been de- A.C.7, dealing with single, two and three signed especially for phase induction type motors, ranging from portable sets, and are a h.p. to 120 h.p., also starter equipment made in three sizes, for same. viz., 63 volts, 99 0000 volts, and 108 volts, Snap Switches, Ltd., controlled by the prices being 9s., Graham Farish, Ltd., Bromley, Kent. -15- 14s., and 15s. 6d. page illustrated catalogue describing the respectively. new range of Sinlas, Nilos, Litios, and At the time of Ton -A -Kap condensers recently introduced writing a sample by this firm. Red Triangle bat- 0000 tery is undergoing The Edison Swan Electric Co., Ltd., a life test, and 123-5, Queen Victoria Street, London, when this is com- E.C.4.- Publications R583 dealing with pleted we shall be in the full range of Mazda valves, and R594 a position to give describing the Ediswan Sound Reproduc- some first -hand infor- ing Installations designed to distribute mation concerning broadcast matter throughout hospitals, Clarke's Atlas No. 2 and No. 9 short -wave coils. these new batteries. hotels, and residential flats. A 30

www.americanradiohistory.com FEBRUARY 18th, 193í. Wipd200 VWOlsRCI

By Our Special Correspondent. More Money for the B.B.C. -Talk of "Sponsored" Programmes.- Mystery of Northern Regional. - Acoustic Tests at "Broadcasting House. " -Is an Organ Needed ?

New B.B.C. Licence. The " Sponsored " Programme. ably ways of introducing the commercial A new licence for the B.B.C. is being While on the subject of B.B.C. finance, element without transgressing the printed drafted by the Postmaster -General, Mr. it is interesting to note the merest hint rules. Lees- Smith, but it will-be a considerable of a change of view at Savoy Hill in It is quite conceivable that a business time, I understand, before its terms are regard to the question of sponsored pro- house might supply the programmes for arrived at by the Treasury, mainly be- grammes. I believe that the attitude of a whole day without paying a penny to cause its chief clauses will concern the more than one person in authority has the broadcasting organisation. But the amount of money to be paid over to the recently changed from the adamantine said organisation would be saving the Corporation. to the slightly plastic. money which would otherwise have been 0000 0000 required for that day's programme. 0 0 0 A Treasury Grant. A Way Out? 0 The present percentages of the licence The B.B.C. Charter specifically states, Why Revenue Should Increase. receipts of course, that the Corporation must B.B.C. revenue should undoubtedly allowed to the B.B.C. are, it not accept any monetary will be recalled, 90 for the first million, consideration increase very considerably during the for its progranuncs, but there coming 80 for the second, 70 for the third, and are prob- year with the inauguraticin of 60 for each of the succeeding millions. Northern Regional. This station opens The new licence will provide for an up a vastly enlarged service area in the additional grant to the B.B.C. of counties of Lancashire, Yorkshire, Cum- £200,000, which should considerably ease berland, Westmorland, and Durham the strain in regard to the regional where the present proportion of listeners scheme. to the total population does not exceed 7 per 0000 cent. 0000 The B.B.C: s Assets. "This Year, Next Year, Sometime, It was always unlikely, of course, that Never." a loan could be floated. Capitalists who In view of this fact it is all the more might have been inclined to part with mystifying that there should be sq much their money would have looked for assets delay in starting up the new station. as security, and it is surprising how few Week after week slips by and still we of these the Corporation possesses. hear that " great progress " is being 0000 made, that radiation tests are being con- More Money from Licences ? ducted, but that the public tests will not The only alternative that the P.M.G. yet take place. has recognised is the grant of additional Already the station is nicknamed " The funds from the licence receipts, and these Four Cherry Stones." will certainly be needed before much 0000 headway can be made with the Scottish The " Patient " Northerner. regional transmitters. There is surely something disquieting 0 0 0 0 in the fact that the northerners, who are at generally vocal enough if they are kept Salaries Savoy Hill. waiting, are displaying very little sign In the meantime, strict economy is of impatience over the regional delay. the order of the day at Savoy Hill. Is this an indication that after all the Considerable savings are being effected regional scheme is not so tremendously in lighting, heating, and a hundred and popular as was once thought ? Or is the one little details about which the staff North Country breeding a new race? formerly never bothered themselves. 0 000 The annual period for the revision of Harry Tate. staff salaries is now approaching, but I Harry Tate will make his first appear- hear HYJ CALLING. A picturesque view of that suggestions for increases are the new Marconi short -wave station at ance for many years in a studio on Feb- being subjected to the closest scrutiny. the Vatican City. ruary 28th, when he is to broadcast in A 31

www.americanradiohistory.com 184 Wfi gIl®g FEBRUARY 18th, 1931. WoYIld` the vaudeville programme for National Coming " O.B.'s." theatre contract to enable him to broad- listeners. He has been heard only twice A large number of sporting events are cast. - previously on the wireless. entered in the Outside Broadcast I)iiec- A few years ago Denis O'Neil's name 0 0 0 0 tor's diary for running commentary was frequently in the broadcasting pro- Exclusive to the " Radio Times." arrangements on the coming months. grammes, both for London and provincial How far the B.B.C. should go in On February 28th National listeners stations. Before the Alhambra, Leicester advertising their journals by the aid of will hear Captain Wakelanl describe the Square, " went ` talkie,' " he was billed the microphone has for long been a sub- Wales 1'. France Rugby International, as the great B.B.C. artist." ject of comment, but it must have come and Regional listeners will have Mr. 0 0 0 0 as a rude shock to many who were Allison's story of a match in the sixth listening to the Wanted : A Better Interval Signal. broadcast service the round of the F.A. Cup. Several other Sunday evening to hear the bright suggestions have reached The Oxford and Cambridge Boat Race me for a better" interval signal than the announcement that the words of the on March in 21st will be described a run- existing " tick took," which one critic second hymn were to be found " in this ning commentary by Mr. John Snagge, week's copy of has described as " a death tick in the Radio ?'lutes only." while the scenes on the course during the goloshes." The best, to nay mind, is the notion that each B.B.C. station should have its distinctive interval signal, devised to reflect the traditions and sentiments of locality. the 0000 intelligent Suggestion. My correspondent suggests that Northern Regional might adopt a con- tinuous clog- dance. Intervals at Aber- deen would echo with the sizzling of the haggis, while a Welsh station would offer the familiar " drip, drip," indicating a

leek . . .

FUTURE FEATURES. National (261 and 1,554 metres). FEBRUARY 22,1D.-" The Tempest" (Shake- speare). . FEBRUARY 23RD. -" Aida," Act 4, relayed from Halifax. FEBRUARY 25TH. -B.B.C. Symphony Con- cert relayed from Queen's IIall. FEBRUARY 26TH.-" Matinee," a play. FEBRUARY 27TH. -" Requiem Mass," by Frederic d'Erlanger, first perform- AT THE VATICAN CITY. ance. Marchese Marconi and some of the officials inspecting FEBRUARY 28TH. -Running commentary of the short -wave transmitter which was formally opened by the Pope on Thursday Rugby match, Wales v. France (from last in a Latin speech successfully re- broadcast by the B.B.C. Swansea). National (1,554 metres). Acoustic Tests at "Broadcasting House." race will be described by Mr. IIolt FEBRUARY 22N1.- Religious service from Now that " Broadcasting House " has Marvell. St. George's Chapel, Windsor. its lid on, arrangements are in hand FEBRUARY 25TH.- Ot'Chestl'al concert from for Other future arrangements include Bournemouth. testing the studio acoustics, on which so commentaries on the Grand National, the much will London Regional. depend. semi -final and the final of the F. A. Cup, FEBRUARY 22ND.- R.A.F. Band concert. According to the plans. the studios the Derby, and the race for the Schneider FEBRUARY 238D.- Memorial concert to should yield perfect results, but archi- Trophy. Peter Warlock, relayed from Wig - tects have learnt from 0000 atore Hall. bitter experience FEBRUARY 24TH. -Bach choir concert re- that a'ustic theories sometimes suffer Touring Scottish Churches. layed from Queen's Hall. very ce rally when put into practice. FEBRUARY 25TH. -Vaudeville programme. 0000 The Scottish Region of the B.B.C. is; FEBRUARY 26TH. -" I Pagliacci," relayed during the coming year, from Halifax. Is to Make a tour an Organ Necessary? of the churches all over Scotland, and FEBRUARY 27TH. -A Grieg programme, The results of the tests will greatly on February 22nd one of the first vocal and instrumental. Interest two rival factions of these FEBRUARY 28T11.-Running commentary on at head- outside broadcasts will be given from sixth round of F.A. Cup. quarters. One school of the thought con- Old High Church at Inverness. Midland Regional. siders that the B.B.C. should This possess its tour of Scottish churches will take a long FEBRUARY 23Rn. -The Prime Minister own concert organ speaking at annual banquet of and that an instru- time, but it is to be hoped that when the ment of worthy proportions should Birmingham Chamber of Commerce. be end of it is- reached FEBRUARY installed in the large the B.B.C. will, 28TH. -" All the Fun of the studio at Portland under the advice of the Religious Ad- Fair," cokerunt cameos by F. Morton Place. This opinion is opposed Howard. by those visory Committee, have made a represen- who feel that the B.B.C. has proved its tative selection from the country West Regional (Cardiff). ability religious FEBRUARY 27TH. -" Creditors," a play by to obtain all the necessary organ life of Scotland as it is to -day. music front outside sources. Leonard J. Hines, relayed from 0 0 0 0 0 0 0 0 Bristol's Little Theatre. North Regional (Manchester and Leeds). A Severe Test. Theatre Releases Actor for Broadcasting. FEBRUARY 24TH. -Liverpool Philharmonic I understand that there would he no Tons Walls, who is recovering Society concert. from Liverpool. from his FEBRUARY 26TH.- IIallé concert from Free objection to a studio concert organ if- recent accident in the hunting field. has Trade Hall, Manchester. and the if is a big one -it could be shown written from his sick bed to Denis O'Neil, Glasgow. that no interference would be caused to releasing him from his part in " Marry FEBRUARY 28TH. -Mr. Alexander Malay: other studios. But organ music, parti- the Girl " on the nights of March 5th An eye- witness account of a tie in cularly in the lower register, is of a pecu- the fourth round of the Scottish Cup. (Regional) and March 7th (National), to Belfast. liarly penetrating kind, and it seems enable him to take part in the radio play, FEBRUARY 28T15.- Running commentary on highly doubtful whether even the sound - " Rich Girl, Poor Girl." This is the International Rugby match, Ireland insulating walls of Broadcasting House first occasion in the history of the B.B.C. v. Scotland, relayed from Dublin. could withstand it. that an artist has been released from his A 32

www.americanradiohistory.com FEBRUARY 18th, 1931. Wflp®Il@go WOr Rd SOUTHEND'S RECORD RADIO SHOW.

Good Amateur Exhibits : Talking Along a Beam of Light. TF public enthusiasm in the Southend -on -Sea district most outstanding constructional merit was this year won affords an index to the feelings of the country gener- by Mr. Britton for his radio -gramophone set incorporat- ally, the popularity of wireless is still steadily in- ing a four -stage R.C.Ç. amplifier with paralleled pen- creasing. No fewer than 6,281 people visited the ex- tode output. This instrument and another shown by hibition held at the Boys' High School, Victoria Circus, Mr. S. Wilkins, using band -pass tuning and a diode by the Southend Radio Society on Saturday, January detector, gave continuous demonstrations from 2 to 31st last, an advance of about 1,600 compared with 6 p.m. Other attractive exhibits included a small short- the previous year. wave transmitter, made by Mr. Costin, of Billericay, The attractions were many and varied. They in- and an all -mains set encased in glass which had been cluded a collection of receivers built by members in specially constructed by Mr. Knipe for the stand of competition for the silver challenge cup, a vigorous dis- Messrs. E. K. Cole, Ltd., at Olympia. play by the local radio trade, and a striking demon- The judging was carried out by Mr. Frank Garon stration, conducted by NIr. F. H. Haynes, Assistant and by Mr. H. B. Dent of The Wireless World. Cabinets were judged by Mr. H. L. Lobb. The entries showed an increase over those of last year, the total being 53. Many valuable prizes were presented by well-known radio firms. Messrs. Garnett, Whiteley and Co., gave one of their " Lotus " D.C. all -mains sets, valued at &19 1os., for the best entry-by a High School boy, and this was won by C. J. Stockell. The General Electric Company gave an " Osram " Music Magnet Kit com- plete with valves. Great interest was shown in the modulated light beam demonstration. Gramophone music was transmitted by a beam of liglit traversing the hall and falling on a photo- electric cell followed by an amplifier. Visitors were able to convince themselves of the genuine- ness of the display by intercepting the ray of light, thus interrupting the music, to the amusement of the large audience. The upper photograph shows the apparatus used for a demonstration of transmission along a beam of Prize -winners. light at the Southend Radio Society's Exhitltion. The light The full list of prize- beam was focused on a photo- electric cell fifteen yards distant. winners is as follows :- In the lower photograph the Judges by School are seen inspecting home -built Sets High receivers. boys : I, C. J. Stockell ; 2, E. R.. Bessell ; 3, A. Jen- k i n s o n. Transportable sets : consolation, F. C. James. Short -wave sets : Editor of The Wireless World, in conjunction with the I, J. G. Ward. Two- or three -valve sets : Class A : General Electric Company, of music and speech trans- I, J. G. Ward ; 2, G. Crabb ; Class B : 1, Miss mitted along a beam of light. D. Burrows. Four -valve sets : 1, E. W. Lock- The entire proceeds of the Exhibition, which was the hart. Sets with battery eliminator, Class A : I, seventh annual event of its kind, were devoted to charity, F. R. Ford ; 2, E. T. Wiseman. Class B : i, M. Keddie. the main object being to install wireless equipment in Radio -gramophones : I, G. P. Britton ; consolation : S. the New General Hospital. In addition to the admis- R. Wilkins. Loud speakers : i, W. McGeorge. Battery sion receipts, charitable objects also benefited from the eliminators : I, A. H. Gregson. Wave- meters : 1, B. proceeds of the " junk " stand, at which many members Costin. Pick -up carrier arms : r, T. Holbeche ; con- of the public displayed discrimination and judgment in solation : F. C. James. Adaptor units : 1, E. W. Lock- the purchase of spare parts. hart ; special : H. H. Burrows. Various non -mechanical The silver championship cup for the exhibit of the units : I, L. Renall ; special : H. A. Clinton.

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www.americanradiohistory.com WApa®00 MEE FEBRUARY 18th, 1931 TU1OJ9 0 ILE TWSFORMER 11111111110 Iiiii1111111111111111111111111111111111111111111111111

Interactions Between the Windings. By S. O. PEARSON, B.Sc., A.M.I.E.E. (Concluded frtrn page 126, February 4th issue.) AVING found by numerical calculation the When we come to consider the high -frequency trans- effects of the tuned secondary circuit'. on the former as an intervalve coupling it will be seen that primary winding for a specified transformer, it is the value of R,' plays a very important part. We now expedient to develop a simple formula giving the must, therefore, have some means at our disposal of apparent increase of primary resistance caused by the determining its magnitude in a moderately simple way. tuned secondary circuit magnetically coupled to it. This might appear to have already been accomplished This will show us upon what factors the effective increase in the development of equation (3), but, unfortunately, of resistance depends. We already know that the a difficulty arises when we attempt to express the mutual voltage induced in the secondary circuit by the primary inductance M numerically. The value of w is known, current I, is given by E,= wMI, volts, where to =27 x being 27 x frequency, and R, can be fairly closely frequency and M is the mutual inductance in henrys estimated from the dimensions of the secondary winding between the circuits. With the secondary circuit tuned of the transformer ; but the mutual inductance depends to resonance the current in it is then I2=--E,/R, amps., not only on the geometrical properties and the number where R, is the effective series resistance of the circuit, of turns of each winding, but also on the relative posi- and putting E_ = wMI, we get tions of the two coils. Theoretical calculation of the uR mutual inductance or the coupling coefficient between I2 I1 amps. .. .. (2) = R2 the windings is not an easy matter, and for a given transformer, actually having been constructed, Now, reversing the the safest process, the voltage E,' induced course is to find the value of M in by practical measure- the primary coil by the current I, in the tuned ment. This in itself secondary requires special apparatus and a is El' = wMI, volts, and, substituting for I, certain amount of skill. the value given by equation (2), we have El'= °'R'I` Nevertheless, we have gleaned the information (a) that the tuned secondary circuit produces an apparent change volts. It has already been shown that the primary of primary circuit resistance, (b), that there is no effect current I, and the voltage El' produced in the primary on the primary reactance, and (c) as to the factors coil by I, are in exactly opposite phase, so that the determining the apparent change of primary resistance. effect of the secondary current on the primary coil is to oppose the passage of I, at all times to an extent Power Losses in the Windings. proportional to its magnitude: This means, as pre- The fundamental effect of resistance in a circuit when viously explained, that the resistance of the primary a current flows is the generation of heat. have coil is apparently increased by We re- an amount whose value peatedly referred to the calculated increase of is R,' = E,' / I, ohms. Therefore, primary substituting for E,' the circuit resistance as being an apparent increase, the value given by the previous equation, we have for the magnitude of which is given by equation (3). We apparent or effective increase : of primary resistance shall now see why this is only an apparent increase. Its magnitude R,' = " ohms .. was obtained from Ohm's law as regards R, (3) the ratio of voltage to current, and would therefore where (= 2- x frequency. appear to be real enough. But for a given current in It should be borne in mind that this expression for the primary winding the rate at which heat is generated the apparent increase of primary resistance is only there is exactly the same whether the secondary coil is true provided the secondary circuit is tuned to resonance carrying current or not, and so from this point of and that R, is the effective H.F. resistance of the view there would appear to be no change of resistance secondary circuit under these conditions. whatever in the primary circuit. Equation (3) shows that the resistance apparently The power represented by the generation of heat in added to the primary circuit is proportional to the the primary coil is equal to the product of the square square of the mutual inductance M between the coils of the primary current and the actual resistance under and inversely proportional to the effective resistance all conditions. Both methods of resistance calculation of the tuned secondary circuit. are perfectly sound, and yet they seem to render con- tradictory results. However, a little consideration will See The li'ireless World, February 4th, 1931. show that the conclusions are not really contradictory. A 34

www.americanradiohistory.com FEBRUARY 18th, 193r. NWp®Il@zo 187 /nOpIlCa..l The Theory of the H.F. Transformer. - On the other hand, the primary current will not be Under working conditions heat is generated in both the in phase with the resultant voltage determining its magni- primary and secondary circuits simultaneously, the total tude, and will therefore influence both the secondary power represented being I12R1 + I22R2 watts, where resistance, as regards damping and selectivity, and the R, and R, are the actual resistances of the respective secondary reactance, as regards tuning. circuits. The apparent increase of primary resistance arising Now, suppose that the whole of this power could be as a result of the proximity of the tuned secondary is made to occur in the primary circuit alone, the power WR1' in the secondary circuit being reduced to zero, and R,' = ohms, and the apparent impedance of the suppose, further, that the primary current remains un- primary circuit is therefore altered. This feat of imagination means that the Z1' = 1/(R1 -{- R1')2 -I- (wLj)2 ohms .. (5) secondary circuit is removed altogether, and that, there- On the primary side the transformer behaves like a fore, extra resistance is inserted in the primary circuit single circuit having this impedance. Knowing the in order that the total power shall be the same. Then applied voltage the primary current is therefore the extra power expended in the primary circuit must E be just equal to the power originally in the secondary, given by I, = Z', amps. and lags by an angle namely, I32R2 Let denote the extra primary watts. R,' R,' resistance necessary to account for the additional power, where cos =121+ the primary current remaining I, as before. Then Z, I12R1' = IO2R, watts, or Apparent Increase of Secondary Resistance. Referring again to Fig. I, suppose that the supply R1' (Ì1)-R2 ohms _ (4) voltage E, were to be suddenly reduced to zero, the Turning to equation (2), we find that circuit remaining closed. On account of L, wM the energy stored in the electrostatic and substituting this value of Ì = R and, magnetic fields of the tuned secondary the current ratio in equation (4), we get circuit, the current in the latter would (Ö2M2 not cease immediately, but would die R ' = ohms, which is precisely the 1 R away gradually in much the same way same result as given by the other method that the oscillations of a pendulum fall and shown as equation (3). to zero when the driving impulses cease. So, in spite of initial appearances, the The rate at which the current oscillations decay depends on the rate at which the two methods of treatment actually lead Fig. 1.- Circuit arrange- ment of a H.F. transformer stored energy is dissipated as heat in both to the same result. The latter method, with primary connected to however, gives a more definite physical a constant voltage source. circuits. The conditions obtaining meaning to the apparent increase of when the secondary is tuned At the instant the voltage E, is cut off, to resonance are discussed primary resistance R1'. It is equivalent in the text. the secondary current has its normal value to the extra resistance which would have I, amps., but immediately begins to de- to be included in the primary circuit to maintain the crease at a rate depending on the total power at that total power unchanged if the . secondary coil were instant. Now we can apply the same argument as removed or open -circuited, the primary current thus regards the apparent increase of secondary resistance being unaltered in magnitude or phase. as was applied in the case of the primary circuit, namely, the of in second- Effect of Primary Impedance on the Secondary Circuit. apparent gain resistance the ary circuit is equal to that extra resistance which, when Since the secondary circuit has a definite effect on multiplied by the square of the secondary current, gives the primary, it is only reasonable to suppose that the a dissipation of power equal to that in the primary reverse is also true. Let us refer for a moment to circuit. Fig. i, in which a high -frequency transformer is con- Immediately after the primary applied voltage E, nected to a source of alternating current. The primary has been cut off, the secondary current will be I, amps. and secondary resistance R, and R2 are shown con- This will generate a voltage E,' = uMI2 in the primary nected external to the windings for convenience. Comparing the primary and secondary circuits, the coil, and this, in turn, will drive a current Z` amps. main differences are (a) that the secondary is tuned to resonance whereas the primary is not, and (b) there round the (closed) primary circuit, where Z, is the are two E.M.F.s injected into the primary circuit, actual impedance of the latter. The power dissipated namely, the supply voltage E1 and the induced voltage in the primary circuit will, therefore, be equal to the E,' due to the secondary current, whilst in the secondary square of this current multiplied by the primary resist - circuit the only injected voltage is that due to the E'2 current in the primary winding. Since the secondary ance, namely, (Z') x R, watts. circuit is tuned to resonance, the current is in phase with the voltage producing it, and we have seen that The additional resistance R2' in the secondary coil for this reason alone the secondary circuit only affects required to account for this extra power would have to the resistance component of 2 the primary impedance, be such that I,2-R2' = i > R, watts. and, putting the reactance being undisturbed. Z'

A ,,;

www.americanradiohistory.com .SS WAp®Il®00 FEBRUARY 18th, 1931. WopIlCl The Theory of the H.F. Transformer. - fore, the equation will be of exactly the same form as El' _ t,MI we get for the apparent increase of second- equation (6), but with the resistances involved in the ary resistance- latter replaced by reactances. Thus, the apparent 2 change of secondary reactance due to the primary effects R2 Ri ohms .. .. (6) = (Z) will be- From this equation we obtain the important infor- mation that the apparent gain in secondary resistance X2' _ (Z1)2X, ohms .. '(7) is directly proportional i to the primary resistance, and where X1 is the primary reactance inversely proportional e)Ll or 24L, ohms. to the square of the actual pri- But it is necessary to know whether this represents mary circuit impedance. It is also proportional to an increase or decrease of the inductive reactance of the the square of the mutual inductance, and therefore to coil. The current in the primary circuit lags behind the square of the coupling coefficient between the the voltage, and by drawing windings. the representative vector diagram it would be found that the reactive voltage set Effect on the Secondary Tuning. up in the secondary coil by the primary current is in By considering the power dissipation we have been opposition to the ordinary E.M.F. of self -induction in able to develop equation (6) without recourse to the secondary. The effect of inductive reactance in the elaborate deductions involving the phase differences primary circuit is, therefore, to produce an apparent between the various voltages and currents. But the decrease in the secondary inductive reactance. A pre- equation could have been evolved by the vector method ponderance of condensive reactance in the primary cir- cuit employed in the first instance in obtaining equation (3). would cause an apparent increase of secondary in- However, in that case we should have found it neces- ductive reactance. sary to split the primary current up into two imaginary Since inductive reactance is given by r x inductance, if we components, one in phase with the votlage E1' generated divide each side of equation (7) by e we obtain an expression by the secondary current, and the other out of phase for the apparent decrease of inductance of the by qo °. It is the component of primary current in phase secondary coil, namely- with E1' that accounts for the apparent change R2' of 2 L2' = (Zr) L1 henrys .. .. (8) secondary resistance. The other component, out of step with respect to E,' by a quarter of a cycle, generates The secondary inductance which is effective as regards in the secondary circuit a voltage which is just go° out tuning is, therefore, L2 -L,' henrys, and for this reason of phase with respect to the secondary current, and rather more capacity will be required to tune the therefore trans- represents an apparent change of reactance in former to a given wavelength than for a single coil with the secondary coil. the same inductance as the secondary of the transformer. In obtaining an equation for the change of secondary In an article to follow a complete numerical calcula- reactance, the only difference we have to contend with, tion will be given for a high- frequency transformer compared with the determination of the apparent rise in when used as an intervalve coupling, and the theoretical resistance, is this go° angle of phase difference. There- results obtained here will be utilised.

To Beginners and Advanced Transmitters. tending to take part should send notice In our issue of February 4th, under to GSIIM not later than February " Books Received," we drew attention TRANSMITTERS' NOTES. 0000 21st. to the seventh edition of "The Radio International Short -wave Radio League. Amateur's Handbook," issued by the metres) for the purpose of investigating We have A.R.R.L., received from Mr. Mander and containing much useful the possibilities of this wavelength for Barnett, the vice- president and European advice and information, both to experi- reliable long- distance communication Manager of the League, enced amateurs and to those who wish the February with moderate power, and to note how issue of the " International Short -wave to qualify for a transmitting licence. The much the reliability is affected by weather Radio News," from which book, of we are glad to course, deals mainly with conditions, statics, fading, etc. The see that the membership is American practice, which differs increasing. some- competition is open to all members of The current issue gives particulars of what from that observed in Great Britain, the R.S.G.B. in the British Isles, and transmissions from the Westinghouse but most of the valuable information is the trophies at present held by G6ZH Electric and Manufacturing Company's equally applicable to both countries. and G6YL will be presented to the station W1XIZ, which broadcasts Mr. F. T. pro- Carter, Flat A, Gleneagle winning transmitting and receiving station' grammes on 31.35 metres daily from Mansions, Streatham, tells us lie that respectively. No contact will count for Boston, Mass, as stated in our issue of keeps a supply of these handbooks, which points unless the range exceeds 100 miles, February 4th, in " Current he can sell post Topics." at 4s. each, free, and and competitors should specially record The transmissions intended for Great that he also has a few copies left of the the differences in signal strengths during Britain are sent from 1900 to 2100 and sixth edition, which he can dispose of at periods of light and darkness. The from 2300 to 0200 G.M.T., half price. and reports this Those interested will he power will be limited to 10 watts, and of the reception of these signals, glad to hear that copies are obtainable especi- each competitor will be allotted four - ally with regard to their strength, fading in this country, as thereby they avoid letter codes, which will be changed for or other transmission phenomena, should the delay consequent upon ordering each of the four Sundays. be sent either to the headquarters of the direct from Hertford, Conn, and the Mr. J. Hum, G5UM. 17, Eastwood League, Box 22, Jamaica Plain Station, trouble of remitting in 0000 foreign money. Road, Muswell Hill, N.10, is in charge Boston, Mass, or to Mr. Mander Barnett, of the organisation of these tests and 106, Lord Street, Southport, who will R..CJ.B. Tests. will allot the code letters to intending also receive applications The for membership, four Sundays in March will he competitors and receive their reports on for which the annual subscription is only devoted to tests on 2 megacycles (150 each Sunday's working. Transmitters in- 4s. 2d.

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www.americanradiohistory.com FEBRUARY 18th, 1931. lagIl@m 189 Wor Ilcl

Replies to Readers' Questions of General Interest.

Technical enquiries addressed to our Information Department are used as the basis of the replies which we publish in these pages, a selection being made from amongst those questions which are of general interest. Coupling Condenser Values. Neutralised Filter Coupling. the tuning condenser is mounted on a I hare some coils which I should like to If it is practicable, 1 should like to use metal panel or in a screening box, but, use in the construction of a capacity a capacity- coupled band -pass filter as of course, the metal work must not be coupled band -pass filter, although I a coupling between a neutraliser) joined to the mains, except through a believe that their H.F. resistance is triode ILA'. valve and a grid detector. condenser; otherwise your object would rather lower than that of those Indirectly heated A.U. valves are be defeated. ordinarily employed. Does this mean used; there is to be no reaction, and, 0000 that I must use a smaller coupling if possible, I should prefer to use condenser than usual' ganged tuning condensers with earthed Motor-boating. No; as your coils are " better " than rotors. Will you please give me'a I have a four -valve A.C. -operated receiver usual, the coupling condenser must have circuit diagram of this arrangement? which, after working satisfactorily a somewhat higher value than is generally The circuit diagram you require is for nearly a year, has developed specified. given in Fig. 1. As the tuning controls uncontrollable L.P. oscillation or peco are to be ganged and reaction is not to "motor- boating." Before attempting Helping the Pigeon Fancier. be used) it will be almost essential to to trace the source of the trouble, I To avoid risk of injury to my neighbour's " tap down " the detector grid connec- should like to know to what you think Screening be pigeons, have threaded a number of tion in the manner indicated. it might reasonably ascribed. I this is the corks on my aerial wire. Unfortu- between the component circuits of the The most obvious cause for nately, this has had the effect of filter must, of course, be provided. failure of one of the by -pass condensers impairing my reception ; in fact, the transmissions from my favourite H.T. t TO L.F. stations are non) heard at less than A half the normal strength. Is there anything I can do to avoid this loss of efficiency? We would assure you that the fitting of corks to your aerial wire cannot in itself be responsible for any falling -off in efficiency of the aerial system. It must be assumed that your trouble is due to DET. some other cause, or possibly that when the aerial was lowered a joint was dis- the result this resist- ANODE turbed, with that BY -PASS ance has increased, or that a partial COUPLING moo CONDENSER short- circuit to earth has inadvertently f CONDENSER been introduced. 0000 /, Non -uniform Coils. A band -pass filter has just been added to my existing 1 -v -1 receiver. Results are excellent as far as medium -wave reception is concerned, but on the long waves it is impossible to find an arl- Fig. 1.- Intervalve filter circuit in conjunction with a neutralised triode H.F. valve. ju.stntent that will enable nie to obtain single -knob control over more than a " Live " Condenser Spindles. forming part of the decoupling scheme; small part of the tuning range. This In designing an "all -D.C." receiver is it an internal or external disconnection may is not a matter of coupling, as various possible to arrange matters so that have developed. Similarly, a short -cir -' capacities have been tried. TVill. you the condenser rotors are " dead " as cuit. may have developed across one of please suggest what I should do in far as the mains are concerned? the decoupling resistances, but this is order to improve long -wave reception? The simplest way of attaining this hardly probable. As your ganged -control system appar- object is to insert a large condenser (its Cases have come to our notice where ently works quite satisfactorily on the actual value is not important, provided L.F. oscillation has been due to decrease medium band, it seems certain that the that it is many times that of the tuning of emission in the output valve, with the inductance values of the long -wave coils condenser) between the rotor of the vari- result that the voltage applied to the are not properly matched ; attention to able condenser and the low -potential end earlier valves -and consequently their this matter should overcome your diffi- of the tuning coil with which it is asso- magnification and tendency towards self- culty. ciated. This plan is -applicable when oscilla tion-has increased. A '17

www.americanradiohistory.com .190 IAhpdstg FEBRUARY i8ih, 1931 'Wyk]. An Unconsidered Resistance. the method in which a very small vari- Earth Connections. you please - ll'ill examine the circuit dia- able condenser is joined between the high Is it not a fact there gram of my that should always output stage and say if potential ends of the two coils is slightly be a direct connection between either there is any reoson why the automatic better for your particular purpose. the L.'1'. positive or negative bus -bars bias arrangement as shown should not A tuner of this sort may best be oper- and the earth terminal of a receiver? work properly. The valve seems to ated by tuning -in signals to maximum I have just been examining a com- be over- biased, as anode current is strength with loose coupling -sufficiently mercial three -valve battery -operated rather less than it should be, accord- loose to give no trace of the " double- set of which the components are ing to the manufacturers' figures. humped " effect -and then, without sornewhgt inaccessible, The resistance seems and find that bias to be of the touching the tuned controls, increasing there is no electrical continuity be- right vague for the valve you are using, the value of the coupling condenser till tween either of the L.T. terminals and assuming that the maximum permis- intensity is reduced to a slight extent. and the earth terminal. sible anode voltage is applied. We notice This procedure is hardly as simple as Almost invariably there is electrical that you are using a 400 -ohm. potentio- it would appear, and, if it can be continuity between these points, but very meter which is connected across the valve managed, we would advise you to adopt rarely an internal condenser is included we filament, and think you have ignored single -knob tuning. in the earth lead, so that the set may the fact that this introduces an extra 0 0 0 0 he operated from a D.C. mains elimina- 100 ohms into the circuit. Bias is thus tor with safety. More rarely still, a increased by about 2.5 re- Binocular Chokes. volts. This When building a receiver with two H.P. double -wound, aerial -grid transformer sistance, of course, is that of the two without direct connection to the filament halves stages one generally anticipates a cer- of the potentiometer in parallel. tain amount of trouble through H.P. circuit is provided for the same reason. The matter will be made clear refer - on instability; in order to minimise the ence'to Fig. 2. 0000 risk of uncontrollable 11.1'. oscilla- .tion, I propose, if you think it worth Battery Valves for Power Detection. while, to obtain fieldless or binocular Although indirectly heated A.C. valves Il.F. chokes, in place of those of the are generally used as power -grid de- ordinary type. tectors, I believe there is no reason Conditions sometimes arise where the why those of the battery- operated use of chokes with a restricted external type should not serve this purpose. field is beneficial. This is particularly If so, will you please tell me what likely to be the case when dealing with particular types are likely to be most self- contained frame a erial sets. suitable, and -whether positive bias is 0000 needed OS in an ordinary grid -detector Louder Signals. circuit? At present my anode -bend detector feeds Battery -heated valves, can be used for directly into the output stage through this system of detection ; those of the a resistance coupling. I am thinking "L " type with impedances in the order of changing over to power -grid de- of 10,000 ohms are suitable. Positive tection, and at the same time should grid bias will be required. like to obtain somewhat stronger signals. Accordingly, it is proposed Fig. 2. -One- quarter of the filament poten- to replace tiometer resistance must be added to the the coupling resistance by FOREIGN BROADCAST GUIDE. bias resistance value when calculating an L.P. transformer; will you please automatic bias voltage. give me some idea as to the increase of magnification likely to be brought Front every point of view it would be about by this change? POZNAN better to use a potentiometer of very Provided that the conversion is pro- (Poland). much lower value. perly made, it is reasonable to anticipate Geographical position : 52` 25' N. ; 16° 55' E. o on o a gain magnification in approximately Approximate air line from London : 728 Overheated Resistance. equal to the step -up ratio of the L.F. miles. So far I have been unable to a transformer chosen. Actually, it is to trace fault Wavelength : 335 m. -'Frequency : 896 kc. in my "Band he anticipated that the actual gain will -Pass Four" receiver. Power : 1.9 kW. The fact that the resistances R and be slightly greater than this. R6 become quite warm may possibly 0000 Time : Central European Time (one hour in advance of G.M.T.). suggest to you where the trouble is H.F. or L.F. Amplification ? likely to be found. I am about to rebuild a somewhat out -of- Standard Daily Transmissions. The resistance R will naturally become 'date H.P. -der. -2 L.I'. receiver, and 06.15, G.M.T. news ; 08.00, concert (Sun.) ; warm, as it carries about 40 milliamperes. intend to adopt a 2 -stage H.P. am- 09.15, relays Wilno (Sun.) ; 12.00, time R6, however, should remain cool, and the plifier; A.O. valves will be used signal, fanfare from Town Hall ; 16.45, fact that it heats up may be due either throughout. As these valves are concert ; 19.00, main evening programme; to the application of excessively high much more efficient than those origin- 21.00, time signal, news, dance music (exc. voltages or to the existence of a short ally used, do you consider that it Mon., Thurs.). dreuit. 0000 would be as well to omit one of my Woman announcer. Call : Rhalo ! Rhalo ! present L.P. stages? Radio Poznanskie and Ici Radio Poznan Aerial Coupling. Yes ; we strongly advise you fo do so. (phon.: Paus-narn). trill you please tell me what is the Two L.F. stages in cónjunction with best' Opening signal : as under ; imitation of form of coupling for a two- circuit modern "A.C. valves will generally pro- carillon. tuner which is to be operated in such vide rather more L.F. magnification than a way that broad tuning -the band - can comfortably be dealt with. Unless pass effect, more or less -may be ob- one decides either to sacrifice a good deal tained without going to the length of of the available magnification, or else to proniding ganged condensers? Any adopt special precautions, theme is a very hints on the operation of the tuner real risk that instability will he pro- Interval signal : Metronome. would be welcomed. duced. By adding an extra H.F. ampli- Announcements as well as news bulletin are From the electrical point of view there fying stage, losses in overall amplifica- frequently given in both the Polish and is very little to choose between the vari- tion, due to the omission of one of the French languages. ous methods of coupling between the L.F. valves, will be more than made Closes down as Warsaw (q.v.). aerial and secondary circuits, but possibly good. A 38

www.americanradiohistory.com ) re AN D RADIO REVIEW (18/h Year of Publication)

WEDNESDAY, FEBRUARY 25TH, 1931. VOL . XXVIII. No. B.

Editor : HUGH S. POCOCK. Assistant Editor : F. H. H AYNES. Editorial Offices : 116 -117, FLEET STREET, LONDON, E.C.4. - - Editorial Telephone : City 9472 (5 lines) Advertising and Publishing Offices : DORSET HOUSE, TUDOR STREET, LONDON, E.C.4. Telephone: City 2847-(13 lines). Telegrams: "Ethaworld, fleet, London." COVENTRY: Hertford St. BIRMINGHAM: Guildhall Bldgs., Navigation St. MANCHESTER: 260, Deansgate. Gl.nscow : 101, St. Vincent St., C.2. Telegram: '0Vriet,Coventry." Telegrams: " Antopress, Birmingham." Telegrams: "tu ffe, Manchester." Telephone: Telegrams: "IIife, Glasgow." 6210 Coventry. Telephone: 207) Midland (8 lines). Telephone: 8970 City (4 lines). Telephone: Central 4867. PUBLISHED WEEKLY. ENTERED AS SECOND CLASS MATTER AT NEW YORK, N.Y. Subscription Rates : Home, L1 s. 8d. ; Canada, Ls Is. 8d. ; other countries abroa d, LI 3s. sod. per annum. As many of the circuits and apparatus described in these pages are covered by patents, readers are advised, before making use of them, to satisfy themselves (ha) they would not be infringing patents.

New eaders9 Numberv

ik" NEW Readers' Number " is an innovation circuit for attaining selectivity with one high -frequency for The Wireless World, so that we think a valve stage. An interesting refinement is introduced, word of explanation might here be given as to to give constant selectivity over the tuning range. The why it has been introduced. second constructional set is not novel in itself, but is of Very frequently, but more particularly about this time a type which we believe will prove extremely popular of the year, our correspondence with readers indicates with those of our readers who may not have attempted that Those who have become regular readers for the first set construction before, or who want a stand-by receiver time during the winter months, and a number besides for general home use. The construction is reduced to who have been constant readers over a longer period, the simplest possible limits, and the set is self -contained. find that the pace set by The Wireless World is at times A feature which should be of very general interest is a little too hot for them. They have difficulty in keeping the Station Identification Chart, supplied as a supple- up to date, because new subject matter is frequently in- ment. This gives general information on most of the cluded, and, in their opinion, without as much explana- European stations, which can be satisfactorily received, tion or simplification as they would like to see. 'It will and space is left for readers' additions, as required. The be our endeavour, therefore, in this issue and in subse- approximate tuning positions for various types of quent numbers, to maintain the same standard as in the receiver are indicated. past, but it will also be our aim to interpret more simply The first of a new series of simple articles entitled much of the subject matter with which we deal. We " From Aerial to Loud Speaker" is included in this believe that this can be done without interfering with number. We believe that this series will supply a long - our policy of maintaining a high standard of accuracy felt want amongst many of our readers. The aim has and being right up to date in everything affecting the been to take a standard modern four -valve receiver and interests of our readers. No other wireless paper in this describe it section by section, explaining the purpose of country has earned such a reputa- every component. This method tion for dependability, and, as our of treatment is new, and should, regular readers know, this position In This Issue we think, prove extremely helpful. has been attained because the paper FROM AERIAL TO LOUD SPEAKER. To emphasise the purpose which is one which can be trusted not to UNBIASED OPINIONS. components serve in the set, the PRE- SELECTION A.C. THREE. exaggerate claims for anything de- CURRENT TOPICS. author discusses the effect of scribed, but to present all wireless CONTROLLING VOLUME. changing values above and below topics, whether technical or general, WITHIN RANGE. those chosen as correct, in THE VALVE AT ITS BEST. and in their proper perspective. MARCONIPHONE A.C. RECEIVER. this way gives the reader a very In this issue two constructional BROADCAST BREVITIES. complete explanation of circuit receivers are described a THE EVERYMAN TWO. principles. The will con- -one CORRESPONDENCE. series be three -valve A.C. mains set, which READERS' PROBLEMS. tinued through several issues con- makes use of the most. up -tc -date cluding- with the output stage B I[)

www.americanradiohistory.com 92 paßOA t Wald FEBRUARY 25th, 1931.

The Purpose of Every Component. By A. L. M. SOWERBY, M.Sc.

l us make the arbitrary decision that the receiver offer is a greater simplicity in use, due to the reduction L of our discussions shall have four valves, but we in the number of the tuned circuits. The increased have not yet settled what the functions of these selectivity offered by the 2 -v -1 set is more than adequate four valves shall be. Modern practice is very definitely in compensation for sacrificing this simplicity ; the other favour of the set having two high- frequency stages gains, not the least of which is the distortionless and one low- frequency stage as against the other type detector, are all pure profit. of four -valve set with one high- frequency stage and The circuit which the writer has chosen as being two low- frequency stages. (These two types are re- representative of all that is best in modern circuit ferred to, shortly, as " 2 -v -I " and " I -v -2 " respec- design -including, of course, all his own pet fads -is tively, the " v " standing for the valve detector, an reproduced, in slightly simplified form, in the accom- abbreviation adopted by The Wireless World.). One panying full -page illustration. The simplifications made of the reasons for preferring the 2 -v -I type is the fact amount to no more than omitting wave -range switching the new high -ratio transformer and and leaving the filament circuits the pentode, used in combination, present incomplete. provide least THE article is the first of a short at as much amplifica- series in which the circuit of a modern Either battery- heated or mains tion as one needs to have after a four-valve receiver will be analysed in some valves can be used without altera- detector of the modern " distortion - detail. Each component used in the com- tion of the circuit ; for the former, less " type. pleted receiver will be separately examined, the L.T. plus leads must be added The fact that the detector has thus the reasons for its inclusion discussed, and to the diagram, on which only L.T. arbitrarily settled the point for us the actual value that the component has in minus leads are shown, while for need not be regretted. It is not the set justified. indirectly heated valves the L.T. unfair to say that the most urgent The series will almost amount to a short minus leads are to be connected to need of the present day is selec- treatise on receiver- design, and readers are the cathodes, and complete heater - tivity, and it is self -evident that warned that however great the care taken wiring has to be added. Whichever when there are two valves amplify- to be strictly objective, the writer's favourite type of valve is adopted, the circuit ing at high -frequency we shall be fads, fallacies, and prejudices will inevit- as drawn may be used either with able to use more tuned circuits than ably lie behind the whole. It is desirable, an H.T. battery or with any when there is but one valve ful- therefore, that every reader of these arti- eliminator which will provide a cles should consume them with the addition filling this function ; the 2 -v -I re- smoothed current at the necessary ceiver is thus inherently more of as many grains of salt as his individual voltage. As all decoupling and selective than its rival, the i -v -z. It taste demands. voltage- dropping resistances are will not, however, be so very highly included in the set itself (which, in selective by nature as to absolve us from paying a very the writer's opinion, is the proper place for them) there great deal of attention to selectivity in working out the is no need to make any provision in the eliminator for details of the design. The only advantage, in fact, that voltages below the maximum which it will give. This the receiver with the singlé H.F. stage' could possibly simplifies the eliminator design. B 20

www.americanradiohistory.com FEBRUARY 25th, 1931. WhpsIln,g Walt 193

POWER GRID DETECTOR CIRCUIT

COMPENSATED PENTODE OUTPUT

H T

- GB

CAPACITY BAND -PASS 1 POST DETECTOR VOLUME FILTER CONTROL AND PARALLEL FED L F COUPLING

TUNED GRID COUPLING

PRESENT -DAY PRINCIPLES IN RECEIVER DESIGN. An analysis of a modern receiver circuit showing the essential features. They are, reading from left to right across the circuit (1) the capacity -coupled band -pass filter as used in the " Band -pass Unit." (2) The screen -grid feed circuits, (3) the tuned grid H.R. intervalve coupling and (4) the grid and anode circuits of the power grid detector as shown in The Wireless World Four," (5) the parallel -fed L.F. circuit with its post -detector volume control and (6) the compensated pentode output as arranged in the " Power Pentode Two." 11 21

www.americanradiohistory.com 194. WApagg FEBRUARY 251h, 1931. Would, From Aerial to Loud Speaker. - in the roof. The necessity for a low- resistance earth For' purposes of illustration, it will be assumed will be appreciated when it is remembered that the that battery -heated valves are to be used, in conjunc- aerial and the ground beneath it form two plates of a tion with a mains unit giving 200 volts H.T. ; such condenser, and that the high -frequency currents small alterations as are necessary for mains valves or resulting from the reception of signals have to pass from other H.T. voltages can readily be made by those who aerial to earth and back again in about a millionth are sufficiently interested to do so. part of a second. Even a small resistance, inserted in . In going through the set in detail, one naturally begins with the aerial. This is necessary to pick up energy from the ether waves radiated by the trans- mitting station, from which it follows that a long aerial is likely to be more efficient than a short one. On the other hand, greater selectivity is usually ob- tained if the aerial is not too long. With the receiver shown, selectivity will be quite high even if a hundred - foot aerial is used, though in the immediate neighbour- hood of a powerful transmitting station some sixty feet would Fig. 2.-In (a) C5 is in parallel with L_ ; in (b) it is in parallel with be preferable in this respect. C1. The former position, in which the coupling is slightly loosened at low wavelengths owing to part only of the current flowing Aerial and Earth. through C7, is theoretically less perfect. In practice, the differ- ence is too small to be detected with any certainty. Currents induced in the aerial pass through the coil L which consists of some 20 turns of fine wire wound a path so frequently travelled as this, is to be avoided in close proximity to the earth end of the tuning coil L9. if possible. The coupling between L, and L, is close enough to The band -pass filter, built up from the two coils ensure that in passing through L, the aerial current L, and L the tuning condensers C, and C2, and the sets up an induced current in L,. thereby feeding the coupling condenser C7, may be regarded as one unit. signals into the set. If L, were short -circuited prac- Much has already been published in these pages on the tically no signals would be received, while if it were design of filters ; for present purposes it will be enough open- circuited by breakage of a connection the aerial to say that such a filter gives both better selectivity currents could only pass to earth by way of stray and better quality than can be had from a single coil capacities, so that signal strength would be very poor. and condenser. The two coils L, and L, should be With io turns in place of 20, signal strength would identical, and for such a set as that under discussion fall off very considerably, but the selectivity of the they should be as " low -loss " as is convenient. They aerial circuit would be greatly enhanced ; with 40 turns, should, therefore, be as large as space will permit, and on the other hand, selectivity would become extremely should be wound on formers of good dielectric quality with wire of fairly heavy gauge. Sixty -eight turns of No. 24 D.S.C. wire wound on a 2 ¡in former of ebonite or first -grade paxolin will he very suitable. Each of these coils has a dual function ; besides being tuned by its condenser to the required wave- length it gives a very considerable step -up of signals. If, therefore, either of them is short -circuited or dis- connected, signals will vanish altogether. The wind- ings suggested give an inductance of .about 23o micro- henrys, which can be tuned comfortably over the wave - range 200 to 55o metres. If the inductance were halved, the wave-range covered would become 14o to 390 metres, while if it were doubled the receiver would not tune much below 300 metres. i.- Variation of signal strength with primary turns, using average aerial. The dotted line gives some rough idea of the Wave -range Covered. variation of selectivity. Tha tuning condensers C, and C, have a maximum poor, and volume would again suffer, though to a less capacity of 0.00035 mfd., which serves to take the extent (see Fig. Z). It is not unusual to have aerial - coils L, and L, over the correct wave- range. If either primaries tapped, so that the number of turns most of these should be disconnected, or short -circuited suited to the particular aerial in use can be determined through contact between fixed and moving plates, or by experiment. In general, the smaller the aerial the any other cause, the circuit concerned will cease to more turns must be included in circuit between it and tune, and signals will vanish exactly as if the same the earth connection. fault had developed in the associated coil.. Condensers Of the earth connection itself there is not much to of lower maximum capacity would restrict the waveband be said, except to reiterate the well -known advice to at the upper end, so that with the condensers " all in " use a thick, short wire running to a water-pipe that the maximum wavelength would be below the 550 really fines to earth -and not, for example, to a cistern metres which we have taken as the highest wavelength B 22

www.americanradiohistory.com FEBRUARY 25th, 1931. WApsIlsfflo WopIlcl 195 From Aerial to Loud Speaker. - than this would give too loose a coupling between the to which we are likely to wish to tune, while if larger two tuned circuits, leading to over -sharp tuning and a condensers were used the waveband would be extended considerable loss in signal strength, especially at the to well over 600 metres. This would be harmless lower wavelengths. Too small a capacity, on the other enough were it not for the fact that the useful portion of hand, will broaden the tuning, especially at the higher the tuning range would then be restricted to part only wavelengths, to an unpleasant degree, and, in a bad of the dial, thus making accurate tuning more difficult case, would lead to the appearance of two separate than it need be. A change of condenser capacity in points on the tuning dial, at either of which the same either direction would have but little effect at the lower station could be heard, though at reduced strength. The end of the waveband. advantage of using a capacity band -pass filter in the input of a receiver Ganging the Band-pass Filter. having tuned high -frequency stages is seen by referring to Fig. 3. It is intended that C, and C2 should be ganged to- If C, is short -circuited, there will be no coupling gether what- and operated by one knob ; for ease in com- ever between the two tuned circuits, so that signals will pensating for any slight difference in inductance between only get through to the grid of the first valve by acci- the coils L. and La, it is desirable that log-law con- dent and in small amount. Disconnecting C, would densers should be chosen, and that at least one of open both tuned circuits, altogether preventing them the two sets of moving plates should be movable on from tuning. the shaft. Since C, has to provide the coupling between In the two addition to making allowance in this way for halves of the filter, and is given the right capacity to possible discrepancies be- produce the exact degree tween the inductances of of coupling required, it is the two coils it is neces- essential that no source of sary, if the two circuits accidental coupling shall are to tune accurately to- exist. By setting gether L, and over the whole L3 at right angles to oiie wave- range, that the mini- another, and putting be- mum capacity (that is, the tween them a sheet of capacity remaining in the metal to form a capacity - circuit when the condensers screen (shown dotted are on set to " o ") should be the diagram), stray coup- the same in both circuits. lings can be reduced to For this purpose there Fig. 3. -(a) shows a typical freq uency- response curve of a band- negligible proportions. It is provided a s m a l l pass filter, and. (b) that of an am plifier without band -pass filter. If two is to be noted that if Ct and compensating, the " humps " in (a) are hi gh enough, the insertion of curve or "tririm- (b) into the central dip will" give some approximation to the ideal C2, which are on opposite m i n g," flat -topped curve. The humps " in (a) are raised by using low- condenser C, resistance coils in the filter. sides of the screen, are in which need have a maxi- metallic contact with it, C, mum capacity no more than 0.00003 mfd. (four o's 3), will be short- circuited. The two tuning condensers and should have a very low minimum. An air -dielectric should therefore be mounted on insulating bushes. In neutralising condenser, possibly with a few of the vanes next week's issue consideration will be given to the stripped away, is ideal for the purpose. In the diagram choice of the screen -grid valve and the methods of C; is shown in parallel with the first tuning coil L_ ; it feeding its screening grid and anode. depends very much on the details of the individual 0000 receiver whether it is required here or in parallel with the second tuning coil L3. A filter which refuses to be OBTAINING SMOOTH REACTION WITH ganged with the trimmer on the aerial side will usually AN ANODE BEND RECTIFIER. behave at once if the trimmer is shifted to the secondary r g HE complaint is not infrequently heard that it is side and is put in parallel with L3. There is, perhaps, 1L extremely difficult to obtain the same smooth re- a slight theoretical advantage to be gained by putting action effects with an anode ,bend rectifier the as with the trimmer across the tuning condenser rather than conventional leaky -grid arrangement, and in some across the cases coil, as shown in Fig. 2, but in practice no it is difficult to obtain reaction at all. The latter state difference can be detected in a normal case. The choice of affairs usually exists in cases where the detector is between these alternatives is largely a matter of con- followed by a stage of resistance coupling and the grid venience in wiring. voltage has been adjusted to give maximum 'rectification The condenser C which is common to the two tuned efficiency. Under such conditions the power available circuits L_C,C, and L3C2C,, provides the coupling in the anode circuit is small in comparison with that exist- between them. In theory it should 'be varied in capa- ing in the anode circuit of a leaky grid city rectifier. It is as the tuning is changed ; in practice a fixed con- necessary, therefore, if smooth reaction is desired, denser is used, to choosing a capacity which gives a satis- compromise in the matter of grid voltage and use less factory, if not quite perfect, compromise over the negative bias than is necessary for best rectification con- whole waveband. For the coils and condensers already ditions. A little experiment will speedily enable a value described a capacity of about o.or mfd. will provide to be found at which reasonably good detection effi- about the best possible compromise. A larger capacity ciency and also reaction control are both obtainable. R 23

www.americanradiohistory.com 196 Wh. @hog FEBRUARY 25th, 1931. Wal.cl fulness. The case is analogous to that of the H.T. battery and H.T. eliminator. No serious amateur would think of conducting experi- UNIA3D mental work in receiver design using By FREE GRID any form of battery eliminator as his source of H.T. supply, not only Shocking. convinced, and feebly protested that because he would have to be con- theory was all against this, as shock One frequently reads of various stantly changing the values of volt- in the case of A.C. mains was due novel methods of ascertaining the age dropping resistances, but be- to an R.M.S. effect. However, on nature of light supply, cause he would never know to what the electric home, my first job was to ranging from the homely glass of reaching extent his experiments were influ- connect up 240 volts' worth of accu- water to the stroboscope, but, per- enced by stray coupling effects in the mulators to a lamp socket and stick sonally, I always the sim- eliminator. It must be the bias bat- think that my thumb into this and then into plest and quickest method is to look tery every time for the experi- one of the household lamp sockets. at the meter, even though in some menter, but in the case of a com- I was convinced. You are defi- cases this involves crawling under plete mains set built for the sole advised to try this un- the stairs and striking matches in nitely not purpose of receiving broadcast pro- less you wish to bring grist to the the neighbourhood of the gas meter. grammes nothing is more absurd Now, it so happens that I live in a than the presence of a battery of district in part of which the supply any kind, even for the purpose of is A.C. and in other parts D.C., operating a remote control relay. although the voltage- 24o -is the 0 0 0 0 same in each case. A friend of mine who is an electrical engineer re- A Matter of Opinion. cently moved into the district, and This does not necessarily mean, on the evening after his arrival we however, that personally I entirely both sat in his den amid the havoc favour the particular method of created by the " plain -van " men, mains bias which consists of putting discussing the details of a new re- resistances in the anode return lead ceiver which he intended building. -usually known as " automatic The question of mains operation by the way- although there is nq- cropped up, and I suddenly called "Elementary, ty dear Watson." . thing whatever to be said against to mind the fact that I was in a part the system if you are prepared to, of the district unfamiliar to me and mill of the mortician (see American change resistance values every time did not know whether A.C. had yet dictionary), since a medical friend of valves are changed for others of displaced D.C., as it had done in my mine tells me that this test, besides somewhat differerft characteristics. part of the town. The cellar door determining the nature of the supply, A far more satisfactory method from was still blocked by large packing - is also invaluable for detecting weak my point of view, however, is to cases, and I commenced pushing, hearts and various other bodily ail- employ an entirely separate gr them to one side in order to get at ments. Incidentally, the foregoing bias eliminator, which cons.sts of the meter. probably explains why A.C. and not nothing more than an H.T. battery With a muttered " Don't bother D.C. is used at the electrical treat- eliminator in miniature. A separate about that," my friend rose from his ment centres maintained by various power transformer need not be used seat, removed a glowing lamp from State Governments in the U.S.A. as you can now obtain transformers its socket, and inserted his thumb. 0000 with an extra winding for this pur- " A.C.," said he nonchalantly, and pose. resumed his seat without more ado. This Freedom. Since only a very small current " But how on earth " I began, So called " free " grid bias is be- will be taken from the instrument thoroughly mystified. " Element- ing condemned in certain quarters in order to produce a fall in voltage ary, my dear Watson," said he on the grounds that nothing worth across a potential divider, quite a facetiously. " The fact has prob- having is free, or, alternatively, as small and inexpensive smoothing ably not penetrated to your some- they say in law courts, that it is not choke can be used. The method what adamantine cerebellum that in free, as you have to pay for it in the possesses the advantage that grid the case of A.C. the shock you get is form of lessened H.T. voltage and a bias values can be changed as due to the peak voltage, and not to greater tendency to instability. The quickly and easily as when using a the R.M.S. value." Now, obvi- advocates of it are putting forward battery, and, moreover, the value of ously, if this were so, one would get equally foolish arguments against grid bias can be accurately deter- the benefit of nearly an extra ioo the retention of the grid battery. It mined, which is not always the case volts when the mains were A.C., and seems to me that both parties are with the " automatic " method. It my friend amplified his explanation barking up the wrong tree. Surely is, however, equally important to by saying that experience had taught both methods of obtaining grid bias de- couple each grid circuit if motor him this little trick. I was still un- have entirely separate spheres of use- boating is to be avoided.

www.americanradiohistory.com FEBRUARY 25th, r93r. WAp®Il®24 107 WO114

A New By Filter System H. F. SMITH. Giving High and Constant Selectivity without Complications. SPECIFICATION. OME eighteen months ago it was seriously debated GENERAL. A three -valve general - purpose receiver for whether the neutralised three -electrode H.F. valve A.C. mains operation with an open aerial. should be granted a new lease of life. Selectivity CIRCUIT. Input filter. Screen -grid H.F. valve, coupled was the trouble ; everyone realised that the screen-grid by tuned -grid method to power -grid detector. Choke valve gave much more real amplification -and with L.F. coupling to pentode output valve. Auto- trans- much simpler former feed to loud speaker. H.T. supply through apparatus -than its forerunner, but to 1Vestinghonse metal rectifier. avoid cross -modulation and other drawbacks, while re- taining simplicity CONTROLS. (1) Single -dial control of three tuning con- of operation, it was found that so densers. (2) Input volume control. (3) Reaction. large a proportion of the available aerial input had to .(4) Trimmer for H.F. coupling. (5) Wave -range switch; be thrown away that the average set of the period worked no better than a good " neutrodyne " of 1926 inclination to master the art of operating them. The or 1927. development of single -control Fortunately, band -pass filters entirely the drastic and retrograde step of aban- obviated the need for any special skill. doning the screen grid valve was proved to be quite Now, any kind of input filter is probably better than unnecessary. Pre- selection, or the virtual elimination none ; but, as pointed out in last week's Wireless of interfering World, signals before they are allowed to reach the conventional capacity- or inductively- coupled the first valve of the receiver, ar- was found to offer a prac- rangements have definite shortcomings, as their selec- tical solution of the problem. Admittedly, this principle tivity changes seriously while passing from one end of liad previously been applied by adopting two -circuit the tuning scale to the aerial other, if we assume that coupling tuners, but such devices appealed only to the is fixed. If provision is made for skilled amateur, variation of coupling, and the man in the street showed no the receiver will no longer be easy to operate.

H.T ELIMNATOR

LS

A (.

a b G B -

b0

b1 oYi Fig. 1.- Simplified circuit diagram, omitting wave -range switching, feed resistances and by -pass condensers, reaction, automatic grid bias connections, etc. 13 27

www.americanradiohistory.com ßpGil@g FEBRUARY 25th, 1931. 198 woyIli

LIST OF PARTS. 1 Triple variable condenser, 0.0005 mfd., with drum drive (Polar Tub) 1 Resistance, 100,000 ohms (Ediswan) 1 Variable condenser, 0.0003 mfd., with knob (Utility, W.218) 2 Resistances, 250,000 ohms (Ediswan) 1 Variable condenser, 0.0002 mf d., with knob (Utility, W.219) 2 Grid leak holders (Bulgin, porcelain) 1 Variable condenser, 40 mmfds. (J.B. Midget) 2 L.F. chokes, 40 henrys (Bayliss) 1 Differential condenser, 0.0003 mfd. (Utility, W.211) 1 L.F. choke, 300 henrys (R.I.) 1 Condenser coupler (flexible mechanical link) (Ormond) 1 L.F. choke, centre tapped (R.I., Pentomitejj 3 Condensers, 1 mid. (200 volts working) (T.C.C., No. 50) 1 Power transformer (Bayliss, Type W.W.3 3 Condensers, 4 mfd. (400 volts working) (T.C.C., No. 80) 1 Valve 'screen (Colvern 4 Condensers, 2 mfd. (200 volts working) (T.C.C., No. 50) 3 Coil screens (H. & B.) 1 Condenser, 2 mfd. (400 volts working) (T.C.C., No. 80) 1 Metal rectifier (Westinghouse, H.T.7) 3 Condensers, 0.0003 mfd. ( Dubilier, No. 610) 1 Flash lamp holder 1 Condenser, 0.0001 mfd. (Dubilier, No. 810) 1 Fuse bulb, 100 mA. (Bulgin, Type B.) 1 Condenser, 0.05 mfd. ( Dubilier, No. 1.775) 1 H.F. choke (Lewcos) 1 Semi -variable condenser, 0.00002-0.0002 mid (Lewcos) 1 H.F. choke (McMichael, Binocular Junior) 1 Semi -variable condenser, 0.00015-0.002 mfd (Lewcos) 2 Terminal mounts (Belling -Lee) 1 Resistance, wire -wound, 30,000 ohms, with holder (Varley) 4 Indicating terminals, Aerial, Earth, Output, Output (Belling -Lee 1 Resistance, wire- wound, 50,000 ohms, with holder ( Vazley) 2 Panel brackets, 4in. (Magnum) 2 Resistances, wire-wound, 10,000 ohms, with holder Vazley) 3 Tuning coils, with built -in switches (Type WWTG, Tangent) 1 Resistance, wire -wound, 400 ohms ... (Clarostat, Type F.W., Claude Lyons) 3 Valve holders, 5 -pin (Lotus) 1 Resistance, wire-wound, 100 ohms ... (Clarostat, Type F.W., Claude Lyons) Wire, screws, sleeving, sheet tin, sheet aluminium for brackets, etc., etc.

What is needed is an arrangement whereby filter that correct coupling may be maintained, this condenser, coupling may be automatically maintained at a value which must be very small, is mounted on an extension giving the desired band width, irrespective of wave.- of the main tuning condenser spindle in such a way that length. This can be done by electrical means, as de- coupling capacity and wavelength are increased or scribed last week, or mechanically, as in the case of the decreased together. receiver to be discussed in the present article. Although the theory of this method of coupling is Reference to the simplified circuit diagram, Fig. 1, involved, in practice it is simple and free from shows that the two component circuits of the filter are " snags." Further, it is flexible, and any desired peak coupled by a variable condenser (C.C.) joined between separation may easily be obtained to suit individual the high -potential ends of the tuning coils. In order needs.

Fig. 2.- Complete circuit diagram. Numbers on coil terminals correspond with the coil base terminals. C.C., coupling condenser (see text) ; C.C1, long -wave coupling condenser, semi -variable, 0.00002 -0.0002 mfd. ; C, differential volume control condenser, 0.0003 mfd. C5, balancing condenser, semi -variable, 0.00015 -0.002 mfd. ; C2, C3, 3 ganged tuning condensers, 0.0005 mfd. ; C., Co, C;, 1 mfd. ; CH, C1 , 0.0003 mfd. ; Cfe, 0.0001 mid. ; C14, C52, 0.0003 mfd. ; Cl., 0.05 mfd. ; Ct,, C15, C1 B, C17, C15, 2 mfds. ; CM. C20, Cet, 4 mfds. R, 100,000 ohms ; RI, 30,000 ohms ; R2, 50,000 ohms ; R;s, R 250,000 ohms ; R4, 10,000 ohms ; Rs, 400 ohms ; R-, 100 ohms ; Rs, 10,000 ohms. n 28

www.americanradiohistory.com FEBRUARY 25th, 193r. WAp@Il®00 WopIlcl 199 Pre -selection A.C. Three. - if a satisfactory system of single -knob tuning Before discussing is included, the purpose and performance of the a set cannot fairly be called easy to operate if receiver, it seems selectivity advisable briefly to enumerate the and sensitivity are attained, respectively, by constant basic features of its circuit. The filter output is applied manipulation of aerial coupling and reaction controls. to an indirectly heated screen -grid H.F. amplifying It is hardly necessary valve, to add that the reproduction of a which is coupled to a power grid detector (also modern set must be good enough indirectly heated) to do justice to the by a choke -fed tuned grid circuit. best loud speakers available; and that when In order to reduce A.C. damping, and to avoid the transfer- mains supply is used, there is little excuse for ence of sufficient providing stray capacity to upset the ganged an undistorted output that is insufficient tuning system, to fill a room the detector -grid connection is " tapped of ordinary size, with at least a small down " on the margin of safety. inductance. Reaction (not shown in The Marconi or Osram P.T.425 output valve the skeleton diagram) which is is applied between the detector recommended actually gives about 75o milliwatts. anode and grid circuits. As far as long- distance reception is concerned, the An H.F. choke with two by -pass condensers is in- satisfactory performance of the receiver is almost en- serted in the detector anode circuit as an H.F. stopper, tirely due to the input filter. Amplification conferred .

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E

AMY e9.4111111VerelP ARM' er AY ;Or MEW AMP Weir !VW erePP elnletr",er .7` Awe Rear view of the receiver, with Input coli screening cover removed. and this valve is linked to a pentode output valve by the H.F. stage is but little above the average for through a simple L.F. choke an coupling. To assist in A.C. set ; excessively " good matching " coils are out of place when valve and loud speaker an output choke with ganging is employed. Similarly, a centre the power -grid detector tapping is fitted ; it should be noted, in passing, is sensitive enough, but not exceptionally that a simple so, and it tone -control device, consisting of a con- may be taken that the loss of L.F. magnification denser and variable resistance, due to can easily be added in choke coupling is about balanced out by the extra am- cases where reproduction is likely to be improved by plification of the pentode. The set provides a wider such a refinement ; this is largely a matter of one's choice of programmes than the usual specimen of its personal taste and the loud speaker characteristics. class, mainly because It has been full use can be made of every- the aim of the writer to produce a set thing that is picked up by the aerial. possessing enough true selectivity to be operated under Elaborations to the simplified circuit are shown in average conditions without any need for sacrificing the complete diagram, signal Fig. 2. Starting at the aerial strength. With regard to range, it was con- end, there is a differential sidered essential volume control condenser ; to have sufficient real sensitivity to this, in conjunction with a semi- variable balancing ensure fairly consistent reception of foreign stations condenser, enables regulation without relying to be effected without to any great extent on reaction. Even appreciably changing the tuning. B 29

www.americanradiohistory.com 251h, 193r. 200 Wflpa@mo FEBRUARY WopIlcal Pre -selection A.C. Three. - Wave -range switching is effected by short - circuiting the loading coils, and, as the coup- 2% ling condenser CC is inadequate for the long - wave band, its capacity is supplemented by a semi-variable condenser, CC which auto- matically comes into effect when the switch is " opened." Clearly, it is not suggested that sensibly constant band width is provided on the long waves, as well as on the medium 11y; % band ; in order to do so, mechanical compli- 2.3/s-"`_-238-1 cations would ensue, and, in practice, results T would not be noticeably better than those of 114 the " compromise " arrangement as described. Voltage supply for the H.F. valve screening is obtained through a potentiometer consisting of two resistances in series, and the anode cir- cuit of this valve is fed through a decoupling resistance. e

A, sub- panel, front; B, sub- panel, 1/z - 1y5--2t 13/4 rear; C, baseboard, top; D, base- board, under -side.

The tuned -grid coupling coils are tapped, and the detector -grid connection is transferred to the appropriate points for each wave- band by a change -over switch.

is preferred to . the This plan o simpler one of throwing the grid load across the whole inductance, 2X8 as it restricts grid damping and the transference of ex- prevents 15/8 , r-1 -r cessive stray capacity ; further, it } 11,t assists in the maintenance of lB accurate " ganging " when pass- ing from one waveband to the other. Balancing Capacities. Each of the three tuning con- densers, C., C3, and C5, are fitted with internal trimmers (not indi- cated in the diagram) and, in addi- external tion, there is a small 2 1__ trimming condenser in parallel 4rï1 rr;ri 1 ® with the detector grid circuit. C -i r-11- ¡ E i i Actually, there is little need for CS - '7 r1 I _ _ V_'i¢Lr'rir this component, but on the rare y1 ' occasions when it is necessary to :5t i lHHT -( -t -A1 apply considerable reaction, it is , ¡4 C (¢' s! reassuring to know that this cir- ;-É ---l-i IQ cuit may be tuned with extreme s;I % I'í_; Ï'i t o q ----"--1 ------1--1 precision. 2 _f-_ LI' ' r ,e-) i''--

i i 3_____ 1r -1 ^---`iar5-I 01-. I 1 i Constructional details will be 1 (.!-PJ ii 1 i in issue. ¡ ; } -- t next week's Ì discussed 1 w i' l _ 1 ) Care has been taken to avoid the 1='J need for making special parts, and ------,, +' - the only components needed that a o are not at present available com- L....=.; mercially are three simple brac- == kets for supporting the variable condensers. Fig. 3.- Layout of components on baseboard and sub - panel. B 30

www.americanradiohistory.com FEBRUARY 25th, 1931. WtpgIl®M- 201 Worddl

AN ALL -PURPOSE TRANSMITTER. HAVE YOU HEARD BUCHAREST? Madagascar is to have a short- The Electrical Institute at Buchar- wave wireless station, which will est is now operating a 300 -watt operate the local telegraph service, transmitter every Sunday evening on and transmit news, besides broad- wavelengths of 21 and 50 metres. casting concerts and lectures when it 0000 is not required official for purposes. IN ßRIEF REVIEW R.S.A. LECTURE ON TELEVISION. 0000 Sir Ambrose Fleming, M.A., B.Sc., A POLYGLOT and 60,000 kilocycles (30,000 to 5 metres). F.R.S., will preside at the meeting STATION? There this It is rumoured is hardly a radio station in the evening (Wednesday) of the Royal Society that a German firm has world obtained a contract for the construction with substantial power, whether of Arts, John Street, Adelphi, London, of a 100- kilowatt it is broadcasting by voice or telegraph W.C.2, at which Mr. W. G. W. Mitchell, station at Luxembourg code, which will broadcast in all European that cannot be tuned in. B.Sc., joint hon. secretary of the Tele- languages. 0 0 0 0 vision Society, will deliver a lecture en- : 0 0 0 o PRESS WIRELESS NETWORK IN titled " Developments in . Television. WINDMILLS FOR WIRELESS. U.S.A. The meeting opens at 8 o'clock. Permits to erect 19 short -wave stations 0 0 0 0 The Air Ministry has notified the Mar- to form a network across the United AMERICA'S 45 PER CENT. coni Company that the Marconi- Newton States have been granted by the Federal constant -speed windmills, Having completed its census of Ameri- type 110, 140. Radio Commission to the Wireless Press can radio 160, and 180, are approved for use on civil receiving sets on April 1st, Inc. Two of the stations, situated at 1930, the U.S. Census Bureau hopes to aircraft registered in Great Britain. Los Angeles and Chicago respectively, These aero- generators, which develop have the total compiled by April 1st of will be intended for trans -ocean news t his output of from 100 to 500 watts, accord- year. In the meantime it is issuing, messages. as fast as they can be ing to type, include important improve- 0 0 0 0 compiled, the ments, greater " radio set populations " by States. The strength and efficiency A DIFFERENCE. first being obtained by the use of solid blades State count shows that New Hamp- An Italian correspondent deplores the shire, with 119,660 families, has and a new system of governing. fact that in the 53,111 0000 land of Marconi there with sets. In other words, 44.4 per cent. are only 150,000 broadcast listeners, while of the families of the State have radio MONEY SHORTAGE AND WORLD'S in France, which possesses almost BIGGEST RECEIVING STATION. the sets. The second count released shows same size of population as Italy, there that Delaware, with 59,295 families, has America's magnificent new " policeman are more than three million. of the air," the constant frequency radio 27,183 or 45.8 per cent. owning " radios." Our friend can console himself with the O O O 0 monitoring station recently erected by thought that, whereas the Italian listener SWAN the Radio Division of the Department of MEMORIAL APPEAL. pays a tax of 15s., his French counter- All valve users who are conscious of Commerce near Grand Island, Nebraska, part gets his programmes free. is radio's debt to the inventor of the electric standing idle while traffic incandescent congestion grows more acute lamp will be in- every day along the invisible terested in the Swan Memorial roadways of the ether, writes Appeal which is being, or- our Washington correspondent. ganised to provide a sum of not less than £5,000 for the Although the Fed era I foundation Government has expended of National Swan Memorial Scholarships in Elec- about £50,000 to build the l:aermres .. station, trical Engineering Science. which represents pro- H.T. KNIGHT Sir bably the most sensitive g,WIBbNpNMR Joseph Swan was born in Sunderland in 1828, and a " tadio ears" in the world. t.CIWi^GN+SE11 committee it has thus far failed to representing the appro- town of his birth- and the priate the £20,000 or so neces- Institution sary to reran the station With of Electrical En- gineers, of which he was adequate radio engineer; n staff. Certain president in 1898, is respon- members of sible for Congress are endeavouring to the appeal. Sub- secure the much- needed scriptions should be sent to money Mr. P. F. Rowell, secretary, Thirty -six trained engineers ALL -(RE)MAINS SET. An amusing exhibit at the Southend are required to Radio Society's recent show. Although complicated in appear- The Institution of Electrical enable radio's ance, the set was quite Engineers, " super -traffic cop " to cheap to construct, the most expensive Savoy Place, Lon- furnish item being the lump of coal needed as a detector. don, W.C.2. instructions to radio stations o 0 0 0 in America and the rest of the world by TRANSLATORS AND WIRELESS. TRANSMITTING FROM A MOUNTAIN standing continuous twenty- four -hour Wireless is among those subjects which TOP. watches to measure their signals. Only can play havoc with the reputation of an A running commentary on a climb to six men are available, and they are find- expert translator-if his knowledge of ing the summit of Mount Brévent (8,100ft.), little time away from their routine wireless is nil ! To save distress in the duties in the Franco -Swiss Alps, was carried at the plant to attend the compli- future the Association of Special Lib- out by M. Cheney, director of Radio cated apparatus of this receiving system. raries and Information Bureaux, 16, Lyons, on the afternoon of January 31st The vast network of antennas, cover- Russell Square, London, W.C.1, is con- ing the M. Cheney started in the morning with better part of 50 acres in the sidering a scheme whereby the Associa- four guides, who helped to carry the midst of a sparsely inhabited prairie tion should establish a panel of transla- transmitter. and located near In the afternoon, during the exact geographical tors possessing the double qualifications the final stages of the climb, the guides centre of the United States, is capable of proficiency in one or more languages of receiving each took a turn at the microphone. radio signals over the whole and expert knowledge of one or more The commentators concluded their range of useful frequencies story between 10 subjects. while sitting on a snowdrift 6ft. deep. B 31

www.americanradiohistory.com 202 WllintIlmo FEBRUARY 251hr, 1931. %WpIlt CONTROLLING VOLUME

How to Change Signal Strength Without Affecting Tuning or Quality. By W. T. COCKING.

r-- HE amount of low- frequency amplification in produces rectification, which is a prolific cause of that most up -to -date sets is so chosen that when the form of interference known as cross -modulation. loud speaker is working at normal strength the Apart from not introducing distortion, the volume detector is working under its most favourable conditions. control must not reduce the selectivity to any great In order to maintain this correct operation on signals extent ; a small reduction, however, is not of great of varying strengths a pre- detector volume control is importance, since the amount of selectivity needed in essential, while to control the sound output from the most circumstances depends upon the amplification em- loud speaker a post- detector control is desirable. In ployed. In addition, the control must be smooth and practice, however, it is found that in most cases such noiseless in action, and give an adequate variation of a post- detector control is unnecessary, for the reason volume. that the detector operating conditions are not very critical, and that quite a wide departure from the The H.F. Potentiometer. theoretical input of optimum value makes little, if any, One of the most satisfactory controls of this type is audible difference to the results. the H.F. potentiometer shown in It will be seen, therefore, that Fig. I, which has been used in every receiver should include a volume control is an essential part of the a number of receivers described pre- detector volume control, THEmodern receiver, not only to allow of the desired in the pages of this journal.' while the use of a post- detector degree of loudness being obtained, but also to pre- The tuned circuit is shunted by control in addition is often a vent a strong signal from overloading the valves a high- resistance potentiometer, and so causing distortion. Means can be found matter of personal preference. It for controlling the amplification in almost any and, by the adjustment of the becomes of importance, there- part of the set, but it is obvious that the best slider, a varying proportion of fore, to determine the best type position is before the first valve, since over- the voltage developed across the of pre- detector control, and its loading can then be eliminated on even the tuned circuit is applied to the strongest signal. Many methods of volume best position in the receiver. This control are described and practical hints given grid of the valve. If the re- latter question is easily settled ; as to the best way of introducing them into a sistance of the potentiometer be high- frequency amplifying valves receiver. high, some 500,000 ohms or so, introduce amplitude distortion it damps the input circuit but when they are overloaded, just lightly, and so has little effect as do low- frequency valves, although this distortion is upon either the selectivity or the sensitivity. not quite so evident. The volume control, therefore, In practice, there are certain objections to the control, should precede the first H.F. valve in the set, when but these are not always serious. In the first place, the possibility of distortion through overloading becomes it will be obvious that when the control is set at maxi- small. mum the input capacity of the valve is thrown directly It is worthy of note that this question of amplitude across the tuning condenser in the normal manner. distortion rules out all those forms of volume control When the control is set for low volume, however, this which depend for their action upon a variation in the capacity becomes effectively disconnected from the D.C. voltages applied to the valves. Any increase in the grid bias, or a reduction in the anode voltage, the i The Megavox Three, by N. W. McLachlan, September 12, screen -grid voltage, or the filament current inevitably 1928. The Band -Pass Four, by W. T. Cocking, June 25, 1930. introduces distortion of the H.F. wave -form, and also "The Wireless World" Four, by F. H. Haynes, October 15, 1930. B 32

www.americanradiohistory.com FEBRUARY 25th, 1931. Mfamo 203 pIld Controlling Volume. - that the potentiometer, which can have a resistance of variable condenser, and the tuning of the circuit is some 15,000 to 30,000 ohms, is connected across the altered. The importance of this in a ganged receiver aerial winding, and that the aerial is taken to the depends upon the number of tuned circuits in the set ; slider. With this form of control it is very important if there be many circuits. it is only of slight importance ; that all the tuning coils should be completely screened, if there be but few, however, then it becomes a serious and it is desirable that the valves, variable condensers, objection to this form of control. and wiring should be also. It has been the writer's There is, however, an advantage to be gained, which experience that unless these precautions be taken the is due to this same fact. When the control is set for control is quite useless on the local station, owing to reduced volume, the first tuned circuits' are partially direct pick -up in the coils and wiring following it. It is disconnected from the succeeding ones, and any a satisfactory control, however, where care is taken to tendency towards instability is usually eliminated. avoid direct pick -up. ` This results in an improvement in quality on the It will be found that, just as in the case of the H.F. stronger stations, for which the potentiometer control, its adjust- control need not be set at maxi- ment upsets the tuning of the mum ; at the same time, the first circuit, due to the alteration receiver may be so adjusted that in the amount of aerial capacity it oscillates with the control at thrown on to the tuned circuit. maximum, which may then be This can be reduced to a neg- used as a regeneration control as ligible amount by using only a well as in its normal capacity as small aerial winding, and this is a volume control. the method usually adopted in Unless care be taken, however, the U.S.A. where this type of control is very popular ; but as it reduces the sensitivity consider- ably it is not of universal appli- cation. The Aerial Capacity Control. Another method which has been used in several recent receivers- is very similar. This is shown in Fig. 3a, and it will be seen that, like the method just discussed, it controls the voltage transfer between the aerial and the tuned circuit, and, furthermore, that the method of control is still by means of a potentiometer ; the potentiometer arms, however, are variable condensers instead of variable BIAS resistances. This offers several important advantages ; in the first place, it is possible, by the careful propor- Fig. 1. -The H.F. potentiometer method of volume control, which tioning of the relative values of capacity, to eliminate also makes an extremely good regeneration control. At low volume settings the first tuned circuits become partially disconnected the change of tune of the tuned circuit and, secondly, from the succeeding circuits, with a great reduction in feed -back the control is and more noiseless any of effects. This type of control was embodied in " The Wireless smoother than World" Four receiver, the input end of which is illustrated. the resistance methods. This is simply because a

2 The Band -Pass Three, September 17, 1930. some curious effects may be observed in practice ; for it is often found that minimum volume does not occur when the slider is at the earthed end of the potentio- meter, but at some higher setting. Indeed, instability sometimes occurs when the volume is supposed to be at minimum. This is due to the valve cathode, or filament, not being at earth potential, and the cure is to increase the capacity of the condenser C in Fig. z ; in some cases the writer has found that it is necessary to use a capacity of 4 mfd. in this position. The Aerial Control. The fact that it produces an alteration in the receiver characteristics with its adjustment, although often ex- tremely useful, may, in certain circumstances, render its use undesirable. There is then no alternative Fig. 2.-The potentiometer control of the aerial input is very but to control the actual voltage transferred from the satisfactory, provided that all the tuning coils are well screened to first circuit, one method of to eliminate direct pick -up from the local station. The potentio- aerial the tuned and meter should have a resistance between 15,000 ohms and 30,000 accomplishing this is shown in Fig. 2. It will be seen ohms. n 33

www.americanradiohistory.com 204 FEBRUARY 251h, 1931.

Controlling Volume. - tap and the earthed end. By suit- perfect variable resistance of high ably positioning this tap, the value is extremely difficult to manu- effective amplification of the first facture, while a variable condenser H.F. valve can be reduced to about is comparatively simple. unity, and the normal volume con- At the present time there is no trol still serves to adjust the condenser available with charac- strength to exactly the desired teristics designed to eliminate the value. It is suggested that as a change of tune of the secondary ; start in arriving at the correct but it is possible to make a fairly position for the tap, which will, close approximation to this by naturally, vary in individual cases, adapting the particular circuit some five to ten turns only should shown in Fig. 3b and using an be included between the anode ordinary differential condenser. connection and earth.

BIAS

Fig. 3. -The control of the aerial input by means of a differential condenser Is shown in (a) and a modification in (b) which to a large extent prevents the tuning of the first circuit changing with different settings of the volume control. Such a volume control was used in the Band -Pass Three receiver here illustrated.

The extra condenser C, which can be of the compression In conclusion, therefore, it may be said that for type, is so adjusted that the tuning of the secondary radio purposes a post- detector volume control is often circuit is unchanged whether the control is set at unnecessary, and that the most satisfactory pre- detector maximum or `minimum ; at intermediate settings the control is probably the variation of the aerial input tuning is changed, but the amount of this change need by means of a differential condenser, accompanied by only be small. some method of reducing the H.F. amplification for Now, it will usually be found that any one of these local- station reception. The resistance methods are three methods will give an adequate control of volume usually quite satisfactory, and if in use there is little with even the most sensitive set, except on the local point in changing to the condenser method. station. The control will be satisfactory, even on a strong local station, with only a single H.F. stage, but it fails completely when two stages are used. In a sensitive set, therefore, it is essential to provide some additional means of controlling the volume for use on the local station only. In many cases this can take the form of some altera- tion external to the set, such as disconnecting the aerial or short- circuiting the aerial and earth terminals. It is usually desirable, however, to reduce the actual amplifi- cation in the receiver itself, for the reason that all valves introduce a certain amount of noise into the set, and this may take the form of an objectionable hiss on the local station. Considerations of efficiency and stability usually pro- hibit the switching of one of the valves out of circuit, but there can be no such objection to the adoption of the arrangement shown in Fig. 4. It will be seen that Fig. 4. -A suggested method for reducing the amplification on the local station. When the switch is in the upper position normal the anode tapping on the tuned intervalve coupling is results are obtained, but when It is placed in the lower position transferred to a tapping position at the bottom of the the amplification of the first valve can be reduced to any desired extent. It is used in addition to the normal control and has the coil, so that only a few turns are included between the advantage of eliminating valve hiss on the local station. B 34

www.americanradiohistory.com 25th, MpsIlsoo 205 FEBRUARY 1931. OpIl1 J

iütiüïi u ,° i1u`ni 1 f, rr \ ` BO BO ,Q\\`\ \ 0 100 /// ,ti0

11H IN RANEF The Stations Heard and How to Identify Them.

beO plan our listening in advance by stronger than one at 400 miles, the reference to the published pro- Round the Dial. stated power in each case being about T grammes, using the foreign trans- equal. missions as a refreshing relief to dinner -time talks and Accompanying this issue is a station identification such like, is the only way of obtaining full benefit from folder which has been compiled partly by reference a radio receiver. Now, when we turn to the foreign to international lists of wavelengths and the programmes stations it is safe to say that few of us have any idea given in foreign radio journals, but the main factor as to the identity of the stations to which we tune. as to the inclusion and classification of the stations has We should not search for alternative programmes at been based entirely upon. listening. The conditions of random, as it is preferable to select particular stations reception were a four -valve, two H.F., set, with single - by appreciation of their programmes. Many sets of dial control and input band -pass filter (" The Wireless to -day unquestionably give good foreign- station recep- World Four "), a Soft. aerial, and located 5¡ miles tion, and no longer is the absurd plea advanced that from the Brookmans Park transmitter. Stations were the transmissions from abroad have no programme value principally identified, in the first instance, by their wave- and are to be jammed out of existence by high -power length, with the aid of a wavemeter. In this way the stations situated in arcas of dense population. confusion arising out of relayed transmissions was As a rule, the user of the long -range set merely avoided. rotates the dial until he hits off an item pleasing to his Three styles of type are used in the chart when taste. He has no knowledge denoting the stations, re- of the precise identity of the vealing the comparative station and, in fact, to de- facility with which they are termine the origin of the See Coloured Sulesjaent received. Capital letters are transmission is often a used for the more powerful, matter of difficulty. The STATION IDENTIFICATION CHART. or key, stations, from which better the set the greater others may be identified by the number of stations their relative tuning posi- received and the more difficult does identification become. tions. Smaller type is used for the less powerful In making a guess as to the name of the station stations, light type representing those not usually re- transmitting we are guided first by the programme itself, ceived satisfactorily on sets with less than two H.F. such as the language spoken, characteristics of the music stages. Many other stations than those shown can be or the interval signals, next by the loudness of reception, heard, but reception from them does not possess a pro - and, thirdly, by the tuning position as compared with gramme value, hence they are omitted, Instead of stations already known. Reference to a list of foreign relying entirely upon the ear as a means of estimating stations and their wavelengths affords some assistance, the loudness of reception a wattmeter was used on the yet in only a few instances can stations be recognised output, in addition to a milliammeter in the anode with any certainty. Stated power of transmission may circuit of the detector. The classifications given above be misleading as, for instance, a 1.5 kW. transmitter were represented by outputs of (I) over 1,30o, (2) over may be louder than one of 6 kW. situated at approxi- 500, and (3) over 15o milliwatts when receiving a mately the same distance off. Distance, moreover, is programme of music and using a pentode output valve, but a poor guide, and a station 800 miles away may the P.M.24A. n 37

www.americanradiohistory.com ioo Vebdno FEBRUARY 25th, 1931. Wont], In each station panel appear observations noted in one tuning dial, or be fitted with reaction or volume the course of reception, giving some detail typical and co-itrols which change the tuning positions, the settings peculiar to the station. Phonetic spelling is adopted for these auxiliary adjustments might be noted in the spaces provided.

" Hallo! Rah -dee -o Booda -Pescht Un- gar." The studio of the well received Budapest station.

The best plan is to identify and note the settings of those key stations shown on the chart in large type. Thus, as the dial is rotated, the high -power station at Heilsburg, in East Prussia, is readily identified at a few divisions below the National transmitter. Almost within a degree of this station, and immediately above it, is Bratislava, a station located in Czecho -Slovakia, which, with

"Eh -yah Radio Roma-Na- poli," the familiar call of the Rome station, the aerial system and station buildings of which are shown.

a power of only 14 in these notes in respect kw., is counted among of languages other than the best -heard stations French or German. In- in this country. Pass- terval signals are given where they occur, and form a valuable guide. It is to be regretted that well - defined interval signals are not used by every station to -day. Although the London transmitters are now coming into line in this respect, the feeble sound emitted, re- sembling that of the clanking of a distant shipyard, so lacks brilliance that it may be passed unnoticed, or may be confused with the metronome signals now used by many other stations.

" Hallo! Hallo! Oslo her." The talks studio of the long wave Scan- dinavian station.

Spaces are left on the station panels for entering the precise dial setting ; the actual figure obtained will probably not widely differ from that shown on the ioo- or 18o- ing on, Hotween the two London transmitters a number division scales. The difference between the figures of stati , of medium power are encountered, given and and those obtained will not be uniform across the next high -power foreign station is Mühlacker, the scale ; it will steadily increase or decrease, but the almost overlapping the London change Regional, which is will be progressive and can be estimated.. usually heard in the intervals of the foreign programme. Should the receiver be of an older type with more than Rotating farther, we hear the new transmitter of Lvov. r ;S

www.americanradiohistory.com FEBRUARY 25th, 1931 WpisIloo 207 W DS'l.e Within Range. - by listening on a modern long -range single -dial super- This Polish station has only recently increased its power, heterodyne. and its identity is revealed by the frequent announce- Listening on the long -wave range, we find the stations ment " Ler Woof." Farther on, and almost over- well defined. Most sets give better amplification on lapping, is Toulouse, an unmistakable station. After the long wavelengths, and when, during the hours of an interval of several divi- daylight, the medium wave sions, Katowise is heard, band may be found nearly with its hollow -sounding deserted nine stations can in- IDENTIFICATION metronome and its frequent CHART variably be brought in on announcement " P o l s k i the long waves. Taken in Raadjo Kat- owe- vee- tsee." order round the dial we hear Stockholm, found a little Oslo, Kalundborg, Motala, farther on, is one of the Eiffel Tower, Daventry most powerful of European National, Koenigs- transmitters as regard s wusterhausen, Radio Paris, initial energy, yet it is often Lahti and Huizen, the last overlooked by Brit i s h mentioned being well re- listeners, its signals being ceived in spite of its modest slightly weaker than, say, power. Careful estimation the modest Radio Toulouse. of dial setting assists in the Rome, almost adjoining, picking up of the other needs no introduction, with stations shown. its lady announcer and her At this time of the year frequent " Eh -yah Raadio at least five American Roma Napoli." Just be- broadcasting stations can fore reaching the Mid- be heard on the medium land Regional is Langen- broadcasting band on berg, recognised by its favourable occasions after distinctive chime of five midnight. Details of these bells. Beyond, but well stations have been included clear of the Midland Re- on the chart. They are gional, is Brussels, which, readily recognised by their owing to a recent increase frequent announcements. of power, comes in well. As changes are frequent, Towards the end of the spaces have been left on scale are the two well - the chart for additions and defined stations, Vienna amendments. With the and Budapest. The former knowledge that the stations is recognised by its frequent shown are those normally announcement, ,' R a d i o received one can go ahead Wein," and the continuous fill in THE SUPPLE VIENT. and methodically ticking of its interval signal, positions. Stations are classified according to the facility with which their tuning The while - the musical interval they are received. *Having identified the more powerful trans- chart thus completed and mitters, other stations are recognised by their relative tuning signal of the latter, and the positions. available for reference adds fact that it is clear of other considerably to the utility stations, makes its identity obvious. It might be men- of the radio receiver, particularly in unskilled hands. tioned that the tuning range of certain receivers fitted Using the chart with several different receivers it with large trimming condensers will not go high enough was found that the actual readings given did not differ for the reception of Budapest. widely from the tuning positions obtained. The differ- Between these key stations those of less power can ence being progressive, it is worth while entering on be readily fitted in. It is best to go definitely after the scales the increasing or decreasing difference be- a particular station, having carefully estimated its tween dial setting and scale reading. Thus, with a par- tuning position, hanging on to the transmission until ticular set the dial reading for Cork was two divisions some clue to its identity is heard. The lot of the on a 18o° scale in advance of that shown. Toulouse English listener is a little unfortunate in that the was three divisions in advance, the Midland Regional stations best received mostly fall bunched closely to- four, and Budapest four -and -a -half. Taking into gether between 5o and 75 divisions on the scale. account this steady change, precise dial readings for Stations crop up clearly at almost half -degree fervals; other stations were readily predicted. Another useful so that errors are easily made in identificatioir.''' There hint for the enthusiast is that of projecting a wave- are very few powerful transmitters operating below the length calibration on to the centre red semi -circle using National, and while this was at first thought to be station wavelengths as a guide and coinciding with the due to the falling sensitiveness of the capacity- coupled particular dial reading to wavelength scale of his band -pass filter the evidence of stations was confirmed recei ver.

13 39

www.americanradiohistory.com ro8 WApgNoo FEBRUARY 25th, 1931. %KA cd

is the effect of using an unsuitable value of grid bias WHAT MINIM ri ir. in a receiver 1 Why is it possible for a mains valve to mrr.r.rr...ri become temporarily paralysed if the supply transformer is badly designed 1 These questions and many others are dealt with in 7he í.rr..rr...rrr..r..\ simple terms and rule-of- thumb methods helpful to the newcomer to radio are introduced. r¡r U íriiI AL::III rr rrrr .r.rrrrr...rrrr e No. 1. -The Triode and its ---- onnuii°.. l' C:::r... Working Voltages. ff e ifT is probably safe to hazard the guess that one of the l. S most common faults contributing to unsatisfactory reception in a receiver is the application of wrong By W. I. G. PAGE, B.Sc. voltages to one or more of the valves. Although the number of different valves available to the user grows tions will bring about distortion-low notes being absent. greater with the passage of time, it can be said with con- Better reception will be obtained by bringing the bias fidence that the modern valve, in conjunction with appro- down to C, as high -impedance valves only require small priate components, may be relied upon to give admirable values of grid potential, but this must be considered as results when its filament, anode and grid are fed as a temporary expedient. their makers meant them to be. Had the valve been suitable for H.F. amplification the In these notes it is proposed briefly to take the case drop in filament voltage, which can be imitated by of the three -electrode valve and to make a simple study putting a rheostat in one of the filament leads, would be of the effect of increasing and decreasing the voltages accompanied by a decrease in volume and by better beyond their correct values in the hope that where sets selectivity, because the higher impedance of the valve are not giving of their best the fault may be traced to would act as a smaller load across the tuned circuit. its source. It will be With large battery or assumed that the coup- mains output valves a con- lings between the valves siderable drop in filament have been chosen with due ANODE VOLTS or heater voltage is likely 50 100 1 care, and that at some to reduce their life, as they time or other reception has will be working near been beyond reproach. " saturation." Valves in Let us take Fig. I, where this condition will be par- there is a schematic dia- ticularly sensitive 10- A 10 to fluc- gram of an imaginary low tuation of mains voltage- impedance four -volt valve the volume from the which the maker's pam- speaker changing audibly phlet tells us should have w e` as the lighting supply 8 volts negative bias, with 7 varies. 15o volts on the anode w 6- 8 Next we Z 4 VOLT shall consider a when used as an amplifier VALVE rise of heater volts to point 5- -5 3 -the arrows pointing to JI- 2 A-about 5.5 volts. The p 4- -4 these conditions. > word heater rather than Examining the filament c) 3-B C -3 filament is used advisedly, circuit first, we will assume as it is not likely that the that the L.T. accumulator `1- o B-2 potential of a four -volt is faulty, and that its volt- -1 accumulator will increase age has dropped to the above 4.5 volts, but there value marked C--just A are, unfortunately, mains 1 s 5/ 4 5 6 under 3 volts ; what will FILAMENT OR HEATER VOLTS transformers existing, defi- be the effect? The imped- cient in iron and copper, ance of the valve will rise which if under - loaded considerably, less current Fig. l.- Schematic diagram showing how the voltages applied may make possible this to the various electrodes of a valve interconnect with one will pass through it, and another. A 4 -volt valve is taken, but the considerations apply state of affairs. The only the equally to the 2 -volt or 6 -volt type. When a valve is given volume of sound from a considerable negative bias a resistance R, as shown, does gain - doubtful in its the loud speaker shown as not affect the grid voltage. nature -is an increase in " Power" will drop, say, power output to point A. from the arbitrary figure of 8 to point C. But as the Besides a considerably curtailed life, there is often speaker will have most certainly been matched to the another effect of overrunning the heater of an A.C. valve for quality in the first instance, the new condi- valve. Owing to excessive heat the grid, which is close B 40

www.americanradiohistory.com FEBRUARY 25th, 1931. Mp@iligog 209

The Valve at its Best. - being used. With an indirectly heated valve in this by, begins to rise to a Sufficient temperature to emit position a minimum bias of 1.5 volts is necessary. It electrons like an ordinary filament, and grid current is generally safe to double the normal bias where the flows in the external grid circuit, damping the input to grid potential of an anode bend detector is concerned such a degree that signals drop to a low level. Readers and to give zero bias-that is, connect the grid return may have come across a mains receiver in which the lead direct to filament or cathode -in the case of the volume decreases almost to vanishing point after, say, power grid detector, the new form of distortionless rec- an hour, and only returns to normal when the set is tifier developed by The Wireless World. switched on again after a period of rest. This may be Lost Anode Volts. the result of grid emission. Gradually the cathode dete- riorates, the chances of recuperation grow less and And now to the anode circuit and back to Fig. I. less, and signal strength is permanently lost. The We will suppose that the high-tension battery is get- remedy is the choice of a transformer of reputable make. ting old and that the anode volts have dropped to the Turning our attention now to the grid of the valve, point B -i.e., to 5o volts. The output power will drop we can formulate a rough rule for negative bias under to B, or about a quarter of its normal value, and as be amplifying conditions : divide the anode voltage by the in the case of reduced filament voltage the bias must amplification factor and multiply this by two- thirds. As decreased to about the point B to obtain better recep- an example, the amplification factor of the Mullard tion which in no circumstances will be distortionless PM.254 is 4.2, and the maximum anode voltage 150 ; with such a low anode voltage. It must be remembered by dividing the latter by the former and taking two - that a loud speaker or choke at Z will cause a loss of thirds of the result we get 23, which is the official bias volts between the battery and the actual anode of the figure. Should a milliameter be available, it is rather valve ; this loss is calculated by multiplying the resist- better to set the working conditions by the correct anode ance of Z by the milliamperes passing, and dividing the current, which is now given by most manufacturers, result by a thousand. If, for instance, the battery voltage making the necessary alteration to the bias to effect this. is z8o, the resistance of the loud speaker wired directly A word of warning, however, is necessary here -never in the anode circuit 2,000 ohms and the current 24 break the bias circuit unless the H.T. has been switched milliamps, the volts lost will be 48,000= 1,000 =48 volts. off first, otherwise excessive anode current may flow, There are many amateurs who find themselves called damaging the coating of the filament. Under -biasing or upon to design amplifiers for small dance halls and make the use of an exhausted bias battery with an output valve use of power valves of the DO/ 2o type, having a com- will cause a big drain on the H.T. battery, whereas over - paratively high amplification factor and low impedance biasing will cause distortion and reduction in volume. (high mutual conductance). Owing to the tuned circuit If a milliammeter for measuring anode current cannot formed by the small condenser consisting of the grid be pressed into service, it is best to increase negative bias and anode of the valve and the inductance in the external to a point just short of where noticeable distortion occurs. circuit, self -oscillation at about io When a directly heated output valve is fed from raw metres may take place, causing a A.C. -as is now the custom -three or four volts should peculiar form of distortion, a drop be added to the bias over and above that advised by in volume, and a rise in anode the makers for battery heating. current. The cure is to put a zoo - Those who are still protagonists of the neutralised ohm resistance in the output cir- triode would be well advised to try a o.9-volt bias cuit connected directly to the cell for the grid circuit, provided battery valves are anode terminal of the valve.

TWENTY YEARS OF VALVE PROGRESS. On the left is the early Fleming valve next to the V.24. The two following valves are the R and the DER, on the right of which are a B.S.A. power valve and a WECO valve. More recent additions are the Marconi and Osram P.625, and the Ediswan ES.75 on the right.

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www.americanradiohistory.com 210 Wh Demo FEBRUARY 25th, 1931. WaDirIlc.l

MARCON I PHONE Model 560

SPECIFICATION. Two H.F. stages with tuned grid coupling and S.G. H.F. valves. Power grid detection. Single dial control. Compensated transformer L.F. coupling. Power pentode output. THERE is no outstanding radio set which might potentiometers, Valve rectification. earn for ` itself the title- -` The Rolls-Royce of though the former Permanent magnet moving coil Radio Receivers." This statement assumes that may be regarded al- loud speaker. the set earning such a title is the best that could be most as a simple resist- Walnut console cabinet. produced regardless of cost. Price and performance in ance feed circuit owing Price 38 guineas. radio do not go hand in hand, but it is safe to say to the high value of that the best-built sets to -day from a production and resistance included on assembly standpoint are the best performers. It is the earth side. after an examination of the Marconiphone Model 56o Regulation of screen voltage constitutes receiver the volume that these remarks are made. Here we have control, although the spindle of the variable resistance a set which is a thoroughly first -class production repre- used to control screen potential simultaneously senting operates the bringing together of everything that is right the potential divider connected across the gramophone in radio -receiver design with skilful manufacture, pickup. The inclusion of an additional resistance in sparing nothing in the making of patterns and press series with the pick -up control prevents the tools. maximum pick -up potential being applied to the detector valve. Circuit Details. It is interesting to note that the value of these re- sistances, both the fixed and the variable, across the Conforming to a recent tendency, this receiver makes pick -up is only Io,000 ohms, and, while this is much use of a four -valve circuit, so that reliable local- station lower than the values normally used, experience shows eception is possible with a small aerial under the most that a low value is desirable. Adverse conditions, while, with an average aerial, a choice of thirty reliable programmes results. Two H.F. Ingenious Tone Corrector. stages are employed, followed by a leaky grid detector, In the detector stage we find values of condenser capable of and handling a generous signal voltage without grid leak indicating " power " grid detection, while distortion, followed by a power- pentode output valve. the voltage on the anode of the M.H.L. valve is in Anode current is obtained from a U. io valve rectifier the order of 7o. Bias is introduced into the detector from A.C. supply, while the first three valves, which circuit when switched over to gramophone pick -up. are of the Marconi M.S.4 and M.H.L. types, are in- The biasing potential, as in the case of the directly H.F. heated, and the output. pentode directly heated valves, results from the inclusion of resistance in the with A.C., as is customary. Single -dial control, the cathode lead. In the anode circuit of the detector essential of all modern receivers, results from the use of is the usual H.F. choke, and in order to provide a a triple- sectioned ganged condenser tuning choke -fed filter feed to the high -ratio intervalve transformer there circuits, the aerial being brought into the first tuned are both feed and decoupling resistances. circuit through a small condenser coupling. Generous It is important to note, however, that the decoupling decoupling is found in the anode and screen circuits condenser in the anode circuit returns to earth through of the S.G. valves, and the detection which occurs in part of the pentode biasing resistance. The explana- the H.F. stages is prevented from giving rise to L.F. tion for this is that the biasing resistance, being shunted oscillation by the use of high -value feed resistances. by a condenser, has the -bass frequencies developed Both anode and screen voltages are derived from across it to a larger extent -than frequencies in the B 42

www.americanradiohistory.com FEBRUARY 25th, 1931. `MDIl WcoIlcl

SINGLE KNOB TUNING GANGED TWO SECTION GANGED CONDENSER UNIT VOLUME CONTROL

COMBINED WAVE-RANCE AND GRAMO. SWITCH. i.liü".....

to,

OUTPUT 'TRANSFORMER

OF CONDENSERS LBANK I _SMOOTHING CHOKE

MAINS TRANSFORMER

`^{RECTIFY/NG VALVE (U /O) ï---. } ------IGQAMO MOTOR SOCKETS

I % FUSES

MA /NS PLUG

AERIAL SERIES CONDENSER

MAINS TRANSFO121fEQ1 TERMINAL PLATT

VOLTAGE REGULATING RESISTANCES IRON FRAME

CADMIUM P/ATED I

50 OHM LOAD RESISTANCE ON OUTPUT TQ7NSFOR4IERI

TERMINAL STRIP ON CONDENSER. BANK]

Uonstructionat features of the Mareoniphone, Model 560.

B 43

www.americanradiohistory.com 212 VitTeIlsw OpIl FEBRUARY 25th, 1931. Marconiphone Model 560. - A solid -walnut cabinet houses the set, al. offending upper register. Thus, part of the amplified potential controls being hidden in a recess at the top, to which representing the lower frequencies is fed back into access is gained through sliding doors. Th , the anode dial is circuit of the detector, so that when passed illuminated, and carries a wavelength calibratì'n. A on again through the transformer to the grid of the moving -coil loud speaker, making use of a large per- pentode a boosting up of the bass results. This in- manent magnet of quite recent design, ensures the best genious method of introducing tone control corrects the possible loud speaker performance, free normal from hum characteristic of the pentode with its tendency which might arise were a rectifier employed for field to attenuate the bass frequencies in its anode circuit. excitation. A step -down output transformer is fitted, and one notes On performance, it was noted that this the receiver wise precaution of permanently connecting a re- covered as wide a range of audio frequencies as any sistance across the loud speaker terminals in order to set we have tested. Reproduction possessed avoid the charac- the chance of damage to the pentode should teristic brilliance due to the pentode combined with a

215 V 216 235V

6 VOLT PILOT LAMP MAINS AERIAL PLUG

o MOTOR EARTH

Tone correction and loaded output are interesting details found in the circuit of the Marconiphone, Model 560, A.C. operated receiver. signals be tuned in with one of the loud speaker leads well -defined bass devoid 01 boom. Used on a Soft. disconnected. Circuit refinements such as these, which, aerial at a distance of 51 miles from Brookmans Park strangely enough, are rarely found in present -day one finds that the nearby transmissions are only equal receivers, give confidence in the skill of the designers. in their programme value to, say, Brussels or Budapest. Likewise, on the production side, we find a die - Dial readings start just above Zoo metres, and as one cast mounted variable condenser operated through turns the control Cologne and Helsinki are almost at worm- driven cords, positive and free from backlash, once encountered. These stations are free from the and a clever rotation stop device. Single -knob National transmitter, and before reaching the London control, with four positions, actuates the entire Regional there are seven well -defined programmes. mechanism of " on " and " off," wave- change switch- Above the Regional are about ten stations, Katowise, ing and change -over to gramophone pick -up. All com- Rome, Stockholm, Lagenberg, Brussels, Vienna and ponents are screened, and the housing is of pressed Budapest being particularly good. The little station metal, while the voltage -regulating and feed resistances of Rabat, in Northern Africa, almost overlapping on are carried in the ideal position, a recess under the base, Dublin, came in well. .so that all wiring is hidden. All the framing is of At a price of 38 guineas this set does not qualify for iron, which is stiffer and, incidentally, cheaper than the title mentioned at the commencement of this article, the aluminium customarily used, and, being dull though we would call for no change in its design, cadmium- plated, it is rustless and of good appearance. construction or performance.

13 44

www.americanradiohistory.com FEBRUARY 251h, 1931 Mp@D®go 1 3 G7ordlcl

By Our Special Correspondent. New Waves for Broadcast- ing ? -The Power Problem. - Welshmen's Grievance. -Four Million Licences. - Northern " Patience." - Tackling Elec- trical Interference.

Secrets from Semmering. Several secrets of vital interest to ordinary listeners have been brought back from Semmering, where the Union Inter- nationale de Radiodiffusion have just held their little meeting to ruminate on such unpleasant topics as mutual interference and incompatibility of temperament among broadcasting stations. One out- standing concession has been gained. 0 0 0 An air view of the Breslau broadcasting station which can be heard on 325 metres. Bigger Waveband for Broadcasting ? On excellent authority I learn that the " Hope Springs Eternal ... " his reputation through his appearance; organisers of the World Wireless Confer- In the meantime the members of the any impression he could convey over the ence at Madrid next year have already Union in fifteen European countries have ether, however good, would be quite been persuaded to include on the agenda undertaken to measure the field strength foreign to the personality we associate the discussion of a considerable extension at which some thirty of the principal with the screen. of the existing broadcast waveband. The stations are received intheir countries. 0000 possibility of an expansion of 100 metres It is hoped (I am told) that the know- Michael Faraday. both above . and below the existing ledge gained will lead to a solution of Sir William Bragg, one of the greatest medium waveband is mentioned. the interference difficulty. chemists of the day, will broadcast a 0000 " Man never is but always to be National Lecture on March 4th. His sub- Only a Temporary Solution. blest ! " ject will be the life and work of Michael If the world governments prove far- 0000 Faraday. Sir William holds two of the sighted and generous enough to assent to Wails from Wales. scientific posts which were once filled by this change there is some hope that the " We demand the inclusion on the Faraday. ether of Europe may be reasonably com- B.B.C. staff of a Welshman who will 0 0 0 0 modious for another two or three years. attend to the interests of Welsh What Odds ? Assuming that the existing band covers listeners." The fact that the receiving -licence roughly 300 metres, the increase to a 500 - This from an energetic Welsh Society figuro touched 3,501,000 on January 31st, metre width would permit of a station with whose aspirations on this point I am snakes it a sporting chance that the four separation of approximately 15 kilocycles entirely in sympathy. million mark will be reached by the end as compared with the present separation 0000 of the year. Here is an opportunity for of 9 kilocycles. On the face of it we have Welsh Rare Bit. the sportsman ! a solution to most of our interference " Yes, 1 think we have a Welshman- But the wily punter who has studied troubles. 0000 somewhere, said a B.B.C. official in preceding licence figures will probably answer to my enquiry. He ran his have noted that the increase during the The Power Ogre. finger down the Savoy Hill auto- previous 12 months was only 456,000. He It has been repeatedly urged in these matic telephone directory. But nearly will, therefore, bet against the attainment .loins, however, that no amount of everybody on the list was either Scots or of four million by December 31st next. dazzling jugglery with the wavelengths half- Scots. Sir John Reith, Mr. Glad- 0000 can serve much purpose while the ogre stone Murray, Mr. Lockhead. . Two of unrestricted power output still struts or three lonely Englishmen were found in Four Million Licences by December 31st? about the stage. And all the indications company with an Irishman or two. As Honest but observant onlookers, how- are pointing to an all -round power in- for the rumoured Welshman, he was non- ever, will " take him on," nourishing crease in the near future. existent. only in their private bosoms the reflection 0000 The Welsh have a genuine grievance. that, the opening of Northern Regional is What Madrid Must Do. Savoy Hill must find a Welshman imme- about to bring another vast army of new 1t lies within the scope of the Madrid diately. listeners into the fold. to limit every broadcasting 0000 Yorkshire and Lancashire are among Conference the station to a power not exceeding the Charlie Chaplin. most " radio conscious " among Eng- reasonable maximum necessary to provide Where the " talkies " have failed it lish counties, but at present they are not listeners with a suitable service. Unless seems unlikely that the B.B.C. will adequately served. Madrid exerts this authority we can hope succeed, and I shall be more than sur- 0000 for little more than an interminable series prised if Charlie Chaplin can be induced An Anxious Month. of trifling conferences all actuated by the to approach the microphone even to voice Bearing in mind this increase, it seems one motive, viz., the putting of a quart a charity appeal. that a figure of 3,950,000 may be comfort- into a pint pot. After all, the great little man has made ably reached by the end of November. B 43

www.americanradiohistory.com 214 Web= FEBRUARY 2:5th, VbpIlcl 1931 With only a month to go the nation tus which have been known to interfere Consolation. should become white -hot with excite- with broadcast reception are seeking to Transport ment. authorities have still a long I like to think of those anxious modify their designs. I am told that way to go -before they quit the ranks of little groups. of people gathered round the two. important manufacturers have separ- the " man -made static " brigade, but it post offices at the eleventh hour on ately approached the ,B.B.C. for the views is comforting for the defenceless 'listener December 31st nudging each other into of the-Corporation on -the, electrical silenc- to know that Isis troubles are appreciated the purchase of . extra licences... ing of -traction plant. and that efforts are being made to over- 0000 000.0 come them. Making Our Mouths Water. As regards tramway interference, the Post Office and BiB.C. -Tests. best remedy would be to abolish the The really expert chef commands the trams. salary of a Cabinet Minister and his work An admirable report on interference 0 0 0 0 is one of the great arts. In lar too many caused by electric trams and -trolley homes of Great Britain food is ruined in 'buses was issued by the General Post The Grand National. the cooking. The new series of broad- Office eighteen months ago, as a result of Running commentaries on the Grand casts on Gastronomy, which starts ,next National on March 27th and on the Scot- month, will treat seriously this question land v. England Association Football of making an attractive job of cooking. FUTURE FEATURES. Match on March 28th will be broadcasT on Mr. Wickham Steed opens the series on the National wavelength. March a National (261 and 1,554 metres). 6th with talk entitled " Is Cook- ALARCH 1sT.-Orchestral concert from The Grand National will be described ing an Art ?. " Mr. Evelyn Wrench will Hastings. in two parts, one by Mr. R. C. Lyle for be a later contributor to the series, also Memo 250. -" The Ridgeway Parade," -a that part of the course on the Grandstand M. Boulestin, song and dance show. famed equally as a poet MARCH 5TH. -Hailé concert, relayed from side of Becher's and Valentine's Brooks, of gastronomy and restaurateur, the Free Trade Hall, Manchester. and -the other by Mr. W. Hobbiss for the 0000 MARCH BTH.-Military Band concert. remainder of the course, including MARCH 7TH. -" Rich Girl, Poor Girl," a Becher's Brook, Are Northerners Patient? modern musical comedy. the Canal Turn, and London -Regional. Valentine's Brook. General descriptive A Manchester maxi stepped into my MARCH 18T. -Sunday Orchestral Concert. scenes will be given by Mr. Alan How- office with 'a fiery light in his eyes and MARCH BRD. -" The itidgeway Parade." land. a copy of last .week's Wireless World in MARCH 4TH.-Military Band concert. 0000 his MARCH 5TH.-" Rich Girl, Poor flirt." hand. MARCH 6TH. -Concert of Contemporary " Is the North Country breeding a new Music -3. The Microphone Manner. lace ? " he quoted, tapping page 183 and MARCH 7TH. -Brass Band Concert. Seekers after the ideal microphone Is smiling sardonically. Are we growing Midland Regional. talker, who is neither too " announcerial " patient? " MARCH 3RD. -" The Dream of Gerontius " nor too casual, .should tune in on Monday (Elgar), from Worcester Cathedral. evenings for Dr. He drew himself up. " No, Sir ! " MARCH M.-Two short sketches- T. R. Glover's talks on 0000 " Mademoiselle," by Elizabeth Illing- " Virgil and Iris 'rimes." We may not worth; and "Full Tide," by Jack all be interested in few Homely Muhlacker Heming. Virgil, but of us, MARCH 6TH.-Rugby Male Voice Choir I think, could switch off in a hurry after lie then explained. He put it very Concert, relayed from the Co-opera- hearing a few sentences from Dr. Glover. clearly. There was nothing disquieting, tive Hall, Rugby. He seems to address the microphone as he said, in the fact that northerners were MAaca Tro.-The Leicester Brass Band lie Festival relayed from the de Mont- would talk to a friend sitting at the showing no impatience over the delay in fort Hail, Leicester. other side of a cosy hearth in a study opening Northern Regional. West Regional (Cardiff). lined with books. Perhaps Dr. Glover's " Up North," he said, " we don't MARCH 15T. -A St. David's Day Pro- art is explained by the fact that he is listen to British stations. Our pro- gramme of Welsh music. Public Orator at Cambridge. grammes MARCH 3RD. -Second Concert (Season come from Germany. Muhlacker 1930-31) of the Swansea Orpheus 0000 is our local station ; vie enjoy every minute Choral Society, from the Patti Pavilion, Swansea. of the transmissions, especially the operas, Still Waiting for the . Stille. which come through MARCH 5TH. - "Second Childhood," a splendidly with just comedy in one act by R. G. Berry. The B.B.C. seems to be in no hurry to that extra touch of modulation that North Regional (Manchester and Leeds). begin rehearsal tests with the Stille ma- makes all the difference." MARCH Í55. -The Manchester Municipal chine, about which so -much was said a " But Northern Regional I Chamber Concert, relayed from few weeks ago. The machine has not yet began. -," Platt Hall, Manchester. MARCH 6T11. -" Thirty Thousand Pounds," been delivered. " Northern Regional ? " he said. a play by W. P. Crozier. Although the reproduction given by " I tell you, we listen to German Glasgow. this magnetised -steel recording instru- stations- " MARCH 7TH.-Excerpt from the Annual ment is stated to be definitely inferior to A moment Concert of the Glasgow Caledonian later he had gone. Strathspey and Reel Society, relayed that of a gramophone, an interesting 0000 from the St. Andrew's Hall. " stunt " broadcast could easily be ar- Belfast. ranged, but Savoy Hill apparently fears Radio Drama by Celebrities. MARCH 2ND. -" The Far-Off Hills." n comedy in three acts by Lennox copyright complications, and will give no The dearth of original radio plays is Robinson. promise of such a test in the near future. responsible for the increasing trend 0 0 0 0 towards the adaptation of stage plays and novels to the microphone. Three im- Bedtime Story. portant events in radio drama are already tests carried out jointly by the B.B.C. " Sorry, but we can't cut him out," scheduled. and G.P.O. at Blackpool. said the engineer. " The ' fade out ' " Will Shakespeare," the play by It was found that clicks were induced switch has jammed." Clemence Dane, is to be broadcast during in nearby receivers when the collector- The announcer looked grave and the third week in April. Soon afterwards shoe of the experimental car jumped over glanced at his wrist watch. we shall hear Galsworthy's " The Forest," a suspension ear. The noise was con- " Ghastly shame, but what can a man and this will be followed by G. K. siderably reduced, however, by the use of do? " he muttered as he took up the Chesterton's " The Napoleon of Notting the Fischer [ late -a 4ft. long collector revolver specially provided for such emer- Hill." running transversely to the " live " wire gencies and levelled it at the violinist. 0000 and held against it by spring tension. A moment later a shot rang out and the The Dudgeon shoe was also successfully music ceased. Manufacturers and Broadcast Interfer- tested. This consists of a trailing shoe " Ladies and Gentlemen," said the ence. behind the ordinary trolley wheel, and is announcer, addressing the microphone, It is a healthy sign that the manufac- intended to bridge the gaps in the collec- " we must apologise for being two seconds turers o. those types of electrical appara- tor -wire. late with the Epilogue."

13 .t6°

www.americanradiohistory.com FEBRUARY 231h, 1931. 215 pIlcWoad Laboratory 1 ies ts ®n New Apparatus. CLIX VERTICAL ANODE sary to set the selectivity control at mini- speech coil. A step -down transformer was CONNECTOR. mum, consequently the receiver can be incorporated in the model tested. In a large number of modern receivers used muçh nearer the transmitting site The response between 1,000 and 6,000 the screen -grid valves are encased in metal and still give satisfactory reception. cycles is good, and reaches its maximum containers with a small hole in the top The following procedure was adopted to between 2,500 and 3,000 cycles. Between to pass the anode lead. To afford perfect adjust the set, although in all probability 1,000 and 50 cycles the output falls off insulation for this lead, Clix have intro- this will be done before delivery. First slightly, and as there is no resonance in duced a new vertical type insulated connec- depress the wave- change switch, set the tor. This consists of two separate parts; variable condenser at maximum, and tune a spade terminal carrying a plug and in the National programme, using the two an insulated pillar fitted with one of the end " Formodensers." Now pull up the Clix resilient sockets and á flex connector. switch knob and tune in the London Regional programme, this time using the two condensers mounted on either side of the switch. The selectivity control can then be ad- justed so that each programme is free from interference and, if necessary, final adjustment made to the senti -fixed con- densers. The perikon type crystal detector fitted is a good feature, since normal jolts, such as it might receive in the ordinary course of use, will not displace it or call for readjustment. One point we feel requires a little atten- Clix S.G. anode con- tion is the aerial circuit. Used on an _ hh,, nector for use where the valve is encased aerial about 70 feet long -including down in a metal screen. lead and earth wire -the National pro- gramme on 261.3 metres could not be Perfect insulation is assured so that, the tuned in exactly. Either a small aerial shortening condenser or, possibly better W.B. moving coil loud speaker unit in A.T. supply is protected against acci- which the permanent magnet is of un- dental short circuit. The makers are usually generous proportions. Lectro Linx, Ltd., 254, Vauxhall Bridge Road, London, S.W.1, and the price is 3d. the lower register, the reproduction is free 0000 from any tendency to boom. As might be expected with so large a field magnet, MATLOCK TWIN STATION the sensitivity is good, and not inferior to CRYSTAL SET. mains- excited moving coil instruments. Although designed primarily to meet The price is 6 guineas, and the makers the needs of those residing within a few are Messrs Whiteley Electrical Radio Co., miles of Brookmans Ltd., Nottingham Road, Mansfield. Park, this crystal set 0000 prove equally should BELLING -LEE MIDGET WANDER effective under simi- PLUG. lar conditions else- This new Belling -Lee plug has been where. - designed especially for use where the In brief, the ar- H.T. and the grid -bias batteries have only rangement consist s a very small head room for the accommo of two separately the wander plugs. The lead is tuned circuits ex- dation of well inserted through a hole in the side of the ceedingly plug cap and fixed in position by screwing screened and coupled up the metal inset carrying the prongs. at the high potential The prongs are not formed by longitudinal ends by a small vari- Neat layout, accessibility and saw cuts but consist of three segmental able condenser ; this, pieces brass wire riveted wave good screening are the distinc- of hard drawn and a -change tive features of the Matlock crystal set. Note the semi -fixed con - switch, being t h e densers and t he wave -change switch on the sub -panel. only controls on the panel. The function of the condenser is still, the introduction of a tapping on the to adjust the selectivity to cope with local aerial coil for the aerial, would make a conditions. worth -while improvement. " Formodensers " are used to tune the The makers are the Matlock Radio circuits, four being mounted in line on a Manufacturers, Matlock House, North metal sub -panel with the wave- change Finchley, London, N.12, and the price is between them. 39s. 6d. switch disposed centrally 0000 Belling -Lee midget three -prong wander Access to these condensers is obtained plug with side entry for lead. through an opening at the back normally W.B. MOVING COIL LOUD SPEAKER closed by a hinged trap. UNIT. into the metal inset. This gives good A test was made in North London This unit is notable for the massive self -alignment, and in addition provides about 10 miles from the Brookmans Park proportions of its permanent magnet three separate points of contact. transmitters using an outdoor aerial of which weighs no less than 10 lb. It is The price of these midget plugs is 2d. average size. The set functioned admir- attached to a rigid cast aluminium cone each including engraving, which is avail- ably, no difficulty whatsoever being ex- chassis which also supports a 121in. square able in 12 different styles. The makers perienced in separating the two pro- plywood baffle. The diaphragm is 7 ¡in. are Belling and Lee, Ltd., Queensway grammes. Indeed, it was not even neces- in diameter, and carries a low impedance Works, Ponders End, Middlesex. B 47

www.americanradiohistory.com 216 NWP®Il0Z0 %Ad FEBRUARY 25th, 7937.

A Chassis -built By Transportable Set. N. P. VINCER =M.INTER.

IN0 apology is needed fifth of the total number of for describing a plain turns on this coil are in- two -valve regenera- cluded in the aerial circuit. tive receiver in 1931, for Now L, itself is linked into such an instrument still has the medium -wave induct- a very great sphere of use- ance (Ll +L3) at a fulness, although it cannot which causes one-fifth k be used to roam the ether the total number of turn, at will, as was the case in this coil also to be incluuea the earliest days of broad- in the aerial circuit ; it fol- casting when high -powered lows, therefore, that on both stations were quite un- wavebands a 20 per cent. known. In spite of all that coupling is obtained. Con- has been said to the con- nection No. 3 is tapped into trary, it is still possible for Ai _i 117 L2 at a point which gives 6o a man living within half a 1 per cent. coupling on long dozen miles of a Regional /F l/,/t' wavelengths, this particular station such as Brookmans tapping being only intended Park to be able completely to separate the two pro- for use by readers in certain localities, as will be dis- grammes and listen to either without interference cussed later. from the other. At considerably less than double that Constructional Details. distance from Brookmans Park he can count upon elimin- ating both London transmitters, and adding not only the We can now turn to constructional details. The cabinet Midland Regional station but also a few foreign stations is supplied with the whole of the panel-which is of wood to his bag without calling in the aid of an H.F. stage and is in one piece with the loud- speaker fret-screwed or the complication of a separately tuned aerial circuit. to its front. Our first task is to remove this so that we Before giving the actual constructional details we will may fasten on to it the shelf which is provided with the briefly consider the circuit used. In the first place the cabinet. The shelf is already fitted with two side pieces, variable condenser in series with the aerial circuit is as shown in the photograph. It is necessary, -however, solely for the purpose of regulating selectivity. This can for us to cut into it a slot 6in. long and tin. deep, as be connected up to the top of the coil or to a tapping shown in Fig. 2. This slot is merely to avoid fouling on it. The question of which connection to use depends the rim of the loud speaker chassis, as the diameter of solely upon the electrical characteristics of the aerial and the latter is just too large to permit of it being placed upon the locality in which the receiver is to be used. completely under the shelf. It will be noticed from the This optional connection is arranged for by means of a photograph that the loud speaker is not mounted cen- plug and two sockets. Speaking broadly, the connection trally, but as far to the right as it will go. This is done to the top of the coil is for use only with very short in order to enable a no-volt battery to be used in place indoor aerials. Now, it will be seen that the combined of the 99- or 108 -volt type usually employed in trans- wave change on- and -off switch is arranged so that it will portable receivers. short-circuit the long -wave portion of the tuning coil The fact that the loud speaker is not mounted centrally when medium -wave reception is desired. It will be does not impair quality in the slightest degree. The observed, however, that when this is done the aerial is shelf is fixed at a distance of 4 in. from the top of the still tapped on to No. 4 connection on the long -wave coil. front panel by means of wood screws passing through This connection is tapped at a point on Ll at which one- from the front of the panel into its two side pieces ;

THIS receiver is completely self-contained, and since all components, including the loud speaker, are mounted on a removable chassis, the constructional work is very simple. It is suitable for use with either an indoor or outdoor aerial. The H.T. battery may be replaced if desired by .a portable type H.T. eliminator and trickle charger, for which there is ample space in the cabinet. By comparatively slight alterations the receiver may be converted to all-mains operation using indirectly heated valves and for use as a gramophone amplifier only small modifications are required.

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www.americanradiohistory.com FEBRUARY 25th, 1931. Mp@Ilg00 217 ME-'110 Everyman Two. - graph, are screwed on to the bottom of the cabinet in additional support is given by the fact that the bottom order to help to steady the H.T. and L.T. batteries, of both slow- motion dials is on a level with the shelf which are held quite securely when the back of the and, therefore, the two wood screws which are necessary cabinet is put on. The two 9 -volt grid batteries are to fix the dials in position pass right through the front mounted by screwing their lids on to the base of the p*el into the edge of the shelf. The whole arrange- cabinet. The batteries are then stood in these lids, ment is quite firm and strong. All components can now this method of mounting being specially provided for be mounted in accordance with Fig. 2. by the particular make of grid battery used. If the Care should be taken when mounting the tuning coil valves which are specified are actually employed, the to see that it is placed with its numbered terminals in second bias battery will not be required. It was, how- the position shown in Fig. 3. The actual numbers of ever, put in to permit the use, if desired, of other out- the terminals will be found on the ebonite moulding put valves which require a greater value of bias than underneath the coil. The fact that they cannot be seen can be supplied by one battery. If using other valves, once the coil is screwed in position is no disadvantage, however, remember that the total plate current con- if care is taken to position the coil correctly before sumption should not exceed 12 mA., which is the maxi- screwing it down. Before finally screwing the coil down mum economical discharge rate of the H.T. battery it is necessary to identify the three terminals A, B and C, employed. shown in the practical wiring diagram, because these are on Long Waves. ,t marked by the makers of the coil. We do not want Avoiding Interference ± .,make any connection to C, which does not go to a Before finally putting the chassis into the cabinet it , ,parate connection on the coil as is the case of A and is necessary to make up our minds whether we are going B, but is merely a repetition of one of the terminals at to connect to No. 3 tapping or No. 4 ; or, in other words, the base, it being provided to facilitate wiring when the whether we are going to use 6o per cent. or 20 per coil is used in certain circuits other than the one under cent. coupling on long waves. The locality in which the discussion. This terminal may, therefore, be ruled cart. set is used will definitely decide the point once and for It can be readily identified by the fact that the wire all, and so no trouble has been taken to make this con - which passes down the middle of the coil former to nedtion readily accessible after the chassis is once put connect.to one of the base terminals, is covered by yellow into the cabinet. The. 6o per cent. coupling connection sleeving, it being the only one that is so covered. With regard to the remain- ing two terminals A and B, which, of course, go to the switch as indicated in Fig. I and in Fig. 3, it is pointed out that it is im- material which terminal goes to which switch con-. tact. The combined de- coupling and voltage dropping resistance, R2, actually consists of a ` spaghetti " resistance which connects from the H.T. terminal on the inter - valve transformer to the positive terminal on the loud speaker unit. The aerial and earth connections are taken to two sockets mounted on a piece of ebonite tin. x Iii. x Kin., which is screwed on to the edge of the horizontal shelf of the Fig. 1. -The theoretical circuit diagram. Values are as follows : Ci, 0.0003 mfd. C,, 0.0005 mfd. ; chassis, as shown in Fig. C,3, 0.0003 mfd. differential ; C1, 0.0003 mfd. ; G, 2 mfd.; R1, 2 megohms ; R_, 30,000 ohms. 2. This can be clearly seen in the photograph. These sockets come opposite at No. 3 will be of little use, for instance, to people to the two holes which are already drilled in the back of dwelling on the South Coast, as tuning is so broad that the cabinet. The connections marked K and H also 5XX and Radio Paris will be heard simultaneously. consist of sockets into which a plug is inserted. This The chassis can now be put into the cabinet. It is held is easily understood by reference to Figs. r and 3. in position by the screws which attach the wooden Small battens, which are clearly shown in the photo- handles to the sides of the cabinet. These wood screws

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www.americanradiohistory.com 218 1 p@llQog FEBRUARY 25th, 193r. MpIld. Everyman Two. - switch to SW (see Fig. r). If the aerial is an outdoor pass completely through the side of the cabinet into the or a large indoor one, insert the plug which comes from two upright members of the shelf. No other support is C, into the socket K (see Fig. 3) ; C, should be set at really necessary, although, if desired, the fixing screws maximum, and C3 should be set at about 5 or 6 degrees which will be found attaching the front panel to the on the scale, C2 being rotated until a station is tuned in. More probably than not two stations will be heard simultaneously, a n d re- course must be had to C, and its capacity lessened by turning the knob in an anti -clockwise direction. It will be convenient when re- ferring to the pointer on the control knob of this con- denser to consider it as being the hour hand of a clock, the maximum posi- tion being at 3 o'clock and the minimum at 9 o'clock. If interference is bad, put the pointer at 12 o'clock and retune with C2, not forget- ting to use the reaction control to assist in obtain- ing selectivity. The move- ment of C, will naturally affect the setting of C2 somewhat, but C, must in no sense be considered as a tuning control, and, in- deed, it would be impos- sible to employ it as such because hand - capacity effects are naturally preva- lent when it Fig. 2. -The layout of the panel and baseboard. Drilling sizes are as follows s A, din. dia. ; B, using towards 5/16in. dia. its minimum position. If it is not wrongfully used as remainder of the cabinet when it is purchased may be a tuning control these are quite immaterial. restored to their place. The batteries are, of course, If selectivity is insufficient at the 12 o'clock setting inserted after the chassis is put in. move it to Io o'clock ; and in the case of a good outdoor If desired, a simple H.T. eliminator, either D.C. or aerial move it to absolute minimum, since under such A.C., may be inserted in place of the H.T. battery. There conditions the capacity of the condenser is not zero, is, in fact, ample space for one of the combined H.T. and there is quite a useful degree of coupling remain- eliminator and trickle chargers which are now upon the ing. If using a small indoor aerial the plug associated market. For those who desire it, the conversion of this with C, should be inserted into the socket H (see Fig. 3), set to all mains working, using indirectly heated valves, and under such conditions it will be found that signal is not a very difficult matter. strength is very much greater. It may be found, how- With regard to tuning it is necessary to conduct our ever, that C, has to be set almost to its minimum posi- first experiments with the back of the cabinet off. First tion, and that C, has to be adjusted down to about connect up aerial and earth and put the wave-change 8 degrees on the scale to receive a station at the bottom

e LIST OF PARTS. 1 Variable condenser, 0.0005 mid. (Polar, No. 3) 4 Sockets, insulated with bush heads (C(lix No. 24) 1 Variable condenser, 0.0003 mid. (Ready Radio, Brookmans 2 Hook terminals (Cli x, No. 4) 2 Vernier dials, brown (Formo 6 Wander plugs, H.T.+, H.T. -, 2 G.B. +, 2 G.B.- (Belling -Lee, "Midget ") 1 Differential reaction condenser, 0.0003 mid. (J.B. 1 Piece of ebonite, 2 x 1 x ?rin. 1 Fixed condenser, 0.0003 mid., and grid leak dips (Sovereign 1 H.T. battery, 120 -volt (Pertrix) i Fixed condenser, 2 mfd (Dubilier 2 Grid bias batteries, 9-s'olt (Pertrix) i Tuning coil unit, base mounting, tapped 20% and 00% (l'nnewell, Dual X 1 L.T. accwnulator, 2-volt (Exide, D.F.G.) 1 H.F. choke (Ready Radio, " Hilo " 1 Loud speaker unit (Ormond, R.450) 2 Valve holders (W.B., " Universal ") 1 Small chassis and cone (Ormond, R.451). 1 "Spaghetti" resistance, 30,000 ohms (Magnum) 1 Cabinet (Cameo) 1 Grid leak, 2 megohms (Sovereign) 2 Valves (Mullard PM1 HL and PM2A) 1 Switch, three -pole, double -throw (Eureka 1 Pkt. insulated connecting links (Ready Radio, "Jiffilinx") 1 L.F. transformer (Ferranti, A.F.8) Wood, screws, flex, etc. 3 Solid plugs (Clix, No. 25)

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www.americanradiohistory.com FEBRUARY 25th, 193r. Witp@IlM30 219 WarIld Everyman Two. - would frankly point out that on long waves 5XX will of the normal broadcasting band. At first this might be the only satisfactory transmission except for those be thought to indicate that there are too many turns dwelling in localities which are fortunately situated for on the coil. the reception of long-wave Continental stations. On It must not be forgotten, however, that'the coil is not the medium band, stations between the two London designed for use with a connection at its high- potential transmitters will not be receivable. In fact, at halt a end, and this feature has only been put in by the writer because he has found it very convenient under certain conditions, and thinks that others may find the same thing. It will not take long to de- cide which socket to put the plug in, and the back of the set can then be locked up. With a little practice with the delicate adjustment of reaction necessary to get the ut- most selectivity, it is sur- prising what a lot can be done with a set of this kind, although one must remember that results can- not possibly come up to those obtainable with a re- ceiver having a stage of high frequency or a band - pass tuner. This receiver will be found to give much better results on the medium band, from the point of view of selec- tivity, and the writer

Fig. 3. -The practical wiring plan.

dozen miles from Brookmans Park there will be no silent space on the tuning dial between the two transmitters, and both will be heard simultaneously. This does not matter, however, but what does matter is that when the set is fully tuned to the London Regional there should be no sound of the London National. In the writer's opinion the leading feature of this re- ceiver is the excellent quality' it gives on the two Lon - dons and the two Daventrys. This is largely accounted for by the good qualities of the intervalve transformer, the loud speaker and the valves. There appears to be negligible box resonance but obviously anybody can test this for himself by trying the set with the back re- moved, and if in any particular case better results are obtained, a large circular hole may be bored through the back of the cabinet, although in his own case the writer has not found this necessary.

The receiver viewed from the back. Ample space is provided for receiver is available for inspection at the offices of "The the housing of the batteries or a portable eliminator and trickle This charger. TPireless World," 116, Fleet Street, London, E.C.4.

13 51 '

www.americanradiohistory.com

220 Wfiriamo FEBRUARY 25th, r93r. MTN,

H'JÌdiliihnil.

41-1111_1b. ¡¡'N I7ENC E n, CORR ; ' ..:.

The Editor does not hold himself responsible for the opinions of his correspondents. Correspondence should be addressed to the Editor, " The Wireless World," Dorset House, Tudor Street, E.C.4, and must be accompanied by the writer's name and address.

RADIO SOCIETIES AND SERVICE. I feel that a great service could be done to radio if legislation could be passed to compel the users of interfering apparatus to Sir, -I was very much interested in the letter of Mr. J. provide the Baggs in the February 11th issue of The World. necessary filter device; but nothing will be done Wireless while sufferers continue to Personally, I think that the reason for the falling off in put up with the present state of membership affairs as being inevitable. of the various radio societies is due to the intro- should duction by the radio dealers of the " Kit set." The 20 wires It be urged upon everyone whose entertainment is spoiled in any way by this form of nuisance to report its existence and no solder idea has taken the interest away, and until the to set goes wrong the " Kit set " owner requires no knowledge the B.B.C., marking the letter " Interference." The Post all. be if manufac- Office officials who deal with these complaints are anxious to at More interest would stimulated the help in turers gave technical details of their products, i.e., response the matter -I have found them most courteous and curves for speakers, pick -ups, transformers, the grid swing obliging -and the more evidence they can accumulate the sooner will there be a hope of the necessary legislation being power valves would handle, etc. An increase in membership passed. of radio societies would also have a big effect on the radio F. E. GODFREY. trade. ROBERT REID JONES. Hampstead, N.W.3. Lives-pool. HUM IN A.C. SETS. Sir, -The suggestion put forward by Mr. Beggs in your cor- Sir, -There is one cause of modulation hum which does not respondence columns is, in my opinion, an excellent one. appear to be mentioned in the article on " Tracing Hum in As a long- standing member of one of the oldest radio societies Mains Sets." The leads con- in the country, it has always appeared surprising to me that necting the set to the mains may local dealers and their staffs do not take advantage of the excel- be in such a position as to cause lent opportunities for the acquiring or furthering of technical induction in the aerial lead -in; knowledge, and the interchange of experiences and opinions relat- this will cause trouble if there ing to current developments afforded by membership of a radio is no path of low audio -fre- society. quency impedance between To give a signal illustration, it is the practice of my society, aerial and earth. A case in before proceeding with the lecture of the evening, for the point is the filter circuit with members to be asked " Any questions ? " Someone present will, the aerial connected to a tapping in most cases, be able to answer the troubled ones. Knotty or on the coil and the coupling debatable matters that cannot be dealt with at once are held condenser in the coil leads; the over to be dealt with at an early date, when a whole evening is remedy, of course, is to use a devoted to fully threshing out the matter. separate aerial coupling coil. I hold that by following these " questions and answers " D. A. BELL. dealers whose pride it is to advertise service can in course of Magdalen College, time glean more practical information of actual difficulties ex- Oxford. perienced with every type of apparatus than they can hope to acquire by attending the short, intensive " service courses " THE STENODE. arranged by set manufacturers. Other advantages must be Sir,- Unlike Mr. Yates -Fish, I lack the boldness to criticise apparent to the dealer who is prepared to give the matter a or comment on Dr. Robinson's paper until I have had an oppor- few minutes' consideration. tunity of reading it. Though I am thus prevented from entering Mr. Baggs is right when he says the membership of the into technical arguments, I can give the results of practical societies has dropped to the " really hard- boiled enthusiasts." experiences with a Stenode Radiostat receiving set. Some weeks All the people who wanted to know how to wind a variometer ago I had a seven -valve experimental model made up, and this have been shown -and who wants to, anyway, in these days of has been in constant use ever since. cheap components and blue prints ? Those that remain repre- At a range of 15 miles from Brookmans Park this set will sent the " intelligentsia " of the radio fraternity, many of whom, bring in either Stuttgart or Graz quite free from interference although outside the industry, possess a fund of knowledge that from the London Regional ; it is equally successful with the would surprise (and embarrass) many " technical engineers." " next -door neighbours " of the London National. On both The dealer cannot fail to benefit by their acquaintance. medium- and long -wave bands it enables one to obtain excellent, The radio societies are not supported by the technical Press reception from stations which, with any other set that I have with one notable exception, and for this reason my society is tried (and their number is legion !), are hopelessly jammed or distributing a leaflet in the hope of reaching enthusiasts heterodyned. who may not have heard of its activities. As regards Mr. Yates- Fish's " analysis of the behaviour of South Hackney, E.9. G. W. HEATH, the set," may I say that the Stenode eliminates both of the Assistant Hon. Secretary, audible effects of interference that he mentions. Both the inter- Hackney Radio and Physical Society. ference due to audio- frequency modulation and that due to heterodyne tones are definitely cut out. AVOIDABLE INTERFERENCE. With all its selectivity the Stenode provides reproduction. Sir -Your Editorial Comments in the February 11th issue of of the very highest quality, the moving -coil loud speaker used The Wireless World raise an issue of the utmost importance to with it bringing out the lowest notes of the double -bass or the the whole wireless universe -listener, dealer and manufacturer. highest of the violin E- string equally well. Unfortunately, there is no statutory power by which interfer- Why does Mr. Yates -Fish suggest that the Stenode may provo ence, however bad, may be dealt with in the Courts. to be a useful alternative to the band -pass filter ? Surely by As a manufacturer of specialised apparatus, I have investi- simplifying receiving circuits and providing real selectivity in gated scores of complaints of interference caused by electrical combination with the highest quality in reproduction, it is a machinery, and quite agree with your statement that the only step far beyond the band -pass filter, and offers an entirely new satisfactory way of dealing with it is to tackle it at the source. and much -improved method of designing wireless receiving sets. There it is the most easily and cheaply eradicated. Berkhamsted. R. W. HALLOWS. ' B 52

www.americanradiohistory.com FEBRUARY 25th, 193r. Mpd@oorA 2zt

Replies to Readers' Questions Technical enquiries addressed to our Information Department are used as of the basis of the replies which we publish in these pages, a selection being made General Interest. from amongst those questions which are of general interest. Why Coils are "Potted." "Local- distance" Switch. Will any improvement be effected by Some time ago you published a circuit mounting the turning coils of my re- diagram showing holy aerial coupling ceiver (of which I am sending you a could be reduced, by operation of a circuit diagram) under screening throw -over switch, to a value giving covers, as used in the construction of adequate signal strength for local - most modern receivers? station reception without introduc- Your receiver apparently includes .t ing any change in tuning. I should simple vertical screen, and, as its H.F. like to fit a device of this kind to my amplifier is presumably stable, extra Interaction. own set, if possible, without making screening will not confer any benefit. I have just noticed that my 2 -v -1 any internal alterations. The re- The point about receiver screening is that it en- can be made to oscillate when the reiver includes a double -wound aerial - ables one to obtain much more H.F. loud speaker is brought into proximity grid transformer with medium - and amplification -with stability -than woiild with the aerial long -wave windings in series; there otherwise lead -in wire. Must be attainable. this be taken as an indication that i8 also a variable aerial series con- 0000 there is something wrong with the denser. Tuning condensers are Easy Pick -up Adaption. receiver? ganged; hence my desire to avoid Due to the fact that a very small con- This would suggest that the H.F. filter- change of tuning. denser - 0.0001 mfd. -is used in con - ing arrangements in the anode circuit Can you suggest a suitable scheme? junctiòn with a power grid detector, of your detector are not entirely satisfac- The arrangement shown in Fig. 2 will would it be permissible to connect a tory, but it must be admitted that it is be found to work satisfactorily. The pick -up directly across the grid cir- virtually impossible to keep all traces of extra series condenser, which should have cuit in the manner shown in the H.F. energy out of the L.F. amplifier, a rri ximmn capacity of about 0.0001 mfd. accompanying diagram? particularly when dealing with " 2 -H.F." A consideration of your diagram (re- sets. We do not think that you need be EXTRA SERIES CONDENSER produced in Fig. 1) shows that the pick- concerned, provided that there is no trace up is in effect shunted by the grid con- of instability when the aerial and loud denser and leak. As the reactance of speaker leads are in their normal posi- AERIAL the condenser over the whole range cf tions. "SHORT TERMINAL 0000 DISTANCE Heterodyne Wavemeter. When a heterodyne wavemeter is operated by the absorption method, a " flicker" LONG is produced in the anode milliamineter DISTANCE" as the external circuit is tuned 1 through resonance. I have observed BALANCING CONDENSER that this change of current is some- times in an upward and sontetimr.s in a downward position, depending on the wavelength being dealt with. As energy is being taken out of the oscillating circuit, should not the de tiection always be in a downward EARTH RECEIVER direction? TERMINAL - - - Everything depends on the extent of coupling between the grid and plate cir- Fig. Simple, 1.- but generally effective, cuits of the oscillating valve in the wave - method of connecting a pick -up to a power Fig. 2. - Changing sensitivity without grid detector. meter. As the effective coupling changes affecting tuning ; a useful arrangement considerably from one end of the tuning when local signals are overpoweringly audible frequencies is high, it will not scale to the other, it is quite usual that strong. impair the output characteristics of the the effect you have noticed should be and a low minimum, is placed in circuit average pick -up. Similarly, the grid produced. 0000 when the switch is thrown to the " short - leak, if of about 0.25 megohm, as is usual, distance " position ; at the same time will have no appreciable0000 effect. Dry Battery Limitations. compensation for the removal of a part of In your opinion, what is the greatest cur- the aerial capacity is made by intro- Inductive Filters. rent that can be taken with economy ducing the balancing condenser, which Do you consider that the input band -pass from H.T. dry batteries of the may be of the semi -variable the variety, with filter of " D.C. Band -Pass Fite " super -capacity" type? a maximum capacity of 0.0003 mfd. would be suitable for inclusion in a We can hardly be dogmatic on this Having set the extra series condenser simple detector -L.F. set without any matter, and in any case everything de- to give the desired reduction of input, H.F. amplification? pends o0 one's definition of " economy." the balancing capacity should be ad. This system of coupling will be quite The maintenance of a dry battery becomes justed (without touching any other con- satisfactory, except that it is particu- somewhat expensive if current consump- trol) so that signal strength is at maxi- larly susceptible to the disturbing effect tion is allowed to exceed some 15 milli- mum, thus indicating that the input of reaction. amperes. circuit is again accurately tuned. B 53

www.americanradiohistory.com 222 FEBRUARY 25th, 1931.

Economy in Anode Current. Detector grid operating conditions are A8 I have to depend on dry batteries for important, and it is often found that an H.T. supply, it is necessary for me to appreciable reduction in positive grid FOREIGN BROADCAST GUIDE. exercise care in Vie choice of valves. voltage can be made without noticeably 1 f 1 replace my neutralised triode impairing rectification efficiency, but with 114'. valve by a screen -grid valve, is a decided improvement from the point of any appreciable increase in current view of smoothness of reaction control. (INA) 0000 NAPLES consumption to be anticipated? (Italy). A three -electrode valve, of the type Simplified Coil Switching. normally used for amplification, will con- I ant engaged in planning a " Geographical position : 40' 49' 58" N.; 25" sume not far short of 2 milliamperes. A A.C. mains set, intended to give a 14° 16' E. suitably chosen S.G. valve of high im- particularly good performance on the Approximate air line from London : 1,003 pedance will pass about the same current long waves, and cannot quite see horn miles. in its anode circuit, and, in addition, the desired results are to be achieved Wavelength :331.4m. Frequency : 905 kc. from 0.5 to 0.75 milliamp. will be re- without undue complication of the Power : 1.7 kW. quired for the screening grid. It will wave -range switching. Time : Central European (one hour in thus be seen that the extra consumption Ganged tuning is to be employed, advance of G.M.T.). is quite negligible, particularly when the and my aim is to ensure that the 'pro- current drawn by the output valve is portion of the valve capacities trans- Standard Daily Transmissions. taken into consideration. ferred to the tuned circuits shall be (The majority of the programmes are S.B. 0000 practically identical on both wave- with Rome.) bands. 7'he position is complicated Woman announcer. Opening Call : E /AR Reaction Adjustments. by the fact that it will be necessary (phon.: Eh-gar) Signori Buon giorno-Radio There is a good deal in connec- of " overlap" the to " tap down" the anode Napoli : if with Rome, Radio Roma - reaction condenser control of my tion on the tuned grid coils, which Napoli (phon.: Nar- polly). 11.F.-det. -L.F. set. Will you please are to be used as couplings. enumerate briefly the alterations Any suggestions, and, if possible, Opening signal: melody played in several keys (Pipes of Pan). that can be made in an attempt to a circuit diagram would be appre- improve matters in this respect! I ciated. Quick tempo have tried various values of detector We suggest that you should work on anode voltage. the lines indicated in Fig. 3. Medium - We advise you to try the effect of re- and long-wave coils are joined in series, moving a number of turns from the re- and short -circuiting switches are Con- action coil, and also of moving the nected in the usual way. A single -pole physical position of this winding in 'rla- switch is arranged to transfer the anode

H.T.4- TO L.F Closes down with the words: Fine della trasmissione ; Buona none a tutti, followed by the Fascist hymn, " Giovinezza." and the National Anthem, " Marcia Reale."

changing over a second connection. We have not completed the aerial input cir- cuit, as you do not say if you intend to use a filter or two -circuit tuner. In any case, this part of the receiver should pre- sent no special difficulties.0000 Litz Wire. 1 have some Litz wire, comprising thirty strands of No. 42 wire, and, if you approve, would like to use it for wind- ing tuning coils for the " hand Pass Three" receiver. H.F. This cannot be real Litz wire; of which the total number of strands must always be to the power of 3, i.e., 3x3 =9; 3 x 3 x 3=27, etc. In the circumstances we can hardly advise you to use it ; even though it were effective, a complete re- design of the coils would be necessary, as the diameter will differ considerably specified. G.B.- GB.- from that of the 0000wire Better Valves for a Portable. Is there any risk of producing instability by replacing the existing H.F. valves itt a portable (two aperiodic Fig. 3.- Maintaining a balance on both medium and long waves; how excessively stages) by fitting modern valves complicated switching may be avoided. of appreciably the same impedance but with greatly improved mutual tion to the tuned coil. An H.F. choke connection to appropriate points on the conductance? of different characteristics from that at first coupling coil L,. With regard to We have not had an opportunity of present used might be beneficial, and it the coupling between the second H.F. testing this set (which is now obsolete) would also be worth while to alter the valve and the detector, matters will be with modern valves. By fitting better anode circuit load by joining a small greatly simplified by joining both plate valves than those provided for in the fixed condenser between detector anode and grid to the same tapping points, as original design, uncontrollable self - and earth. shown, and thus avoiding the need for. oscillation may certainly be provoked.

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