Wireless-World-1956
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ANITA RI" 1956 TWO S1111,I.I Wireless World evmon *:::: 1'/;'-- --\\" x\ \\ //it 1 1 1,L.,*"'--.: I 1I Pr; '11,1.1111111 1 1 ifilifi( 1 1g i I j i (1 t11.4.\-.'''''''liiiij1)r ( \ \\ \s:ss..**::://I1/If 1 -..:: 1\\ \\ \ \ --**://'//// 4/ .,,//do, .... , "ilk ...., .., ........ .....- 7 .. ........ --:::0.,/ ........., ............ --.......0"0.,/"" V( .....,,................. ..... ,.....4. Arr ... ,,,,,.....- ..-..., ....-...-......, ..... / ' - "-----........./ Ni.' (-------------.....-----..... ..... ,...._ -,_._ ......- PIT ACATION 11 WIRELESS WORLD JANUARY. 1956 STANDARD E D I SWAN METAL -GLASS SEALS 1-0-t 0-005 3 4 Ediswan, who have specialized for many years in the production of hermetic seals, offer a wide range of Standard Metal -Glass Seals of first-class quality. Two main types-multiple and single-are available. Both are equally efficient, the type selected depending on the particular application. These seals are easy tofit(usually soft soldering).They have high insulation resistance and provide vacuum -tight joints 45 which will withstand all normal variations in pressure and temperature. Experience has proved that these standard seals meet the majority of requirements, but our Engineers will be pleased to discuss applications outside the scope of this range. I N FORMATIVE CATALOGUE AVAILABLE ON REQUEST THE EDISON SWAN ELECTRIC Co. Ltd., 155 Charing Cross Road, London, W.C.2 and Branches MG.to6 Telephone: Gerrard 8660 Member of the A.E.I. Group of Companies Telegrams: Ediswan, Westcent, London 1 110. iss97 TiliniliaazMTN ELECTRONICS, RADIO, TELEVISION Managing Editor: HUGH S. POCOCK, M.I.E.E. Editor: H. F. SMITH JANUARY 1956 Assistant Editor:F. L. DEVEREUX, B.SC. Editorial Comment 1 Further Notes on the Sensitive In This Issue Three -Valve T.R.F. Receiver H. E. Styles 2 East-West Hemisphere V.H.F. Link 3 World of Wireless 4 Loudspeaker Enclosure Design -1 ..E. J. Jordan 8 Ninth R.S.G.B. Exhibition 15 Books Received .. 17 Future of European Broadcasting G. H. Russell 18 Automatic Circuit Production .. 23 Tetrodes with Screen Feedback 24 Television Waveform.. 26 Letters to the Editor 27 UL Output Transformers D. M. Leakey and R. B. Gilson 29 Technical Man -Power 32 Television in Switzerland 33 Ionospheric Scattering at V.H.F. J. A. Saxton 36 VOLUME 62NO. 1 Manufacturers' Products 40 PRICE : TWO SHILLINGS Telegee .. D. A. Levell 41 The Nyquist Diagram at Work " Cathode Ray" 43 FORTY-FIFTH YEAR Short -Wave Conditions 48 OF PUBLICATION January Meetings 49 Random Radiations " Diallist" 50 Unbiased .. " Free Grid" 52 PUBLISHED MONTHLY (4th Tuesday of preceding month) by ILIFFE & SONS LTD., Dorset House, Stamford Street, London, S.E.1 Telephone: Waterloo 3333 (60 lines). Telegrams: " Ethaworld, Sedist, London."Annual Subscription: Home and Overseas. 21 12s. 6d. U.S.A. and Canada $5.00.BRANCH OFFICES: BIRMINGHAM: King Edward House, New Street, 2. Telephone: Midland 7191. COVENTRY: 8-10. Corporation Street.Telephone: Coventry 5210. GLASGOW: 26n. Renfield Street, C.2.Telephone: Central 1265. MANCHESTER: 260, Deansgate,8. Telephone:Blackfriars 4412. OVERSEAS OFFICES: U.S.A.:111, Broadway, New York,6, N.Y Telephone: Digby 9-1197. CANADA74,CollegeStreet,Toronto, 2,Ontario. Telephone: Walnut 45631. b A98 WIRELESS WORLD JANUARY, 1956 VALVES., TUBES E 37. "DISTRIBUTED LOADING" FOR TWO MULLARD EL84's IN PUSH-PULL OPERATING CONDITIONS A pentode push-pull output stage is conveniently operated with 'distributed load- VALVE ing' by connecting the two screen grids to tappings on the primary of the output Each screen grid tapped into anode load transformer. The screen grid load is common with part of the anode load. Instead at 43% of turns from centre tap (h.t.)-- of being bypassed at a.f., as they are for normal pentode push-pull, the screen grids V,, 300V are fed With a voltage which varies during the a.c. cycle. In effect feedback is ap- Vg2 300V plied in the output stage itself, and the operation of the stage, in principle, is some- Ik(o) 2 x 40mA where between that of a triode and a pentode. Connecting the output pentodes as Ik(max. sig.) 2 x 45mA triodes is equivalent to moving the screen grid taps to the anode ends of the Rk (per valve) 27012 primary. For the normal pentode connection, the screen grids would effectively Vie (gi-gi) r.m.s. 18V be connected to the centre -tap. Power output is inevitably slightly less than can R a -a 8k1) be obtained with conventional pentode operation, but distortion within the power Pout 11W range of the distributed load stage is very much lower than at the same levels in Dtot 0.7% the normal push-pull pentode stage. In practice the distributed load stage results in PERFORMANCE OF 45-10' CIRCUIT a good compromise between the low distortion of a triode and the high output of a A.Conventional pentode push-pull out- pentode, whilst retaining the high sensitivity of the pentode stage. Because of the vol- put stage tage feedback via the screen grids, the output impedance of the stage is considerably B.Distributed load output stage less than with normal pentode operation, being about 80000 in this arrangement. A A distributed load output stage for two Mullard EL34's has already been described Rated power output lOW lOW by W. A. Ferguson in his article Overload point =14W-t-, 11W R18 in the May and June, 1955 issues of Sensitivityacross AAAr---0 4r- "Wireless World". A similar type of 5.6 kfl volume control 40mV 40mV C'O output stage can also be used for two Harmonicdistor- Mullard EL84's in push-pull with tion (10W, 400c/s) 0.3% +0.1% an output transformer having the Intermodulation appropriate anode -to -anode loading distortion (at 10W, and screen grid taps. The operating for40ctsand conditions are given in the table, 10kc/s in 4:1 ampli- the cathode currentIkbeing the tude ratio) 2.0% +1.0% sum of the anode and screen grid 47!). 'Beat -note' distor- currents. tion at lOW for: Thecircuitdiagram showsthe (i)9kc/sa n d R20 output stage of the Mullard '5-10' 10kc/swith amplifier adapted for distributed equalampli- loading. The screen grids are taken tudes +0.25%0.25% to the taps on the primary of the (ii) 1 4kc/s and output transformer via the existing 15kc/swith stopper resistors R19 and R20 of equalampli- 122751 4751.The centre -tap is fed from tudes 0.4%0.33% the reservoir capacitor Cl. The Loop gain at dropper resistor R18 in the h.t. line must be increased from 1.2k12 to 5.6k52 1000c/s 26dB20.5dB to maintain the same d.c. conditions in the first two stages, as it no longer carries the screen grid current. The anode -to -anode loading should be 8k52, corresponding to the normal loading published for the original circuit.Best results are obtained with each half of the output transformer primary tapped at about 43% of its number of turns, counting from the centre -tap. Suitable output transformers are the Parmeko P2642 and the Partridge P4014. In the feedback loop C12 will normally be 100pF for a 15(1 loudspeaker or 220pF for 3.750. The lower part of the table gives a comparison between the performance of the '5-10' circuit with the original pentode push-pull (A) and distributed load operation (B). The measurements were made on circuits modified according to the information given in the "High Quality Sound Reproduction" booklet. Distortion is very much reduced in the distributed load circuit whilst retaining the original design rating of 10 watts. The maximum power output of 11 watts at the overload point (onset of clipping with sinewave input) is somewhat less than for the original circuit. However, the rate at which distortion increases beyond the 11 -watt point (that is, the slope of the Pout/Dtot curve) is very much less than for the basic circuit driven beyond 14 watts. There is virtually no change in the frequency response. Overall stability is considerably better than in the basic design, partly because the lower distortion is obtained with reduced loop gain. Reprints of this series of advertisements with additional notes can be obtained free from MULLARD LTD., Technical Service Department, Century House, Shaftesbury Avenue, London, W.C.2 MVM 341 JAN U AR Y 1956 Wireless World Vol. 62 No. 1 New Style Electronics Exhibition IN last month's issue we deplored the prolifera- Sound Recording Association's exhibition comes to tion of exhibitions catering for radio and all its mind as one that has filled a need for many years. electronic offshoots and pleaded for a new kind ofHowever, the position here has been complicated show which, for want of a better word, we des- by a proposal that has just been made to organize cribed as professional.This might embrace vir- an Audio Fair in London next April.This is tually everything within the electronic field except planned to be wider in scope and to appeal to a domestic broadcast receivers and their ancillaries, larger public than the B.S.R.A. show, but some which are already well catered for each year at overlapping seems inevitable.Anyway, this inci- Earls Court. dent serves to point the moral that the organization Why suggest still another exhibition, when there of shows should be carefully considered and freely are already so many? The fact is that some of the discussed. smaller private and semi -private shows have out- grown themselves and in the process have changed their character. A case in point is that of the,Physi-Mobile Radio Economy cal Society's annual exhibition, at which commerci- ally made electronic equipment for measurementTHE announcement that a " communal " mobile and research has been predominant for many years.