elerence /jnnti

1948

ED. r PHONE 4 "3 - Ei 0 X 42. 1.

RADIO -CRAFT 25 'West Broadway New York 7, N. Y. qet 411to

Here are a total of 640 pages of downright practical knowledge to help you keep punching away at the unknown factors of radio until you have mastered them.Every one of the 10 books is worth many times 50e if you will but apply a small portion of the information gained to your daily work.You'll be glad you have the complete set handy whenever you are trying to work out a problem in construction, testing or servicing.You'll like the easy -to -read type-the smart, modern flexible covers, and the generous supply of illustrations. They all help to make learning a pleasure as well as a profit.Order as many as you need today.Better still, order all 10.

$5.00 FOR ALL 10-50c EACH No. 29-HANDY KINKS and SHORT CUTS. No. 34-RADIO-ELECTRONIC CIRCUITS. An A carefully edited compilation of time savers. extensivecollectionofcircuitdiagrams,with Kinks on Antennas-Power Supplies-Test Equip- brief, to -the -point descriptions of each, includ- ment-Servicing-In the Shop-Phonographs and ing: Intercommunications Systems - Power Sup- -Coils and -Miscellane- plies-Balancing Circuits-V. T.Voltmeters- See your ous Subjects. Amplifiers-Receivers - Phono Amplifiers-Short Wave Adaptors-Electronic Relays. No. 30 PATENTED CIRCUITS, 1944-46. A gold mine in new hook-ups! A selec- No. 35-AMATEUR RADIO BUILDER'S GUIDE. tion of unusual circuits, many aresult of war- For the "ham" who builds his own.Practical, radio dealer. time research-of great value to anyone inter- down-to-earth,it tells how to Build: A 2 Meter estedinelectronics. Control Circuits-Detec- Band Acorn Tube Set-Miniature Communica- torsand Amplifiers-PowerSupplies-Miscel- tions Receivers-LongLines Transmitter-Begin- laneous Circuits-Foreign Circuits. ner'sTransmitter - Preselector - Rotary Beam Ifhecan't Aerial-Power Supplies-and other ham gear. No. 31-RADIO QUESTIONS AND ANSWERS. Answerstomostfrequently askedquestions. No. 36-RADIO TEST INSTRUMENTS. A MUST Clear circuit diagrams.Amplifiers-Receivers- for every radio man.This latest book on build- Transmitters-MetersandTestEquipment-In- ingtest equipment places emphasis on practi- supply you terphones-Power Supplies-Phonograph Ampli- calconstructionof:Signal Tracers-Capacity fiers-P. A. Systems. Meters-Portable and Bench Multi-checker-Sig- nal Generators-Tube Checkers-Electronic Volt- No. 32-ADVANCED SERVICE TECHNIQUE. meters-and many others. An up-to-date collection of information for the use the advanced serviceman: Visual Alignment-Track- No.37 - ELEMENTARY RADIO SERVICING. ingProblems-Negative Feedback-Tone Con- For the radio man who knows little about ser- trolCircuits-AddingMicrophones toSets- vicing.How to get started, and even more im- MatchingLoudspeakers-Remote JukeBoxes- portant how to keep going: Planning the Ser- Servicing Record Changers-Hum Elimination- vice Shop-Equipping the Shop-Systematic Cir- coupon FM Servicing. cuit Checks-Signal Tracing Methods-Servicing the Midget-Soldering-Servicing Volume Con- No. 33- BUILDER'S GUIDE. In- trols-How to Service Speakers. valuable for the designer and builder of audio equipment.Tells how to builda wide variety No. 38-HOW TO BUILD RADIO RECEIVERS. on page 42 of amplifiers with power outputs ranging from This book contains a sufficient variety of sets to 8 to 30 watts,Detailed design information on: appealtoeveryradiofan. Includes:Short - Push -Pull - PhaseInversion - BassandTreble Wave,Broadcast,Portable,A.C.Operated, Boosters-Volume Expanders - Resistance Coup- Miniatureand StandardReceivers. Complete and mailto ling-Internal Feedback. coil -winding dataisfurnished. IltAl) I 4, RADIO -CRAFT 25 WEST BROADWAYCRAFTNEW YORK 7, N. Y.

handy uuusoul radio Mobs uuuuuu ed advanced amplifier radio amateur how tobuild questions radio elementary Cr circuits service builder's el ttttt nic radio builder's test radio radio guide circuits guide receivers short cuts to44rl94b answers technique instruments servicing

11010-C1111 11111 1111 11111 55111 1010.01/11 II01111 tI111 [1111 111 11011[1111 01111 11111 11 11010.t 0100111, 11010 1110 C. 11010.01111 1010 -[1111 50u 50 ,n; 50 <, 50 ARIi to" 50," Ill% 50 «r11 t0". 11033 50 N0.38 z t`: \ \ \N\ , N \\\\ k CONTENTS

A Stable V. F. 0. 2 An Electronic Timer 4 - 2 -Tube Portable 5 Important Uses of Transitron Oscillators 6 Construct an FM -AM Superregen'r 8 A 4 -Tube Reflex Superhet 9 New Direct -Coupled Amplifier 10 Direct -Reading V. T. V. M. 11 Class A and B Amplifier 12 Hi-Fi Tuner 13 Tone Control Circuits 14 Good FM Receiver 16 Loudspeaker Fidelity 18 Experimenters' Clip Set 20 Feedback Amplifier 23 New Circuits from Old 24 Supersonic Door Opener 26 Low Level Tone -Control 27 Power Stabilizer 27 Tracer Receiver 28 Check Your Frequency! 28 Identify That Station! 29 Useful Small Circuits 30 V. 0. M. Condenser Checker 33 Frequency Drift 35 Modern Metal Locator 36 FM and Television Antenna Chart 39 "Radio -Craft" Index (Vol. XVIII, Oct. 1946 -Sept. 1947) 46

Copyright 1948 by H. GERNSBACK 1948 A STABLE V. F. O. No Battery is Needed in This Instrument

THEbeatfrequencyoscillator, a calibration; strength of the outputcertain frequency, in this case about though not absolutely necessarymust be under control; the note at all200 kc.The signal from oscillator 2, in the radio service shop, is im-times must be fundamentally pure; andcomprising L2, VC2, and the sec- Inensely helpful in many tests. Even athe circuit must be designed to preventtion of V2, will combine with the signal battery instrument is helpful, but to"pull -in" of the 2 frequencies whichfrom V1 and any difference between overcomethedrawbacksofbatterycombine to produce the third 'or beattheir frequencies will appear at the power, the instrument described herenote, even when thedifferencefre-mixer plate. If the difference is small, was designed. quency is only a few cycles. say a few hundred cycles, then this If a b.f.o.isto serve its purpose Referring to Fig.1,the oscillatorsignal will come well into the audible properly as a test instrument, it mustcomprising V1, L1, VC1, and its asso-range and can be passed on to an audio deliver output over the entire audiociated components feeds into the signal amplifier in the usual way. range; it must be convenient to selectgrid of the mixer tube V2. The adjustment of the tuning circuit any particular frequency by means of This oscillatorisfixed -tuned to aL2-VC2 is such that when VC2, the main control, isat minimum capaci- tance, the frequency is the same as that to which Li-VC1 is tuned. Under these conditions there is no difference frequency and therefore no signal. But immediately that the capacitance of VC2 is increased, a signal of a few cycles will be produced and further in- crease will cause the signal to rise in pitch until, at the maximum position of VC2, it is at the highest wanted fre- quency, or, if required, just out of the audible range.Although the first os- cillator is fixed -tuned, it is necessary to provide for some limited manual ad- justment of its frequency by making VC1 variable.This control makes it possible for the instrument to be subse- quentlyadjustedtozerofrequency when the pointer of the main control VC2 is in that position. Referring to Fig. 1, tube V2 is utilized

Fig. I (below)-Schematic of audio test instrument.Suitable U. S. tubes are indicated. as a mixer (heptode section) electron - Fig. 2 (above)-Front-panel layout.Space around edge of main dial is for calibration. coupled to the variable oscillator (triode

SHIELD SHIELD 20 H 200 V 000

41{c,--° 40K OUTPUT +8 'TERMINALS% o

25K 100 .001 CI 100µµf I300µpf 100µpf .3MEG I II

100µof

I00µµf LI 100K VI V2 V3 6C5 6K8 6C5 C4 6J 5 37 8 -"' 300µµ f R I I ook 50 K PRE-SET) 250K l --J

vol R2 VC2 2000Af 300µµf 15K 300µµf 500ppf 2 50K K OUTPUT CONTROL

2 1948 .RADIO -CRAFT REFERENCE ANNUAL section). The second triode tube V3 VCI -SHIELDED LEAD n OUTPUT operates as a straight audio amplifier. FROM Ct CONTROL The audio output of V2 is fed to V3 through a high -frequency filter made RI up of RFC1, RFC2, a 100-, and a 300-Awf V.C.2 CHASSIS L2 R2 condenser. A 0.3-meg. , in series SINGLE GANG V2 SHIELDED with the output , limits CONDENSER MIXER the input to V3 to maintain good wave- 500 µid form by preventing overload of the 0 TOP CA tube. If V3 is to be -coupled VI le el to a piece of following equipment, to FIXED e / SHIELD maintain good output quality it is es- OSC. SHIELD sential that the transformer be of high- FIXED VANES TO PRE-SET, C2 fidelity type. 0 COND. The shields shown in Fig. 3 (chassis, Elp.F top view) were included as a precaution, while those shown in Fig. 6 (chassis, , e underside) are essential. Material may TO 13+ be aluminum, brass, or copper, well bonded tochassis.Chassis material may be 16 or even 14 gauge, for all POWER TRANS. 0 construction must be rigid.Drill the required holes for leads before fastening shields in place.Mica coup- ling condensers may be used. The 0.25- megoutputcontrolpotentiometer should have an audio taper. It is ganged Fig. 3-Parts layout of original v. f. o.Layout may be altered to fit parts available. to the on -off , $1. VC2 must be of rigid constructionstep is to remove the ground from theamplifier under test) must be constant and securely mounted on the chassis.grid of Vl. at all frequencies. See Fig. 5. A useful The requirements for "set zero" vari- The b.f.o. may be adjusted by earinstrument for this purpose is the vac-. able condenser VC1 are less severe since quite easily. Set VC1 at halfway posi- thisunit,once set,seldom requirestion and VC2 at minimum as before. COIL CORE 4GOT # 32B.aS. adjustment. Now, adjust C3 and C4 for zero sound. 2,/ex ENAMOR DSC Referring once again to Fig. 3, noteHaving adjusted the instrument to a WIRE, C -T that output control is mounted on apoint where no sound can be heard, the shield.Extend its shaft with an exten-main control VC2 should be increased sion shaft and coupling, to reach thevery slowly until a "pop pop pop" sound front panel.Use shielded 2 -conductoris heard in the speaker, indicating that INSULATING wirefor heater connections to tubethe b.f.o. is functioning on a frequency WASHERS sockets, with the shielding grounded.so low that the beats do not resolve 16/8.0.DIA. Since the wiring must be rigid, No. 14themselves into a note.Further in- DIATOPIT bus bar may be used for the remainingcrease of VC2, however, will increase FORM connections; or, at least, wire no small-the frequency and therefore the pitch er than No. 16 B. & S. gauge.Shieldof the note, so that at maximum capaci- the control -grid lead from V2. Supporttance the note just passes out of audi- any or wired in bility.If this occurs before the con- FINISH OF CENTER TAP WINDINGTO TO GROUND suspension, to prevent movement. Shielddenser is at maximum, or if, with the PLATE COND chokes RFC1 and RFC2. condenser in that position, the note is STARTOF WINDINGTO The completed instrument should benot high enough, V2's preset condenser will need adjusting.This component GRID COND housed in a metal cabinet to maintain Fig. 4-Coil winding details for LI and L2. the stability achieved by shielding com-alters the effective capacitance of the ponents and wiring, etc. main control and a point will be found where, by rotating VC2 from minimum uum-tubevoltmeter, since it is indepen- to maximum, the note starts from zerodent of frequency and, in fact, is often 'Preliminary Adjustments and rises gradually to a pitch where itincorporated in the better class b.f.o.'s Tune both oscillators to the samebecomes only just audible. as one complete instrument.The re- frequency. A signal generator tuned to The completed b.f.o. should be cali-sultant output from the amplifier under 200 kc is coupled to the control gridbrated in cycles per second. A conven-testisthen recorded on an output of V1, the fixed oscillator, through aient method is to plot a graph, usingmeter, which may consist of an a.c. coupling of small capacitance.as check points the notes of a properlyvoltmeter of suitable range.If the Temporarily ground the triode grid oftuned piano or the notes of a phono-amplifier is large it is necessary only to the mixer tube, and adjust VC1, thegraph frequencyrecord;adjustthefeed a small signal into it.If the out- "set zero" condenser, to approximatelyb.f.o. to zero -beat with the known fre-put meter used has no provision for an one-half its capacitance.Now adjust quency. artificial load it would be necessary, the 300-/.44 preset condenser so that its Accurate check frequencies are al- (Continued on page 40) circuitresonates(heterodynes)withways available in the form of the 60 - the test oscillator.The exact wavecycle line supply, and the continuously BEATFREO.OSC. length used is immaterial, so long asbroadcast audio frequencies (4,000 and both oscillatorsare evenually tuned440 cycles) over station WWV. 0 to it. Next ground the of V1Applications and remove the ground on the control One of the most useful applications

grid of variable oscillator V2, and re-of the b.f.o. is in checking the frequency 0 00 011 peat the same resonating procedureresponse of an audio amplifier or the 0 0 V. T. V.M. AMPL. OUTPUT with this tube, except that Ira shouldaudio section of a receiver.For this UNDER METER be set at minimum before the V2 pre-test the output level of the b.f.o. (and TEST set condenser isadjusted.The finaltherefore the input signalfed into the Fig. 5-Making frequency run on an amplifier.

1945 RADIO -CRAFT REFERENCE ANNUAL 3 AN ELECTRONIC TIME A convenient means of making photographic prints where exposuretimes range from a fraction of a second to 45 sec- onds. Accuracyunobtainable by manual means ispossible.

repeated, makingitunnecessaryto switch is "thrown over", the capacitor listen, count, watch, or otherwise markis then connected in series with a resis- the passage of time during exposure. tor across the + and - poles of the In some scientific work it is very im- d.c. supply. portant to know and to repeat exposure On releasing the key, condenser Cl times with accuracy which may notcauses the grid of tube V1 to go nega- always be possible with cruder methods,tive with respect to its cathode. Owing particularly where times are less thanto the trigger action of the pair of several seconds. tubes, current in the common cathode The switch unit consists of a 2 -tuberesistor abruptly transfers the bias vol- Photo timer set up for use with an enlarger. electronic switch, controlled by the ex-tage from V1 to V2. The resulting plate THE photo shows the timer unit.ponentially decreasing potential differ-current actuatesrelay RE, thereby It is built into the baseboard ofence across the terminals of acapaci-lighting the printing lamp. a vertical enlarger, but may betance shunted by a discharging resis- Capacitor Cl is now discharged by built as a separate unit. tance. In circuit with the switch is aresistor R, the grid potential of V1 con- The preset control shown on the left whose contacts are in series withtinuing to rise until the relative poten- of this instrument operates the stan- the printing lamp.Various exposuretials on the grids of the 2 tubes cause dardizingpotentiometer;thecenter times may beselected by a rotaryanother abrupt change -overoftube knob, operates the switch selecting the switch which changes the value of thecurrent, this time from V2 to V1. On time -interval discharging resistor. Alsodischarging resistance. The 2 tubes are the cessation of current in the plate cir- on the sloping front panel are a toggle connectedinacircuitarrangementcuit of V2, the relay ceases to be ener- switch for sustained illumination, andcovered by British Patent 552,301. gized, the contacts open, and the print- the operating pushbutton which may be Normally the relative potentials. oning lamp is extinguished.C2 and R4 extended by flexible to any re-the control grids of the 2 tubes areserve to quench any arc due to opera- mote position. The tubes are accessible such that V1 is conducting and V2 isting the lamp from a d.c. source. from the side of the unit, while the con-not conducting. Set uses English EF50 The time during which the lamp is nections to the power line and the lamptubes, similar to U. S. 6AB7, 6AC7.lighted is determined by the time con- are brought ,out at the back. The grid voltage of V1 is establishedstant of the resistance -capacitance com- As shown in the diagram, this timer,by connecting the potential dividingbination in the grid circuit of V1 and developed by the English firm Cinemanetwork 1, 2, 3, through a selected dis-the setting of the tapping point on the Television,consistsof an electronicchargingresistorR.Whend.p.d.t. potential divider resistor R2. - switch for controlling the duration ofswitch S1isin the positionshown, Manually operated switch S3 is used the exposing light. It allows accuratelycanacitqr Cl is connected in shunt withfor exposure times longer than those known exposing times to be given and resistor R. When this (push-button key)supplied by the automatic unit, or for .31c RECT. operations requiring continuous illumi- nation of the printing lamp. The values of resistors R give opera- RI R4 500 tion times from 1 to 45 seconds with a 330K 200 100railF ratio of -0between successive steps C 2 numbered according to the f/ numbers of lenses. Calibrat% by comparing the exposure CI8 time on the longest range, i.e., 45 sec- S I onds, with a suitable clock, watch, or R2 S3 / seconds pendulum, and adjusting R2 20 K until the required interval of 45 sec- R onds is obtained.It is easily possible o-V\A"- to set the 45 -second interval to within MA, 230 second, i.e., within about 1%. S2 VI V2 0 Anyelectricalprocessrequiring R3 6AB7 6AB7 PRINTING 117 V operating times covered by -the unit 8K 62K LAMP 68K 0-AINA9- or or can be controlled readily either directly

' R 6AC7 6AC7 from relay RE, or from a subsidiary 0-MN-- relay if greater power -handling capac- R 0-11/0e-- ity is required. TT Condensed from anarticleby N.Phelps andF. Tappenden, in Electronic Engineering (London, Eng.), Schematic of the timer.Equivalent U. S. tube types have been substituted for originals. Oetober, 1946.

4 1948 RADIO -CRAFT REFERENCE ANNUAL BACK TO FIXED PLATES OF REGEN. COND.

TO PLATE r TO GROUND AND MOVING PLATES OF TUNING COND.

TO ANT. AND FIXED PLATES FRONT OF TUNING COND. Fig. 4-Loop data. Form is rectangular. t-- 5%.

I 1

3' I

2./4'

Fig. 1-Loop is wound over outside of the case. Fig. 2-B -batteries mounted on sides at bottom. (A) (b) Fig. 5-Layout of case. (a )-Left side. (b)- 2 -Tube Portable Front. INaddition to being portable, thisinches, 2 pieces.Drilling details are: set features the use of low voltage, Fig. 5-a, drilling holes in left-hand side regeneration, and a loop antennaof set for phone jacks; Fig. 5-b, front which makes it useful as a directional of set, showing mounting holes for the receiver. variable condensers and off -on switch S. Fig.1,front viewof the2 -tube At this point, check the fit of the parts portable, shows 2 controls: regeneration in the case, then fasten the wood sides, 3;4%; 34 (upper left), and tuning; both are varia-top and bottom together; for this pur- ble condensers of the same capacitance pose wood dowels and glue are recom- and therefore may be reversed for left- mended.Next, cut a plug shelf, 33. handed owners. Volume iscontrolled inches x 5' inches, as A battery shelf, by the regeneration knob, by detuning,for fastening in position later along and by turning the set to point in awith the front panel. A ground shield, different direction. The rear view, (Fig.not shown, is required. For it, snip a 2) shows how two 7.5 -volt C -batteries piece of metal 43.30 inches, drill holes HOLES FOR (lying flat), in series,operate as ato pass the shafts and mountings of M'T'G. S'K'TS. B -supply; - 7.5 volts on the first bat-the variable condensers. Fig. 6-Tube-sockets chassis cutout details. terybecomes B- inthecircuit, A metal chassis for the wafer tube and -I-of the second battery becomessockets is made as shown in Fig. 6. between both may the required B+15 v of thecircuit, Bend on dotted lines. A holder for one be about 1/8 to Adjust B -voltage for best results.The A -cell is shown in Fig. 7; the other is 1/16 inches. The 4 A -cells are shown upright.Currentheld in place by the back plate of one leads are brought consumption is very low and few parts variable condenser. Mount the sockets, through small are needed. then socket chassis and A holder to holes and pegged The carrying case may be made ofshelf; and make sure all parts, includ- with toothpicks. 3 -ply wood, with wood handle placed ing tubes, are fitted together properly, For protection, the cater-cornered as shown in photos. and that back and front plates will fit outside of the Dimensions are: top and bottom, 51/2properly on frame. The variable con- i_frame,over the x334 inches, 2 pieces; front and back, densers hold in place the ground shield, er windings, may be 53x61/s inches, 2 pieces; sides, 61/2x3% which eventually must be connected covered with 9+ tOA -, B -. To leatherette fasten - REGENAPART OF 13'4-1" mount top of socket Fig. 7-Batteryholder. edon with cellu- LOOP WINDING) chassis when ready, lose cement. Four 5 TURNS fasten it by means rubber feet complete this part of the job. of wood screws to a The parts may now be wired -in. piece of wood which Keep leads short.Remember to con- I 5-GT/G may be glued to necttheanti -hand -capacitanceplate top of frame; to ground (B-), so -that the rotors LOOP ANT. The loop antennaof the variable condensers will be con- which is woundnected to the circuit, too.Note also around this framethat the leads from the regeneration consists of 29 turns coil must be properly connected; make of No. 23 (B. & S.) twist connections until certain that re- d.s.c. wire; the re-generation is being obtained. The A - generationc oil,cells may be connected by flexible leads 29 TURNS alongside, consistsor arranged to plug into spring con- ON FRAME OFF -ON of 5 turns, woundtacts. - -0B - in the same direc- Adapted from an article by F. Caspers, in Practical Fig. 3-Schematicof2 -tube portable receiver. t..)n; the space Wireless(London, England) October, 1946.

1948 RADIO -CRAFT REFERENCE ANNUAL 5 Important Uses of Transitron Oscillators THE transitron is an oscillator of As shown in Fig. 2, the suppresserers the ratio should be better. great simplicity, flexibility, andis biased to point A where the negative When investigating the possibilities stability of operation.Further-' slope is obtained. The value may varyof a tube of unknown suppressor/screen more, it is almost entirely independentwith different tubes.If an impedancecharacteristics, it is advisable to have of any change in tube characteristicsis connected in series with the screena suppressor bias control capable of a due to ageing of the tube.In thisgrid, the circuit will oscillate freely orwide range, say + or - 100 volts or at respect it is infinitely superior to theas a triggered circuit,.Oscillation canleast + 10 v to - 50 v.It is always well-known dynatron, which is very sus-be maintained only as long as, the totalhelpful to take the actual curve by ceptible to changing secondary emis-circuit resistance is zero or negative. plotting screen current against suppres- sion,characteristicof ascreen -grid To keep the harmonic content small,sor voltage, feeding the screen from a tube during its life.In addition, thewe must arrange for a very small am-fixed high -voltagesource through a transitron is not critical in adjustmentplitude of oscillation; that is, the oscil-droppingresistorof the order that even when great purity of wave formlations must not build up very much.would be used in practice. is required, such as with an audio -fre-This is accomplished by arranging the One method of amplitude control is quency sinusoidal oscillator. point of operation to occur where theto vary the coupling between g2 and, g3 As will be shown later, the methoddynamic negative impedance increases (as in Fig. 3). The same effect may be of operation may be applied to an r.f.(toward zero) with amplitude.If weachieved by changing the ratio C1: C2. oscillator with either a sine -wave form, start off with just enough static nega-Generally speaking (particularly where output or one rich in harmonics, withtive impedance to permit oscillation,the oscillator is not required to gate r.f.) the circuit elements consisting of Lthis arrangement will prevent oscilla-it is preferable and more convenient and C; or, if desired, in anR -C,circuit tions from building up, due to the in-to use control -grid bias for amplitude capable of operation up to several mega-creasing, effective damping as the am-control. A further point of interest is cycles (Fig. 1). plitude increases. The particular pointthat any resistor placed in the plate cB+ required is usually that of maximumlead tends to stop the circuit function- slope (Fig. 2); the essential requisiteing, unless it is decoupled, but under R2- 1, , isthattheabove -and -belowpointscertaincircumstances may be used T across which the tube is swung mustwhere satisfactory operation can be , have a lower slope than that at the achieved. point of bend. The frequency range of the circuit In the case of an oscillator employingin Fig. 3 is about 650 to 1,600 cycles. V2 an L -C circuit, the best wave form is Another useful application ofthis -C2 obtained by lightly coupling direct intocircuit is that it may be used to pro- that circuit. vide tone modulation of r.f. by replac- ing R1 (500k pot.). by an r.f. choke, Fig. I-Fundamental R -C transitron oscillator. The R -C Transitron placing a second choke in the grid, and simply feeding r.f. in at the grid and Another feature of the transitron is TheR -Ctransitron is most useful fartaking modulated r.f. out of the plate. that the output voltage may be con- the lower frequencies, and is not usual- trolled by 3 main methods, oneof ly treated in any detail. The principle B+ which (negative bias on the controlis no different, merely being a specific grid) does not affect the frequency, case in which we can use a frequency - 500K 22-47K the other methods doing so under cer-selective network. 0.I tain conditions. From an analysis of the basic circuit and Fig. 1 it can be shown that OUTPUT Principle of Operation 1 The negative-transconductance (tran- f = sitron) oscillator operates by virtue of 27VR1R2 Cl C2 a property common to most Then in the special case where we make in which an increase in suppressor -grid 500AA voltage causes a decrease in screen -R1 = R2 and Cl = C2, grid current, and thus an increase in 1 screen voltage.This effect, in which f = an increase in the suppressor voltage 27, -RC 25K 500K has a greater effect on screen current than the screen voltage itself, occurs There are certain obvious limiting a - only over a limited range, in which thefactors to be considered:R1,R2may Fig. 3-R -C transitron covering a.f. range. screen exhibits negative resistance. not be much higher than 0.5 megohm unlessabnormald.c.conditionsare A Although the matter of harmonic 1g2 used; also, due to difficulty in obtainingcontent has been mentioned, it is im- ganged that track prop-portant to point out, particularly for POINT OF OPERATION erly, to vary the frequency over a widethose who wish to make up a variable - range it is better to use a 2 -gang con-frequency audio oscillator, thatitis denser to vary C1, C2. This does, how-necessary to use large (but low -leak- ever, have the disadvantage that C age)500-wifcondensersforpurest min to C max for, say, an averagewaveform. Undertheseconditions -20 0 +20 500-Agf, 2 -gang condenser may be 12.5:- quite large outputs of pure sine waves Eg 3 500 or 40:1, whereas with a good pairare obtainable; values up to 0.5 or 1µf Fig. 2-Suppressor bias vs. screen current. ofganged 0.5-megohm potentiomet-can be used in these positions. Further,

6 1948 RADIO -CRAFT REFERENCE ANNUAL if a very small variation of frequencynegative slope mid -point at a little un-point of bend (by suppressor bias) and is desired, it may be accomplished byder minus 2 v on the suppressor of theturning the output up full; the relation- varying one R or C only. sample measured; and the effect ofship of the screen resistor to the high - The circuit may be used also for theshifting the point of bend down thevoltage lead may also be used to pro- productionof saw -tooth and squareslope very slightly by operating at 0duce limiting and consequent distortion. waves. This use will not be treated involts on the suppressor makes a negli- detailhere,exceptto mention thatgible difference and is most convenient.Transmitter Control (saw -tooth wave forms excluded) out- [For a 6AB'7 or 6SJ7 tube, values of From the above we see that here is puts of over 100 volts may be obtained.R1, R2, and C1 must be changed experi- an excellent variable frequency oscilla- In most r.f. applications it is prefer-mentally.] tor for the transmitter. Since the v.f.o. able to use the L -C version both for In Fig. 6, L is 18 turns No. 16 B. & S. frequency would always be checked, it convenience and the greater stabilitywire, d.c.c., wound for 2 inches on awould be permissible to reduce the high of the frequency determining circuit,ceramic form 1 inch in diameter. specification demanded in the previous such as shown in Fig. 4. Resistor R1 The frequency stability versus high -section by allowing the input capaci- drops the plate voltage to 20-50 volts,voltage variation is excellent; raisingtance of tube V2 to be "hung" straight B+ B+ from 92 to 250 v caused less than 3across the resonant circuit, as shown in L cycles change at 3.5 mc. Nevertheless,Fig. 7. As long as the buffer was ope- the author uses a neon stabilizer torated class A, well clear of grid current, regulate the high voltage. The warm-it ,would be quite satisfactory.Grid ing -up period should be about 15-20current would not start till V2 grid was minutes, after which the drift due to swung to reduce effective bias on peaks C3 changing interelectrode capacitances ofto about -1.5 v. or less. the tube are negligible; drift is also The de luxe version should include, compensated for by a part of the tuningof course, the cathode follower, feeding capacitance being ,of the negative tem-into a high -mutual -conductance, sharp perature -coefficient variety. The induc-cut-off operated as required. tance itself will not change much, pro-

R R3 vided it is wound tightly and a ceramicCathode Follower Buffer form is used. A tube must be chosen that is free +92V B 8+ from grid current and in which the Fig. 4-L -C transitron for radio frequencies. 150 V "leakage" due to the ultimate C does not carry the r.f. straight through from and C2 is r.f. decoupling; R2 provides the input circuit to the cathode load, suppressor -grid bias, and C4 decoup- making the tube a minor participant. ling.Condenser C3 must have a low A suitable circuit is shown in Fig. 8. reactance compared with R3 at the The efficiency of this circuit is about resonant frequency of L -C1. 80%. The parasitic suppressor R2 is This arrangement has several dis- advisable as there are certain (frequent- advantages, one of which is that the ly obtained) conditions under which a tuned circuit has d.c. on it, preventing cathode follower becomes unstable. the use of a grounded rotor -type con- If :,a greater input impedance is re- denser. The author prefers to have one quired, the grid leak mu'st be returned end of the tuned circuit at d.c. ground to the cathode via a bypassed bias re- as well as a.c. ground. This is possible by placing the tuned circuit inthe sistor inserted above the cathode load. suppressor, as shown in Fig. 5. Article by H. De Laistre Denting,in Short Wave; Magazine (London, Eng.), June, 1947,is reprinted by Constructional details of an oscillator B- - courtesy of the publishers. suitablefor frequency measurement 5K or for a signal generator may be of Fig. 6-Practical circuit for L -C transitron. interest. The requirements are stability Output is taken directly from the WW 250V of operation and a small tuning range. tuned circuit, with a high input imped- NEON In the circuit of Fig. 6.. the amplitudeance at the succeeding buffer stage, to poten. may be omitted, unless one isavoid undue loading. Both the type of anxious to reduce the harmonic content buffer and the coupling condenser (ef- VI V2 fectively in series as far as stray capac- ,6AB7 6V6 itance is concerned) are arranged to 6SJ7 have the least possible effect upon the tuning. This is done by using 1µµf be- tween the hot end of coil L and a suit- able cathode follower, the output from thecathode followerbeing 6volts peak -to -peak across a 1,000 -ohm load. The frequency of oscillation of all 220 the L -C transitron circuits considered may be taken as o g 1 Fig. 7-Transitron and buffer used asV.F.O.

B Fig. 5-Fig. 4 with L -C in suppressor circuit. 2 ir V LC 150 V although if the frequency is made very toa minute amount, since an oscillo-low by large inductances having ap- gram reveals no perceptible distortionpreciable d.c. resistance, the resultant when this control is at maximum. frequency will be lower than that given Grid and cathode may be grounded by this simple expression. if amplitude controlisnot required. It is strongly recommended that if The output voltage then will be aboutharmonics nre required they be genera- 18volts peak -to -peak,and constant't.ed by a separate tube, say a saturated over the range 3.390 to 3.940 mc. Thepentode fed by the oscillator. It can be ea tube chosen (an English EF39) had itsdone in the oscillator by shifting the Fig. 8-Cathode follower buffer for transitron.

1948 RADIO -CRAFT REFERENCE ANNUAL 7 Two views ofv. h. f.superregenerativereceiver. Rods at back of the set are dipole antenna. Construct an FM -AM Superregen'r THE great merit of the superre-stricted. An acorn will do, of course. wire, enamel or cotton insulation, close - generative type of receiver is thatOther suitable tubes are 7A4, 6C4, 9002, wound in a single layer on aU. -in. it gives so much for so little, par- 1/2 of a 6J6 or 7F8, tube.If moving the phone and power ticularly as the frequency range over The circuit is a self -quenching Col-leads affects the tuning, another choke which it operates is increased. pittsoscillator;values need not bemay be inserted at X. The "breadboard"constructionof exact. The optimum quenching frequen- The completed receiver may be used this set is only one evidence that itcy is not critical, but increases withas a v.h.f. tuner by connecting its out- may be built easily and cheaply; justthe radio frequency.Connecting theput to the PHONO terminals of a stan- how much so, we shall see. Further, itgrid leak to B+ raises the quenchingdard radio set, instead of to headphones is very sensitive and has no criticalfrequency, and at the same time en-PH. The required poiVer supply may be adjustments. courages quenching.In fact, with ataken from this set, or a suitable power The unfairly maligned backgroundhigh -mutual -conductance tube, such assupply unit may be made as shown by noise not only indicates when this setthe 6J6 and 7F8, quenching may bethe diagram in Fig. 2. Is working properly, but simplifies tu-rather excessive, as shown by the plate SELENIUM ning to unmodulated carriers, whichcurrent falling to perhaps only 0.1 ma; B+ show up as "holes" of silence. Finally-therefore, pre-set condenser C3 is used and this may be news to some - it to control oscillation. works with either AM or FM, and indi- A geared tuning drive is shown and cates which is which. So altogether itrecommended. A nonconducting exten- is just about ideal for a preliminary sionisessential for reducing body - survey of the v.h.f. band. capacitance effects, since neither side of the tuning condenser is grounded. Circuit and Construction Tuning L1, of #16 wire, Fig. 1 is the circuit diagram of thefastensinto2screw connectors on Fig. 2-Power supply using selenium rectifier. receiver. The meter is very useful forthe supports of the variable condenser. calibration and for tuning the aerial.Examples: 37-57 mc: 9 turns 25/32 in.. A suitable power unit may be built Most can be made to work overto 13/16 in. diam., 1 3/8 in. long; 65-100from oddments, says the author. With the low -frequency half of thev.h.f. mc:4 turns }between 21/32 in. andcurrent consumptionof perhaps0.3 band, but for exploration around 30011/16 in. diam. x 29/32 in. long; 90-140amp. at 6.3 volts, and only 0.5 ma at mc or higher, the choice is more re -mc: 1 turn between 25/32 in. and 13/16about 90 volts, a bell -ringing trans- in.diam.; 110-170 former may be used for A supply and mc: short circuit. a class -B driver transformer for B - voltage. ECH .005 For higher fre- 8 MEG quencies, it is usual V to eliminate leadsAntenna 7A4 2 in the tuning cir- A pair of tubular metal telescoping 9002 cuit, to remove C3,camera tripod legs makes an excellent CI 17F8 and,if necessary,dipole antenna.They contract to a 20µµf to cut out C1 alto-reasonable size when not being used, MAX gether, tuning byand may be adjusted to resonate at any C2 C3 means of a variablefrequency over a wide band. The top condenser C2. ends are frequently provided with lugs 25µµf 20µµf Choke CH is notthrough which screws can be passed to MAX 3 + critical; it may have serve as terminals for a coupling coil about 70 turns ofof 1 or 2 turns, and also to hold strips Fig. I-Schematic diagram using one of several U. S. tube types. No. 36 B. & S. ga. (Continued on page 40)

8 1948 RADIO -CRAFT REFIRENCE ANNUAL A Four -Tube Reflex Superhet This receiver is stable on signal inputs front I microvolt to I volt

THE reflex circuit is given a newadvantages; the fulli.f.gain can bewith a tapped -coilcathode feedback lease onlifeinthis carefullyrealized, and the minimum volume ef-oscillator. It is interesting to note that engineered broadcast -band super-fect is reduced to negligible proportionswith the padding condenser placed as heterodyne using 3 tubes plus rectifier.for all but the most severe listening itis,the oscillator voltage between It has amble sensitivity for use in mostconditions. With ana.f.plate loadcathode and ground is higher at the country areas, yet it is free from over-of 15,000 ohms, the high mutual con-low -frequency end of the band-quite loading in regions of high signal level.ductance of the 6SF7-GT results in anthereverseof the usualcondition. The circuit provides i.f. neutralization,audio gain of about 12.5 times for veryWhen the receiver is designed to cover hum neutralization, and negative feed-little sacrifice of i.f. gain. This extraonly the broadcast band, there is the back on the power stage. amplificationbringsthesensitivityuseful possibility of making the grid The idea of using 1 of the tubes in afigure down to the region of 40 micro-condenser small enough in relation to radio receiver simultaneously to amplifyvolts input for 50 milliwatts output. the grid leak to minimize the potential inputs of 2 different frequencies is not The receiver is completely stable onrise for low frequencies. In this- way by any means new. Some of the verysignal inputs from 1 microvolt to1 it is feasible to preserve almost opti- early receivers of the triode r.f. ampli-volt.For high volume levels,over-mum oscillator voltage over the band. fier era quite commonly employed thisloading takes place first in the speaker Theintermediate -frequencytrans- device. In more recent years, consider- able thought has been given to elimina- ANT. 6SA7-GT 6SF7-GT 6V6-GT ating the fundamental failings of the reflexed amplifier. c1 One of these defects has been the CONVERTER I.E, ED:1 so-called"minimum volumeeffect," 2nd DET. SPKR. !Oka PRIM.) which causes a very distorted output aA.F. PWR from the speaker when the volume AMR controlisturned nearly off, and a 50110 substantial output when the control is turned right off.It results from the curvature of the tube mutual charac- teristic partially rectifying a large in- termediate -frequencyvoltage. Then, becauseof the audio -frequency load and capacitance in the plate circuit, the tube acts as a plate -bend detector, which passes audio output directly to VOL. the power tube.This output is quite CO NT. independent of the , and is un- 7K affected by the volume control. 2W A further serious disadvantage of MEG the usual reflexed amplifier has been the tendency to overload on moderate G ND. and strong input signals.This is be- 6X5-GT 10000 cause the presence of an audio load of OFFS ON 61-I 550ft the order of 0.1-megohm in the plate 6.3 V circuit makes it necessary to reduce 117 V the plate current, by means of lower A.C. 50 MA screen voltages and/or increased nega- 200 V tive grid voltages, to avoid excessive 250 8 drop in the plate load. The best com- promise considerably reduces the trans - conductance of the tube. As the gain between the grid of the converter tubeand then in the power tube, which isformers are of medium quality and and the diode is not high enough todesigned to deliver a little more than 1 small dimensions, asis usual for a provide a good a.v.c. characteristic, thewatt to minimize heat dissipation. smallreceiver.Improved selectivity, i.f. input to the reflexed tube is there- The power amplifier tube is operatedwith some increase in sensitivity, could fore quite high. Furthermore, the peakwith low plate and screen voltages andreadilybeachievedwithhigher -Q a.c. potential at the plate is thesum ofwith increased negative bias on the gridtransformers. the i.f. and a.f. signals, while the d.c.to reduce unnecessary heat dissipation. The antenna coil has the usual high - potential is only. of the order of halfThe audio output exceeds the power - impedance primary, and is unshielded. the B -supply.Thus, a tube which ishandling capabilities of the usual small The oscillator coilisalso unshielded not capable of handling large signalsspeaker generally supplied for use withbut placed about 3 inches away and at without distortion has, by virtue of the small receivers, so there is no sacrificeright-angletothe antennacoil(or circuit arrangement, large signal volt- of performance for the saving in powerunderneath chassis-Editor). ages applied to it, thereby resulting in consumption and heat dissipation. The simplest test for optimum oscil- overloading. lator amplitude is to connect a vacuum - The arrangement used in this receiverGeneral Description tube voltmeter between cathode and employs a very small a.f. load in the The 6SAI-GT tube is used as a con-ground. When this shows a potential plate circuit.This has 2 importantverter in a conventional arrangement (Continued on page 41)

1948 RADIO -CRAFT REFERENCE ANNUAL 9 New Direet-CoupledAmplifier A circuit comparisonbetweena conventional resistance -capacitance coupledamplifier,. an old-style direct -coupled amplifierof the Loftin- White type. anda push-pull type designed to overcome faults in the first 2 arrangements.

REFERRING to Fig. 1, it will be tanceofthislargercomponentto at low frequencies, right down to zero. notedthatseveralelementsground, plus the value of Ca, would To understand the d.c. voltage re- serve to control or establish theequal 125 Auf or more, resulting inlationships in the amplifier, let's look extent of bass and treble frequencyserious attenuation of the highs. at the circuit and tube potentials of amplification possible in an audio am- From this, the reader may have con-Fig. 1 again. When we short out Cg to plifier employing capacitance coupling cludedthatanexcellentexpedientmake a direct -coupled amplifier, the Cg between stages. would be to make direct connection orcontrol grid of V2 assumes the poten- Although there is considerable meritcoupling of tube V1 plate to tube V2tial of VI's plate( +62v). To main- in a circuit of this type-for example,grid, thereby entirely eliminating thetain the required negative grid bias of the high -voltage power supply in thisproblems presented by use of condenser 14v on V2, its cathode voltage must be case delivers current at only 264 voltsCg. This we can do, after altering theraised by 62 volts. This added to the (and all calculations are based on thispower supply circuit to apply properoriginal 14 volts makes +76 volts. But figure, although other supplies may bevoltages tothe tube elements afterthis also reduces effective plate and used if due allowance is made for anycounteracting the effect of connectingscreen voltage on V2 by 62 volts. We difference in voltage)-from the stand-the V2 control grid, which must bemust increase the plate supply voltage point of reproduction fidelity it leavesmaintained negative, to the V1 plate,by 62 volts (from 264 to 326) to com- much to be desired.If the circuit iswhich must remain positive. pensate. Now look at Fig. 2, the result designed for optimum response over Theresultingcircuitcompromise of these changes.A voltage divider some mid -frequency range, then there (Fig. 2), was introduced around 1928 (R3-4-5) has been placed from plus is necessarily a reduction of amplifica-and designated by the namesofits326 volts to ground; R1 goes to +252 tion at the extremities of this rangeoriginators as the Loftin -White (direct- volts(point B) as it did in Fig.1. for various reasons. coupled amplifier), here modernizedas Control grid of V2 goes to plate of V1 Tube interelectrode capacitances, andto choice of tubes, the 6Q7 voltage am- ( +62v). Cathode return of V2 goes to component and wiring capacitances toplifier, and 6L6 power amplifier. It is +62 v (point A). This 62 volts added ground, represented in the diagram as"d.c." in another way, for it also willto 14 -volt drop across R2 makes V2 Ca, serve to limit circuit gain at theamplify direct current in contrast tocathode +76 v as required. higher audio frequencies.While thethe R/C circuit of Fig. 1 which will So much for Fig. 2, designed to show gain at low frequencies may be in-amplify only alternating current. thed.c.voltage distributioninthis creased by increasing thevalueof Thus at one stroke, we are able todirect -coupled amplifier. To permit the coupling condenser Cg to as much asimprove the output at high frequenciesamplifier to function correctly when 1 to 1.5 '4,the resulting static capaci-and get substantially uniform outputalternating voltages are used, the cir- VI V2 cuits must be bypassed with condensers 6Q7 +62V at several critical points. If the conden- 6L6 -G sers are too small, amplification will be + 264 V reduced at low frequencies. Electrolytic condensers are used. With the values given inFig.3, +14V there is no appreciable 1.f. attenuation until we reach frequencies of the order INPUT of 10 cycles, which makes it very satis- III 4SPKR, factory for all the usual applications.

I Below 10 cycles, the currents circula- ting in the voltage divider and coming + 252 V t +264V from the plates of the 2 tubes go in Fig.I --Simplified circuit of conventionalr -c amplifier, showing important electrode voltages. opposite directions, causing degenera- VI tion which increases as frequency is lowered. This reduces gain which is also diminished by the drop in effective capacitance of the condensers across the cathode resistances. An apparent reduction in amplification also takes place as a result of the cutoff charac- teristicoftheoutputtransformer, INPUT which takes place at some low value, SPKR. perhaps 15 cycles, depending upon the quality of this component. To cancel the effects of the degenera- tion pointed out above, the only com- pletely effective solution is to use push - Fig. 2-Simplified circuit of originalLoftin -White direct -coupled amplifier, with new tubes. (Continued on page 41)

10 1948 RADIO -CRAFT REFERENCE ANNUAL _Direct -Reading V. T. V.

ONE of thedrawbacksof the balancehasbeen direct -reading vacuum -tube volt- obtained, the meter meter is the necessity of main-is switched to the tainingthesupplyvoltageswithin potential divider fairlycloselimitstomaintainthe and itsreading accuracy of readings. noted. In the v.t.v.m. shown here the cali- For high accura- brationisindependent of the plate c yacalibration voltage, and a range of 50 volts or more curve may be can be covered with a B -supply of only drawn or the meter 9 or 10 volts.Satisfactory operation canbescaledto is obtained over a range of frequencyread volts directly. from 50 cycles to several megacycles T h e calibration per second. curveissubstan- The principle on which the instru-tially linear apart from a slight initialS1, control Z is "cheek zero" switch S2, ment operates will be understood frombend on the low range. and P is the potential divider. When Fig.1.The voltage to be measured The higher Rl's value (Fig. 1), the balanceisobtained,switch B -M is is applied to the diode tube V1 so thatcloser will the voltmeter reading be toturned from Balance to Meter, and the condenser C1 is charged to nearly the the peak voltage and, what isalso meter then shows the input voltage. peak voltage.This voltage which ap-important, the smaller the load takenInput terminals are shown atleft; pears across the resistor R1 is appliedby theinstrument.Components R2those on the right are normally not to the grid of tube V2 and causes aand C2 form a low-pass filter to reduceused, being connected to the meter so reduction in the plate current.If anthe high -frequency voltage appearing that it may be used for other purposes. equal and opposite voltage is appliedon the grid of tube V2. CI to the tube from the potential divider With the range switch S1A on posi- P, the plate current will be restored totion x, the instrument measures from its original value. The reading on the0.1 to 9 volts peak; and with the switch voltmeter V is then a measure of theon y, the range is from 1 to 90 volts peak voltage applied to the input ter- peak.If higher voltages are to be minals. measured, use a higher -voltage battery Ifthe range switch S1is movedin place of B2 or use a lower tapping from position x to position y, only aon Rl.The upper limit is set by .the fraction of the voltage appearing acrossdiode V1 and is usually not less than RI is applied to the grid of tube V2.about 200 volts peak and may be much If y is connected to a point, say, one -higher if a special tube is used. tenth the way along the resistance R1, An additional refinement would be then the maximum peak voltage whichto have a coarse and fine adjustment can be measured will be 10 times theon the potential divider. However, the Fig. I-Fundamental circuit of 2-rangeV.T.V.M. voltage of battery B1. voltage can be adjusted, using a single As the negative terminals of bat- resistance,to within about .05volt. Tube V1 is a diode but a triode will, teries B1 and B2 are common, the Condenser Cl must have high insu-serve equally well, plate and grid being potential divider can be connected tolation resistance if the reading is notconnected together. V2 is a tube of battery B2, and battery B1 can beto be affected by d.c. in the circuit tothe low -power class, as a high mutual dispensed with, as shown in Fig.2. which the voltmeter is connected. conductance is desirable for maximum This is generally satisfactory, but as In the experimental model shownsensitivity. (cover removed)inthe photograph, thepotentialdividerrepresentsa AdaptedfromanartielcebyH. W.Baxter. in variable load it is important that thisR is combined range and on -off switch Wireless World, (London, England), January, 1947. should not cause any noticeable varia- tion in the battery voltage since this (1/4,2,0.5 MA

might lead to a slight error due to the CI VI V2 *BALANCE4,11 'METER' balance point on the meter M changing .01 slightly during the measurement. The : ° error is usually negligible if the current 0 SID taken by the voltmeter isnot more SI A

than, say, a milliampere. Where the I 0- load taken by the voltmeterisnot negligible, the initial reading on the -r meter M with no input voltage can be SMEG R3 S3 B. checked, after balance has been ob- 2 MEG 9V tained, by moving the switch S2 in 02 20K Fig. 2 from position v to w, when the INPUT :02 true balance position on the meter will P be found. 2-5K Note that this meter isalso made to serve as the voltmeter, the resistance R3 being substituted for the meter in order nottovary theload.After Fig. 2-Schematic diagram. VI and V2 may beI H4 -G's. Value of R3 equals meter's resistance.

194R RADIO -CRAFT REFERENCE ANNUAL I1. want to do.Therefore, it is common practice to use a unity or slight step- Class A and B down ratio transformer. Class AB As the only essential difference be- tween classes A and B is the potential at which the grids are biased, it follows AMPLIFIER that some suitable bias point between the two extremes could be used to ad- vantage.This is referred to as class THE constructor who decides tooutput from this arrangement is ampleAB, and we now have classes A, AB1, use a couple of 6L6's in push-for most domestic needs. AB2, and B (although AB2 and B are pull and thinks he will get very considered the same). How far back we good quality on low volume because theClass It Amplification bias our tubes determines what class we amplifier will handle large inputs with- However, when even larger outputsare working under. out distress (delivering 15 watts andare required, one invariably juggles By definition, in a class A amplifier, upward of output power) may well findwith the idea of going in for some kindgrid bias and a. c. voltages cause plate that the reproduction on 'low volumeof class B amplification. current (in a specific tube) to flow at sounds no better than did his previous Although the schematic diagram of aall times; in class AB, both these vol- set which perhaps used a modest 3 -watttypical class B amplifier would look thetages cause plate current to flow for pentode. same as one of a class A amplifier,appreciably more than half but less True, he can raise the volume andthere is one very important difference:than an entire electrical cycle; and, in note with pride and satisfaction thateach tube is operative only during one- classB, grid biasisapproximately his 12 -inch speaker overloads before hishalf of each cycle. As the 2 plates areequal to cutoff (value at which plate amplifier does or pulls the coil rightconnected in opposite phase to the pri-current is some very small value) so away from theconeofhis7 -inch mary of the output transformer, thethat plate currentisapproximately speaker! complete cycle appears across it. zero when an exciting grid voltage is But if he wants really good quality What is probably not quite so evi-not applied, and flows for approximate- he will invariably have to turn to triodedent is that by biasing the output tubesly one-half of each cycle when it is. output tubes.Admittedly this is ratherfor class B operation a very muchAdding a numerical suffix, as AB1, a bold statement to make, as there arelarger input voltage may be applied merely indicates that plate current does many pentode enthusiasts who claimwith a consequent greater variation ofnot flow, or does flow (suffix 2), during they can, by employing negative feed-plate current.This, of course, results some part of the cycle, and hence, AB2 back, get results equal to that of tri-in a larger signal voltage across thewould be equivalent to B. odes. output transformer, which means more The nearer we get to class A, the volume from the speaker. further away from the unwanted bot- Power Outputs Required The chief disadvantage is that be-tom bend we get; and in turn the maxi- For ordinary quiet room listening,cause the tube operates over the bottommum undistorted output gets less. using a reasonably efficient speaker, thebend of its characteristic curve as well The nearer we get to class volume produced by an average outputasthestraightportion,acertaingreater is the distortion at low volume of 100 milliwatts could be very safelyamount of distortion, mainly at lowlevels; and the maximum undistorted handled by a 2 -watt output tube. How-volume levels, is inevitable. When oper- output increases. What to do ? ever, there are occasions when louderated at high volume levels, quality is volume is needed. excellent. Class A or B at Will In this case, 2 of these tubes may A certain amount of power must be With all of these pros and cons in be used, either in parallel or push-pull.drawn from the preceding stage tomind, the writer built an amplifier The push-pull arrangement is usuallymake up for the grid current losses,capable of employing any class at will preferred, because of the cancellationand as the preceding tube has to sup-by simply making provision for vary- of even harmonics and the fact that theply that current to the grids of the out- ing the grid bias on the output tubes. output transformer, having equal andput tubes, it is pointless to use a step- This was not intended to be a con- opposite voltages impressed across itsup transformer, because in stepping upstructional article, and the schematic primary, has a higher effective induc-the voltage, this would step down theshown here should serve as a guide to tance, all other things being equal. Thecurrent, which is just what we do notthose wishing to experiment along these lines. The main requirement isa power V I. V 2 V3 supply having good regulation; choke input (so often neglected these days) is 6C5 -G 6J5 6F6 -G(2) ideal for the purpose. )1- 0.1 Tube V1 can be the detector in a T2 straight receiver, or the triode section of a double -diode -triode in a superheter- SPKR. odyne, or the preamplifier in a phono amplifier. The second tube V2 must be (Continued on page 42) V5

5Y3 -G FUSE

0 2K Z 0.5 MEG INPUT 117 V + 1 50 T16 16 _ A.C.

I MEG HEATERS.' 6.3 V0) 1KP -a I OFFS ON Schematic of adjustable bias audio amplifier.Power supply is choke input for good regulation. Note that 6P6 -G's are connected as triodes.

12 1948 RADIO -CRAFT REFERENCE ANNUAL HI-FI TUNER The output of this tuner will drive a 10 -watt audio amplifier

THE average commercial receiverback takes place at audio frequenciesconjunction with a suitable capacitor is of necessity a compromise be-and thereby ensures that harmonic dis-C1 discriminates in favor of the higher tween station -getting ability andtortion shall be almost entirely absent,frequencies. An over-all response curve quality, with the accent on the former.even at high modulation levels. is thus obtained which lifts very gently To the listener who has a rather critical Shielded transformers L1 and L2 arefrom about 1,000 cycles to reach its ear and a desire for purity of repro-the standardt.r.f.type, L1 for themaximum at 8,000cycles, and then duction it is, to say the least, most un-antenna stage and L2 for the r.f. orgently falls again until at 10,000 cycles satisfying. detector stage.Either a high- or low -the output is at the same level as at Itisthe higher audio frequenciesimpedance primary may be used, de- 1,000 cycles.Actually, the maximum that suffer most through the side -bandpending upon local reception conditionsdeviation from straight-line response cutting that is done in an endeavor toand the type of antenna. The 2 -gang,of the whole unit is + or - 2 db be- obtain adequate selectivity 365-Atif variable condenser must havetween 20 and 10,000 cycles. Although the majority of listenersa maximum -minimum capacity range A similar response curve would have tune to the local stations for 90% of which,withassociatedcapacitancesbeen possible without the- treble com- theirlisteningtime,thedemand, (wiring, etc.), will permit coverage ofpensatory circuit if smaller -value damp- strangely enough, is nearly always forthe 550-1600-kc broadcast band whening resistors had been used across the a receiver that will pull in a large num- connected to L1, L2. tuned circuits.But the danger then ber of stations. Sufficient r.f. gain will be obtained would have been of mutual interference between the local stations, and a possi- ' It has often been said that high -qual-by using the tubes specified, and damp- ity reproduction can be obtained withing resistors R1, R2 connected in shuntbility that insufficient gain would be asuperheterodyneifvariable -selec- to the respective secondaries. These obtained to give efficient detection. tivityi.f. transformers are used, butresistors counteract a decided droop in The r.f. plate decoupling circuit is this is only true in part because withthe response curve above 5,000 cycles. includedasapurelyprecautionary this system the i.f. response curve is In spite' of the fact that the 6SK7 ismeasure against instability, and may be almost bound to be uneven or peaked.slightly overbiased and the tuning in-quite unnecessaryifawell -filtered ductances are shunted with comparative-high -voltage supply is available. Probably the best way to get good stationfield quality from a superheterodyne isto lylow -valueresistors,theresponse Insomeinstances curve willstill show a small droopstrengths at the receiver locality may of fairly low value.But assuming thatabove 7,000 cycles.True, this droopmake it advisable to build the tuner we can, by good design (and complica-amounts to only about 4 to 5 decibelson a metalchassis,rather than a ted switching) get a good i.f. curve, at 10,000 cycles and may be considered"breadboard"(literally),andtofit it will be found in all probability thatgood enough by the majority of listen-this into a metal cabinet which will af- true quality, or realism, is still absent.ers, especially those who have a vari-ford good shielding of the wiring and This, at least, is the writer's experi-able treble -boost control in their a.f.all components. ence, and the reason is a matter foramplifier. The output of the tuner is intended conjecture-probably something to do However, for the sake of the some-to drive a 10 -watt audio amplifier; but, what more critical listener who mayif the builder wishes, it may be fed with transients. into headphones, with or without a The above remarks are not meantnot be quite so well placed as regards tonal compensation, the detector has itsmatching transformer,asnecessary. to imply that good quality cannot be Required voltages may beobtained obtained from a superheterodyne, butload resistance split into 2 parts as shown, to give compensation in the unit.from a separate supply unit or its as- rather that quality of the superlative sociated audio amplifier. kind is extremely difficult to obtain The larger part of this load, R3, pre- with one of these receivers. Ideally, ofsents an equal impedance to all frequen- Adapted from an article by C. Summerford, in Prac- course, 2 receivers should be used, onecies while the smaller resistor R4 in tical Wireless (London,England) May/June, 1947. for distant stations and one for the high -quality reception of the local sta- 365µµf GANGED tions. The circuit diagram shows a high- fidelity tuner for local stations.Con- nected to a 10 -watt amplifier it takes care of fidelity requirements not met by the more selective superheterodyne. True, this receiver design with only a single stage of tuned r.f. amplification ahead of the tuned detector, may tune "as broad as a house," but in many lo- calities reception conditions will permit the receiver to operate satisfactorily. If a diode detector had been used, gain would not have been sufficient, - R3 except for those who live almost under 0,1--. 500 0.IT60K1 3K TOUTPUT the shadow of a transmitter.But the 3007.tia f infinite -impedance detector chosen gives considerable gain with low distortion. oB+ By taking the output from the cath- 250V ode circuit, very heavy negative feed - Schematic of broad band t. r. 4. broadcast receiver. RIarnd R2 broaden selectivity curve.

1948 RADIO -CRAFT REFERENCE ANNUAL 13 Tone Control Circuits Methods to accentuate or reduce bass or treble are shown here

THE average tone control reducesstantially constant if the tone control is tiveoutputresistanceofthetone - treble response and nothing more. to work satisfactorily.It is very in-control tube V, and is made up of the Controls to accentuate or reduceconvenient to have to readjust the vol-output resistance of the tube, bass or treble are too seldom provided. ume control every time the tone control Rp x R3 Except with the aid of resonance ef-is varied! fects (usually undesirable), the bass or The remedy is to fix amplification Rp R3 treble cannot be accentuated withoutover the middle range of frequencies atplus the resistor Rl. a general loss of amplification.Tone - a small percentage of the gain which For convenience let us call the tube's control systems do not increase the ab-can normally be secured. output resistance R4, where soluteamplificationat low or high When level response is required, the Rp x R3 circuit takes the form of Fig. 1.R3 R4 = is the normal coupling resistance of an Rp R3 R -C amplifier and C1 is the couplingThen the effective output resistance condenser. R = R1 + R4. The normal stage gain is secured be- tween the grid and plate circuits of the"Cut" Curves tube, but only a fraction of the tubers The simplest curve is the bass cut output voltage is taken off to feed thearrangement of Fig. 2-d, for the per- next stage.This is done by the voltageformance obtained depends only on the. divider R1 -R2. product C (R R2).This is shown in Basic Tone Circuits Fig. 3.The curve is plotted in time - constant terms of T1 = 274C (R R2) To obtain a bass -boost effect it isto make it universally applicable.2..f necessary only to include a condenseris 6.28 times frequency in cycles per C in series with R2, as shown in Fig.second, and C and R are in farads and 2-a.Reactance of the condenser in- ohms, respectively.As an example, Fig. Basic flat -response amplifier stage. creases as frequency falls, and so thesuppose we want a drop of 20 db, at 50 impedance of shunt arm R2 -C increases; cycles; for -20 db, the curve shows frequencies; they reduce the amplifica-the effective ratio of the voltage dividerT1 = 0.1.At 50 cycles 6.28f = 314. tion over the middle range so that thevaries with frequency. bass and treble are relatively increased. To obtain treble accentuation we con- Then 0.1 = 314C (R R2) and Apparent volume is chiefly dependentnect in series with R2 a reactive ele- 0.1 upon the amplification over the middlement which has a reactance rising with C (R + R2) -= .000318. range of frequencies and should be sub- frequency, in other words an inductance 314 L (Fig. 2-b). Capacitance in farads times resistance in ohms must equal .000318 farad -ohm s+. B+ Fora responseor 318 microfarad-ohms.If C is made falling at high fre- .01 ktf, R R2 must be 31,800 ohms. R3 R3 quenciesR 2 i s Response at other frequencies can be shunted by a con-read off from the curve. Thus if T1 = V CI V CI denser so that im-0.1 at 50 cycles as above, then at T1 = R1 R pedance falls with1, the frequency is 500 cycles and the response is -3 db. ( IIHMj:\\) MIIIM frequency( F i g . 2-c). Treble cut with the circuit of Fig. 2-c is equally simple and the response r/ R2 R2 For aresponse curve is given in Fig. 4.The curve is falling at low fre- - plotted in terms of quencies insert con- 6.28fC (R x R2) denser C in series T2 - with R1 (Fig. 2-d). R + R2 a) b) Thechartsof It is again dependent on an R -C prod- B+ B+ Figs. 3, 4, 5, and 6 make it easy to cal- -a R3 R3 culate the values of Cl,. capacitance, induc- -9

CO 12 V CI tance, and resist- RI 7-1. R ance for any given W is amount of boost or mom cut. They are plot- F.. -20 ted in terms of the 4 0-z circuit elements in o. cr+ Figs. 1 and 2: C, W 32 C R2 R2 L, R, and R2. R CC does not appear in -36 -40 thefiguresasa 0.01 0 1 0 c.) separateresistor. Fig. 2-a) Bassboost; b) treble boost; c)treble cut;d)basscut Itistheeffec- Fig. 3-Design curve for bass cutting response.

14 1948 RADIO -CRAFT REFERENCE ANNUAL uct, but the resistance is now the valueA Practical Tone Stage The coil is liable to hum pick-up and of R and R2 in parallel instead of in Suppose we design a stage to giveshould be carefully positioned away series. about 17-db boost at 50 and 10,000from power transformers and chokes. cycles and to have an over-all gain of 5. Magnetic shielding may be advisable. "Boost" Curves Figs. 5 and 6 show that B must equal For treble cut the circuit is Fig. 2-c The circuits giving bass or treble lift 0.1.GR4must be and we use Fig. 4.Taking the same db are slightly more complicated because 5 and frequency values asbefore, T2 there axe 2 variable factors.Not only - or 50. comes out at 6.7 and 2.4.The corre- the time constant of the condenser (or 0.1 sponding capacitances are .0235 /4 and inductance) and resistance, but also theA 6SH7 will give a gain of a little less .00875 Af.For the larger, .025 af is amount of attenuation introduced bythan that figure with R3 = 10,000 near enough; .015 Af in series with this R and R2 has to be considered.Forohms, 250 volts on the plate and 150 on gives.0094pf, which isquiteclose simplicity we shall denote this attenua- the screen grid, as calculated by theenough for the smaller.The circuit is tion by formula above, using 900,000 ohms as shown in Fig. 7-d.

R2 Rp, 10,000 as R3, and .0049 mho for G. The whole arrangement can be con - B= This means B will be between 0.1 and R R2 0.11, which is close enough. p,28 and we now have a family of curves, With R3 10,000 ohms and Rp 900,000, -130.07 one for each different value of B, ascalculation shows that the value of R4 22.080.1 different values of R and R2 are se-is so little different from that of R3 B .0.15 lected.There are given in Fig. 5 foras to introduce negligible error.Tak- - 8.0.2 bass lift with the circuit of Fig. 2-a; asing R4 as 10,000 ohms, R1 should be UJ 12 before, the curves are plotted against40,000 ohms to make R 5 times as great 8 a. 1'3 = 6.28fCR2.At high frequencies, as R3.If B is to be approximately 0.1, 4 the circuit is that of Fig. 2-b, and the a convenient value for R2 is 5,000 ohms. cr aw equation is 00,01 01 10 L Choosing Circuit Values T3 2.rfCR2 T4 = 6.28f - . Use Fig. 5 to get the value of 17-db Fig. 5 -Designcurve for bass boost response. R2 boost at 50 cycles with B equal to 0.1 and R2 5,000 ohms. The curve indicates 1 1 10 B has to be considered as before.The 0.28 B.0.05 response curves are given in Fig. 6. ,the required value of T3 to be about 0.1. _J Using 0.1 = 6.28fCR2, m.24 8.0.075 There is a maximum value of B which B.0.1 0.1 2.20 must not be exceeded if the desired a 6.0.15 attentuation is to be obtained, as the C x 5,000 Z +16 8.0.2 charts clearly show. We cannot work 6.28 x 50 W412 on the flat portions of the curves, as wewhich makes C equal to 6.36 X10-8 z farads or .0636 al. 0+8to want the attenuation to continue to vary W 4 as the frequency rises or falls.Since a An intermediate stage between flat CC

0 low value of B simply means that theresponse and full bass lift is usually 0.1 100 output is being tapped lower down on desirable.Let us make this 8-9 db. T4 2.f L the voltage divider R1 -R2, a tone -con- The new value obtained from the curve R2 trol tube of sufficient gain must be se-is 0.39 for 50 cycles and C = 0.25 /4 Fig. 6 -Curve for treble boosting response. lected, if we are to have any gain in for a bass lift of 8.4 db. A 0.1 14 con-trolled by two 5 -position , as the stage -or at least not suffer a loss.denser in series with it gives a capaci- shown in Fig. 7.This gives 5 degrees Gainofthestageiscomputed: tance of .0715 Af, slightly greater than of bass and 5 of treble response, making Gain = G x R4, where G is the trans -.0636 and making the fulllift at 5025 tone combinations possible.Note conductance of the tube in mhos. Valuecycles 16.8 db, with a lift of only 2.5 that all resistance values in Fig. 7 are of GR4 is given for many tubes underdb at 500 cycles.See A in Fig. 7. converted to nearest RMA standard "voltage gain" in the resistance -coupled For bass cut use the circuit of Fig. values.This will have but little effect design data tables 2-d, and from Fig. 3 we find that T1 on response curves. of tube manuals.should be 0.147 and 0.41 for drops of Multiply this figure by the selected B Adapted from an article by W. T. Cocking is Fire- factor to get over-all gain. 16.8 and 8.4 db at 50 cycles.The corre- less World, London, England. sponding capacitan- In selecting values for R1 and 113, ces are .0085 pf and TO GRID R3 should be small (10,000 to 50,000 OF FOLLOWING TUBE .0238µf. We can use ohms), and R should be at least 5 times .025 pf with .015 in R3. R1 should not be much greaterseries with it, as at than 200,000 ohms to avoid stray capaci- B in Fig. 7, giving tance effects.All this simply meanscuts of 8 and 16 db. RI .25 that a tube must be selected whose vol- AAA 0 At high frequen- 39K tage gain GR4 will be the desired figure BASS cies adopt the same 1.0251 when a low value of R3 is used. procedure. For lift, .25 0 use the circuit of6SH7 .015 .4 Fig. 2-b in conjunc-

En tion withFig.6. s LEE The value of B is L 1,0 -12 ILA Z2.2 MEG 0 unchanged, and for aw -16 lifts of +16.8 db 0.214 l .75H 0 TREBLEs 0 -20 and + 8.4 db at 10,- LEE 000 cycles T4 is 9.3 0 z0-24 and 2.5, respective- tn ly. The inductances 0 cew -32 required are 0.736 .015 R2 36 h and 0.198 h, or 5.6K 4.7K 40. say 0.75 h and 0.2 .0257 0.1 100 h.The circuitis Tg.2R+R22I° arranged as in Fig. B+ t!. 250 -500V Fig. 4 -Design curve for treble cutresponse. 7-c. Fig. 7 -Practical tone control stage with bass and treble cut or boost.

1948RADIO -CRAFT REFERENCE ANNUAL 15 GOOD FM RECEIVER

WHETHER used as an FM re-R.F. Stage of the band, where the tracking error ceiver complete with audio tendstobe greatest,isonly about system and power supply, as This stage requireslittle comment except that the value of the coupling 1/2db. The complete tuning range is designed, or as a self -powered tunercondenser between the r.f. choke and87.5 to108.5 mc, and the oscillator with the audio system of an existingthe tuned circuit is 10 Nuf.This value tunes from 98.2 to 119.2 mc. radio set, this FM design will proveoffers the most satisfactory comprom- The next point is the elimination of useful. The complete set will tune inise between gain variation and stray allr.f.filament chokes, which were allof the 100 frequency -modulationcapacitance loaded across the tunedfound to add na noticeable improvement. channels from No. 201 (88.1 mc) tocircuit. Bypassing the heater leads to ground No. 300 (107.9 mc) in the new band. The r.f. choke is wound on a formwith a .01-4 condenser (one at each Details are presented with referenceas shown in Fig. 3. The plate lead isr.f. and i.f. may be neces- to Fig. 1, the schematic circuit.Coil 11/2 inches, and the lead nearest to thesary) is desirable, as it reduces coup- constructiondetailsin the textare bypass condenser, is 1 inch. ling between stages through the heater illustrated. Parts placement is left to The r.f. coil (Fig. 4), does not use aleads. individualchoiceasdeterminedby chassis and components. form. Total length of this coil, includ- The initial frequency change of the ing the leads which are soldered direct- oscillator is about + or- 5 kc; but, Antenna Stage ly to the gang condenser, is 1% inches.when the heater temperature has stabi- To vary inductance, adjust the distancelized after approximately 30 seconds, Antenna coil is constructed as shownbetween adjacent turns. it drops to less than + or in Fig. 2. No form is used. The half - 2 kc. - From this description of the high- The frequency change does not remain turn secondary is spread slightly forfrequency tuning circuitsitcan beconstant, but varies about the reference connection to the a.v.c. junction. Over- seen that the inductances do not consistpoint within these limits. all length of the coil is 1 inch. Leadsmerely of the coil windings but are Changes in oscillation frequency due of the 11/2 -turn primary winding are formed as well by leads and gang induc- to changes in tube temperature are each 11% inches long for connection to tance. For this reason coil adjustmentsminimized by mounting the tuning con- antenna terminals. An electrical centermust be made experimentally to obtain denser and coilsunder the chassis. tap on the antenna coil primary isthe desired results. These adjustmentsThis arrangement isalso helpful in found by connecting a lead from thenaturally depend on the type of tuning preventing microphonic effects caused ground terminal, located next to thecondenser used and the receiver lay-by vibration of the tuning condenser antenna terminals, and moving the po-out employed. plates. sition of this lead along the primary Since the change in frequency during until equal signal outputs are obtainedSignal and Oscillator the warm-up period when the receiver when the input signalisapplied toCircuits is first switched on, or over a long eitherantennaterminal.Normally The first point to be noted in theperiod of operation, is small, retuning this tapping point will not be at theoscillator circuit is the elimination ofis unnecessary when the receiver is put mechanical center of the winding. the padding condenser to permit con-into operation on subsequent occasions. Coupling between the primary andnecting the oscillator coil straight across Two other points are of interest in secondary is obtained by arranging thethe tuning condenser. Deterioration inthe oscillator circuit. The first is the 2 windings along a common axis, andtracking between the signal and oscil- desirability of keeping the cathode -tap then adjusting the spacing by bendinglator circuits is remarkably small. Two- lead as short as possible. Even a lead the primary leads until the receiverpoint tracking is obtained at 88 and 108 %-inch long will cause undesirable vari- gives approximately maximum output. mc; and the errornear the centeration in oscillator grid current.This

VI V2 V3 V4 V5 V6 6BA6 6BE6 V7 6BA6 6BA6 6 H 6 6SJ7-GT 6V6 -GT P11. LOAD 5000n

11

SFS

0.5

0.211511 50 0.21050 f0.51150 .25 505 0 NEC

.01 0 5NEC .005

0.56E5 0.61E04 001.00105. 1.51 FIXED NEC. FEEDIA

06 0.202.5 V

Fig.I-Schematic diagram of the 7 -tube -plus -rectifier FM receiver. An Australian experimental design, the radiois excellently engineered and receives the American FM band,

8.511 I4On I. F. - 10.7 MC 125 NA FOSE ° T

16 1948 RADIO -CRAFT REFERENCE ANNUAL difficulty is overcome by bending the istic.In this receiver partial a.v.c. in oscillator coil so that it may be soldered the ratio 2/7 is applied. For an input ANT. } 15A BAL. directly to the tube socket. A shortvariation from 10 ANT to 0.1 volt, the ANT. lead may be helpful, initially, in deter-change in audio voltage input to the T ? GROUND mining the best tapping point on theamplifier is from 0.2 volt to 5.7 volts.I. DIAMETER'/." oscillator coil. The oscillator coil, made The bias is a combination of grid SECONDARY as shown in Fig. 5, is soldered directly PRI. TO SEC.SPACI NG leak and a.v.c.voltages.The actual ADJ. AS DESCRIBED. across the tuning condenser. The tap- biasis0.9volt(antenna terminals A.V.C. ping point can be found readily by mov- shorted), 1 volt (terminals open), and ing the cathode lead along the tinned WINDING TURNS WIRE 1.15 volts (signal input of 50 pv). These (7FrirtiTry: " is B. & copper wire until the point is foundvoltages may be measured directly on Air Core Solenoid 11/2 Tinned Copper (2) Electrical. Adj. as 22 B.&.S. Enam. which gives optimum signal-to-noisethe grids with a d.c. vacuum -tube volt- Center -Tap. Described (approx. PA" long) ratio or the desired oscillator grid cur-meter. (3) Secondary. Air Core Solenoid 11/2 19 B.&.S. Enam. rent (see below).The oscillator coil Soldered to Gang. inductance is readily altered merely byAudio System changing the length of wire soldered A relatively high -gain a.f. amplifier Flg. 2-Antenna Coil construction. across the gang. In the design (Fig. 5),is used. A 6SJ7-GT pentode feeds a the tap is located 15/16 -inch from the6V6-GT beam -power output stage.In- bottom end of the coil. verse feedback isapplied from the FORM DIAMETER 444. The other point of interest is theplate of the 6V6 to cathode of the 6SJ7. ElPLATE value of the bypass condenser used inFeedback is used to reduce harmonic the screen grid circuit.If its value isdistortion and alsoto provide tone WINDING TURNS WIRE Solenoid I-f-r;ii 22 B. & S. greater than about 500 nnf, large varia-control.Thetonecontrolprovides Total Length %" tions in oscillator grid current occurvariable bass boost. 61/4 Enam. when tuning across the required fre- Bass boost seemed particularly de- Fig. 3-R.F. Choke construction. quency range. In this circuit, the maxi-sirable because of the improved high- GRID mum variation in oscillator grid currentfrequency response.This theory was GROUND is from 180 na to 225 Aa. adequately borne out by a number of No.3 listening tests, particularly on recorded _DIAMETER 94: I.F. Stages music.Treble attenuation also isde- The i.f. amplifier has 2 stages, the WINDING TURNS WIRE sirable when listening to some ordinary Air Core Solenoid 18 B.&S. Enam. second stage serving as a driver fortypes of recordings as record hiss is Soldered to Gang. 11/2 or Tinned Copper theratiodetector;i.f.transformersrather objectionable. Fig. 4-R.F. Coil construction. are double -slug -tuned to 10.7 mc. [When The total r.m.s. distortion of the audio standard U.S. transformers are used,systemisless than 1%.Push-pull 22A: values of shunting resistors must be output would still further reduce the GRID / adjusted to conform to manufacturer'sdistortion percentage. CONDENSER P CATHODE GROUND recommendationsfordesiredband II tn width of 150-200 kc.] Power Supply This portion of the receiver iscon- WINDING WIRE Ratio Detector ventional,except thatthe chokeis Bend in Arc of Circle 18 B. & S. A 6H6 ratio detector is used following tuned to reduce to a minimum hum to Fit Across Gang. Tinned Copper the i.f. amplifier. (Use of the highervoltages in the output. Because of the Fig. 5-Oscillator Coil construction. perveance 6AL5 tube instead of thegood low -frequency responseof the 6H6 may give better results.) amplifier, the 3 electrolytic condensers tagebeing that smallvariations in The sensitivity of the ratio detector, shown are necessary; and in somecases settings are more difficult to from the driver stage, is almost directlyeven better filtering may be desirable.observe. A standard output meter, or proportional to the value of load resis-Increasing the 16-4 condenser to 32suitable a.c. voltmeter, is connected to tance used. Distortion becomes rather4 ishelpful ifthe audio amplifierthe receiver output in the usual man- severe for very high load -resistanceshows any tendency to motorboat. ner. values. Values under 50,000 ohms may Hum neutralization in the audioam- be advantageous because of these fac- plifierisachieved by adjusting theI.F. Amplifier tors and the desirability of a more.014-4 condenser across the 3-meg Assuming that the audio amplifier rapid time constant for the a.v.c. cir-resistor in the screen grid circuit ofis operating satisfactorily, a signal at cuit; but this is left to the individualthe 6SJ7-GT for minimum hum output.10.7 mc is applied through a blocking to decide experimentally for himself. Incorrect values for this condensercan condenser to the signal grid of the The band width of the complete de-lead to instability, particularly when6BA6 driver stage. The i.f. signal is tector circuit, measured statically usingthe values are too high. The hum levelamplitude -modulated (say 50% or any a normal AM generator with 50% in the final circuit is almost completelyconvenient value).First adjust the modulation, is 320 kc. inaudible on normal listeningtests,trimmer condenser on the primary of When setting the band width, it willeven with the tone control in the bass- the discriminator transformer for maxi- be found that the slug in the halfboost position. mum current through diode load re- secondary may be adjusted to have sistance, next adjust thesecondary some effect on the high -frequency peak, Receiver Alignment trimmer until 2 peaks of audio output and the primary tuning condenser may The ideal method of alignment isare found. The secondary trimmer is be made to affect the position of thewith a cathode-ray oscilloscope and FMthen set so that audio output falls to low -frequency peak. signal generator.However, completeits minimum value between these 2 alignment of the receiver can be carriedpeaks.This completes the initial ad- A.V.C. Circuit out with ordinary AM generators cover- justments. 'The a.v.c. circuit presents some dif-ing the required frequency ranges. Next, tune the signal generator above ficulty because there is no delay volt- If a sensitive microammeter is avail-and be:ow center frequency and note the age. If the full a.v.c. voltage is appliedable, say 0-25 this is shunted withpeak amplitudes of audio output at to the controlled stages, the gain ofa 3,000 -ohm variable resistor (or someabout 10.55 mc and 10.85 me (there the receiver drops so rapidly that evenother suitable value) to allow variationsshould be practically zero audio output for strong signals there is availablein sensitivity, and is placed in seriesat 10.7 mc).The magnitudes of the insufficient audio voltage to give maxi-with the ground end of the 50,000 -ohmpeaks should be approximately equal mum output.For this reasonitis resistor in the diode load circuit of the and symmetrically spaced from center necessary to compromise between audio ratio detector. A 0-1 ma meter, un-frequency. If the high -frequency peak output and a suitable a.v.c. character- shunted, can be used, the only disadvan- (Continued on page 42) 1948 RADIO -CRAFT REFERENCE ANNUAL 17 what is variously known as the "bass - reflex" or "vented baffle" loudspeaker, 80- 40- which may be applied to any ordinary 2 dynamic speaker.It has very little 060- 01 effect on the high -frequency perform- 23°- 2 anceoftheloudspeaker, but when 0 0 3 n correctly adjusted has 2 very beneficial

4 effects on the bass region. 'Ui 020- 0 First, it imposes considerable acous- 0. tic damping on the loudspeaker over a a. 20- fro - fairly wide range of frequencies below about twice the bass resonant frequen- cy; second, it has the effect of replacing

0 the very sharp impedance -frequency ° /0 2C 30

All this is brought about by the ex- power tube. RL= 2,500a 0 tremely wide variations in impedance - ohms isnormalload ;1= Zz' presented by the loudspeaker, hence the E - 4 most logical approach is to investigate resistance cutting zero the possibility of restricting the varia- biascurve a+ knee. tions in loudspeaker impedance with frequency. MO 200 300 400 500 600 700 800 900 The problem then is: how to reduce PLATE VOLTS

18 1948 RADIO -CRAFT. REFERENCE ANNUAL INSIDE CORNER VIEW

00 200 300 400 500 600 PLATE VOLTS

Fig.4,above-Fig. 3 asit appears with the loadlinecorrected. isillustrated in Fig.3,in which the damping, down to normal load resistance is 2,500 ohms. a certain minimum This normal load -line cuts the zero bias frequency, and thus characteristic approximately at the knee give very satisfac- of the curve.Increasing values of load tory performance. resistance of 2, 4 and 8 times normal Ordinary dynamic are shown on the same figure.It will speakers exhibit in- BACK PLATE be seen how unsymmetrical and dis- sufficientacoustic Fig. 6-Drawing and dimensions of the vented speaker enclosure. torted the performance would be if fulldamping and it is grid excitation is maintained.In prac- therefore desirable to introduce furtherform of an angle so as to fit right in tice, owing to the high degree of non -damping by electrical Means.This isthe corner of a room.Alternatively, it linearity, there is a rectification effectusually provided by the plate resistancecould be Mounted in the corner between which results in the load -line shifting of the output tube in parallel with anythe ceiling and 2 walls, with the speaker its position either up or down. shunt resistor that may be used. pointing downward to approximately Figure 4isdrawntoshowthe Acoustic damping on the other hand,the center of the floor.This has the corrected load lines, after allowing foractsdirectly on the "perfectrepro-advantage of reducing the floor space the shift caused by rectification.As ducer" so that it is all usefully applied.occupied by the cabinet. with Fig. 3, there is considerable dis-We therefore need to search for some If 2 bass -reflex speakers are used, tortion which is of a type particularlymethod of applying acoustic damping,their relative positions will have an ef- unpleasant to listen to. particularly at the bass frequencies, sofect on the impedance charactceristics The waveform of the output currentas to achieve satisfactory performance.of the 2inparallel.Two speakers which occurs with full grid excitation Fortunately, this result is provided by placed closely together and connected with sine -wave input, with a load resis- the bass reflex baffle.Even better re-in parallel give a more satisfactory im- tance 4 times normal, is shown in Fig.sults are gained with 2 such baffles at pedance characteristic than when placed 5. The flattening of one-half of thestaggered frequencies.This has the15 feet apart.In any actual case, it is cycle is severe and results not only in effectofreducing the forward anddesirable for impedance teststo be high harmonic distortion but also in thebackward movement of the cone andtaken with the speakers in position and productionofprominent inharmonicthereby reducing the speaker nonlinearcorrectly phased, and the final location combination tones. distortion which occurs when the voice - should be determined so as to give the One solutionisto reduce the grid coil moves out of the uniform flux 'area.flattest impedance characteristic. excitation until the distortion becomesIn other words, for a given speaker dis- If the impedance characteristic per- sufficiently low at the highest impe- tortion,this kind of baffle increasesmits the 2 speakers to be placed some dance which the speaker develops attheeffectivepower whichagiven distance apart, the writer prefers this any frequency.Another isto reducespeaker will handle under wide -rangeto close spacing, since it provides some load impedance at 400 cycles so that conditions.Moreover, the damping of semblance toa thirddimension.It the loadline for the maximum impe- the speaker is more nearly dead -beat dance of the speaker does not cut soat bass frequencies, so that the speaker 284 much below the knee of the curve. Thedoes not continue emitting sound when best compromise seems to be a combina- the source of such sound ceases.It tion of both methods. therefore gives a better reproduction of transient sounds. Speaker Damping Sufficient has been said to indicate ,o- One of the most important character-the advantages of a bass -reflex baffle 40" istics of loudspeakers is their damping.over aflat baffle, and particularly over Horn speakers have a high degree ofthe type of baffle formed by the aver- 5" age radioreceiver cabinet.Itssize does not require to be excessive, a total 14/., enclosed area of about 8 cubic feet or even less being sufficient. The enclosed back has advantages, not only in ex- Fig. 7-Dimensions of front panel of enclosure. cluding dirt, but also in preventing un- wanted reflections from the wallatmight be possible to have one speaker the back of the set. in eachoftwo adjacent corners of a If a single bass -reflex speaker is used room or even at opposite ends of a in an ordinary room, one very satisfac- room.This spacing of the speakers is tory position is in the corner so thatnot recommended unless a check of the the 2 walls and floor form a sort ofimpedancecharacteristichas been horn for concentrating the sound in themade and shown to be satisfactory. direction where itisrequired.The Construction details of the 2 vented shape of the bass -reflex enclosure isbaffle enclosures are shown in Figs. 6 Fig. 5-Distorted wave with load 4 times norm. immaterial, and it may be made in the (Continued on page 38)

1948 RADIO -CRAFT REFERENCE ANNUAL 19 Experimenters' Clip Set Many circuits may be set up by con- necting wire jumpers between clips

The parts aboveRepeating Relay and below the chas- The circuit of Fig: 3 with a few modi- sisare connectedfications may be used as a repeating permanently in therelay. skeletal circuit of The value of R7 controls the amount Fig. 2. A few re-of actuating current through the relay, movable parts arewhich is now connected in the plate cir- hooked up as re-cuit of V3 instead of the primary wind- quired for the par-ing of transformer T.Changes in val- ticular experiment.ues permit the relay to remain closed PRECAUTIONS: (or open) for an interval.The fre- always turn offquency of this operation is 0.8 to 1.6 equipmentbefore cycles, after changing values to the changing any com- following: R1, 10 megohms; R2, 560,000 ponents; nev erohms and 1 megohm (variable control) stand on dampin series; R3, 100,000 ohms; R4, 56,000 floors or touch ra-ohms; R5, 2.2 megohms; R7, 300,000 diator or pipingohms; Cl, 0.5 ihf; C3, .05 id:remove C4. while working onReversing the positions of Cl and C2 Top view of Magi-Klips set.Clips permanently connect under equipment. chassis to immoveable parts. As aslows the operating frequency to 0.25 safety measure the cycle. ASINGLE chassis equipped withchassis connects to the negative side parts and spring clips as shownof the power supply only through con-Code Practice Oscillator in the photo will provide a radiodenser C and its shunt resistor. "Learn the code! Be a telegraph oper- set, electric -eye burglar alarm, remote The 2 plug-in coils are wound to haveator!"Well, anyway, the circuit of control, phono amplifier,interphone, 1/4 -inch space between windings. Wind- Fig. 3 may help Junior get a Scout wireless record player, and other radioing data: broadcast coil, primary 79merit badge.You'll have to lay out and electronic devices operating on 117turns,secondary151/2 turns;short-some cash for a telegraph key, if you're volts, a.c. or d.c.At least 18 differentwave coil, primary 7 turns, secondarynot content with tapping 2 wires to- circuits can be set up quickly merely51/2 turns. Tight -wind these coils in thegether.Also, make a few changes in by connecting jumpers and small partssame direction with No. 27 enameledparts values, as follows: R1, 100,000 between the spring clips permanentlywire. Coil forms are 114" in diameter. ohms (fixed); R2, 56,000 ohms; R3 and wired in position on the chassis of the R4, each 390,000 ohms; R5, 1 megohm Magi-Klips kit developed by a PacificElectronic Metronome (variable control); R7, 120,000 ohms; coast manufacturer.No soldering is To illustrate the wiring procedure, C1, .005 Af;C2, .002p,f;C3, .01-4;C4 required. the clip set connections for the elec-not used.Insert the telegraph key at The idea so intrigued us that we felttronic metronome shown in Fig. 3 are x in the plate circuit of V2. Note that many readers would like to ransackgiven in Fig. 4.This multivibrator has it is in the B+ lead.Key frame and their own junk box for a few basica frequency range of 1 to 100 cycles.circuit ground should not be touched at componentsthatcouldbesimilarlyVarying the values of R1, R2, R3, R4,the same time. adapted to a wide variety of useful cir- Cl, and C2 will affect the frequency. Vary R5 for volume control, or ex- cuits.(For those who don't want to Possible additional circuits are shown, change its position with R2 and operate go tothisbother, we suggest the but without the clip set connections forit as a tone control. factory -made kit). beginners (since these are available in Tubes used are 12SL7 twin triode,a 51 -page manual, together with somePhono Amplifier 50L6 beam -power tube, and 35Z5 recti-circuits not shown here). (Fig. 5).Add to the low -frequency fier. reproduction of this electric phonograph The 24 spring clips are marked and by mounting the speaker on a 3 -foot R Rc R Gi square baffle, or even in a soap box! located on the chassis, as shown in Fig. 0 0 0 0 1, with connections as follow: 2 3 5 Lowering the capacitance of C1 will in- crease the high -frequency response and I.(R) Fixed contact of relay 2. (Rc) Movable contact of relay vice versa.Increasing C2 will increase Re Kr Cg PI 3. (R) Fixed contact of relay Rw low -frequency response. O O 0 You need a 4.Negative side of power supply 0 .0 (Indicated as ground) 6 7 8 9 10 pickup, motor, and turntable, of course. 5. (GI)Grid of first triode (VI) Stage gain may be altered by a change 5.(Rw) Relay winding in value of R1 or R2. A howl signifies (Rw) Relay winding 7. Op Cb (K1) Cathode of VI VR 8. O O 0 0 0 pickup and speaker are too close. 9. (Cg)Grid terminal of coil 12 13 14 15 10. (PI) Plate of VI II.Plate voltage (least filtered Remote "Wireless" Tuner 02. Plate voltage (most filtered) Hook up the r. f. oscillator shown in 03. (VR) Variable resistor Pa Ga F Pe G2 Fig. 6, connect it to antenna, and run 14. (Cp) Plate terminal of coil 0 0 0 0 0 05.(Cb)High voltage terminal of coil 16 17 18 19 20 a lead from the vicinity of a standard 0 6. (P3) Plate of pentode (V3) broadcast set to the tuner, wrapping 117. (G3) Grid of pentode (V3) 48. (F) Open terminal this end of the wire a few times around 19.,(P2) Plate of second triode (V2) SG3 VR TO the plate lead of V1 as shown.Now, '20.(G2) Grid of second triode (V2) 0 0 0 -21. '(3G3) Screen grid of V3 21 22 2423 stations tuned -in on the tuner will be 22. (VR) Variableresistor picked up, amplified and heard from :23.Negative side of power supply '24. (tO) Variable tuning condenser Fig. I-Marking and locationof clips on deck.the big radio set, when it is tuned to 20 1948 RADIO -CRAFT REFERENCE ANNUAL 12

ANT. 15 22 13 10 19 0 R4 R3 C3 220K 220K .005 50 L6 14 C CI .05 02 TRIMMER 9 O VI PLUG -J TUNING IN .0 COIL COND. SPKR. 4 0 RI 12SL7 R2 R5 1MEG IOMEGS 1100K

THIS RESISTOR IS PERMANENTLY 35Z5 WIRED UNDER THE CHASSIS I .01 V4 Fig. 2-left-Schematic ofpermanentwiring.Fig. 3-above-Circuit 12 0 0 of electronic metronome. 150 V DC DC +110V I 20 K 56K frequencycarrier, plifier and a relay, makes up the cir- FILTER 4, CHASSIS 117 V we arrive at Fig. 7.cuit of Fig. 11. When the circuit values A.C. 20 or are properly adjusted, the relay will or Use only two D.C. three feet of wireoperate, and the equipment acts as an HEATERS 0 PIGTAIL LEAD for the antenna, so approachorproximity alarm when 35Z5 50L6 150o. 125L7 that radiation willsomething approaches the short wire "not exceed the dis- used as antenna. (LINEINCORD) tance to your big The local oscillator of a superhetero- the same frequency as the tuner.Ifradio set, from which a whistle will bedyne receiver connected to its usual an- this tuner is close to the radio set, theheard when key is operated and bothtenna radiates a signal which may be lead will not be needed, in most cases. units are properly tuned to each other.picked up with the unit of Fig. 11. VI Or headphones may be connected in the detects, V2 amplifies and V3 amplifies B supply lead at X and you have aWireless Record Player or detects. With V3 operating as a de- C.W. receiver; or, reduce oscillation un- As with the code transmitter, for thetector, the relay is closed by no -signal til the circuit merely regenerates, as bywireless record player only a very few feet of wire will be needed far an an- ANT. TO REMOTE SET 0 0 tenna, or perhaps none at all (if there is JUMPER WIRES BETWEEN sufficient radiation from the plug-in L2 C3 RFC TERMINALS -4. coil and by back-up through the power V 1500pilf line).(See Fig. 8.)Music picked up 0 0 by the crystal phono pickup is heard in .002 250µf a nearby radio set, when both are in tune, without any direct connection be- C2 tween the two.

I Home Broadcaster LI CI RI Carrying the idea of Fig. 7 one step 56K further leads us to a novel fun maker TUNING CONDENSER IS 12 (Fig. 9).The 4 -inch PM loudspeaker, PERMANENTLY WIRED when spoken into, acts as a microphone; ACROSS COIL. phono records may be played at the Fig. 6-Schematic of "wireless" tuner circuit. same time. C plate current, resulting from a value of Radio Receiver about 220,000 ohms in Ri.The relay Fig. 4-Clip connections for circuit of Fig. 3. Fig. 10 is the circuit of a full-fledgedwill open when a signal is received from broadcast and shortwave loudspeakerthe set's local oscillator. With V3 oper- connecting the (1-megohm) variable re- set.Since only a standard plug-in coilating as an amplifier, and R1 of about sistor across L2, and you have a 1 -tubeL1 -L2 is used in the tuning circuit, due390,000 ohms to maintain relay open radio set. to broad tuning of the resonant circuit (Continued on following page) the design is neces- 12 Code Transmitter sarily suitable only By combining the multivibrator ofwhere there is little Fig. 3 for an audio -frequency tone with likelihood of inter- the oscillator of Fig. 6 for a radio -ference between SHIELD VI V3 stations.Resistor XTAL C2.05 R controls regen- PICKUP SPKR. eration. (Condenser C is also shown in Fig. 2.Adjust for best results.) Vary dis- tance between L1 and L2 as need be. Radio - R2§ 100K Controlled 1207.0.556K Relay

:2 The oscillator of LL Fig. 5-Circuit for simple phono a,plifier. Fig. 6, plus an am - Fig. 7-A simple tone -modulated home -transmitter.

1948 RADIO -CRAFT REFERENCE ANNUAL 21 (no -signalcondition),therelaywill in Fig. 14 will op- LJ ANT 500sµf close when the radio set's local -oscilla- eraterangesbe- tor, or equivalent, signal is received. tween.01second Values for V3: detector operation, R, and 30 seconds, de- 10megohms,asamplifier,560,000 pending upon the ohms. settingof R1; R and C too control Contact Detector timing. Connecting Oscillation, in the circuit, of Fig. 12,the relay between is determined mainly by the value of R, the 2sectionsof usually about 1 megohm. With circuitthe filter (termin- oscillating, the average plate current als11 and 12 as passing through the relay winding is shown)resultsin too small to actuate the relay.If a per- son touches the contact area, which maymore positive be a metal plate or a piece of wire, thethrow-in and drop- oscillator will cease oscillating.The out points.Before resulting plate current rise, due to lack starting another of negative grid bias, will energize thecycle, C must be relay, causing a bell to ring, etc. discharged by 12 shortingitwith Fig. 10-Complete6.c. and shortwave receiver. Electronic Switch switch S. In Fig. 13 is shown how a very small Welding, photo enlarging,etc.,re- 'CONTACT current may control very large amountsquire the momentary operation possible AREA of power-an example of a system nowwith this circuit. 12 RFC ANT.L "Electric Eye" Burglar Alarm A photoelectricrelay -type burglar alarm or door -opener may be devised by making a few changes in Fig. 13. PHONO PICKUP 250µµf Connect a type 868 photocell and a 560,000 -ohm resistor, in series, across 002 switch S (which is to be left open). Connect R to terminal 11 instead of 12. The 1-megohm variable control should Fig. 12-Circuit of asimple electronicalarm. be connectedto ANT. 56K 12 2.2MEG points (a) and (b). f RFC Itcontrolssensi- 390K Fig. 8-Schematic for wireless record player. tivity.Withthe photocell dark, set I500µµf finding scientific and industrial appli-itso relay arma- RELAYr cation. A few millionths of an ampere tureis just held may cause a relay to be actuated, thus down (condition of 002 L closing a circuit carrying, say, 300 watts maximum sensitiv- 250µµf at 117 v. ity). Note that high Note that the control grid of V3 is sensitivityresults left floating(unconnected).With Sfromhavingthe open, the control grids of V1 and V2control grid of V3 also float, and the 2,500 -ohm relay re-unconnected. mains closed; with S closed, these grids lose their negative charge, causing theirIntercom 0 plate currents to rise.The resultingSystem i voltage drop across R causes the screen - Connect the Magi- Fig.I I-Control circuit operated by signal from aremote transmitter. grid voltage on V3 to' drop; reduced Klip chassis as RELAYr plate current through the relay causesshown in Fig. 15, with the modification VI V2 V3 it to release. of Fig. 16, so that 2 loudspeakers are Ito Interval Timer available to serve as transducers (loud- The interval within which the relayspeaker-microphone, optionally), and 12 ANT. you have an inter - communicator or 2- I w a y telephone. 390K 390K 120K 500µp.f .002 SwitchesS1and -=- S2 are preferably press -to -talk o r momentarytype, normally up (list- 12 11 ening). Any tend- Fig. 13-Electronic switch for power control. ency of the ampli-Line Amplifier fierto motorboat Modify Fig. 15 a little and you have may be reduced bythe fundamental principle of telephone increasing the val-repeaters, broadcast line -terminal am- ues of resistors R1plifiers, and such equipment.Connect and R2, or by re-points Y and Z and break leads at x 1 ducing the valuesand x 2.This isolates V2, which is not of R3 and R4. Use used.Now insert condensers of .01 As,/ 0 shielded speakerand .05-4 at X 3 and X 4, respectively.

Fig. 9-Using speaker as microphone for home broadcasting. cable. Change values, to: R, 15,000 ohms; R1,

22 1948 RADIO -CRAFT REFERENCE ANNUAL ates both as volume control and to es- INCLUDES VI V2 tablish detection characteristics in tube OUTPUT section V2.In operation, terminal A is TRANSF: usually a clip, fastened to the chassis L under test; terminal B is a probe, ordin- arily applied first to the output circuit and then to preceding -stage plate and SPKR. SPKR. grid circuits, successively, noting where in circuit the desired signal of a broad- Fig.16-Intercom speaker -switchingcircuit. cast station or signal generator is re- R6 (not used); Cl, .002-Af. RI ceived properly. Condenser C is not used.At X3 and 220K I MEG Following are the required changesX4, connect condensers of 500 gizf and in value (Fig. 15) for use as a signal .05 Af, respectively. tracer: R, 1 megohm; R3, 1 megohm Adapted from Magi-Klips Instruction Book, by per- Fig. 14-Circuit of simple interval timer. (variable control); R4, 100,000 ohms; mission of Deer & Taylor Co., Berkley 9,Calif., mfrs. II0 12 TO TERM 7 OF 12 0 12SL7 SOCKET

RI R2 3'90K 220K GI .01 (.005

VI B 11V2 0 rl

INPUT R6 I MEG R3 VOL. R4 =4": R5220K 120 56K.05 390K 100 K CONT. 0.5 0 0 X OUTPUT A 4 O II Fig. 1S-Intercom amplifier.Speaker switching shown in Ffig.16. Fig. 17-Heat-sensitive relay.Temperature changes unbalance bridge.

220,000ohms; R5, 100,000 ohms. Resis- tor R4 is not used, nor is condenser C. The output connects to the output trans- former. Feedback Amplifier Thermal Relay THE average well -planned audio between top and bass, that tone control amplifier has ample high -fre--except in a most elaborate form- An importantapplicationofelec- quency response and, when usedcannot give. The use of a pentode need tronic techniques is the control of tem-with the average loudspeaker, exhibitsnot dismay the fidelity enthusiast. Con- peratures in industrial processes, thea response which rises with frequency.denser Cl (0.1 AD, permits excellent change of resistance of various metalsThis condition 'may be counteracted bybassresponse,whichprovidesthe being used as an index of its heat underthe use of bass boost to level the re- "body" of recorded music.. test conditions.In principle the circuitsponse, then applying over-all gain con- In this circuit, the value of R1 pre- of Fig. 17 is used. trol to limit the output volume. vents instability or overloading; and, To adjust thiscircuit, balance the Such a plan was followed in thiswhile feedback is made variable up to bridge by increasing the value ofR4 amplifier.There is no tone control28%, it cannot be reduced to a figure until the relay is actuated, then reduceas such, but the constants of the nega-lower than 12%, owing to fixed resistor R4 until the relay drops out.If heat istive feedback circuit have been chosen R1. At 12% the feedback is sufficient now applied to R2, which has a nega-carefully to give just that right balance (Continued on page 44) tive coefficient of resistance, the relay will operate.It may be helpful in find- ing the balance point to connect the speaker at x ; listen for minimum hum. This a. c. voltage is the amplified un- balance voltage applied to the control S grid of V1, through the bridge from VI point A (12 volts).

1VOL.GONT. Signal Tracer 500K Although a modification of the circuit of Fig. 15 concludes this article many readers realize by now that more circuit arrangements are possible.The varia- tion of the circuit is useful to service- FEEDBACK men as an aid in locating radio re- CONTROL ceiver faults.It is an r. f.,i.f., and 50 K a.f.wide -band(virtually aperiodic) amplifier. Any signal at these frequencies will be heard from the loudspeaker.Resis- tor R3 (now a variable resistor) oper- (CHASSIS)

1948 RADIO -CRAFT REFERENCE ANNUAL 23 New Circuits From Old Many ideas in the old circuits could be used profitably today

THE average experimenter givestolerate this clumsy method of varying reduce interference by enablingthe littleheedtopresent-day re-the coupling.So fixed coils and con-equipment to be "beamed" on a given ceiver circuits, usually taking for densers have been made to do the work broadcast station. granted that they are sensitive andin cases given here. The Unidyne was designed to work selective.Thesedesirablequalities Of these circuits,7, as altered forwith extremely low plate voltage, as in- however, were evolutionary, and theimproved operation in the light of more dicated.Note that the usual control - paths they traveled through the yearsmodern, equipment and facilities, aregrid is positive biased, and the signal from the early '20's(when most ofpresented below. isfedinto what isordinarilythe them originated) were devious indeed. These circuits include the Cockadayscreen -grid of the tube. Ahost of old-timecircuits which4 -Circuit Tuner, Unidyne(directional Until the proper number of turns is served a purpose in their day haveloop receiver), Sir Oliver Lodge N Cir-found for the loop antenna, experiment ANT. 8+ cuit, Filadyne, Crystachoke(selectiveto match the particular variable con- J crystal set), and Reflex (with crystaldenser and to suit individual carrying - CI and with tube detector). case dimensions. OO Cockaday 4 -Circuit TunerN Circuit This famous circuit, modernized as Sir Oliver Lodge invented one of the in Fig. 1, is easy to operate.Further,most ingenious hookups of his time, and it exhibits the sensitivity and selectiv-called it the N circuit. A regenerative ity of an average superheterodyne with and non -radiating modern versionis 200µ0 a stage of radio -frequency amplifica- given in Fig. 3. The frequency to which tion.The tube may be almost anythe resonator, C -L2 in the figure, is available receiving type triode, usingtuned is called the N frequency. A+ rated voltages. In the N circuit we have a closed The antenna is center -tapped to theresonator consistingofarelatively A- large inductance coil and a relatively coil A, which consists of 65 turns ofsmall capacity.This is excited by im- Fig.I-Modernized Cockaday 4 -circuit tuner. #18 D.C.C. or S.C.C. enameled wire on a tube 21/2inches in diameter. The pulses it receives from a single wire, gradually faded from the memories ofbottom end of this is connected to aand builds up only those oscillations to all but the oldest experimenters; whilesmall inductance coil B, the bottom end which it is itself in tune.Exceedingly to the newer generation they might toof which is grounded. The coil A mustsmall amounts of energy either from all intents and purposes have neverbe shielded from the rest of the circuit.an antenna or from the grounded plate existed. Coils C andDare identical, and consistof a tube or from both in combination Many enjoyable hours can be spentof 65 turns of the same size wire onare sufficient to energize the circuit. trying out old and almost forgottena form 21/2 inches in diameter.Coil B, The resonator is to be left almost circuits.There is the additional in-which is1 or 2 turns only of #14free from the antenna. It must be con- centive of trying to make the circuitssquare bus bar, is wound over the coilnected to it in some degree, or it would meet modern conditions, for when theyC, about halfway along its length. not respond at all, but the more limited were in vogue very different conditions The action of tuning is as follows:the connection the better. obtained in the way of transmitters, Now, if a suitable choke, Ll, is placed components, and circuit techniques. Rotate condenser C2 until desired sta- tion is heard, however faintly.Then in the plate circuit of the tube and if the grid has alternating potentials of 9V+ rotate C1, which should bring it up in REGEN strength.The circuitwillprobablya frequency N applied to it, and if the LOOP PH. 6V+' oscillate, so C3 (the stabilizer) is ma-plate is also grounded, this plate cir- nipulated until oscillation ceases and the cuit will pulse gently at that frequen- --desired station isreceived free fromcy. (B- is not connected to ground.) RFC 8- TUNING interference.A little practiceisre- Further,ifthese two circuitsare LOOP quired before the 200µµf set can be handled B+ 90y 8+120V 2 MEG -- properly. A -Wr- This set requires 500µµf 8- 500 300 a long, high aerial µµf up to 300 ft. long, *Ff A+ C VI to obtain best re- TUNING COND sults. CI )1' 100µµf Unidyne R EGEN. Fig. 2-The Unidyneuses a9-v B -battery. 100µµf Thecircuitof Allthe circuits given have been re- Fig.2will make LI L2 cently tested and tried, and where im- an interesting little C+ provements suggested themselves these portable, if a bat- A+ have been incorporated and a totally tery -type screen - new circuit evolved.The old circuits grid orpentode ® A 1' used moving coils to produce regenera- tubeisused.Its tion effects, but no one nowadays would use of a loop helpsFig. 3-Modernized version of the N circuit. Itisnon -radiating.

24 1948 RADIO -CRAFT REFERENCE ANNUAL now combined, the N circuit will build up energy both from the antenna by collectionand from thecapacitance effect between the antenna and ground. This feeble regenerative tendency is just what is needed to overcome the resistance and damping in wires, etc. Now, supposethislimitisover- stepped, so as to give true regeneration, and suppose the N circuit is tuned to a frequency different from that of the incoming signals.If the difference is great, the circuit can oscillate only at the natural frequency of the N circuit, TO SEC and therefore it will not respond.But OF T. if the difference is very slight, the in- C coming signalswill build up to an REGEN. amount in accordance with the energy curve of a tuned circuit, and heterodyn- ing will take place in the circuit.The antenna being a collector only, and out of tune with the incoming signals, can- not respond and therefore cannot radi- Fig. 6-Reflexed t.r.f. receiver using crystal detector. XTAL may be aI N34 crystal unit. ate.This is why the circuit is non - radiating. dyne circuit is the use of a choke coilof capacitance without affecting the in the filament circuit.It is wound insignal strength.The other preset, C4, 2 sections, with about No. 24 wire, onhas to be set to about 50 Auf for best an insulating bobbin.This its a dioderesults. detector circuit, but does have the merit By reducing the capacitances of C2 of operating without B -supply.It isand C4, still greater selectivity could not selective nor is it much more sen- be obtained, but at the sacrifice of some sitive than a good crystal set, but atsignal strength.In all cases, the two least itis novel.The best detectioncoils must be set in position of zero point is critical and controlled mainlycoupling, that is with their axes at by the rheostat R. right angle to each other. Tuning may be done by using an iron Crystachoke (Selective slug -tuned coil as Li, or a variable con- Crystal Receiver) denser and plug-in fixed or adjustable Here is a circuit for the crystal en- coil. Fig. 4-The Filadyne is a diode detector. thusiast.In this circuit two preset The use of one of the new crystal Choke Ll may have about 60 turnscondensers, C2 and C4, are employed, in the "1N" 'series is recom- of No. 18 or No. 20 wire on tubingeach with a maximum capacity of 100mended for Xtal. 2 inches in diameter.Coil Ll may have/.44, though in normal use only about 100 to 120 turns on the same size tube.20 or 30tcptfis in use.The function ofReflex (With Crystal The antenna circuit is aperiodic. Con-C2 is to couple together the two tunedDetector) denser Cl controls regeneration. circuits, Li -CI and L2 -C3.Condenser In the t.r.f. receiver, a radio program ANT. C 2 C4 is in the nature of a "tapping" ca-is amplified at r.f., detected, and then pacitance, reducing the crystal damp- amplified at a.f;., using successive tubes ing.Coil L3 is an r.f. choke connected (in cascade) for each stage.In the in series with the headphones across the reflex receiver, signals are amplified at crystal detector, as is common in allr.f. in the first stage, detected in the Crystachoke circuits. second stage, and then amplified at a.f. FIXED In practical test the preset condenser LI L3 in the first stage instead of a third. XTAL C2 could be set to a very small value (Continued on page 44) C

PH. VI V2 Fig. 5-The Crystachoke, a novel crystal set. .0001 /JL The wave trap in the antenna lead-in is of the rejector type.Its purpose is to prevent strong local stations break- LI ing through, when other stations are tuned in.The coil is tapped one-third 85 RFC the way down, and with the trimmer in I MEG MH shunt should tune across that portion of the broadcast band in which inter- ference is greatest.It should be wound 50K x .001 VQL. 85 11 with about No. 16 or 18 wire. TO SEC CONT. Tube V1 should have a plate impe- OF T MI4 )1. dance of about 20,000 ohms, or even less 2 (the 1G4 -G, for example, operated at - 25K 3 v grid bias) for best control of re- B +135 V generation; also try reducing the B 0 voltage. Tube V2 is a pentode or beam - B+ 90 V° 0.1 power tube. A+o Filadyne As reference to Fig. 4 will show, the - ca,B-,A-0 outstanding circuit element of the File - Fag. 7-Reflexedt.r.f. receiver using a grid -leak triode detector. VIisa beam -power tube.

1948RADIO -CRAFT REFERENCE ANNUAL 25 Left-Driver operates dashboard pushbutton, sounding supersonicwhistle. Center-and right-two views of microphone above garage door.Sensitive side faces down. whistle frequency is above20,000.cyc- les and substantially constant over wide ranges of vacuum, but varies with air temperature. Theweather -protectedmicrophone installed outside the garage, about 7 to 9 feet above the ground, is of the PN crystal type,its weatherproofed element being mounted on a resonated duraluminum diaphragm. It connects to a 5 -tube amplifier and stepper relay inside the garage (see diagram). Tubes remain on continuous- ly. Tube V1 and tube section V2A are voltage amplifiers; V2B is a limiter to maintain constant voltage, feed to the discriminator V3 (to compensate for volume variation caused by automobile Supersonic position, wind and air conditions, and other factors); Li -C2 resonate to the 'whistle frequency, enabling tube sec- tion V3A to rectify and deliver a nega- Door Opener tive voltage to the grid of tube section V4A, operating the relay circuit and THE Vendo Automatic Door Open- whistle and control in an automobile;starting (or stopping) the door opener. er is an ingenious device whicha microphone and door -control key out- To avoid erratic operation caused by presents to radio servicemen elec-side the garage; an amplifier, motorpick-up of unwanted supersonic sounds, tronic and mechanical problems roughly and drive mechanism, and door -controlsuch as jingling coins or keys, rustling button inside;and asecond button <7, -,RESONANT CHAMBER of cellophane wrappings, etc., a dis- within the home (if desired). criminator circuit is used; L2 -C1 reso- The "silent whistle" in back of thenate at a different frequency (usually \s, car's radiator grille faces into a reflect-higher) than L1 -C2. Signals passed by or which beams the vibrations forward, L2 -C1 and rectified by V3B develop a while protecting the whistle from rainpositive bias on the grid of V4A. But TO VACUUM and foreign matter. A flexible tube and most unwanted sounds include a wide -,:fA1R PASSAGE tee join the whistle to the vacuum lineband of frequencies and actuate both Cross-sectionaldetailof supersonic whistle. of the window wiper; a button -actuatedhalves of the discriminator V3A and valve on the instrument board controls V3B. Resultant bias on V4A will then equivalent to usual servicing.In thethevacuum pressureactuatingthebe zero or positive. Since V4A actuates particular use describeditinvolves, whistle. Or, a vacuum source may bethe relay circuit only when its grid is mainly, the installation of a supersonicobtained at the intake manifold. The (Continued onpage 45)

.0001 .0001 )1--- 30µ0F

L2 47 K 1.8 MEG 10 OK 80mh,L 80mb

500/40F

r AM PL. --, LIMITER--- DISC RIM. --1 RELAY RECTIFIER -7 CONTROL VI 100K 1MEG V2A V2B'.001 V3A V3B V4A V4B V5 6SJ7 6SL7 61-16 6SN7 6X5

FUSE 2A 210v B.

MICROPHONE 2201<470K 210 V V 0.1= STEPPER 117 V INPUT 6.5K RELAY T5 A.C. )1- 0 6.3 V .03 10 AMP. ,mEG!T40 2470 -10 1 MEG 10K CONTACTS 4.7K 10 - TO MOTOR

Circuit diagram of supersonic amplifier.Operating frequency isabove 20,000 cycles. Steppingrelay operates the door -opening motor.

26 1948 RADIO -CRAFT REFERENCE ANNUAL Low Level Tone -Control

ASOLUTION to the problem ofthrough V2 has been reversed in phaseusual resistance -network method. maintainingtonequalityac- with respect to the signal being passed In the phase-splitter tube the grid ceptable to the listener, wherethrough the treble control circuit.By returns have been decoupled to ground reproduction is held to low volume lev-feeding these signals into the oppositeto prevent any signal being fed from els, is offered by this circuit arrange-grids of the phase splitter, the 2 sig-the cathode to the grids, as this would ment. The controls afford linear volume control, treble lift, bass cut, and bass +400 V lift; the latter control tends to make unnecessary any provision for treble 50 K 50 KZ 10K cut. At low volume levelsthe earis lesssensitivetoextreme bassand 50K 50K > I 30K treble.Consequently, for good repro- TO OUTPUT - duction it is necessary to have a very STAGE GRIDS great lift of the extreme bass, and also VI 0.I V2 5 V3 a fairly large lift of the extreme treble. It is also an advantage if the tone con- 6J5 6J5 trol circuit has a position that is truly linear and not just halfway between treble cut and treble lift. These requirements haveallbeen .05 met in the circuit shown here, which is FM- designed to work straight into a push- pull output circuit. The circuit gives a INPUT .002 Z270K large over-all gain, the input from a pickup being sufficient to load fully an (2A3's are R output stage of PX25's. R2 500 similar U. S. tubes.) TREBLE BASS VOLUMERI 100K The signal is amplified in the first CONTROL 0.5 MEG I MEG tube after passing through the volume .1 MEG control R1, and from the plate it is 10 K .02 taken to 3 potentiometer circuits. The 1K 20 K first 2 chains are connected to the ends potentiometer R2. The value of R2 is high enough effec- tively to isolate the networks connectednals are brought into phase at thecause a bad misbalance of the signals at the 2 ends, and consequently move-plates.A further advantage. of thisat the plates. ment of the contact arm of R2 gives a method of mixing is that there is no Condensed from an article by Hill,courtesy variation from a truly linear signal toattenuation, as there isin the more 71" iretess World (London, Eng.), December, 1946. a large lift of the higher register. An attenuation ratio of 5:1 is incorporated in the linear position to give constant volume over the whole range. Power Stabilizer The bass control is incorporated in a normal bass lift circuit, the bass voltage developedacross condenser C being THE voltage of a.c. power lines is This circuit removed almost the last tapped off on the bass -control poten- varying all the time, by smalltrace of ripple from the output of a tiometer. To get sufficient bass lift it amounts and with considerable power pack.Also, it removed all but has been found necessary to amplifyrapidity, due to changes in the loadsthe slowest of those variations due to this bass signal in a separate bass am-connected to the system. These changespower -line voltage fluctuations. plifier tube V2. find their way to the output side of a The components needed are standard The two signals, the treble and mid-power pack.It is very interesting to and easily procurable.In essence, the dle frequencies from R2 and the ampli-connect a power -pack output througharrangement consists of a normal power fied bass signal from the plate of V2, a blocking condenser and amplifier to apack, with ordinary filtering design to are mixed together in the correct phasecathode-ray tube; if the amplifier has reduce the ripple to a value well within in the last tube which also acts as aa reasonable performance down to 10 (Continued on page 45) phase -inverter amplifier.The signals cycles or less, the output voltage will at the plates are equal in amplitude be seen to be subject to violent and ran- and opposite in phase, each being the dom variations.It would be a bad sum of the treble and bass frequenciessituation in which the variations ex- fed into the 2 control grids. ceeded a fraction of a volt, but sudden In this type of amplifier a currentsmall changes can be a greater nuis- of larger change in the first half of the tubeance than slow variations INPUT FROM causes a change in the common cathode amount. voltage, which produces an approxi- There are several well-known stabil- POWER PACK mately equal and opposite change ofizing circuits, but the output current in current in the second half of the tube. many cases has to be passed by a large In the present circuit the 2 tone -tube, or by a battery of tubes in paral- controlled inputs are fed into the gridslel; or gas -discharge stabilizing tubes of the phase splitter simultaneously.are needed; or the load is paralleled The reason for mixing in this mannerby a large tube so that the total current isthat thebasssignalinpassingalways equals the full -load rating. Fig. I-Simple regulator. See text for values.

1948 RADIO -CRAFT REFERENCE ANNUAL 27 TRACER RECEIVER OPERATABLE as a t.r.f. receiver, The operating voltages are standardAlternatively, a front panel can be this 5 -tube circuit acts also, re-for this type of tube selected for theadded and the chassis housed in a cabi- versibly, as a signal tracer inr.f. circuits, and variable cathode biasnet to give it a more commercial ap- following through a defective radio sethas been added to permit control overpearance as an important piece of test a signal from loudspeaker to antenna,the r.f. gain. equipment. or as an r.f., i.f., or a.f. circuit feeding The audio section of the receiver is Adapted from anarticle by H., M. Turner, Lock- any of these frequencies into their re-of straightforward design. port, N.S.W., in Radio and Hobbies spective sections of a faulty radio set, Coupling the antenna to the tracer (Australia). as a time -saving servicing facility. Forallows signals to be tuned in the norm- example, a well-known commercial re-al way, the instrument behaving exactly ceiver developed the fault of droppingas a t.r.f. receiver with 2 tuned stages. in volume, but was so touchy that ap- The signal from the can beCheck Your plication of the test prods in any posi-diverted to the audio end of a receiver tion restored normal volume. By eon- under test.This section of a set can GANGED 1 Frequency! T.0001 .00025 HE Central Radio Propagation /or .0005 Laboratory of the National Bu- reau of Standards' precision radio 6U7 -G 6U7 -G 6V6 -G transmissions are carried on 8 frequen- cies that insure reliable coverage of the United States and extensive coverage 0001 SPKR of the world (see table, pg. 29).Vertical, nondirectional transmitter antennas are used. To be certain of receiving the sig- nals of WWV uninterruptedly for 24 JI hours, it may be necessary to use sev- INPUT 2 MEG eral receivers, combining their output; O however, skilled operators can antici- patetimesforshiftingfrequency, .25 MEG thereby reducing the requirement to 2 receivers. TheBureau'sstation WWV near Washington, D. C. continuously trans- mits radio and audio frequencies that 100V serve as national standards for pre- cision measurements. 25K V.D. .25MEG 0.5 MEG 0.1 T.000I Standard Radio Frequency The national standard of frequency 5Y3 -G isof value in radio, electronic, acou- 6.3 V 2000n. stic, and other measurements requiring 0 0 8 an accurate frequency. Any desired ra- dio frequency, including fre- T quencies, may be measured accurately 000000 0.9 ZOO in terms of the standard frequencies. This may be done with the aid of one or more auxiliary oscillators, harmonic generators, and radio receivers. netting the tracer to the audio stages,therefore be checked for intermittent The detector, and i.f. stages in turn, the accuracy of each of the radio carrier faults while other work is proceedingfrequencies, as transmitted, isbetter fault was eventually found in the 6A8.on the bench.In this use alone the than one part in 50,000,000. The tracer circuit is a simple t.r.f.tracer will pay for itself almost at once. arrangement which uses standard r.f. Alternatively, the functions can beTime Announcements coils. Two fixed mica condensers arereversed and the signal from the tuner The audio frequencies are interrupted switched in parallel with the tuningportion of a receiver can be fed throughprecisely on the hour and each 5 min- gang to extend the tuning range be-the audio system of the tracer. Testsutes thereafter; after an interval of yond the low -frequency end of theare possible on pickups, high -outputprecisely 1 minute they are resumed. broadcast band and at the usual 450-microphones, and loudspeakers. The beginnings of the periods, when 470-kc i.f. setting for superheterodyne The original tracer, incidentally, was the audio frequencies are interrupted, receivers. built from parts salvaged from an old-are in agreement with the basic time Coil switching could be provided tostyle t.r.f. receiver.In fact this typeservice of the U. S. Naval Observatory cover additional frequency ranges, butof receiver can be adapted for the pur-so that they mark accurately the hour. has not been considered essential so far. pose with very little alteration. Mod- and the successive 5 -minute periods. The r.f. sections of a receiver can beern tube types are suggested in the Eastern Standard Time is announced checked by plugging the test lead intocircuit, but any equivalent older. typesin telegraphic code each5minutes. jack 1, which feeds the signal to the would suit. This provides a quick reference to cor- grid of a 6U7 -G r.f. amplifier.The The mechanical construction can fol-rect time where a timepiece may be in signal is then tuned in at the approp-low the inclinations of the individuall error by a few minutes. The zero -to -24 - riate frequency by rotating the main serviceman.It is possible to add thehour system is used, starting with 0000 tuning dial.Itis passed on to the 1or 2extra terminals and controlsat midnight. The time announcement second 6U7 -G r.f. amplifier and thencenecessary to a t.r.f. receiver chassis andrefers to the start of an announcement to the diode detector. leave it in much the same original form.interval, i.e., when the audio frequen-

28 1948 RADIO -CRAFT REFERENCE ANNUAL cies are interrupted. It occurs immedi- ately after the beginning of each 5 -min- Identify that Station! ute interval. At the hour and half-hour it is followed by the station announce- THE nations which signed the General Radio Regulations agreed upon at ment in voice. the International Telecommunication Convention in Madrid, Spain, in 1932, and the revisions at Cairo, Egypt, in 1938, use the prefixes listed Standard Time Intervals here to identify every radio transmitter as to its nationality. The first letter There is on each carrier frequencyor first two letters of the call signal serve this purpose. a pulse of .005 -second duration which occurs at intervals of precisely 1 sec- Country Call SignalsFinland OFA-OJZ ond. The pulse consists of 5cycles, Chile CAA-CEZCzechoslovakia OKA-OMZ each of .001 -second duration, and is Canada CFA-CKZ Belgium and Colonies ONA-OTZ heard as a faint tick when listening to Cuba CLA-CMZ Denmark OUA-OZZ the broadcast.It provides a useful Morocco CNA-CNZ Netherlands PAA-PIZ standard time interval, for purposes of Cuba COA-COZ Curacao PJA-PJZ physical measurements, and for quickBolivia CPA-CPZ Netherlands Indies PKA-POZ and accurate measurement or calibra- Portuguese Colonies CQA-CRZ Brazil PPA-PYZ tion of timing devices or very -low -fre- Portugal CSA-CUZ Surinam PZA-PZZ quency oscillators. It may be used as anUruguay CVA-CXZ U. S. S. R. accurate time signal. On the 59th sec- Canada CYA-CZZ Sweden SAA-SMZ ond of every minute the pulse is omit- Germany Poland SNA-SRZ ted. The 1 -minute, 4 -minute, and 5 -min-Spain EAA-EHZEgypt SSA-SUZ uteintervals, synchronized with the Ireland EIA-EJZ Greece SVA-SZZ seconds pulses, are marked by the be-Japan EKA-EKZTurkey TAA-TCZ ginning or ending of the periods whenLiberia ELA-ELZ Guatemala TDA-TDZ the audio frequencies are off. Japan EMA-EOZ Costa Rica TEA-TEZ A time interval of 1 second markedIran EPA-EQZ Iceland TFA-TFZ by the pulse is accurate, as transmitted,Japan ERA-ERZ Guatemala TGA-TGZ to 1 microsecond (.000,001 second). AEthiopia ETA-ETZFrance, Col. and Protect THA-THZ 2 -minute or longer interval is accurateJapan EUA-EYZCosta Rica TIA-TIZ to one part in 50,000,000. Germany EZA-EZZFrance, Col. and Protect.._. TJA-TZZ The 1 -minute interval is provided inFrance, Colonies and Protectorates.... F U. S. S: R. ordertogive time and station an- Great Britain Canada VAA-VGZ nouncements and to afford an intervalHungary HAA-HAZ Australia VHA-VNZ forthecheckingof radio -frequency Switzerland al3A-HBZNewfoundland VOA-VOZ measurements free from the presence Ecuador HCA-HDZ British Colonies and Prot. VPA-VSZ of the audio frequencies. Switzerland HEA-HEZIndia VTA-VWZ Poland HFA-HFZ Canada VXA-VYZ Standard Audio Japan HGA-HGZ Australia VZA-VZZ Frequencies Haiti HH A.-HHZ U. S. A. Two standard audio frequencies, 440Dominican Republic HIA-HIZ Mexico XAA-XFZ cycles per second and 4,000 cycles per Colombia HJA-HKZ China XGA-XUZ second, are broadcast on radio carrierJapan HLA-HMZ France, Col. and Protect XVA-XWZ frequencies. Iraq HNA-HNZPortuguese Colonies XXA-XXZ The 2 standard audio frequencies arePanama HOA-HPZ Burma XYA-XZZ useful for accurate measurement orHonduras HQA-HRZ Afghanistan YAA-YAZ calibration of instruments operating in Siam HSA-HSZ Netherlands Indies YBA-YHZ the audio or supersonic regions of theNicaragua HTA-HTZ Iraq YIA-YIZ frequency spectrum. They may also beEl Salvador HUA-HUZ New Hebrides YJA-YJZ used for accurate measurement of shortVatican City HVA-HVZU. S.' S. R. YKA-YKZ time intervals. France,Col. and Protect HWA-HYZ Nicaragua YNA-YNZ The accuracy of the audio frequen-Saudi Arabia HZA-HZZ Rumania YOA-YRZ cies, as transmitted, is better than 1Italy and Colonies El Salvador YSA-YSZ part in 50,000,000. Transmission effectsJapan Yugoslavia YTA-YUZ in the medium (Doppler effect, etc.) U. S. A. Venezuela YVA-YWZ may result at times in slight fluctua- Norway LAA-LNZU. S. S. R. YXA-YZZ tions in the audio frequencies as re-Argentina LOA-LWZAlbania ZAA-ZAZ ceived; the average frequency receivedLuxemburg LXA-LXZ British Colonies and Prot. ZBA-ZJZ is, however, accurate. Bulgaria LZA-LZZ New Zealand ZKA-ZMZ The 440 -cycles -per -second signal hasGreat Britain British Colonies and Prot. ZNA-ZOZ been since 1925 the standard U. S.U. S. A. Paraguay ZPA-ZPZ musical pitch, A above middle C. It isPeru OAA-OCZ British Colonies and Prot ZQA-ZQZ broadcast for 4 minutes and interruptedSyria and Lebanon ODA-ODZ Union of South Africa ZRA-ZUZ for 1 minute, and this sequence is re-Austria OEA-OEZ Brazil ZVA-ZZZ peated continuously on eachof the radio carrier frequencies. This service AUDIO is useful to musicians and those con- FREQUENCY TIME BROAD- POWER OUT- FREQUENCY cerned with the manufacture or main- (mc) CAST ( EST ) PUT (kw) (cycles) tenance of musical instruments. A warning of radio propagation con- 2.5 7 pm - 9 pm I 440 5 7 pm - 7 am 10. 440 ditions is broadcast in. code at 20 and 440 and 4,000 50 minutes past the hour. If a warning 5 7 am -7 pm 10. 10 continuously 10. 440 and 4,000 is in effect, a series of W's (in code) 15 continuously 10. 440 and 4,000 follow the time announcement; a series 20 continuously 0.1 440 and 4,000 of N signals indicates no warning. 25 continuously 0.1 440 and 4,000 30 continuously 0.1 440 The table at right shows radio and audio fre- 35 continuously 0.1 440 quencies, time and power of WWV broadcasts

1948 RADIO -CRAFT REFERENCE ANNUAL 29 USEFUL SMALL CIRCUITS A large number of hookupsfor the serviceman andconstructor

==== Voltage Indicator 000 In the radio shack or service shop, it Electronic Metronome is often desirable to know whether the FROM A 2,500 -ohm d.c. relay connected as line voltage is above or below certain RECT. LOAD shown will operate as a timer for limits.For this purpose two Y4 -watt periods of 40 to 100 per second, with neon lamps, that fit standard pilot - L-047 PILOT LAMPS 160 µf (each condenser) at C; at 80 light assemblies may be made into a Af, the range is 90 to 208 per second, plug-in unit as shown and arranged on isnegligible.If a condenser short- the servicing instrument panel in thecircuits, the bulbs will light, saving the SW.ON POT. shop-or even built into a piece oYpower transformer from possible burn- equipment. out due to overload. Ehrick H. Wright Low -impedance Output Coupling The circuit shown, covered by British patent No. 564250 of E. L. G. White, shows how television signals may be coupled to a low -impedance line, using a cathode -coupling type of connection in place of the transformer ordinarily required.It is useful as well in manyapproximately,cepending uponindi- PLUG TO RECEPTACLE services other than television. vidual condenser leakage and whether AC POWER LINE the relay contacts are clean. It may be FOR LOAD necessary to adjust the relay spring To establish limits between 100 and tension. 120 volts, for example, the potentio- meter A is adjusted so that its neon Dr. Angelo. Montani lamp starts to glow when the line is 100 Tubeless FM Converter volts or more.Potentiometer B is ad- A novel circuit, which is credited to justed so that its lamp glows when line Henry R. Kaiser of FM station WMOT, voltage reaches 120, thus maximum and is shown in the diagram. It makes pos- minimum limits are set. sible the reception of new -band FM Stanley E. Weber stations on receivers designed for the old band. Buzzer Probe The unit is merely connected in series A low -resistance buzzer, ora relay with the new high -frequency antenna's connected as a buzzer, may be wiredas transmission line near the old -band FM r - 1 receiver. No other connectionsare ;W. ; 1 necessary. The input circuit is tuned + TEST PROD 7.73v. I 17 to the transmitter frequency, and the 6001 IGND CLIP. output is tuned to a frequency in the I lf I J a old band. This latter frequency is the "SMALL BUZZER 2 SMALL FLASHLIGHT CELLS result of radiation from the local oscil- The low -impedance load(line)islator in the receiver mixing with the represented by R1. transmitter frequency and producing a b A positive -going impulse applied toconverter output signal which falls in the range of the old band. R.F. COIL SHIELD the input terminals increases thecur- The self-supporting coils are space- SHIELDED LEAD rent from tube Vi to the line.Simul- TEST PROD taneously the drop in the plate potentialwound, 7/16 inch in diameter. Coil L1 of that tube is applied through conden-is 2 turns of No. 18 insulated hookup ser C to the grid of tube V2 to reducewire interwound with L2; L2 and L3 are GND CLIP its output, so that the 2. amplifiers workof No. 10 solid enameled wire, L2 hav- in opposite phase. Both grids are ini- XTAL shown in a and mounted in shield tially biased so that for positive signals can the whole load is taken by one tube, TRANS.LINE TO ANT.TERM. as at b. Its use in checking circuits and H.F. ANT. OLD F M RECT. components is apparent to servicemen.and for negative signals by the other, the mean plate current being kept ata L I L4 Leon G. Brown minimum. The arrangement thus com- "Short" Indicator pares favorably with a single cathode- Wheneverthereisanyquestion follower tube giving the samepower whether condensers will develop output. When the input is a televisioning 3 turns and L3 10 turns; L4 is 3 a shortsignal of the usual wave form, one ofturns of the same wire as Ll, and is circuit in service, or in lab work where interwound with L3. overloads may accidentally occur, it isthe tubes can be made to take the pic- excellent practice to connect protectiveture signals only, while the other passes For XTAL, a germanium type 1N34 lamps as shown in the diagram. Thethe synchronizing impulses whichare crystal, with pigtails, is recommended. resistance of the filaments when coldin a different range of amplitude. Condensers Cl and C2are8 -plate (Wireless World, London, Eng.) midget air -trimmer type. 30 1948 RADIO -CRAFT REFERENCE ANNUAL Electronic Tone Control A 1.0 -ma moving -coil meter is usedA.C.-D.C. Power Pack for M2. The zero adjuster is turned A humless A- and B -voltage supply, This circuit affords individual trebleuntil the pointer indicates .05 ma withfor use with tubes drawing up to 50 and bass tone control. It has the fur -no current flowing through the instru-ma with filaments in series, is shown 6C8 -G .01 ment-this permits a backward move-below." Resistor R1 controls high vol- 25K NEXT7 ment of the pointer to be easily seen.tage and R2 controls low. STAGE MI is a moving -coil voltmeter; it should Lehman M. Hauger GRID be of reasonably high resistance so that .311EG LEAK R1 does not need to pass much current. II7Z6.20041/10H .05A It must cover the voltage of the supply A -1-7V being tested, of course. The resistance A- of R1 and voltage source depend upon the voltage being tested. A wire -wound B+6Z5V potentiometer of about 50,000 ohms with B- ther merit of neither motorboating nora high -voltage battery will do for volt- 117 kl interacting between controls. ages between 20 and 120 volts. In opera- F. C. Hoffmantion, rotate RI. until M2 shows no cur-Capacitance-type rent; M1 then gives the voltage. Burglar Alarm Tone Control Be careful in using the circuit.Do An intruder alarm which will operate A circuit with individual high- andnot reverse the polarity of the test(flash lamp, ring bell, etc.) when any- low -frequency attenuation is shown inleads, or M2 will be damaged. The cir-one approaches within a short distance the diagram below as designed for usecuit being tested must have an internal TO NEXT resistance(asitusually will have), RY-I CAPACITY WIRE. HI PASS FILTER !t. otherwise M2 will show no current for SCREEN.PLATE, any voltage .given by Ml. A resistor SW.I 0A4 -G 1.0001 .0001 of about 100,000 ohms (for high, vol- IMEGVG j tages) may be fitted within one of the test leads for safety and to protect M2. V..AC

INPUT F. G. Rayer 6N7 (Practical Wireless, London,Eng.) C4 'MEG °1 TO ADDITIONAL UNITS - T.C. Low-cost Signal Tracer An inexpensive circuit tester may be RY 2

I 50K made as shown. A germanium crystal I .05 .05 84 of a capacitance screen or wire may be I. I 4 made as shown. A grounded metal wire LO PASS FILTER BODY OF PROBE or screen may be placed about 6 inches PHONES TIP OF PROBE j.006 IN341 behind the capacitance plate to act as a with a phono amplifier.. The 6N7 acts 0 as 'a mixer. shield and reduce sensitivity from one George A. France, W3LTA direction. CLIP TO G ND OF SET UNDER TEST A gas triode is shown, and should be B -less Radio Set shielded from light, since it has a pho- Here's a little space -charge type ofdetector is shown; it has pigtail leads. toelectriccharacteristic. Afterthe detector, with which you can have aEverything fits inside most probes. Use 5,000 -ohm relay Ry-1 has been tripped, lot of fun.It is both selective and, toit also as a local -station radio set bythe alarm will ring until SW2 is opened connecting to antenna and ground. to reset the 6-v d.c. relay Ry-2. 0. B. Miller H. W. Schendel Novel Audio Oscillator Aningeniousapplicationofthe Incremental , utilizing its negative -resis-Frequency Change tancecharacteristicto produce sus- Although this circuit, the subject of tained oscillations, opens many possi- a British patent application, is intended 4.5V bilities for other experiments. It is thefor use with oscillators, it should prove considerable degree, sensitive. Note itsubject of British patent No. 562705. interesting as a tone control. uses no B -battery; tryincreasing A - A thermistorisa semiconducting In all resistance -capacitance -coupled voltage to 6. mixtureofselectedmetallicoxides, oscillators the frequency depends upon Emmitt A. Barnesand has the property of altering itsthe value of one or more capacitated. electricalresistance with changes ofand resistance elements, and the fre- Novel Voltage Tester temperature produced by passing a cur-quency can be expressed as Fairly high voltages, ranging fromrent directly through the device, or by 1 20 to 120 volts and higher, may beheating it indirectly. fo tested with the circuit shown without 2,rRC taking any current from the supply This frequency is the point where under test. some critical phase relation is obtained Potentiometer R1 allows any voltage in R -C sections which may be zero or to be applied across the voltmeter Ml; but whichever it is, the critical fre- when the voltage across Ml is the same quency can be expressed in the above as the voltage across thetest leads, form. An example of one such arrange- meter M2 shows no current to be flow- ment is shown in the diagram. The ef- ing throughit.The voltagebeing fect of connecting in series with the in: tested is then the same as that shown finitely variable condenser C some fixed by Ml. In the diagram, the voltage acrosscondenser Clis to change fo to fl V M2 0 thermistor R changes as the currentwhere: through filament F isadjusted by C Cl TO rheostat Rl. At a critical value the fl M TESTING VOLTAGE 1 2vRCC1 SOURCE POINT circuit of L -C can be made to oscillate, particularly at a. f. (and i. f.). (Continued on following page) -T 0 (Wireless World, London,Eng.) 31 1948 RADIO -CRAFT REFERENCE ANNUAL 1 Solution: Therefore, fl - fo Grid -bias volts = drop across cathode 27rRC1 resistor = 16.5 volts (from tube man- The increase of frequency is there-ual). fore independent of the setting of the Averageplatecurrent = 34ma ALL PIES infinitely variable condenser C. (.034 amp.) 1/8" WIDE If the resistance R is changed in Average screen current = 6.5 ma order to obtain ranges, as is usually the (.0065 amp.) case in R -C oscillators, then, of course, 16.5 16.5 R - -= - 407 ohme ; a different increment is obtained for I .034 + .0065 .0405 each range, but this is frequently con- (use390 ohms) venient.For example, suppose the fre-R = EI = 16.5 X (.034 + .0065)=-- Fig. 2 -High -voltage transformer construction. 16.5 x .0405 = 0.668 watt. Note: This is the actual power dissi-the screen -grid voltage of the oscilla- pated in the resistor.It is commontor tube. practice to use a resistor having a rat- Precise adjustments are made ing of 4 to 6 times the dissipated powerwith the condensers across Ll. to handle surges, etc. The positive output of this circuit is grounded.If grounded negative is re- The above calculation used the usualquired, 4,000 -volt insulation should be formulas for Ohm's law used on the filament transformer, or a E E battery,well -insulated from ground, E = IR ; R = - ; I =- may be employed. I (Radiotronrcs, Sydney, Australia) where R is resistance in ohms, I is cur- rent in amperes, and E is electromotive force in volts) and power, P= I2R; Driver Stage quency scale is divided into 3 ranges:P = El; P = E2R where P is powerPhase -Inverter 20-200cycles,200-2,000cycles,andin watts, and I, .R, E are as above. (Courtesy Bud Radio, Inc.) A twin triode may be used to secure 2,000-20,000 cycles; and the increments phase inversion in the driver stage of can be arranged in each range as 0.2ILF., 2,000-v Power Supplyan audioamplifier,inthe manner cycle, 2 cycles, and 20 cycles. As many An r. f. power supply, much lesscum- shown. No decoupling is required since increments as desired can be arrangedbersome than other types, may be madecomplete push-pull operation is secured. by providing the require, values of Cl.as in Fig. 1.The high -voltage trans-Note that the value of R is not critical. For example, a total of 10 may be de-former is made according to Fig. 2.The gain is about equal to that ofa sired and the oscillator then can beThe circuit was designed experimen-normal, medium -mu triode. calibrated with an increment dial oftally by Amalgamated Wireless Valve + or - 5 separate increments.For Co. (Australia).Operation is at ap- the frequency ranges indicated in theproximately 1 megacycle. The r.f. vol- range, we should get: 5 increments oftage applied to the 8016 rectifier is + or - 0.2 cycle in the middle range; about 2,000;other suitablerectifiers 5 of + or - 2 cycles in the top range; are: 879, 2Y2, 2X2. 5 increments of + or - 20 cycles. After the coil has been wound, it A. R. A. Rendall, Ph.D closely resembles the harmless oscilla- (Electronic Engineering, London, Eng.) tor coil of a broadcast receiver. Do not FM Channels let this appearance deceiveyou - the The new frequency -modulation bandhigh voltages across L2 are sufficient allocates a channel number for eachto cause serious injury or DEATH. 200 kc difference in frequency as shown The core of the coils is a polystyrene in the following table. rod 3. inch in diameter and about 11/2 inches long. FM Broadcasting band frequencies The secondary winding vs. channel numbers consists of a total of 500 turns of No. Channel Channel Channel Channel 9/44 Litz wire wound in 3 pies topre- mc No. mc No. mc No. mc No. vent high -voltage breakdown between .1 201 93.1 226 98.1 251 03.1 276 turns.The feedback and plate wind- The resulting balance of this circuit 88.3 202 93.3 227 98.3 252 03.3 277 is very good, and frequency distortion 88.5 203 93.5 228 98.5 253 03.5 278 ings, each consisting of 60 turns,are 88.7 204 93.7 229 98.7 254 03.7 279 is very low, because of a form of cur- 88.9 205 pie -wound on opposite ends of L2.All 93.9 230 98.9 255 03.9 280 rent feedback inherent in thd action of 89.1 206 94.1 231 99.1 256 04.1 281 dimensions are given in the illustration, 89.3 207 94.3 232 99.3 257 04.3 282 Fig. 2. the circuit. 89.5 208 94.5 233 99.5 258 04.5 283 A valuable and interesting property 89.7 209 94.7 234 99.7 259 04.7 284 The coils should be provided witha 89.9 210 94.9 235 99.9 260 04.9 285 shield that will allow at least 1/2 -inchof this circuit is that of being able to 90.1 211 95.1 236 00.1 261 05.1 286 reject in -phase signals on the principle 90.3 212 spacing from the nearest conductor. All 95.3 237 00.3 262 05.3 287 used in biological amplifiers. 90.5 213 95.5 238 00.5 263 05.5 288 leads should be kept as shortas possible 90.7 214 95.7 239 00.7 264 05.7 289 90.9 This property ensures that phaseIn- 215 95.9 240 00.9 265 05.9 290 and sharp bends in the high -voltage 91.1 216 version is complete, and the 2 driven 96.1 241 01.1 266 06.1 291 wiring should be avoided to prevent 91.3 217 96.3 242 01.3 267 06.3 292 tubes are presented with a push-pull 91.5 218 power losses through corona discharges. 96.5 243 01.5 268 06.5 293 91.7 219 signal automatically balanced. 96.7 244 01.7 269 06.7 294 The high -voltage output of the unit 91.9 220 96.9 245 01.9 270 06.9 295 L.A.Saunders 92.1 may be adjusted roughly by varying 221 97.1 246 02.1 271 07.1 296 (Electronic Engineering, London, Eng.) 92.3 222 97.3 247 02.3 272 07.3 297 92.5 223 97.5 248 02.5 273 07.5 298 92.7 224 97.7 249 02.7 274 07.7 299 92.9 225 97.9 50K I 250 02.9 275 07.9 300 L2 200imif TOTAL 100K rig.I -Circuit of high - Grid -bias Calculation voltage supply. Speci- You may wish to keep the following fications for coils are example handy as a reminder. given in the text and Problem: the arrangement of the Calculatethesizeand H.V.-+- power rating of the cathode biasre- coils inFig. 2, above. sistor for a type 6F6 tube. 6V6- GT

32 1948 RADIO -CRAFT REFERENCE ANNUAL V. 0. M. Condenser Checker A r. o. milliammeter with a built-in condenser checker is a eery efficient and useful piece of test equipment

Avolt-ohm-milliammeter is con- The a.c. and d.c. ranges are 0-5-25- sidered to be the most valuable 125-250-500-1,250 volts.Si, the left- piece of test equipment for thehand toggle .switch, is used only as a serviceman, ham, or experimenter; with protective measure for the a.c. recti- a built-in condenser checker it becomesfier.This switch should always be left indispensable. initsshorted(d.c.)positionunless This meter has the following ranges:measuring a.c. voltages; and even then 6 voltage, a.c.; 6 output; 6 voltage, d.c.;itisbetter that the switch be left 5 ohmmeter; 8 current, d.c.; and a con-shorted until after test leads have been denser checker that will check paper,connected to a high and safe a.c. range. mica, and electrolytic condensers.The A voltage surge is placed across the rec- photograph shows that 24 tip jacks aretifierastheblockingcondenseris used, 20 red and 4 black. A multiple -charged.Although this particular rec- jack type test meter has several advan-tifier has a maximum d.c. output of tages over a pushbutton type. 5 ma, these precautions are given only With this circuit a single meter mayto lengthen the life of the meter rec- be used to measure more rapidly voltagetifier. and current in a single circuit, or in dif- No external voltage blocking con- ferent circuits. For instance, the properdenser is needed when using the a.c. Front view of V.O.M. Condenser Checker. milliammeter range tip jack is connect-voltage ranges, or when used as an out-negative and (ley volts pin jacks and ed in series in a d.c. circuit, and theput meter when direct current is alsoplacing the meter selector switch S4 in proper d.c. voltmeter range jack is con-present. The condenser is connected in-the DCV position.In this way the 2 - nected across the same or another cir-ternally. When used as an output meter,meter terminals are brought out direct- cuit; then by simply turning the circuitselect the proper a.c. range and use as ly.Two: a 12.5 -ampere range is avail- selector switch S4 to the milliammeteran a.c. voltmeter.A.c. scales- are ac-able by using the MILLS negative pin or voltmeter setting, either can be readcurate only at 60 cycles. jack and the 12.5A pin jack, and placing instantly without changing the hookup.Ohmmeter the meter selector switch in 12.5A posi- The milliammeter shunt resistors re- The ohmmeter and megohmmeter tion.The 0 -25 -ma range is also con- main in the circuit when the voltmeterjacks are located on the left side andnected internally with the condenser position is used.Also if a circuit con-up around the upper left-hand corner.checker, as will be explained below. tains both a.c. and d.c., each can be There are 5 ranges: 0-2,000-20,000- measured by simply turning the circuit200,000 ohms (which obtain their d.c.Condenser Checker selector S4 to the a.c.-volts or d.c.-voltsfrom a small 45-v battery); 0-2 meg The two condenser checker tip jacks position and opening the rectifier short-and 0-20 meg (which obtain their powerare at the bottom right-hand corner. ing switch Si. from the built-in a.c. power supply). On Only two tip jacks are used to check all The tip jacks have been arrangedthe lowest scale the first division is 1condensers, whether mica, paper,or around the outside edge of the panel soohm and at mid -scale, 35 ohms.The electrolytic, by the use of a continuous- that test leads will not interfere withtest prods are always shorted togetherly variable voltage from 0 to 500 volts control knobs, or hinder reading of thefirst and the meter pointer adjusted to d.c. meter scales. The meter is easy to readread 0 (on the right-hand side of the Condenser charging and discharging and the 4 meter scales are even mul-scale). When the 2- or 20-meg. scalesability is checked through a sensitive tiples of 5 or 10: 0-2,000 ohms, 0-5, 0-25, are used, the X100 COM tip jack is used 14 -watt neon bulb, (sensitivity over 20 and 0-125. as the common connection for the me-rnegohrns). Leakage current of electro- The meter has a 0 -1 -ma movement, gohmmeter.When the 0-2-meg scalelytics can be measured by switching S2 making it a 1,000 -ohms -per -volt meter,is used a tap connected internally to theto the ELE position. which is ample for all-around testingpower supply bleeder resistor supplies Let us begin by testing a mica or and will withstand a lot of hard knocks.45-v; with the 20-meg. scale, 450 v arepaper condenser.Plug the a.c. cord The a.c. cord is connected into the backused, as shown in Fig. 1. into a 110-v a.c. outlet, turn the meter of the tester where it is out of the way. From Fig. 1 it can be seen that theswitch S4 to the PSV position, and this It is needed only when the two highestoutput of the ohmmeter battery can bewill indicate the output voltage of the ohmmeter rangesorthecondensertested, or its output used externally ifpower supply as read on the 0-5 scale checker are used, or if the power supply desired, by using the common ohms andmultiplied by 100. Now adjust the fila- is used externally, as will be explainedthe positive condenser checker tip jacks. ment control rheostat to correspond to later. The multiplier resistors and their paral-the working voltage of the condenser. lel condensers can be tested with the Flip S2 to the PAPER position and con- Voltmeter ohmmeter by merely plugging into the nect the condenser tothe condenser The same tip jacks are used for theproper tip jacks. checker tip jacks.By depressing S3, a.c.andd.c.voltage measurements. thecondenserwillbecome charged Since these will probably be used moreMilliammeter through the flashing neon bulb.If the than any other they are located in the The current tipjacks are located neon flashes just once or twice and goes most convenient place.Looking at thearound the upper right-hand side without, the condenser is good, but if the top of the tester they begin with thethe following ranges: 0-5-25-125-250-flashing persists at a steady rate the negative -volts jack at the left side, pro-500-1,250 milliamperes. Two additional condenser has a high resistance leakage. ceed down, and then to the right alongranges are available: One: a 0-1 maIf the neon does not go out the con - the bottom edge. full-scale reading, obtained by using the (Continued on following page)

1948 RADIO -CRAFT REFERENCE ANNUAL 33 denser is shorted.Upon releasing S3have a greater leakage current than denser is tested the high leakage current the condenser discharges through theabout 0.2 -ma per pf of capacity. Whenwill not damage the meter or the neon. neon, if it is a good condenser.If theS3 is released. the discharging current If desired, electrolytics can also be neon does not flash when trying towill not flow through the meter in thetested with S2 in the PAPER position, charge or discharge, the condenser isreverse direction and damage it, butthereby observing the charging and dis- bad. Condensers whose working voltagewill flow through the neon and its paral-charging current through the neon.If is below 60 volts will either have to belel resistor where it is desired. In thisthere is a steady glow of the neon on checked with the ohmmeter or checkedway the condenser is discharged andthe CHARGE, but no glow on theDIS- as an electrolytic because about 60 voltswill not "bite" if its terminals are ac- CHARGE,the condenser will not hold a of d.c. is required to "strike" the neon cidentally touched.If an electrolyticcharge and is open. bulb. does not indicate any leakage current When S3 is in the D position and S2 When checking electrolytics, adjuston the charge, or if there is no flash ofin the PAPER position a condenser will the d.c.' output voltage to the workingthe neon on the discharge, the conden- discharge only until the neon glow goes voltage of the condenser and flip S2 toser is open.The 20,000 -ohm resistor out, and a charge of about 60 volts will the ELE position. When S3 is depressedbypassessomecurrentaroundthestill remain across the condenser.To charging current flows through the neon so large currents will be availableget away from this condition, Fig. 2 is electrolytic condenser and then returnswhen checking high -capacity conden- suggested.In this circuit, the conden- through the 25 -ma range of themilliam-sers.The 20K and 25Kresistorsser will discharge completely through meter. A good electrolytic should notare so rated that if a shorted con -the 15K ohm resistor.It is important Si 01250V that one can see the discharge through 750K neon; for this reason this circuit may AC -OPEN IW not appeal. The resistor is used so that 0500V the condenser will have to discharge 250K IW slowly and not injure the contact points 0250V of S3.By the use of S5, Fig. 3, all I25K these difficulties are overcome.S5 is 5K IW a single -pole, 3 -position switch and can ACV0- 0125V WW be a rotary type or toggle switch that 100K ..0185 IW will automatically return to the D (dis- 025V charge) or S (short) position, which- 20K - .075 ever is desired.After a condenser has C I IW been charged by placing S5 in the C o 5 V (charge) position, it will have to be .73 4.7 K IW turned to the S position for a complete 0 o 0.3V -!MA discharge, although it is not necessary that S5 be turned to the S position when S2 is in'the E position because the con- 0 V denser completely discharges through 0-1 MA the 20,00.0 -ohm resistor. 05MA. To sum up Fig. 3, it has all the needed 60 features as follows: (1) the charging W and discharging current of any con- o 25 MA denser can be observed by use of the 12 VeW neon bulb; (2) condensers can be com- 0125 MA pletely discharged; (3) electrolytics can 1.5 be checked with the neon and also their Vs W MA 0 250MA leakage current measured on the 25 -ma .75 scale of the meter by flipping switch VeW S2 to the E position; and, (4) the d.c. 0 500MA power supply can be used externally by turning S5 to the C position (positive 12.5 A 0- - O 01250 MA. polarity) and using the negative mil-

liammeter tip jack. The meter switch S4 selects the cir- 0 MA cuits in this order, starting at the ex- treme left: ACV; DCV; OHMS;MILLS; -0] PSV 0- 012.5A PSV (power supply voltage); and 12.5A (d.c. amp.). 500K Power Supply 2W One feature of the power supply is CONS CHEIIER that the output voltage can be varied OHMS - 0- 0+ from 0 to 500 volts. The output voltage 4.5V is automatically measured by switch- 111=-0 OHMS ing the meter switch S4 to the PSV position.The positive tip jack of the 0 X t condenser checker will be the positive 6, 4 W polarity terminal and the negative tip WW 50,1/2W jack of the milliammeter will be the X 10 negative terminal. But by plugging the negative or return lead into the proper X 100, COM. milliammeter range tip jack, the current drainon the power supply can be I0.5K measured conveniently by turning the 110V 16",--0 X 1000 meter switch to the MILLS position. 700V C7/90MA AC 225K I/2 W (2 MEG) The construction of this tester is not 5Y3/GT 2W 315K 0 X 10,000 difficult and the use of precision resis- 1W (20 MEG) tors will pay dividends.The measure- Fig. I-Wiring of the combined volt-ohm-milliammeter and condenser checker is not complex.ments of the 3 -layer (black, white, and

34 1948 RAbIO-CRAFT REFERENCE ANNUAL ohms), plus the meter resistance, mustma milliammeter used originally in the 1/4 W, NEON always equal 300 ohms. Supreme No.339 DeLuxe Analyzer. S3 If reasonable care and good qualityOther 1 -ma meters can of course be parts are used the d.c. and ohmmeterused if milliammeter shunts are adapted ranges will have an accuracy of 2% to fit the meter's internal .resistance. and the a.c. ranges 5%. The meter used was a Supreme 1 - -Harold Kreiger

Fig. 2-Optionalcondenser testcircuit. Frequency Drift I/4W, NEON 25K,2W TO reduce frequency drift in v.h.f. COMPENSATING TO TANK CIRCUIT receivers use a good grade of mica filled rubber or ceramic socket for the oscillator. Ordinary phe- nolic wafer sockets cause greater drift during warm-up. Connecting a negative temperature coefficient capacitordi- rectly across two of the socket termi- nals will almost completely compensate Fig. 3-An improved condenser test circuit.for any remaining drift caused by os- cillator tube and socket warm-up. Long- black) bakelite panel are as follows:er time drift due to warm-up of other %-inch thick, 8% incheslong,and oscillator circuit components such as 7% inches wide (the width is considered coils and tuning condensers is not much to be in the same plane as the handle).improved by this treatment. That type The outside dimensions of the box, with of drift is generally due to faulty de- cover removed, are: 4% inches deep, sign of the tuning condenser and coil. 8% inches long, and 10 inches wide. TheIt is important to first reduce all basic small compartment at the top of the causes of frequency drift (such as poor- meter is very convenient for the a.c. lydesignedparts)beforeapplying power cord and miscellaneous cords.temperature compensating condensers. Fig. 3 The outside thickness of the cover is Fig. 1 shows the circuit far a con- 1% inches and is mounted with two slip ventionaloscillatorcircuitusinga 6C4 B hinges. 6BE6. Fig. 2 shows frequency drift The following items are mounted inbefore and after compensation.Fig. 3 the bottom of the box: power trans-shows how to mount thecapacitor former, rectifier tube, C, R, RI, and theacross the screen grid (6) and heater 4.5-v ohmmeter battery.Five 12 -inch - (3) terminals of the 6BE6 socket. This long flexible leads are necessary to isequivalenttoconnecting between connect these parts to the parts mount-cathode tap on the coil and ground. ed on the underside of the panel. Make capacitor leads as short as pos- Due to the same pin jacks being used sible.The capacitor will receive more for a.c. and.d.c. voltage readings, paral- heat connected to the heater terminal 6.3V lelcondensers are placed across the Fig.4 series voltage multiplier resistors. This becausethisterminalrunshotter. is done so that the same scales can be For Fig. 1 the capacitor is about 8 /.4µf, used for both a.c. and d.c. voltage read- with an N750 temperature coefficient. ings, which is a very good feature.If Try different values near this for C it becomes necessary to change the con- in Fig. 4.If C is too large, too much densers in thea.c. voltmeter circuit, detuning will result. start calibration on the 5 -volt scale by [When using a separate triode oscilla- trying various sizes of condensers for tor tube such as a 6C4, connect C as Cl and, using a source of 5 -volt, 60 - shown in Fig. 4.Pin 2 is blank in the cycle a.c., choose a condenser to make 6C4.Ground it.-Editor.] the meter read full scale, the same as Fig. I In Figs. 1 and 4 the cathode and one the known source of voltage; and so on side of the heater are connected to- up thescale.No trouble, however, 0 gether and returned to ground through should be experiencedifthevalues the cathode tap of the tank coil.The given are used. CURVE 2 other side of the heater goes through The 12.5 -amp shunt was made by -J 40 a 1-microhenry choke to the heater paralleling two No. 18 Manganin wires. 5- CURVE I-COLD START- PHENOLIC SOCKET supply.This arrangement minimizes By making two trough -like lugs out of0 small variations in heater -cathode capa- -J CURVE 2-COLD START-MICA FLABRUBBER sheet copper, resistance could be addedY 80 SOCKET WITHCOMP. CAP. citance which cause microphonics in or taken out easily by shortening or DRIFT EXPRESSEDAS FREOUENCTCHANCEAFTER reception.Wire used in the tank coil lengthening the shunt wires until the FIRST 30 SECONDS and r.f. choke must be heavy enough LL meter reads 12.5 amp at full scale. Each 120 to carry the heater current of the oscil- of the two parallel wires was 6 inches0 lator tube, since it flows through them. long and fitted properly between its 5- Keep choke and tank coil resistance 2 tip jacks. z low to maintain proper heater voltage. 1+.1 160 If it should be difficult to obtain the O Be sure to place the r.f. choke in one various sizeresistors and condensers CC CURVE I, leg of the heater circuit. If it is omitted, the oscillator may fail to operate. This they can always be made up by placing 200 one or more condensers or resistors in 0 2 4 6 10 choke must be connected direct to the parallel or in series.Resistor R2 in TIME - MINUTES tube socket heater terminal. series with the meter (in this case, 170 Fig. 2 From data in RCA Application Note..

19415 RADIO -CRAFT REFERENCE ANNUAL 35 Modern Metal Locator To satisfy the demands ofmany readers for copies of the April 1943 Metal Locator article it is reprinted here.

Practical Uses Many operators of metal locators will The practical uses of this locator aresay you can't do any good in a spot many.To mention a few: Exact loca-of that kind.You can, if you get ac- tion of buried pipe lines, such as oil,quainted with this locator, because we gas, and water pipelines;checking did find those lines, and staked them location of construction projects for any to where a ditching machine went on buried metal orlines;checking andthe job and uncovered them. The ditch- searching for metal and ore deposits;ing machine operator said one of the tracing pipe lines, or veins of ore, once 50 stakes was about a foot off the cen- they are located, etc. ter of the line.The lines were 51/2 feet The unit is very easy to operate and deep under sand and mud. use, as simplicity and efficiency were The method used to do this job is the paramount ideas when it was de- shown under Operating Instructions. signed.The most practical way to get acquainted with it is to study all theFeatures details in this article and then check the Some of the features of the equip- locator on known buried objects andment are: metal lines.Results will be easy to 1. Highly efficient use of the 1T4, produce if you build it properly and1S5, and 1S4 type tubes, from 1.5 -volt learn how to make it respond to theA supply, and only 45 volts of B supply. different situations under various and Three t.r.f. stages, 2 audio stages, and adverse conditions. one vacuum -tube -voltmeter stage. Just as an example of what one may 2. Plate modulation. Uses two 1G6- be called on to do with a locator-I wasGT twin -triode tubes, one as a Hartley The author, G. M. Bettis, withtheLocator. asked to locate two sections of a pipeelectron -coupled oscillator, the other as an audio oscillator used for modulation. stream in a river bed.The river, theThis type modulation increases the out- THIS Metal Locator is not an ex- Prairie Dog Branch of the Red River input of the transmitter tremendously perimental model but one thatNorth Texas, was 7,000 feet wide andand sharpens the tuning of the carrier hasactuallydoneproductiveusually could be waded. Floods caused also.Distinct advantages over the self - work.Tests made with it have beentwo sections of line about 900 feet longmodulated type transmitters. more than satisfactory.The first loca-to be broken out and lost.Dry sand, 3. Not an experiment. Makes use of tor of this exact type was made in 1941wet sand, fresh water, salt water, andthe proven radio balance principle, re- for a pipe line engineer.This engineervarious combinations of soil were foundsulting in high sensitivity, stability and made several suggestions astothe in this particular location as there wasefficency, wth the advantages of two design, which have been proven in the a lot of salt water flowing into theindications-visual and audible. field, many times under adverse con-river just a few miles above the pipe 4. Lightweight.High-gradecon- ditions. line. struction of standard parts.No trick I T4 I T4 IS5 I S4

.01 0 )1

1.F.T.

48T.No.25 C 3MEG IMEG cIMEG 20K ENAMEL 175-500 PIO rwf

BIAS CELLS 2 - IV 50K 500K1.0002 IOMEG 10.1 .006 2 I.5V 0.1 VOL.CONT.

F1LS- 200 +

3 MEG 58)

0 0 0 I T4 A+1.5V A- B- B+ 45V The receiver isa t.r.f. job and uses i.f.t's.as radio frequency transformers. Indicationis eitherbymeterorsoundinthephones.

36 1948 RADIO -CRAFT REFERENCE ANNUAL Leff-Rear view of receiver. Right-Rear view of transmitter. Note the neat construction, and the compact arrangement of the batteries. circuits nor special unobtainable parts The chassis is wired with flexibleallel to, and away from, the receiver so needed. connections to the miniature tube sock- as to hear a faint signal in the 'phones. 5. Alternate tubes may be used. Theets because of danger of breaking the Then readjust for maximum signal in 1N5-GT, 1H5-GT, and the 1T5, respec-glass base of these tubes when placing the receiver.If there is to be quite a tively can be used in place of the 1T4,them in sockets.Leads are left so asbit of use of the locator with other 1S5, and the 1S4.The miniature typeto reach parts mounted on front panel.power control positions, place the trans- tubes are recommended because of theirThe coil and chassis strip are fastened mitter on the ground and the receiver high efficiency on low plate voltage. to the case with brass wood screws slightly above the ground, facing each 6. Flexibility of operation. Use as aand the wiring is completed. The wiring other, as shown in Fig. 1, and as far single unit by joining the transmitterof both the receiver and transmitterapart as possible to still hear a weak and receiver with the two handles inis conventional. signal in the phones.Then adjust the order toexactly spot some desired The meter I like to use in the re-trimming condenser Cl of the transmit- metal object, and as two separate unitsceiver when available isa 0-100 d.c. ter for the maximum signalin the in tracing metal or ore after spottingmicroammeter, but any good 0-1 d.c.phones and the greatest deflection of with the single unit, or when at leastmilliammeter will do the work. the meter with the power control in one point of the desired metal or ore the position on which it is desired to is known. When all wiring is finished, check operate. 7. Results, or the type of work thisand double check, as there is danger of locator will do.This is the most im-burning out a complete set of tubes byOperating Instructions portant feature.This locator has easilyhaving one wire in the wrong place. located the following, which may serveConnect the batteries, and measure the When not in use the controls must be as a guide of what to expect:1/2 -inch voltages at the tube sockets before in the OFF position.The transmitter water line, 2 feet deep, more than half -placing tubes in sockets. controls are the ON -OFF switch and scale deflection of meter; 4 -inch water It will be worth while to test the the power control with 4 positions. The line, 414 feet deep, more than half -audio oscillator with headphones to see receiver controls are the power switch scale deflection of meter; 6 -inch oil line,that it is producing a good signal ofand meter sensitivity control.To test, 11 -feet deep, about third -scale deflec-about 400 cycles.It may be necessaryplug headphones in jacks of receiver, tion of meter.Six- and 10 -inch pipeto switch the ground or grid leads ofturn receiver switch on, adjust meter lines that were located in river bedthe transformer. indicator to full scale, and then advance under 5112 feet of sand, salt water, and First test the receiver by aligningthe sensitivity control to just below mud, show from half to almost fullthe i.f. transformers and the trimmingthe point where a whistle is heard in scale deflection of meter.I have madecondenser C for greatest signal in thethe headphones.This is the most sen- many other tests, and know that thisphones, and greatest deflection of thesitive spot to operate. Turn the trans- locator will work well on aluminum,meter when using a calibrated oscilla- mitter switchon, then advance the brass, copper, lead, and silver as welltor set at 175 kc.Make these adjust-power control to positions 1,2, 3, and as it does on iron. ments with the gain control advanced (Continued on following page) to just under the `Construction regeneration point. The cases shown in the pictures wereA competent radio made from Philippine mahogany; theserviceman can do panels and chassis strips from temperedthis, if a calibrated Masonite. This exact material may not oscillatorisnot be available but any good dry wood, available. fibre, bakelite, or non-conductive ma- terial will be suitable. The transmitter The coils are wound with 48 turns of isturned on and No. 25 enamelled wire, 24 turns eachthe trimming con- 48T. No 28 side of center tap, andwoundin thedenser Cl adjusted ENAMEL same direction.Not critical as to sizefor greatest signal of wire or covering of wire as long as inreceiver.This coils will tune to 175 kc, the operatingadjustment should frequency of this locator. be made with the The tube sockets are mounted first in powerswitchin 0 the chassis strips and then all otherposition 2 (the 22.5 90V 67.5V 45V22.5 V parts in both chassis strip and frontvolt position). Move A- 9- A+ B+ panels. the transmitter par - Transmitter uses parallel 166-GT'S as oscillator and modulator.

1948RADIO -CRAFT REFERENCE ANNUAL 37 line and bring the transmitter to this position and place over line and pro- ceed as before.In this way two opera- MAX. DISTANCE tors can trace and mark a long line to gel very efficiently. SIGNAL IN PHONES Another method to exactly locate a line is to place the receiver horizontally over the line where the peak signal was heard (See Fig. 3), with the top of the receiver toward the transmitter and close to the ground.Move back and forth across the line.When you are directly above the line there will be a Fig. I-How the instrument is used.It should be balanced to suit null (no signal) in the headphones. the soil.

""\"""\""\\\ Loudspeaker Fidelity or moreATRANSMITTER. (Continued from page19)

and 7.These enclosures are for 12" speakers with cone resonances between 55 and 65 cycles. No provision has been made for accurate adjustment of the size of each cabinet to suit the cone resonanceofdifferentloudspeakers since the dimensions are fortunately riot Fig. 2-Maximum-loudness method of tracingpipeline. Receiver at allcritical.While the impedance aloneis moved. characteristic may notbeperfectly symmetrical, the general improvements VER TURNI in the performance over the whole bass frequency range will be almost equal to those obtainable with ideal adjustments. MM. UUMMV -Jk The 2 enclosures are identical except WNW% WUWN for dimensions of the venting slot and use of the venting pipe. Use %" ply- wood dimensioned and assembled as shown.Fig. 7 shows the cutouts in the front piece of plywood.Dimension A of the vent slot depends on the bass resonance frequency of the loudspeaker selected.In the original experiments 1 speaker had a bass resonance of 55 Fig. 3-Alternate method of tracing pipe. Receiver positioned fo cycles in free space.Dimension A for minimum signal. this enclosure was 251/4". Note that the vent pipe shown in Fig. 6 was elimina- 4. There will be a loud signal in 'phones well to metal objects that are on topted with this speaker.Only the slot and the meter pointer will deflect down- was used.The other speaker had a of the ground., It will be a disappoint-bass resonance of 62 cycles. Dimension ward toward zero. ment except on buried objects. A for this enclosure was 15,/4" and the Assemble thetransmitter andre- Set up the locator as shown in Fig.vent pipe shown was used. Vent pipe ceiver as shown, between wooden han- 1. When a metal object or pipe lineis also built of %" plywood. dles, attaching wire extension handles istobe located somewhere between and turnbuckle.Turn the receiver on,points A and B with different types Inconstructing theenclosuresall turn transmitter switch on, and thenof soil, it is best to determine the exactjoints must be glued with the exception place the power control on position 2. of the bottom.Triangular fillets are area of each type of soil with the loca-screwed and glued to the joints of sides With the locator inthepositionin tor.This will show by the rise andand top on the interior as shown in which it is to be used, adjust the turn-fall of the meter pointer.Mark these Fig. 6. buckleuntilthe highest readingof areas.It will be necessary to balance The 2" x 2" stay with fillets meterinreceiverisobtained,then the locator over each type of soil. Thenis firmly screwed and glued to the front tighten turnbuckle so there is a slight cover each type of soil with the locatorand back panels to withstand internal, deflection of the meter, and a low sig-balanced for that particular type. pressure when the speaker is operating. nal heard in the phones, then tighten When it is desired to trace a line orThe vent pipe is screwed and glued to the lock -nut on the turnbuckle.The the front panel. The removable bottom a vein that has been located (Fig. 2.) plateof %" plywoodis top of the transmitter case willhe or if there is a known spot, the trans- glued and slightly forward.This is called "bal-mitter should be placed over and paral-screwed to a 2" x 1" frame. This frame ancing the locator."It must be bal-lel to the line.Then with the switchis attached to the main portion of the anced for each position of power con- cabinet using 6 screws on each long on and the power control on position 2side and 4- screws on each short side. trol.Power control in position 2is the receiver should be turned on 25 feet usually the best way to use the locator. or more away from the transmitter andThe bottom plate is made removable When crossing buried metal or a pipemoved back and forth above and paral-for getting at the interior, should this line there will be a loud signal in phones lel to the apparent course of the line.be necessary. andthemeter willdeflectsharplyThere will be a peak signal heard, and Cover the inside of each enclosure downward.The metal object will bethe loudest spot is directly above thewith thick felt, leaving a clear ring 2" directly under the transmitter at the line or vein. When the signal gets too widearoundthe1.03/4" loudspeaker lowest meter reading and the loudest weak as the receiver is moved along themounting hole.Do not place felt on signal in phones.Metal objects or pipeline, advance the power control to posi-the 2" x 2" stay. lines are located easily when crossed. tions 3 and 4, as far as the line can Adapted fromanarticle by F. Langford -Smith,in RodiotronicsNo.124,(Sydney,Australia) ;courtesy The instrument does not respond verybe followed. Mark the exact spot of the Amalgamated Wireless Valve Co.,Pty.,Ltd.

38 '948 RADIO -CRAFT REFERENCEANNUAL FREQ IN MC LENGTH IN INCHES 140 50 55 60 65 70 75 80 85 I 90 I 95 1 100 1 105 I 110 1213 I it will intersect Place To the left endFind of Length a straightedge of Dipole on Elements the chart On so Chartthat the desired frequency in the mc scale at the - - 10 thedipole,left. dimension Note reflector where straightedge intersectsExample: the - To desired find the curve length - of a dipole to tune to center correspondingor directorto the intersecting - and at the vertical top of line. the chart read - 99 upward,straightedgetheof 98 the mc 88 intersects the dipole curve. Glancing directly scaleto 108 reading mc FM is pointfound band. on toplace the be 56.7"scale end of at(563/4'). left of theThis chart; is note where straight edge at .. 8 toTheorlength 28%".have chart gives its length as 59.4" (59Read V2 length " ) of a reflector using curve markedof reflector.the completea length dipole; of each leg is one-half this - 54.4" (541/2") using . A directorreflector is curve. found 7 - ,9et,040 qs C1 0 09,... DIPOLE+5% (DIPOLE X1.05) 6 Oh, .`resci, 0,9 044, 5 DIRECTOR= .96X DIPOLE (4% SHORTER) I/2A6% (FT)END CORRECTION ALLOWED) 462 fMC (DIPOLE ) 50 . I I I v.h.f. signals. It also effectively demon- bands spaced at intervals of f q(the FM -AM SUPERREGEN strates the relative immunity of thequenching frequency), so each of these (Continued from page 8) superregenerative receiver from igni-is frequency -modulated by an incoming tion interference and also its automaticFM carrier. Iffq is considerably higher of insulating material as a mechanicalgain control properties atallsignalthan the frequency deviation of the support, as shown in Fig. 3. The legsstrengths great enough to suppress thecarrier-say 200 kc-so that any one are attached to the baseboard by swivelbackground noise. side band can be selected to the ex- wood supports that permit the dipole to FM transmitters can be distinguishedclusion of all others, it can be applied be twisted to vary the coupling withfrom AM transmitters,becausethe to the usual FM discriminator(see the tuning coil. A standard dipole withmodulation becomes almost or entirelyFig. 4). It may be necessary to use a 72 -ohm twin lead or co -axial trans-inaudible when the receiver is tunedfrequency changer to lower the fre- mission line can also be used. With theexactly to the carrier frequency.To quency of the superregenerative out- dipole mounted on the roof, pickup willhear it, the receiver must be slightlyput. And a buffer stage is desirable to be much greater. mistuned, when the variations in trans-prevent radiation.So it begins to ap- mitter frequency on the slope of theproach the orthodox superhet in com- receiver response curve give amplitudeplexity. But with an equivalent amount variations, at fair fidelity. of gear it seems likely to have a higher sensitivity and freedom from ignition Superregeneration interference. (The ordinary FM receiver Amplifier is not too good at coping with the latter; While on the subject of FM, it maybut a superregenerative stage in front be worth mentioning that a superre-ought to reduce it to manageable pro- generative stage has been found veryportions.) Fig. 3-Details of dipole insulating support. effective as an r.f. amplifier. Whether you go in for this fancy The theory is that a superregenera-hybrid or stick to the plain superhet, When thesetisswitched on, thetive oscillator is synchronized by an in-however, thissingle -tuberisa good plate current first rises and then dropscoming signal of as little as 10 micro- introduction to v.h.f. once at the start of oscillation andvolts. Being modulated by the quench- A digest of articleby "Cathode Ray," writingin again at the start of quenching.The ing, the oscillator has a series of side Wireless World (London, Eng.), January, 1947, characteristic rushing sound can then issue. be heard. If only one drop occurs, os- cillation is not strong enough to quench itself. Try raising the B -voltage or re- A STABLE V. F. 0. ducing the capacitance of C3. (Continued from page 3) The easiest way tocalibratethe tuning scale is with an absorption waveassuming the speaker was not in use, By connecting the b.f.o. to the "ex- meter, held no closer to Ll than isto connect a resistor across the output ternal -modulation" terminals of a ser- necessary for the meter to show a per-meter in the diagram, the a.c. resistancevice oscillator, the latter instrument is ceptible flicker when the wave meter isvalue being the same as the voice coil then provided with variable modulation tuned through resonance. of the speaker normally used in con-on any r.f. signal generated.To do fq junction with the amplifier'soutputthis, the b.f.o. must be provided with transformer. resistance -capacitance output by con- Another use for the b.f.o. is in detect- necting a 3- or 4 -watt plate resistor ing the frequency at which a loud-across the output terminals and a 2-4 SELECTED (0) speaker has developed a rattle. Not that condenser to the "plate" output termi- SI DE BAND we want to know the frequency particu- nal.The input to the service oscillator larly, but it is useful to be able to feed is then taken from the other side of FILTER into the speaker a sustained note at thethis condenser and the ground of the frequency at which it vibrates.Underb.f.o., respectively; CHARACTERISTIC (b) these conditions it is most convenient Adaptedfrom anarticlebyStanleyBrasier,in to work on until the rattle is cured. PracticalWireless(London, England)October,1945.

DISCRIMINATOR (C) CHARACTERISTIC POWER Fig. 4-Superrenegerative amplifier character- TRANSFORMER istic.Fq is separation equal to quench fre- quency (not less than 200 kilocycles). Baf The antenna is tuned in the same way, SHIELD ELEC.CONDS. as if it were an absorption wave meter, by noting the setting of C1 at which maximum absorption isindicated by the risein plate current, and then shortening or lengthening the antenna rods as required. One can make sure that it is the antenna that is absorbing by touching one or both of its ends; the meter should respond. If no meter is available, antenna resonance can be de- tected generally by a rise in noise. The antenna effectively covers a band of at least 3 me each side of resonance. This little set has possibilities as a school demonstrator, for use in examin- C4 C3 ing the polarization and direction of Fig. 6-Underside parts layout of original v.f.o.Location of the shields is important.

40 1948 RADIO -CRAFT REFERENCE ANNUAL I 4 -TUBE SUPERHET DIRECT -COUPLED AMPLIFIER (Continued from page 9) (Continued from page 10) of the order of 1.4 volts r.m.s., maximum pull for the 2 stages, as shown in Fig. If, as a result of adjusting the pre- transconductance will be obtained. 4 (note lack of condensers). Here thevious stage, the 100,000 -ohm resistor The most interesting stageistheopposed currents of the 2 tubes of eachhad been replaced by a resistor of a reflexed intermediate- and audio -fre- quency amplifier. Feedback is neutral- VI V2 ized. The values of the capacitors in 6Q7 6L6 -G this case are small, because of the need to make thecapacitor between the lower end of the i.f. transformer and ground a high impedancetoaudio frequencies. Neutralizingcapacitors Cl and C2 are very small indeed, and with a well -designed layout they might INPUT even consist of stray wiring capacitance. In one model of this receiver a 3 -inch length of shielded wire was run from the lower end of the i.f. transformer, the shielding being grounded, with a 3. -inch unshielded section at the end 0.1 32 0.132 placed about %-inch from the plate lug 500V T 75.00V of the 6SF7-GT tube socket. Five turns of No. 26 B. & S. enameled wire were wound over the rubber insulation of Fig. 3-Circuit of Fig. 2 with addition of necessary bypassing for goodI.f. response. the unshielded portion and connected to the diode socket lug to form a small 6Q7(2) I capacitor. Thecapacitancesofthereceiver shouldbeadjustedduringthe lining -upprocesstogive minimum side -band hiss as the signal generator is tuned through resonance.This re- 1 MEG sults in minimum gain, through reduc- tion of regeneration. Resistor R1 pre- INPUT

vents the volume control affecting the 1 MEG Y neutralization, as well as providing i.f. filtering of the diode output.While the maximum possible a.v.c. is applied to the converter tube, only about 1/9th i of this voltage can be used safely for the reflex stage. This is because any bias applied to the 6SF7-GT is doubly effective in that it reduces both i.f. and -/0A,---~f-4-We385 30K 7775 a.f. gain. 12W 2W 2W B v+326V The 6SF7-GT is given initial negative bias of approximately 2.5 volts.It is Fig. 4-Improved d -c amp ifier designed for push-pull operation. Bypassing is unnecessary. important to bias this stage with apair, cancel out in the voltage divider potential of not less than that provid-and in the cathode resistances, provided ing maximum gain, otherwise a smallof course that the two tubes are identi- increase of a.v.c. potential will resultcal in characteristics. in increasing gain-which is equivalent To balance this push-pull setup prop- to positive feedback and leads to in-erly, apply to the amplifier a voltage of stability. a fixed frequency,say,400cycles. INPUT The value of R2 takes advantage of With a vacuum -tube voltmeter, meas- the neutralization of hum output pos-ure across each of the 6Q7 tubes from sible when the 2 audio amplifiers haveeach plate to point M to determine the correct gain relationship.It sowhich tube amplifies the most. Having happens that this value satisfies theidentified the tube, reduce itsplate other requirements of thea.f.load. load resistor (100k) so that the am- The method used for selection was toplification will be the same for both indicate visually the hum output from tubes. Fig. 5-Gain control circuit for Fig.4 the plate of the 6V6-GT on an oscillo- Finally, check the 6L6 plate-curreqt scope and to vary the 6SF7-GT balance by using an adjustable resist-lower value, the correct potentiometer plate load to show minimum hum. value may be obtained by shunting ance (of about 10 ohms) in the Z or U across the 100,000 -ohm potentiometer a Condenser C provides negative volt- cathode lead, whichever has the strong-fixed resistor calculated according to age feedback which is greater for higher current. the formula of resistances in parallel. frequencies than for low, thus serving The output a.c. voltages then will be as a "top limiting" device.This hasbalanced by measuring potentials be- Having made the adjustment, meas- the added advantage of negative feed-tween 6L6 plates and point N. Havingure each part of the potentiometer and back over the alternative system usingfound which tube amplifies more thanreplace by 2 fixed resistors of the same a much larger condenser between platethe other, replace one of the 100,000 -values.In this `way the amplifier will and screen of the 6V6-GT. ohm 6Q7 plate load resistors by a po-be adjusted perfectly. tentiometer of the same value and con- Gain may be controlled in a- number Adapted from anarticleinRadiotronicsNo.120, nect the control grid of the respectiveof ways, depending upon the use of this Sydney,Australia;courtesyAmalgamatedWireless (Continued on page 42) Valve Co.,Pty., Ltd. 6L6 to the slide. Regulate for balance.

1948 RADIO -CRAFT REFERENCEANNUAL 41 NOW -A REALLY HIGH -POWERED- NEW DIRECT -COUPLED AMPLIFIER (Continued from page41)

Radio Engineering amplifier and its connection to relatedfier; 6.3 v, 1 a, for 6Q7 tubes (center- equipment. One arrangement is shown tapped totheir cathode connection); Library in Fig. 5. To maintain dynamic balance 6.3v,2a,for 6L6 tubes(center - the parts placement and wiring shouldtapped to their cathode connection). be symmetrical. The power supply may If these construction details and ad- utilize a 15-h choke not exceeding 300 - justments have been properly executed, ohm, d.c. resistance and filter conden-it will be unnecessary to use feedback sers of 30 Af(input) and 30-60id to improve the frequency response. (d.c. output), 600 w.v.Power trans- NOTE: former secondaries are: 800 v (center- Excerpts of a translation by P. Minor of an article The Library comprises a selec- tapped), 200 ma; 5 v, 3 a, for 5Z3 recti- by F. Justerin Toute la Radio (E. Aisberg, Director, tion of books culled from lead- Paris, France), February, 1947. ingMcGraw-Hill publications in the radio field. especially selected by radio specialists of McGraw- Hill publications. CLASS A AND B AMPLIFIER. togivemostcomplete,dependablecoverageof (Continued from page12) facts needed by all whose fields are grounded on radio fundamentals available at a special price and terms a low -impedance triode as it will be way, i.e., operating under approximate- THESE books cover circuit phenomena, tube theory, called upon to supply a certain amount ly class AB1 conditions, the volume"was networks, measurements, and other subjects-give specialized treatments of all fieldsof practical design of power to drive the output tubes whenincreased, and the quality greatly im- and application.They are books of recognized position approaching class B conditions. proved. in the literature-books you will refer to and be re- Leaving the volume well on, ferredtooften. Ifyouareapracticaldesigner, The transformer T1 is any good qual- the bias control was turned back to its researcher or engineer inany fieldbased onradio, ity class B driver transformer. you want these books for the help they give on hun original class A condition, and, as was dreds of problems throughout the whole field of radio The choice of output tubes depends expected, the output tubes were hope- engineering. entirely upon the constructor's require-lessly overloaded, and thedistortion 5 Volumes, 3319 Pages. 2289 Illustrations ments.As an illustration and guide,was serious. I. Eastman's FUNDAMENTALS OF VACUUM TUBES 2. Terman's RADIO ENGINEERING the writer used 6F6's as triodes (screen It is suggested that the bias poten- 3. Everitt's COMMUNICATION ENGINEERING grid connected to anode). When ascer- 4. Hund's HIGH FREQUENCY MEASUREMENTS tiometer be treated as a sub -control and 5. Henney's RADIO ENGINEERING HANDBOOK taining the output transformer ratio forlocated out of reach of the less techni- 10 days'examination. Easy terms. Special price under correct matching to the speaker it is ad-cal members of the family, and set to this offer less than books bought separately. Add these visable to get the matching correct for standard worksto your library now; pay small monthly sucn a value as required for the occa- installments, while you use the books. class AB conditions. sion, i.e., ordinarily possibly class A will 10 DAYS' FREE EXAMINATION-SEND COUPON Under these conditions, it was esti-be sufficient; for a noisy party, well ----- mated that the maximum undistorted McGRAW-HILL BOOK CO. Inc.,330 W. 42d St.,N.Y.18 down near the class AB2 end, and so on. Send me Radio Engineering Library, 5 vols.,for 10 output under class A is 1.5 watts, with With the circuit shown, true classBor days' examination on approval. In 10 days I will send bias of -12v and a steady plate cur- $2.50, plus few cents postage, and $5.00 monthly till AB2 cannot be quite reached, as self $27.50 is paid, or return books postpaid. (We pay post- rent of 50 ma.Quality was extremelybias is used. age on orders accompanied by remittance of the first good, and a pleasure to listen to. installment.) To get really true class B operation, Still leaving the amplifier at quietfixed biasisessential.Incidentally, Name room volume, the grid -bias potentiome- larger outputs can be obtained with Address ter P was adjusted to apply more nega-fixed bias, but the self -bias method used tive bias to the grids of the output tubes City and State here should give ample output for most and the deterioration in quality wasrequirements. Company noticeable, getting worse as the bias Position RQ-I.48 was increased. With the bias control set about half- Condensed from an article by fl. Miller,in Practical SeeAd on Inside Front Cover`'''""al Wireless magazine (London, England). I MAIL THIS COUPON RADCRAFT PUBLICATIONS, Dept. RA48 GOOD FM RECEIVER I 25 West Broadway, New York 7, N. Y. (Continued from page17) Send methe volumes(50c each, postpaid) Ichecked. My dealer isunable to supply. is too large and further out than the I at the center frequency and the primary I enclose $ low -frequency peak, set the generatortrimmer adjusted on the low -frequency I ALL 10 - $5.00 I at the desired high -frequency peak andpeak to give approximately equal high- I No. 29-HANDY KINKS & SHORT CUTS alter the setting of the slug to give aand low -frequency output peaks. No. 30-UNUSUAL PATENTED definite increase in output at this new CIRCUITS frequency. Now retune the" generator Regetting the primary trimmer should I No.31-RADIO QUESTIONSAND have slight effect on the maximum ANSWERS to 10.7 mc and adjust the secondary trimmer to give minimum audio output. Value of diode circuit current. Larger No. 32-ADVANCED SERVICE TECH- diode currents can be obtained by de- NIQUE Recheck the peaks again; if they are spaced approximately symmetrical, buttuning the secondary trimmer, but this No. 33-AMPLIFIER BUILDER'S GUIDE with high -frequency peak larger thanis not the desired condition. The sec- No. 34-RADIO-ELECTRONIC CIRCUITS low -frequency peak, tune the generatorondary trimmer may be considered as No. 35-AMATEUR RADIO BUILDER'S to the low -frequency peak and retunebalancing a bridge circuit to give zero GUIDE the primary trimmer to peak 'at thisoutput. No. 36-RADIO TEST INSTRUMENTS frequency.This should require only Having completed alignment of the No. 37-ELEMENTARY RADIO SERVICING very slight resetting of the trimmer. discriminator, apply a signal at 10.7 mc No. 38-HOW TO BUILD RADIO RE- If the low -frequency peak does not fallto the grid of the 6BA6 i.f. amplifier. CEIVERS where desired use the same procedure.The signal should be just sufficient to YourName The whole process may seem rathergive good indication on the current (Print Clearly) tedious, but with experience the desiredmeter, as the a.v.c. circuit is operating Address results are obtained very quickly. Onceand is not disconnected because of the Dealer'sName a setting is fixed for the slug, there isbias arrangements. The slugs in the i.f. Address no difficulty since the secondary trim-transformer are now peaked for maxi- mer is always adjusted for zero outputmum current through the diode resistor.

42 1948 RADIO -CRAFT REFERENCE ANNUAL Next, apply i.f. signal to the grid ofvantageous as the decrease in high - the converter, removing the grid leadfrequency response was less noticeable Every Radioman Needs to the tuned circuit, as this will short-than the decrease in background noise. circuit generator output.A blocking Positioning of the speaker was im- condenser is not used in the generatorportant in this regard and if placed "RANGE MASTER" MODEL10 lead for this test(to provide ad.c. near the floor, as is usual in ordinary The 8 -in -I Service Instrument pathtoground from the converterreceivers, both the desired high fre- signal grid). The slugs are now peakedquencies and the noise present were A precision instru- in the 1st i.f. transformer again to give almost inaudible.Therefore,if good mentforRADIO maximum diode current. The completehigh -frequency responseisrequired, TESTING, Appli- i.f. a n c e Repairing, alignment of the 2 transformerspositioning of the loudspeaker is an Service Calls, Am- can be rechecked if desired, as is usually important factor, and is a requirement ateur andExperi- done in an ordinary AM receiver. Thenot fulfilled by ordinary radio cabinets. mental Work. zero output setting of the secondary A further point of interest was the trimmer ofthediscriminatortrans- lack of noise when detuned from a "RANGE - former can be checked also if desired. carrier.The general noise levelap- MASTER" Only a small audio output signal willpeared far lower than with AM broad- covers these be heard during alignment. The smallercast receivers having similar sensitivity, 25 ranges. this residual audio output the betterand was very much less than with FM (1) CAPACITY .001-.1,.01.1,.1-10 Mfd. (2)A.C'. CURRENT 0-.150,0.-1.5, 0-15 amps. the balance of the ratio detector circuit. receiversusingordinarylirniter-dis- (3) A.C. VOLTAGE 110 100 500 1000 volts. From this it is seen that for receiverscriminator combinations having lower (4) D.C. VOLTAGE 10 100 500 1000 volts. (5) D.C. CURRENT 1 10 100 1000 milliamps. using ratio detectors alignment is not over-all sensitivities.This is probably (6) RESISTANCE 0 to 10,000 100,000 1 megohm. much more difficult than for an AMdue to the ratio detector as this circuit (7) SPECIAL High Range ohmmeter to 2 megs and 20 megs. receiver. provides appreciable amplitude rejection (8)SensitiveA.C. microammeter to900microamps.

with low signal inputs. TELEVISION SERVICEMEN .. why guess, meas- R.F. Circuits Good oscillator stability is of great ure!This is the only instrument in its class that An amplitude -modulated signal gen- can be used as an OSCILLOSCOPE CALIBRA- assistance in any FM receiver using a TOR. A standard bleeder will deliver1-10-100 erator tunable over a range of aboutbalanced discriminator whether of the accurate RMS volts from A. C. line. 80-130 me is required to align theseratio detector type or when preceded Available in KIT form, or assembled circuits. The procedure is almost exactlyby limiters. Very appreciable improve- Complete KIT and Instruction $17.95 net the same as for an AM receiver, but 2ment in amplitude rejection is secured Bench Model (assembled) 23.50net FoundationMeter with 3 -color scale, audio output peaks occur just as withwhen the i.f.is correctly centered at schematic and operating instructions.. $7.85 net the i.f. alignment. Adjust receiver tothe null point of the discriminator cir- Testleads .85 only 1 of these peaks and align r.f.cuit; and the less the variation from stages for maximum outputinthethis center point, the less will be the Ask your local distributor, or write to Dept. R.C.R. usual manner.Oscillator coverage isinterference from amplitude variations. set as in an AM receiver, and imageFurther, a well-balanced discriminator BRADSHAW INSTRUMENTS CO. points should be checked. circuitgreatlyimprovesamplitude 942 Kings Highway, Brooklyn 23, N. Y. Signal levels should be low duringrejectionquite independently of the alignment because of a.v.c. action. Theother amplitude rejection circuits used error in frequency setting using thisin the receiver. Good discriminator and procedure is only about 150 kc. i.f.tunedcircuitstabilityoffersa PRACTICAL FOR Output meter indications are easiernumber of advantages. TECHNICAL to read than current meter changes As a final point for those designers TRAININGYOU when adjusting r.f.and i.f.circuits,who feelthat anr.f.stageisnot and will usually lead to more accurateessential in an FM receiver; not only SPECIALIZE in 6 Months to 3 Yrs. alignment. is image rejection so low as to be un- Train in Electronics, General satisfactory but cases have been re- COURSES Radio,Electricity for ported where radiation at the oscillator SERVICE a substantial career in Field tests with this receiver indica-frequency of the receiver has been 6 to 12 Months First, Electricity industry - or a busF- ted several interesting points. received at distances up to4 miles Welding ness of your own. Pre- tuning was found to be little or nofrom the receiver location.This un- Refrigeration pare in one year to be more difficult than for an ordinarydesirable interference could be a seri- Heating broadcast receiver. The tuning action TECHNICIAN a Technician - or in ous problem when using receivers in Ito2 Years two additional years

Electricity is somewhat different, however, as 3densely populated areas. The explana- secure,a Bachelor of response points can be found by careful Radio tion of the effect is simple. There is Electronics Science Degree in dial setting. This does not lead to anyappreciable voltage at 'theoscillator Refrigeration, ElectricalEngineering ambiguity,however,asthecorrectfrequency developed across thecon- Heating and withmajor in Elec- centerfrequencygivesvery much Air Conditioning verter input circuit and this, combined PROFESSIONAL tronics.M ore than greater output than the side responses,with an efficient aerial system, allows 3 Years 35,000 former students which aresodistortedasto makeundesirable radiation to occur. Electrical Engin- in industries the world completelyunsatisfactory. eeringBachelor students reception A suitable antenna and lead-in, even ofScienceDe- over. 1,538 Correct tuning is indicated by lack offor indoor use, is made from 75 -ohm gree now enrolled from 48 distortion and noise, and sharpness is Major in Mach- states and 13 overseas cable (twin lead). A piece of the cable inery or Elec- sufficiently defined so 'as not to leadis cut into 2 sections, each 28% inches tronics countries.Faculty cf to any serious error. A tuning indica-long, to form a half -wave dipole. The 72 specialists. tor did not appear to be any more2 conductors are soldered together in Terms start January, necessary than with ordinary receivers. each half section.These halves are April,July,September With the receiver located in a verythen soldered directlyto the lead-in bad area as regards electrical noise, cable. Write for catalog and Personal reception was stillsatisfactory using The antenna is very directional and Guidance Questionnaire. an indoor aerial. Only a very occasionalshould be rotated togive maximum `1111* ---M I LWA U E E-' car ignition system gave any severesignal strength as almost a complete 501tai, of ENGINEERING interference.It was noticed, however, null point can be obtained when the A TECHNICAL INSTITUTE that the output from the receiver is aerialpoints inthedirectionofa somewhat distorted when strong inter-transmitter. ference is present. Most of the noise Founded in 1903 heard was high pitched, and a tone Adapted from an articlein Radiotronias-No. 127, Dept. RC -48A North Broadway Sydney,Australia;courtesyAmalgamatedWireless control giving treble cut would be ad- Valve Co.,Pty., Ltd. Milwaukee, Wisconsin

1948 RADIO -CRAFT REFERENCE ANNUAL 43 FEEDBACK AMPLIFIER (Continued from page 23) to preserve stability and yet keep thedue to harmonic content. The matching gain high enough for crystal pickupof the output tube to the speaker, too, work. is much less critical than would be the Resistor R2, rated to carry 40 ma,case without feedback. must connect directly at the socket to In choosing the volume level for a the tube plate to prevent parasitic os-given record or radio program, it is a cillation. Hum pick-up is prevented bygood plan to vary feedback and volume shielding leads as shown; use care incontrols tentatively, until a fine tonal placing the components on the chassis balance is achieved with good strength. and arrange them for short leads. For a level response, work with the The audio output of the amplifier isvolume control as near as possible to much greater than might be expectedits full position and with the negative from an examination of the character- feedback fully applied. istics of the tubes, since negative feed- back increases the efficiency of the Adapted from anarticle by G. W. Brown, in Practi- pentode by diminishing the distortion calWireless(London, England), August,1946.

NEW CIRCUITS FROM OLD (Continued from page 25) One tube does double duty.Regen- other factors.Lower B -voltages may eration sometimes is added. be tried, with corresponding changes in COIL KIT The reflex receiver slipped into dis- C -voltage. use during the years following the ad- vent of the screen -grid tube, but men- Reflex (With Tube 100 RADIO FREQUENCY COILS ALL NEW! Detector) tion of the circuit still arouses a tingle Fig.7,the concluding illustration, ALL USABLE! TREMENDOUS VARIETY! in old-timers who recall the surprising (Continued on page 46) Thousands of Applications in Each Kit sensitivity and power output of even the- 1- and 2 -tube receivers. Fig. 6 Frequency ranges from 2 meters to 25 kilocycles. Both iron core types and air types included. Many shows a 1 -tube reflex set With some HERE IT IS! units have mica condensers which can be salvaged new tricks. for other purposes. Although the tuning condensers are THE FAMOUS CHOKES WAVE TRAPS shown ganged, it may be advisable, at TRICRAFT ANTENNA COILS first, to use separate condensers, until OSCILLATOR COILS ANTENNA R.F. COILS TANK COILS values for inductance and capacitance can be found which will afford matched aswrittenupInthe I.F. TRANSFORMERS October issue of Radio ::t'" and many other types M kit tuning across theentire band. The News. With proper trimmers and associated parts every reason for this suggestion is that while perfectfor Allwave Televisionand FM coilcanbe used. TheseKITSare Ll and L2 are represented as standard antenna complete with mount- amateurs, experimenters, servicemen, schools, lab- ingpole, guy wires and 300 oratories, etc. "antenna" and "detector" coils, respec- ohm line lead-in,all waves. tively, for use with whatever value of $11.60 tuning capacitance is specified, the use of L2 in the circuit as shown will usual- BC229 RECEIVER (USED) $2.49 pleced at $2.9 8ly result in lack of resonance at one MICRO AMMETER-Beede 0-200 D.C $3.00 MILLIAMP METER-Beede 0-1000 D.C. $1.95 If these coils were to be extreme of the tuning range, due to dif- purchasedseparatelythey ference in effective capacitances now METER RECTIFIERS -Full Wave .95 would cost over $75.00. in shunt to the resonant circuit. Half Wave .65 Almost any one ,ofthesecoilsisworth more Since this situation may require some PHOTO -CELL AMPLIFIER 110V.60 than the price of the entire kitl "juggling" of coil and condenser valises, Cy. panel type has Micro Am- 20% Deposit with Order. Balance C.O.D, meter on front panel with tube to secure ganged tuning, either home- 6V6 6J7, 45, 80, VR105 $9.95 made or commercially available plug-in BUTTERFLY TUNERS -80 to 300 Mc., coils may be used for Ll and L2. This 300 to 1000 Mc. New in Cartons $1.89 BUYERS SYNDICATE PARABOLIC REFLECTORS-15"spun will make it convenient to try an alter- alum. Alzak fin. for 1200 Mc. up.pr $3.00 786 CAREW ST.. SPRINGFIELD, MASS. native method of obtaining regenera- SPECIAL-5BM or SCP1 Tube..$2.89 tion by using a "3 -circuit" coil as L1, Microphones -T-24 Carbon 79 T-17 $1.35 I I I 1 I 1 I IMatilleI Carbon connected as shown in Fig. 6-b. ASTATIC XTAL R3 MIKE $4.75 An adjustable detector (Xtal), or a RESISTORS standard sizes1/2 and I Watt BE YOUR OWN BOSS! fixed type in the "1N-" series, may be 100 assorted $1.79 500assorted $7.95 used. The 1N34 crystal detector is par- CONDENSERS MAKE MORE MONEY ticularly sensitive. Mica and Disc 50 assorted $1.95 In"CASH IN"you Tubular 50 assorted $2.69 $1.00 now get THE real Note that the tuned circuit of L2 is Bath Tubs 25 assorted $1.00 VALUE money-makers - doz- connected in the plate circuit of V. The Electrolytic 10 assorted $1.89 ens ofprofitable tested Trimmer and Padder-20 assorted $1.00 mail order plans,con- condensers shown dotted may not be $1.49 fidentialbusiness se- VOLUME CONTROLS 10assorted 25ccrets,dozensofprac- needed.Tube V may be a pentode or TUBE SOCKETS 20 assorted $1.29 ticaltested formulas, MIKE BUTTONS carbon8assorted $1.00 successful t eat e d beam -power tube, such as 1T4, 1Q5, Midget Variable Cond. 40.000schemes - actualex- 140 mmfd. 3/4 shaft 3for $1.00 WORDS periencesof men who 3Q4, etc.It may be necessary to con- have started on a shoe- SPEAKER 12" dynamic 6000 ohm $3.95 IN string -withless than nect a 0.25-megohm resistor across the TEXT $10 capital. 250 a secondary of audio transformer T, to Pr. Headphones" or "Lip or copy postpaid. Send FREEThroat -"I Microphone" with every order. U. S.stamps, money prevent parasitic oscillation. The usual NO ADSorder, or coin. New 1948 Circular Available - Write- Today ALL Money Back ratio of T is about 1:3, although ratios "MEAT"! Guarantee up to 1:10, and even higher, have been AMERICAN SALES CO. NATIONAL PLANS COMPANY used successfully, depending upon the 1811 W. 47th St. CHIC -AGO 9, ILL. Box 26RA. Ansonia Station New York 23. N. Y. characteristics of the transformer and

44 1948 RADIO -CRAFT REFERENCE ANNUAL SUPERSONIC DOOR OPENER TEACH YOURSELF (Continued from page 26) Radio & Electronics negative, unwanted soundswillnotthe door by key switch, for example, cause the door mechanism to operate. you may back the car out so that the The relay control circuit is made upwhistle blast will be directed toward of double triode tube V4A and V4B.the microphone, and the door lowered A time -delay, R -C network in this cir-and stopped by whistle, up to distances cuit prevents relay operation until aof about 20 feet. 3 -second signal has been received. This The motor rotates a bronze bushing Magi-Klips Kit further reduces possibility of erraticwhich causes a lead screw to travel. A Build 18 Experiments at Home-No Tools linkage to the screw moves the door, RADIO RECEIVER, HOME BROADCASTER, operation caused by unwanted sounds PHOTOELECTRIC RELAY, CODE PRACTICE and supply -line voltage changes. which, in the closed position, is held by OSCILLATOR, SIGNAL TRACER, REMOTE a force of 600 pounds per square inch, CONTROL RELAY, Phonograph Transmitter, The stepper relay contacts, the key Intercommunication Amplifier, Code Trans- making any other lock unnecessary. If mitter,RadioFrequencyOscillator,Tele- switch, and the pushbuttons, are all con-an object impedes motion of the moving phoneLineAmplifier,ElectronicSwitch, nectedinparallel,and operatethe Phonograph Amplifier, Temperature Control door,it automatically stops and re- Relay, Contact Detector, Electronic Metro- motor. Successive whistle blasts causeverses its direction. Either electric or nome,IntervalTimer(one-shot),Interval the garage door to raise, lower, andmanual door operation is possible. Times (repeating). With aMagi-Klipskit you cover the entire field of stop (lock); the key switch and push- radio and electronic engineering theory quickly in Photosanddiagramscourtesy VendoCo.,Kansas your spare time. It's simple to arrange the compo- buttons act similarly. Having opened City 3, Mo. nents for each circuit.No soldering.No tedious wiring.Kit operates on 110 v. AC or DC and in- cludes 35Z5 rectifier, 50L6 power amplifier, 12SL7 double triode, powerful 4" speaker -mike, plate re- lay, broadcast and SW coils, tuning condenser and POWER STABILIZER generous supply ofresistors,condensers,chokes, extra wire. Parts worth double the price of kit. (Continued from page 27) Kit's 48 -page manual has complete instructions and diagrams easily followed by the beginner. Re- without further adjustment.The per- member, you need no tools, except $2975 the filtering ability of an ordinary tri- possibly a screwdriver, with aMagi- ode. This triode has a low plate -circuitformance is independent of the load, but Klipselectronicexperimenter'skit. Complete thinis at the expense of voltage regu- COMPLETEMAGI-KLIPSKITSunassembledare resistance and acts as an amplifier, giv- also available.If you wish to assemble ing phase reversal but neither loss norlation.Resistor Ri should now have a yourown MAGI-KLIPSKit,specify c value of 1/g. +R3(1 + 1//.4).Using "knocked -down" on your order only... I gain. By this means the normal ripple Sendtodayforyoureopyof FREE complete is neutralized.Simultaneously, if thea 6J5, R3 can be 1000 ohms; and R1 booklet, "Electronics Made Easy." will have to be 1,300 ohms. But again, Schools - MAGI-KLIPS Kits are excellent time constant of the grid circuit of the equipment forlaboratoryandclassroom. triode is large, relatively slow varia-for best results, adjustment should be Write for special discounts. made with a cathode-ray oscilloscope. Dealers-Some territoriesopen. Write for tions superimposed on the ripple are your trade discounts. also neutralized. The resistance r is included to limit DEER & TAYLOR COMPANY grid current when the load is suddenly Dept. C Berkeley 9, Calif. Circuit Analysis increased; it can be 50,000 ohms. The 1342 Milvia St. Analysis shows that in Fig. 1 Ri =voltage regulation is here made more 1volt for every 1/gm where gm is the mutual conduc-difficult by about Ecteszei Radio tance of the tube. R2 must have a valuemilliampere in the load. designed to give a suitable grid bias, In both these circuits there is little Senceeeefitaa 71,eeeid and this value clearly depends on theobjection to using output voltages up to 350.The 6J5 is not likely to be fixed load current. Condenser C shoulddamaged so long as the plate dissipa- have very low leakage, and should betion is kept below 2.5 watts, because the as large as possible, say up to 2µf; plate voltage variation is small. Rshould be 1 megohm. A large con- The author's samples built to these denser or a further decoupling circuitcircuits have shown a residual ripple isessential,acrosstheoutputter- minals, to lower the impedance pre-of not more than 1 millivolt in 300 sented to voltages arising in the load.volts.Moreover, the random jumpiness For a 6J5, R1 should be about 390of output voltage has completely dis- AC -DC ohms, and for the best results the finalappeared. adjustment of value should be made BALLAST rear Application SUE with a cathode-ray- oscilloscope. Due 51,51 to the presence of R1, the voltage regu- The chief application has been the lation is made more difficult by aboutsupply to oscillators.In this case the TUBE MANUAL 1 volt every 3 ma in the load. oscillator portion of the instrument is To get YOUR FREE COPY of this in- Fig. 2 shows a modification suitableconnected to the semi -stabilized output, dispensable manual(value $1.50) for, say, a laboratory power pack whichwhile the plate circuit, grid circuit, and merely mailin to JFD Factory 12 may be used on various fixed loadscathode circuit of the power stage are flaps from JFD Radio Dial Belt en- connected totheunstabilized power pack direct.For safety, the chassis velopes; include 100 in postage to and case are also connected to the nega- cover mailing cost.(Buy your JFD tive end of the power -stage supply and Dial Belts and Belt Kits from your to one output terminal. nearest parts jobber.) The circuit of Fig. 1, which is almost JFD's new 68 -page Ballast Manual isa treas- INPUT FROM a traditional method of "electronic fil- uryofinformationforRadio Servicemen POWER PACK tering," was brought to the writer's and Dealers - Lists more than3000radio attention byhiscolleague,Mr.C. ballasts - ACDC BallastsforFluorescent Stokes. We have not been able to trace lights and electricalappliances - 220 volt any actual reference to it, but the full to110 volt Stepdown Ballasts. implications brought out by this note Send 12 Envelope Flaps to: are believed to have some novelty. J.F.D. MANUFACTURING CO. INC. Adapted from an article by E. A. Hanney inWireless 4109.4123 FT HAMILTON PKWAY. BKLYN 19. N.Y Fig. 2-Modification for laboratory pack. World, (London, England), April 1945.

1948 RADIO -CRAFT REFERENCE ANNUAL 41. -PROFESSIONAL TRAINING- WILL ADD TO YOUR SUCCESS "RADIO -CRAFT" INDEX The efforts of many arecom- Volume XVIII October, 1946 to September, 1947 bined to make radio the-suc- cessful business it is today. To AMATEUR RADIO ELECTRONIC MUSICAL INSTRUMENTS issuepg. issuepg. mention only a few, are the Amateur Radio W2IXY (D. D. Hall) Dec. 20 BC -624 on 2 Meters Metronome, Simple (Dr. A. Montani) ..Jan. 88 artists, writers, mixers, record- (L. W. May, Jr., W5AJG) Sept. 24 New "Prismatone" Organ Apr. 22 BC -625 on 144 Mc Organ, Simple Electronic (H. Conroy)Jan. 70 ing and transmitting engin- (L. W. May, Jr., W5AJG) Apr. 35 Economy Transmitter eers, as well as those respon- (R. F. Scott, W2PWG) May - 20 ELECTRONICS Frequency Monitor, Simple Airborne Metal Seeker Aug. 73 sible for production and sales. (I. Queen, W2OUX) Feb. 21 Electronic Counting (Jordan McQuay) Aug. 26 In Ham Antenna Tower for V.H.F.&U.H.F. Eectronic Egg Grader (S. R. Winters)Sept. 22 draftingourcurriculum, (W. Hoisington, W2BAV) Aug. 22 Electronic Photometer (J. G. Reed) July 27 Ham Station from Surplus (M. Lindley) Feb. 30 Multivibrators (0. B. Mitchell) May 25 these factors were taken into Medium Power Transmitter Radio Frequency Lamp Feb. 46 (R. L. Parmenter, W1JXF) Jan. 67 Robot Makes Radios consideration. We offer such Mobile Xmitter for 2 Meters (E.L.Byrd) Sept. 26 (Major Ralph W. Hallows) Sept. 20 Narrow -band FM for Ham Radios Switching and Triggering (j. McQuay) Mar. 23 traininginourschool,and (N. L. Chalfin) July 23 Oscillations, Parasitic Teleguided Missiles (James A. Niland) Feb. 24 maintain a free job placement (L Queen, W2OUX) July 37 Test Holder for Crystal Grinding FEATURES for our graduates. (L.R. Robbins) Jan. 88 250 -Watt FM -AM Transmitter Birth of the (Frank E. Butler). Jan. 22 Approved for Veterans (Harry D. Hooton, W3KPX)' Canberra's Mobile Radios May 34 Part I July 31 De Forest the Inventor (H. W. Secor)Jan. 46 SCHOOL OF RADIO ARTS Part II Aug. 28 De Forest and the Navy (G. H. Clark) Jan. 32 Part III Sept. 30 De Forest Patents... Jan. 50 1655 N. Cherokee, Dept. C 250-300 Mc Radiophone Dutch Underground Radios Hollywood 28, Calif. (R. F. Scott, W2PWG) Feb. 20 (J.Maquerinck) Mar. 26 Please send free descriptive literature on Universal 1 -Kw Amplifier Early Radiophone, An Jan. 36 the following courses: (H. D. Hooton, W3KPX) Nov. 22 E. I. Co. Sold First V. F. 0. Exciter or Transmitter (H. Gernsback) Jan. 45 Combination Announcing and Radio (L Queen, W2OUX) Fighting Amateurs ("Ton") Oct. 26 PartI Theory (Prep forFCClicense) June 30 First Phone Broadcast (F. E. Butler) Jan. 38 PartII July 32 First Radio Association Announcing Working on 50 and 420 Mc (Hugo Gernsback) Jan. 37 Radio Telephone & Telegraph (Edward D. Padgett) Mar, 35 How the Audion Was Invented (Lee de Forest) Jan. 19 Sound Recording & Transmission How Audions Were Built Engineering ANTENNAS (Gerald F.J. Tyne) Jan. 27 Name Motion Picture Engineers Honor Dr Antenna Principles (I. Queen) de Forest Jan. 51 PartI Dec. 23 Address New: Crystron Lapel Radio PartII Jan. 72 Mohammed Ulysses Fips) Apr. 24 PartIII Feb. 33 New French Radio Parts (E. Aisberg)May 40 Part IV Mar. 30 Pioneer Atom Splitter June 22 Antenna Principles (Jordan McQuay) Radio Inventions of Lee de Forest Part V Apr. 20 (Fred Shunaman) Jan. 41 Part VI May 29 Radio Pen in Europe, The Feb. 71 Part VII June 27 To the Father of Radio Jan. 52 PartVIII July 39 Folded Dipole Antenna (L. S. Weaker) Dec. 43 New Circuits From Old 300 -Ohm Folded Dipole May 54 Tunable Aerial, A Aug. 74 INTERCOMMUNICATORS (Continued from page 44) Home Intercommunicator (George W. Schultze) Nov. 17 AVIATION RADIO Multi -Station Intercoms (R. H. Dorf) shows the use of a beam -power tube, Private Aircraft Radio (R. F. Scott)...... Dec. Part I Mar. 37 22 Part II Apr. 33 V1, and a medium power triode, V2, in Part III May 33 a stable circuit (unlike earlier circuits CONSTRUCTION using triodes in both positions).This Audio Oscillatok with Band Spread arrangement is presented for those who (C. F. van L. Weiland) MEASURING AND TEST EQUIPMENT PartI Aug. 23 Beat -Frequency Oscillator prefer tube detection, in a reflex re- PartII Sept. 32 (John W. Straede) Nov. 28 ceiver, over the more common use of Carrier Transmitter Dec. 41 Capacitance -Resistance Bridge Easy -to -Build Oscilloscope (McMurdo Silver Model 904) Nov. 18 crystal rectification. (B. W. Southwell) May 23 Cinematic Analyzer (E. Aisberg) Standard broadcast coil and variable French Radio -Model Auto (S. Coudrier) Jply 21 Part I Dec. 18 High Speed Photo -Flash Part II Jan. 65 condenser combinations are used for the (Lyman E. Greenlee) Feb. 22 Clip -On Radio Tester (H. L. Davidson) Jan. 103 tuning circuits.The maximum ratio Lightweight Portable Photoflash Compensator for A. C. Meters (W. G. Many) Sept. 23 (P. Hemardinquer) .., Aug. 37 for T is about 1:3, or 1:5. Miniature -Tube Amplifier Converting the BC -412 (L. Greenlee) Oct. 18 Adequate decoupling of the detector (J.C. Hoadley) Aug. 54 Decade Box for Capacity Tests Oscillator, Code (Bernd Falk) May 82 (R.L. Parmenter) Aug. 33 stage ensures stability under all condi- Phono Oscillator, DeLuxe (W.G.Eslick) Apr. 42 Detector for the Oscilloscope tions, while volume can be controlled Portable Recorder -Player (K. C. Dike) Feb. 74 (Joseph G. Karnath, W2RRA) Aug. 20 Intermittent Alarm (J. Jacobson) June 34 by means of the 50,000 -ohm potentio- Reflexed Four -Tuber (W. T. Connatser) July 20 King of Tube Checkers May 22 meter across the primary of the audio Soldering Gun, A (Ral Collins) Aug. 48 Modulation Meter, Simple Superregenerator, New (M. Black) Aug. 56 (A. B. Kaufman) Feb. 32 transformer.Regeneration is smooth Trainee Five Radio (Edwin Bohr) Jan, 86 Multitester Plus V.T.V.M. (J. Simrin) Jan. 83 and under perfect control by means of Transmitters, Demonstration Probe Tracer (Hubert L. Bailey) Feb. 45 (Harold McDuff and George Pyle)...June 35 "Q" (Raymond G. Johnson) July 38 the 150 Ackf variable condenser C. Travelers' 2 -Tuber (W. G. Beaman).... Aug. 37 Rapid Checker for Capacity - Volume Expander (J. Kwietinskas) Aug. 67 Continuity (Alfred Shortcut) July 37 As with the reflex receiver previous- V. T. Voltmeter, New Ideas in a Scope, The-A Repair Tool ly described, the tuned coils should be (C. F. van L. Weiland) Apr. 30 (S. Prensky and I. Jacobson) Part I Jan. 68 shielded, or plated one above and one Part II Max. 28 below the chassis.Great care must be EDITORIALS by HUGO GERNSBACK "Submarine" Signal Tracer (M. E. Blaisdell) July 26 taken with respect to parts placement "Brand" vs. "Orphan" Radios July 17 Vest -Pocket A.C.-D.C. Volt-Ohm- and wiring of any reflex.Otherwise De Forest-Father of Radio Jan. 17 Millicmimeter (William Lyon) Apr. 26 FM and Altitude June 17 Volt -Ohmmeter Without Switches, A uncontrollable oscillation is likely to re- FM Boom Aug. 17 (Ralph Bloom) Aug. 36 sult, or in rarer cases some form of 400 Million U. S. Radios? May 17 Wave and Pulse Counter (R. Blitzer) Dee. 25 Printed Radios Sept. 17 degeneration which may reduce the re- Radioless Train Hazards Oct. 13 ceiver's sensitivity. Radio Progress is Slow Feb. 17 Servicemen-Wake Up! Dec. 13 MISCELLANEOUS Superadio Apr. Adapted from an article by W. Nimmons, in Practical Tele-Theatre, The 17 German Brainwcrve, A Nov. 44 Wireless (London, Eng.), April -July, 1946. Nov. 13 Heat -Operated "" Apr. 46 Why Radio Specialization? Mar. 17 High -Voltage Power Supply May 52 46 1948 RADIO -CRAFT REFERENCE ANNUAL Names for Wavebands (E. Aisberg) Dec. 63 Modern Crystal Set, A (W. LE. Spain) Dec. 48 Radio arid Racetracks Dec. 51 Postwar Radios sew Transmission With Light The Belmont Boulevard Mar. 24 11p'4Iluupl (Richard H. Milburn) ' Jan. 62 Hallicrafters S-40 Apr. 28 SERIES Telechron Musalarm Nov. 30 NEW RADIO -ELECTRONIC PATENTS Radio Data Sheet IG No. 340 (Crosley Model 106CP) Oct. 35 IMPROVED Ihrl Airplane Antenna Dec. 46 No. 341 (Garod Model 6AU-1) Nov. 33 NO -DRIFT IfiI Control System ' Nov. 58 No. 342 (Howard Radio Model 901)Dec. 35 5 D.C. Amplifier Aug. 52 No. 343 (Zenith Radio Models 8H032, 8H033, 8H050, 8H052, 8H061) Jan. 84 WAVE Dual Control Dec. 46 BANDS Electronic Capacitance Feb. 70 No. 344 (National Union Model FM Electronic DFDT Switch Aug. 52 G-619) Feb. 34 in the Sensational1948 Expanded Sawtooth Wave Dec. 46 No. 345 (Admiral Model 6RT44-7131)Apr. 40 Frequency Meter Feb. 70 No. 346 (General Electric Farm Frequency Multiplier Oct. 63 Model 28a) May 36 Improved Superregenerator July 48 No. 347 (Motorola Auto Radio 1946 .. 38 MIDWEST Light Radar Aug. 52 CR-6) June Magnetic Measurement June 54 Radio Service Set Data Modulation Step -Up Nov. 58 (Browning FM -AM Tuner RJ-12) July 34 RADIOS Multi -Channel U.H.F. Receptiton June 55 Radio Set and Service Review Neutron Detector Apr. 72 (Some Unusual Designs and Devel- Separate Chassis Like This Powerful opments) Aug. 24 Photographic Phonograph Sept. 54 SERIES16 AM -FM MODEL OR Preselector, Crystal Feb. 70 Q -Controlled Insulation Dec. 46 RECEIVERS, CONSTRUCTION Beautiful Radio-Phono Consoles Available Radiotelegraph A.G.C. May 60 R. F. Energy Control Oct. 63 Battery Superhet Uses 3 Tubes R. F. Oscillator Sept. 54 Hechich) June 31 The SYMPHONY GRAND gub-Harmonic Generator July 48 Building a Set Is Easy RADIO -PHONOGRAPH Telegraph Repeater May 60 (H. A. Nickerson) Oct. 24 II Television Sweep Circuit June 54 Miniature 3 -Tube Receiver (E. Bohr).... Dec. 37 Time -Interval Measurement May 60 AM -FM CONSOLE Trigger Circuit Feb. 70 A truly magnificent instrument that offers TV Color Control Sept. 54 SERVICING new, improved No -Drift FM, 5 Wave Bands, Underwater Detection June 54 Automatic Intermix Record Changer that Voltage Regulation July 48 Airline Radio Service (E. D. Padgett) .Oct. 20 handles both 10" and 12" records, 14 V," Wideband Coupling Apr. 72 Cabinet Refinishing (H. A. Nickerson) .Jan. 87 Dealer Service Builds Volume Panasonie Speaker, and exclusive Midwest 30 Tri-Magnadyne Coil System and (Jack Tannehill) Aug. 30 Color -Ray Tone Selection. Other EASY DAYS RADAR Efficient Test and Repair Bench luxurious Console and Table Models (C. A. Brown) May 22 TERMS TRIAL Anti -Collision Radio English Repair Methods withSeries 16,12, and S C,sis.ha (H. W. Secor and Eric Leslie) Oct. 17 (Eugene A. Conklin) Sept. 56 SEND FOR New Radar for Air Safety Mar. 22 Equipping the Shop (Jack King) Mar. 27 'BUY DIRECT FROM FACTORY AND SAVE! Radar Waves Harmless Dec. 45 Freak Repair Problem, A Sept. 78 FREE "Shooting Stars" Tracked by Radio How Does the Signal Get Through? Catalog FILL EN COUPON AND MAD TODAY OR JUST 26 SENDYOUR.NAME AND ADDRESS ON Sr POSTCARD (S. R. Winters) June (Zygmunt Hof).... . Nov. 48 Install a Wave Trap (0. Parsons) May 86 Lamp Bulb Resistors (J. B. Parchman). Apr. 23 MIDWEST RADIO CORPORATION RADIO -ELECTRONIC CIRCUITS Midget Radio, Improving the dept. 43-K,909 Broadway, Cincinnati 2, Ohio (John Kwietinskas) ... July 51 Please send me your new FREE 1948 Catalog. A.C.-D.C. Phono Motor (G. A. Felix) .. Apr. 54 Neglected Servicemen (E. H. Ehle) Apr. 44 A.C.-D.C. V.T.V.M. (Leon Medler) May 70 Noise Elimination (Jack King) June 29 NAME Adding a Phono Pickup (F. W. Rodey) May 71 July 58 Adding an "5" Meter (D. Stapleton)July 42 Notes on Servicing (H. A. Nickerson) Oscillators (Jack King) Aug. 31 STREET Battery Signal Generator (0. Lefebvre) Oct. 36 Dec. 28 Carrier Communicator (R. K. Cobb) Mar. 47 Police Radio Maintenance Problems of Instability (T. Powell) Feb. 25 CITY..., ZONE .STATE Carrier Transceiver (H. 0. Northern),May 70 Quick Fault Finding (H. E. Leeper) Aug. 29 Cleaning Condenser Plates Replacing the Rectifier Jan. 64 (C.J.Whitton) May 71 Code Oscillator (Ralph Bloom) July 42 Servicing Movie Sound, Code Oscillator (Arthur A. Day) Oct. 36 (Hubert W. Watkins) Every tube guaranteed. Every tube in carton PartI Nov. 23 Lots Compact Intercom PartII Dec. 30 Lots (F. C. Hoffman, W9VVU) Feb. 38 Signal Tracer -Injector for Efficient of 10 of 10 Converter Control (C. E. Henderson) June 44 Servicing (Lyle C. Treakle) Sept. 31 Type EachEach Type Each Each Duplex Detector (H. M. Harvey) ..... Feb. 38 Speedy A.C.-D.C. Servicing (J. Bowles) May 28 II.T5 36c30c 7AF7 44 35 Filter Circuit, Novel (C. H. Schuppee) Oct. 36 Static Hunting Is Profitable! IV 45 40 14X7 44 35 Heaterless B Supply (W. G. Murray) .July 42 1L4 55 49 12A8GT 45 37 (Paul W. Streeter) ' Sept. 28 High -Fidelity Tuner (Saul Sherman) Dec. 36 Successful Service Shop (C. J. Whitton) Mar. 20 2A5 65 55 12AT6 50 45 Intercom, Simple (Don Russell) . .... Jan. 110 Transformer Winder Feb. 48 2A6 79 69 12BA6 50 45 Low -Drain Amplifier (R. H. Rethorel)Oct. 36 Use Your Auto Radio (J. C. Hoadley)Oct. 25 2X2 79 72 12BE6 50 45 Low -Voltage Receiver (E. A. Barnes)May 70 3A4 69 59 12C8 49 45 )(Ake Preamplifier (Robert J. Hall) Mar. 46 3Q5GT 79 70 12J5GT 49 39 Modulation Indicator Sept. 42 SOUND 5U4G 50 40 12K7GT 45 39 Multi.Station Tuning (J. O'Connell) Dec. 36 5W4GT 40 36 IRKS 65 59 115 -Volt D.C. Receiver (J. R. Young) Nov. 34 Adding a Crystal Pickup (Geo. Duff) Dec. 32 Dynamic Pickup (P. H. Russell) July 28 5Y3GT 40 35 12Q7GT 45 39 One -Tube Tracer (A. Malinick) Oct. 36 5Y4G 40 37 12SA7GT 40 32 One -Tuber, Simple (Gray Trembly) ...July 42 Full Frequency Range Recording (R. W. Hallows) June 33 5A7 50 45 12SQ7GT 40 32 Oscillator, A New Aug. 44 45 35 Phono Amplifier (Floyd R, Harris) ....Feb. 38 Home -Built Sound Effects May 26 6AEGT 50 44 12SK7GT Phono Amplifier, Simple (L. M. Good)June 44 Magnetostriction Pickup (R..F. Scott) Nov. 16 6AC7 55 49 12SJ7GT 55 49 Phono Oscillator (G. Borchert) June 42 Postwar Features of Phonograph 6AK5 79 69 26 39 30 Phono Oscillator (R. R. Smith) Jan. 111 Pickups (I. Queen, W2OUX) Sept. 36 6AG7/6AK7 89 79 27 42 37 34 6B7 55 49 41 40 35 Power Supply, Novel (F. W. Burdon) . Nov. 34 Response Equalization (J. W. Straede) Sept. Power Supply, Safety (J. A. Dewar) ..May 71 Small Recording Studio, (J. C. Hoadley) 6C5GT 55 45 42 42 38 Power Supply, Simple (R. L. Allman)Jan. 110 Part I Feb. 26 6C6 45 37 43 54 49 Power Supply, Useful (W.M.Finleyjr.)July 42 Part II Aprt 34. _6Q8,G 45 37 45 49 39 Receiver Conversion (A. Thomas, Jr.)Mar. 46 Part III May 31 6D6 45 37 47 49 39 Receiver, Novel (John Justin) Feb. 38 Part IV June 32 6F6GT 45 39 56 49 39 Record Player Pre -Amp (0. G. Waters)Mar. 46 Part V July 36 6116GT 45 39 57 45 39 Regenerator, Novel (H. R. Newell) June 42 Sound Engineering (A. C. Shaney) 6J5GT 45 39 58 45 39 Speech Amplifier (Wilson Finley) . Dec. 36 No. 24 Oct. 32 6J7GT 42 38 71A 39 29 Test Amplifier (F. U. Dillion) July 42 No. 25 Nov. 31 6K6GT 45 39 75 50 39 Time -Delay Circuit (I. A. Siderman) Apr. 54 No. 26 Jan. 89 6K7GT 49 40 76 45 39 Tone Compensator (Ted Hannah) Jan. 111 No. 27 Feb. 36 6L6G 95 89 77 40 32 Tone Control (John Kwietinskas) Feb. 38 Sound System Improvement 6Q7GT 47 39 78 40 32 Tone Control, Novel (J. C. Hoadley) Nov. 20 6U7G 40 35 80 39 31 (George A. France, W3LTA) Mar. 46 Supersonic Applications 6V6GT 45 40 83V 99 89 Tone Control Revised (T. F. Lo Giudice) Dec. 16 6X5GT 49 40 84/6Z4 45 36 (E. A. Chapman) Jan. 111 Ultrasonic Principles and Circuits 6SA7GT 44 37 85 49 45 Tone Control Systems Aug. 44 (J. W. Straede) Aug. 34 6SK7GT 44 37 25L6GT 49 39 Tracer from Blooper (W. Neelands) ..Apr. 54 6SJJ7GT 44 37 25Z5 49 45 Transmitter Monitor TELEVISION 6SL7GT 49 47 25Z6GT 45 39 (Douglas K. Vanderwater) Nov. 34 6SN7GT 49 47 35W4 43 40 25 -Watt Amplifier (Frank Gue) Apr. 54 Building a Televises, (Roy Freeland) 6SQ7GT 44 37 35Z3 44 35 Two -Tube Portable (A. Ettinger) June 42 Part I Jan. 60 6SG7 44 39 35Z5GT 43 39 Universal Antenna (R.J. Walsh) Feb. 38 Part II Feb. 28 6ZY5G 45 39 35L6GT 45 39 Variable B -Supply Sept. 42 Part III Mar. 32 7D6 44 35 50L6GT 50 45 Variable D.C. Supply (M. Gier) Dec. 36 Color Television (H. W. Secor) 7C6 44 35 117Z3 55 45 V. H. F. Oscillator (P. F. Egerton, Jr.)Jan. 110 Part I June 20 35 117Z6GT 99 89 34 7C7 44 " (H. W. Merrihew)Nov. Part II July 24 7B7 44 35 50135 42 32 Oil -Film Television Oct. 22 7Y4 44 35 32L7GT 59 49 Television for Today (Milton S. Kiver) 7X7 44 35 82 39 31 RECEIVERS, COMMERCIAL AND Part V Oct. 27 RADIO DATA SHEETS Part VI Nov. 25 Part VII Dec. 29 SENCO RADIO. Inc. Crystal Radio Makes a Comeback Part VIII Jan. 81 96 WARREN ST., NEW YORK 7,N. Y. (Harry= Winfield) July 33 Part IX Feb. 31 low -Priced FM Tuning Unit Sept. 27 Part X Mar. 33. Tel. COrt)andt 7-6065

19481RADIO-CRAFT ,REFERENCE ANNUAL 47 RADIO -CRAFT INDEX (Continued from page 47) Keep Your

30,000 issuepg. Television forToday(Milton S. Kiver) Magazines Part XI Apr.. 37 RADIO SERVICE- Part XII May 32 Part XIII June 36 TECHNICIANS READ Television Over a Light Beam July 22 Clean in this

IVIRTINING IN RADIO AND RNSIONICS FOR THE THEORY,ENGINEERING AND DESIGN RADIO StRVICAJECHNICIAN Coils, Cores &Magnets (H. W.Schendel) DE LUXE BINDER Part I Oct. 30 PartII Nov. 27 Electronic Mathematicians Pie5era6170111 (Robert W. Ehrlich) Dec. 26 tv Guillotine for FM July 70 Miller -Effect FM (HaroldE. Ennes) Aug. 35 RADIO New Subminiature Printed Circuits June 24 Time Delay Control Circuits (R, H.Dorf, W2QMI) Aug. 27 CRAFT BACK ISSUES TRY THIS ONE perm/tee Adding Headphones(Don Beckerleg). Dec. 42 -N.

Aligning Receivers (Otto Woolsey) Feb. 40 Antenna Installation (Amos Babb) Aug. 42 Antenna Mounting (Emerson Payne) July 54 Antenna Static(Jack Samuels) Feb. 40 Battery Eliminator (J.R. Blundin) Nov. 36 Battery Tester(William B. Thorne) May 72 Buzzerator (LeonG. Brown) June 46 Cable Break Detector (D. W.Baird) Mar. 48 Changer Rack(George Murakami) Aug. 42 Changer Supports(R. M.Bosch) June 46 Chassis Supports (R. C. Szumilo) Jan. 10,8 Circle Cutter, Novel(JohnHaynes) May 72 PROTECT your back issue in this NINO B BONE INC., PUNISHERS Coil Winding Kink (Ray Kiefer) June 46 beautiful DeLuxe Binder, tailor-made to Condenser Decade(M. R. Solomon) Mar. 49 Connectors, Simple (E. W. Hutchinson) Dec. 42 fit 8 issues of RADIO -CRAFT. Crystal Diodes, Mounting Well - constructed,finishedin simulated (Walter J. Blazek) Sept. 44 Morocco leather,richrusset brown with Data File(William A. Home) Feb. 40 EVERY MONTH Dial Cords(WilliamA. Home) Feb. 40 gold embossedlettering and design on RADIO MAINTENANCE today fills a breach that Dial, Expanded(AugustineMayer) July 54 front and back.Size 9"x121/2" with13/4" has existed in the radio field for a long time. Al- Discarded Starters (W. Z. Bugielski) . Apr. 56 back. Guitar Amplifier(Kenneth Ballard) Dec. 42 ready30,000servicemen read RADIO MAINTE- No punchingofmagazines necessary. NANCE every month because itis devoted entire- Hand Capacity (Wallace Busby) Oct. 50 ly to the Radio Service Technician. Handy "V" Tool (Joseph S. Napora) July 54 Justslipyour RADIO -CRAFT magazines Indicator, Beam Antenna(H. L.Hardy) Mar. 48 through a metal strap built into the binder. The RADIO MAINTENANCE staff secializes in the Lamp, Novel Panel(Harry Kundrat) Jan. 108 preparation of articles on every phase of Radio Line Booster (Keith W. Mohler) Aug. 42 Inajiffy they are in place and everlast- Maintenance inseries form which may be filed -- Loop Aerials(H.Leeper) Dec. 42 and used for reference. The leading articles cover ingly protected. Magic Eye, New Sight for(J. Potter) May 73 Start now to keep your magazines clean, everything for the radio serviceman on Television, Meter Multipliers (Ernest Turffs) Dec. 42 FM and AM; Test Equipment; Electronic Appli- Noise Eliminator (H.Leeper) Oct. 50 fresh -looking for instant reference-instead ances; Tools; Antennas; Alignment; Troubleshoot- Ohmmeter Kink(Thomas Higgins) Aug, 42 of lying around helter-skelter,torn, dog- ing: Repair; Construction; Pick-ups ad Sound Am- Oscillation, Curing (H. Leeper) Dec. 42 plification andReproductionEquipment. Also,in Panel Finish, Novel (R. G.Berlyn) July 54 eared and soiled ... very likely lost.This RADIO MAINTENANCE each month there are de- PA Speaker Matching(Hart Webber)Aug. 42 DeLuxebinder isattractiveenoughto partments on hints and kinks, the latest news of Power Supply Saver (E.H. Wright) June 46 dress up your living room library shelves. the trade, review of trade literature, radiomen's Protecting Beam Tubes opinions, new products and news from the organi- (S.H. Beverage) July 54 zations. All articles are presentedina step-by- Protecting Drawings(Pat Clemens) May 72 step precision style, clearly illustrated, with sche- Receiver Panel(Real Bronsard) Mar. 49 matics, accurate photographs, specially prepared Record Player, Plug-in(C. L. Holman)Dec. 42 drawings, white on black charts, color diagrams, - Rectifier, 6 -Volt (C. Leval) Nov. 36 IRISA. isometric projections and exploded views. Removing Rivets(Geo. F. Cutress) July 54 Socket, Experimental FREE DO Isguaranteedre RADIO MAINTENANCEis not sold (Edward F. Ziememdorf) Sept. 44 S . Binder li CO', Sockets, Sub -panel(E. R. Matthews)..Dec. 42 Del.uxe your on newsstands lhe satisist. and Bindersinbeautifulgreensimulatedleather Socket Tool (Floyd Fellows) Aug. 42 than days Soldering Kink(Marcel Strudler) Apr. 56 to more S refun stampedingold are now available for your within S. on Soldering Tip(John Zachazewski) Mar. 49 turn the RADIO MAINTENANCE magazines - see the Solder Pencil (E. H. Stutz) Mar. 48 vr111 be in O. currentissueof RADIO MAINTENANCE for money postpaid and information. Speaker, Inductor (R. S. Selleck) Oct. 50 Sent lCanadlart Fmmi Speaker Kink (WalterC. Williams) Mar. 48 ed. 0. IMO WI Nm MR of ...... Speaker Matching (A. W. Adey) Feb. 40 receipt RADIO MAINTENANCE MAGAZINE RCRA II Spotting Mounting Holes ForeignWS.) 460Bloomfield Avenue, I (G. F. Stoneman) June 46 Montclair 5, N. J. Substitution Gadget Please send me RADIO MAINTENANCE for I (Helen M. Douglas, W5LGY) Jan. 108 o Iyearat $3.00 o Bill me later Supply, 12 -Volt I 0 Payment enclosed (Zolan T. Bogar,W3CJM) Mar. 48 0 2 years at $0.00 Test Lead Saver (W. M.Bowlan) Jan. 108 RADIO -CRAFT Tester, Simple (Edward L. Spear) Sept. 44 Name Tracer, Simple (0. B. Miller) Aug. 42 25 West Broadway, New York 7, N. Y. Transformer, Isolation(H. Hermcm) Apr. 56 IAddress Transformers, Replacement MAIL TODAY - - (Carl G. Blanyer) May 72

City-State Trimmer Repairs (H. J. C. Burns) Oct. 50 RADIO -CRAFT,Dept. RA Tube Numbers, Faded(Bill Keene, Jr.) .Apr. 56 25West Broadway *Occupation Tube Numbers, Fading (H. T. Brown)Oct. 50 New York 7, New York Tube Replacement (A. W, Jackson) Jan. 108 Title - Tube Shields (Harry Kundiat) Nov. 36 Gentlemen:Kindly send ...copiesof Vibrator Kink(Bob Melvin) Nov. 36 your DeLuxe binder at $1.50 each.(Canadian Employed by Vibrators, Using Old (H.Leeper) Nov. 36 Voltage Indicators (S. E. Weber) June 46 end Foreign $1.75). I Independent Serviceman - Dealer Serviceman - Volume Control Repairs (G.Evans) Feb. 40

Service Manager-Dealer-Distributor-Jobber. Wire Stripper(Neil S. Higgins) Dec. 42 Name (Print Clearly) TUBES Address BOLAND BOYCE INC, PUBLISHERS Electrode Material, New Dec. 49 I City_ Zone_State Tubes, Postwar Miniature Oct. 46

48 1948 RADIO -CRAFT REFERENCE ANNUAL RADIO PARTS RADIO SETS AMPLIFIERS- TESTERS ELECTRONIC EQUIPMENT HAM GEAR /60 4.41W-P,KAZ9 PA645--

MAIL COUPON BELOW FOR FREE COPY It's ready now-the new Concord Radio Catalog offering 160 pages of everything and anything in Radio and Elec- the greatest, latest and most complete presentation of tronics. Complete showing of Amplifiers and Sound Equip. radio, electronic and television equipment and supplies ment-including famous Concord Multi -Amp Add -A -Unit in radio history! Just off the press, this new Concord cata- Amplifiers. Special bargain section offers hundreds of log is packed from cover to cover with thousands of items money -saving values in top quality, standard -make parts- to fill every radio and electronic need-Radio Parts, Radio including scores of new items from nationally -famous Sets, Amplifiers, Sound Systems, Test Equipment for every makers. Immediate shipment from stocks in two huge purpose, Record Players, Record Changers, Television Concord warehouses centrally located in CHICAGO and Equipment, Ham Gear, Kits, Receivers, Transmitters - ATLANTA Mail the coupon now for your FREE copy. WANTED - TRADE-INS ON COMMUNICATIONS EQUIPMENT-Dependon Concord forhighest trade-in values on communications receivers. Write, phone or call to tell us what you have-or send it in for free appraisal. TIME -PAYMENT PLAN-Bayyour Radio and Electronic Equipment from Concord on EASY PAYMENTS- Communications Receivers, Transmitters, Radios, Radio-Phonos, Sound Equipment, Test Equipment. Vrite us your needs.

Concord Radio Corporation, Dept.RER-48 901 W. Jackson Blvd., Chicago 7, III. Yes, rush FREE COPY of the comprehensive new Concord MI 9 Radio Catalog. RADIO CORPORATIONMr Name LAFAYETTE. RADIO CORPORATION Address CHICAGO 7, ILL. ATLANTA 3, GA. 901 W. Jackson Blvd. 265 Peachtree St. City State 4mplepea aya. 4me4ica Tat24 em 411 1k/eat MAGNETAPE RECORDERS Here's a preview of our complete line of magnetic tape recording and playback instruments, including models capable of recording and reproducing frequencies up to 12500 cycles. Other models feature a continuous 8 -hour playing ti me on a single reel of Magnetape!

14 Send for our illustrated circular No. 4810,whichlistsand features 10 standard and portable modpls, as well as accessory equipment, including the E -Z -CUE for locating within a matter of seconds, any selection, word, syllable, sound effect or musical note on a reel of magnetic tape; the Robot Program Standard model, designed to Timer which automatically starts and With this model you can now stops the recording of any one or more record complete symphonies fit into your livirig room, club, programs from a pretuned broadcast and operas. Conferences and church, camp, hotel, etc.No station; a magnetic tape recorder cabi- voice pickup may be record- sepcial knowledge required net which serves as a convenient reel - ed and played back for eight to operate this unit at peak continuous hours. Eight -hour holder; a crystal microphone which telephone or broadcast moni- efficiency. Never before such will pick up any accoustic vibration toring is now possible.All a simple method of recording within a solid with which it is in con- ofitspossible applications anything you want to hear. tact; etc. are still unknown.

FULL RANGE REPRODUCTION BOOKLETS by A. C. Shaney With America's FinestAmplifier

AMPLIFIER MANUAL MODEL AC A-1 00

The Model ACA -100 Amplifier series represent sound amplification at the peak of its perfection. ELEMENTS OF MAGNETIC TAPE RECORDING- Send for catalog #4806 which describes and gives complete technical specifications for this model, and 999 Applications-A discussion of the funda- as well as for variations which have beenspecifi- mentals and principles of magnetic tape recording, cally designed to accommodate such new and and a carefully classified listing of 999 applica- popular phono pickups as the General Electric tionsfor which magnetic tapeistheideal Variable Reluctance, the Garrard Magtronic, and recording and playback medium. 25c the Pickering Reproducer. MN MIEn =MI =M NM FM -AM AMPLIFIER MANUAL - The most com- plete booklet ever written on the construction and

NEW GE PHONO PICKUP performance of Direct -Coupled Amplifiers.... 25c Now Complete with Pickup Arm 20 STEPS TO PERFECT AMPLIFICATION -A 24 - Your choice of two pickup arms equipped with page illustrated booklet discussing such desirable GE Variable Reluctance pickup cartridge. Guaran- teed clean response from 30 to10,000 cycles. features of perfect amplification as non -frequency Usual ACA high quality. discriminating scratch suppression, signal expan- STUDIO TRANSCRIPTION MODEL 160GE-Plays records up sion, noise reduction, hum elimination, and other to 16" Transcription size. little known details which contribute to perfect PROGRAM PHONO MODEL 120GE-Plays standard records up to 12" diameter. amplification 10c AMPLIFIER CORP. of AMERICA 398-10 BROADWAY NEW YORK13,N. Y.