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RADIO DETECTIVE

HOW TO BUILD A SKI BROADCAST P. A.- RADIO- RECORDING SYSTEM See Page 652

RADIO' S, GREATEST MAGAZINE

RECORD PLAYERS RECORDING- PLAYBACK AMPLIFIER

DISTOR 1!O; IN A. F. AMPLIFIERS ULTRA -MOBILE P. A. SYSTEM WIRELESS "P. A." SAYS EVAN R. RUSHING, SOUND ENGINEER, HOTEL NEW YORKER

AMPERITE P. G. DYNAMIC BRINGS STUDIO QUALITY

TO ORDINARY P. A. JOBS

UNI -DIRECTIONAL. NEW SUPERIOR ELIPSOID PICKUP PATTERN ELIMINATES FEEDBACK TROUBLE BECAUSE IT HAS LOWEST FEEDBACK POINT OF ALL DIAPHRAGM TYPE MICROPHONES FLAT RESPONSE. FREE FROM ANNOYING PEAKS. GIVING REPRODUCTION. ItSrONSt STUDIO -QUALITY Or t. prNAU1C The P.G. diaphragm follows air particle velocity where amplitude is a GRADIENT of the PRESSURE. In ordinary dynamics amplitude is restricted from following air particle velocity. The P.G. DYNAMIC is a radical improvement in this type of microphone. You can actually heat the difference. Case is designed according to malern acoustic principles. Rugged. not affected by temperature. altitrde or humidity. HAS UNUSUALLY HIGH OUTPUT, -55 DB. MODEL PGH (PGL. 200 ohms). Excellent for high fidelity P.A. installations, broadcast studio. cnd professional recording. With switch, cable connector, 25 cable. Chrome knish. LIST $3200 l .0. 1P0(í0. C.YS. MODEL PGAH ( 11=ILD. ). COMBINATION VELOCITY- DYNAMIC AMPERITE KONTAK MIKE ACHIEVED WITH Puts Musical Instruments Across ACOUSTIC COMPENSATOR An exclusive Amperite feature: fly moving up the Acoustic Compensator you change the AMPERITE VELOCITY to a DYNAMIC microphone without peaks. At the some time you reduce the back pickup, making the microphone practically UNI. DIRECTIONAL. So beautiful is tie tone produced with the Kontak Mike, that it was used in the Philadelphia Symphony WITH ACOUSTIC COMPENSATOR: to amplify a mandolin solo. Gives excellent results ohms) with MODEL RBHk; RBMk (200 with any amplifier, radio sets, and record players. switch, cable connector. LIST $12.00 Chrome, LIST $42.00 MODEL SKH (hi -imp) MODEL KKH, with hand volume control LIST 18.00 RSHk; RBSk (200 ohms). Switch, cable Plug extra List 1.50 connector, Acoustic Compensator. FOOT PEDAL, for making beautiful LIST $32.00 Chrome or Gunmetal. crescendos LIST 12.00

WRITE FOR FREE SALES AIDS AMPERITE &. 561 BROADWAY, N.Y. U.S.A. ;,e41eMPERITE (ttir (BILL, YOU'RE ALWAYS / 1 LL TRY, MAR FOOLING WITH RADIO- - I'LL TAKE IT OUR SET WON'T -10M£ TONIGHT A WORK - WILL. YOU FIX IT

RADIO AT HOME, SILL, rSAY- WHERE HELLO, BILL- GOT A HELLO JOE- WSIERE'VE IVE BEEN STUDYING I CAN'T FIND OUT WITH THE NATIONAL RADIO INSTITUTE. YOU DID YOU LEARN YOU BEEN LATELY- WHAT'S WRONG TOUGH ONE OUGHT TOTAKE THEIRCOuRCE.. IVE GoT THAT 1E671 175 AND WHERE DID YOU MAKE A A GOOD ONE GUESS I'LL MAKE A TO FIX? A GOOD RADIO JOS NOW. LETS Wrflt TOOL Of LET ME LEARN CIRCUIT PISTURSANCE TEST -STARTING HELP You ANYTHING, THE AUDIO OUTPUT STAGE MY5ELF G AND TESTING EVERY STAGE ABOUT TO THE WITH MARY RIGHT BACK 'RADIO? ANTENNA. LISTEN TOR THE CLICKS WHEN l 'TAP THE GRID LEADS j

AVIATION RADIO, POLICE BERES THE TROUBLE,BILL, IN THE IVE SEEN THEIR ADS FDA CONVINCED NOW THAT THIS A JOB RAOIO, TELEVISION, STAGE. BUT 1 NEVER TNOUGIfT I IS 'PRACTICAL AND OR GET FIRST I.F. AMPLIFICATION I COURSE ELECTRONIC CONTROLS- THAT TEST EVEN BEFORE COULD LEARN RkAArDIIo Al' I'LL ENROLL NOW WITH A RADIO LEARNED \OMPLETE. RADIO 15 SURELY GOING TAKING THE COURSE HOME --I'LL t;494.1 BROADCASTING OR I STARTED 1 PLACES. AND THE BILL. IT'S 'DESCRIBED IN A MAIL THEIR TRANSMITTING 6 I CAN MAKE NATIONAL 'RADIO TREE LESSON WHICH THE COUPON RIGHT 1 AND THEN STATION HAS TRAINED RADIO INSTITUTE AWAY REAL MONEY FIXING INSTTVTE NATIONAL HUNDREDS Oc MEN SENDS YOU WHEN RADIO SETS -fOR JOBS IN RADIO YOU MAIL A =ROM COUPON OR INSTALL AND SERVICE. X ONE OF THEIR ADS LOUD SPEAKER SYSTEMS

You CERTAINLY KNOW THANKS: R CERTMNLY IS RAPID. SOUnDS AS EASY TO LEARN RADIO THE GOOD AS THE DAY N.R.i. WAY. t STARTED ONLY I SOUGHT IT. A FEW MONTHS AGO, AND I'M I will send you a Lesson on ALREADY MAKING COOP MONEY. IRIS SPARE TIME ), / WORK IS GREAT Radio Servicing Tips FREE FUN AND 'PRETTY SOON 111 DE READY TO SHOW HOW PRACTICAL IT IS TOR A FULL TO TRAIN AT NOME FOR TIME. J08 GOOD JOBS IN RADIO

a slight interest Mille holding other jobs herai:.e ni he ev.a J. E. SMITH. President. Even if you have only WORRIES ARE in Radio, get this sample Lesson. It mobey In tide vt,tit. National Radio Institute ON DILL- I'M SO OUR OYER. is a catalog of common Radio receiver Beginners Quickly Learn to Earn Established 25 years GLAD I ASKED YOU I HAVE A GOOD JOB causes and remedies. 'ro FIX OUR RADIO. NOW, AND THERE'S A troubles, their S5 to Sb0 a Week in Spare Time He has directed the train- Learn how practical IT GOT YOU STARTED BIG FUTURE AHEAD Mail the coupon. Find out hose N. R. I. trains u to make ing of more men for the N. R. I.'s 50.50 method makes learning extra quickly. Many N. a. 1. students THINKING ABOUT FOR us IN the many report a in spare time Radio Industry than any- Radio at home. Discover within a feto months. Many pay for their one RADIO AS A CAREER. RADIO money made in else ornortunities the N. R. I. Course opens entire Course with extra AND YOU'RE well time jobs. spare time while learning. NOW for -paying full GOING AHEAD Why Many Radio Technicians 00 FAST Make $30, $40, $50 a Week You Millions of dollars in defense orders on top a record year for Radio are opening new Get This oppurts for Radio Professional well paid because they use Ttheir t tinti, as well as their hands. It takes both Servicing O repair a home or auto Radio set: to oper- to and maintain a Broadcast or Commercial Instrument ransntltting station: to Install. operate and epair Pollee. Anal toit. Experimental Radio nd Television equipment. That's teby so This instrument makes practically any test tang fellows who iserane Radio Technicians called your are able to jump their pay; why others op- Radio work rh sucre ante and fulll I. E. SMITH. President. Dept. IEX rn full time Radio Service Jobs.etItn tsehenused on the test bench. or National Radio Institute, Washington, D. C. rate their long ea Businesses. Many fix Radios in spare time A.C. and D.C. voltages and currents: tests Dear Mr. Smith Mail me FREE, without obligation, your Sample Lesson - 1 t e.. h ltl I. d ill t r fI uni t: I. page bade "Rich Rewords In Radio" which telle about Ifadiú s spare aligning ea set. old or new. l'ou get this and full tune opportunities and explains your 50 -50 met hay of train instrument tas part of y re N. R. I. Courte. ing men at home to be Radio Technicians. (No salesman will call. Write Plainly.) DRAFT REGISTRANTS! Mall the Coupon Hundreds Of m who knots Radio when Get our SA51pt.E t.E..SSON d a envy of Age they military n n' r .dn, .. Rich Rewards In Radio" FREE. a ai,, perialint raying.. in'the Army. lette from menn who o Into Radio this Marine CoD,a. These ratings Nrau.yNavy telling tlido a timed a private's what they wdoing anti earning. You'll sec Name any in addition to carryings extra k why the N. R. I. Course is easy to stud, and rest,ee! If ARE NOT called. fascinating to learn. practical to v You'll you are getting into Radio when the get facts on Television and other facpgrow)inir Address Governmentt is pouring millions Into lice branches of Radio. Act NOW. Write onétutop C and address on the coupon below. Post of boom civiliancivilian Radio it rie postntposte-ant tir man it in City State business. Whether you or watt envelope-RIGHTlopeC -RIGHT NON'. for conscription -IT'S SMART TO LEARN President RADIO -NOW! J. E. SMITH. 14X1 Dept. I EX. National Radio Institute Washington. D. C. J RADIO -CRAFT f -,r MAY. 1941 641 NEXT Issue!Use to How Detector Leak HUGO GERNSBACK, Editor- in -Chie/ o Rodio Service Selling Minio N. H. LESSEM THOS. D. PENTZ 'Trans- How }O Stable Associate Editor lrt Director P o mittermitten an Radio R. D. WASHBURNE, Managing Editor Oriental "Theory Coils f and De Re eco{ Contents MAY, 1941 Issue Per Oes4e ?Ottabesn S VOLUME XII NUMBER 11

Mailbag 643 DEPARTMENTS, ETC. Editorial: Radiobotage Hugo Gernsback 649 Mailbag 643

The Radio Month in Review 650 The Radio Month in Review 650 SOUND Sound Engineering-No. 17 ....Conducted by A. C. Shaney 679

RADIO SERVICE DATA SHEETS (See Servicing) . 682, 683 How to Build a Modern 30/ 15 -Watt P.A. - Radio - Recording

Console, Part I (cover feature) R. J. Bergemann, Jr. 652 Operating Notes 684. 686

New Circuits in Modern Radio Receivers-No. 44 Measuring Distortion in Audio -Frequency Amplifiers ...... 656 F. L. Sprayberry 685

Broadcasting 658 "Sayville Once More -an attack on The Electrical Experimenter" 687 How to Make Dynamic Tests on Audio Amplifiers Ted Ladd 660 Latest Radio Apparatus 692

Ultra- Mobile P.A. System Edwin F. Longley 664 Classified Radio Directory -Section II (Revised) 696

Index to "Radio- Craft" Articles on Sound 665 Shop Notes- Kinks -Circuits 703

Book Reviews 704 Adding an "Eye" to A.F. Amplifiers -and Other Practical Ideas Harold Davis 666 *

Modern Features a Semi -Professional 10 Re- Circuit of -Watt Published by Radcraft Publications, Inc. Publication office: 29 Worthington cording and Playback A C. Shaney Amplifier 668 Street, Springfield, Mass. Editorial and Advertising Offices: 20 Vesey Street, New York City. Chicago Advertising Office: RADIO -CRAFT, 520 North Microphones -and How They Work J. de Boer 673 Michigan Avenue, Chicagd, Ill. RADIO -CRAFT is published monthly, on the first of the month preceding that Sound Engineering -No. 17 Conducted by A. C. Shaney 679 cf date; subscription price is $2.50 per year in U. S. (In foreign countries, 75c additional per year to cover postage; Canada, 50c additional.) Entered at the post office at Springfield as second -class matter under the act of SERVICING March 3, 1879.

RADIO SERVICE DATA SHEETS: Foreign Agents: London- Gorringe's American News Agency, 9A Green St., Leicester Square. No 306 -RCA Victor Model VHR -307 Home Recording - W. C. 2, England. Phono - Radio Combination (Chassis No. RC -555) 682 Paris -Messageries Dawson, 4 Rue Faubourg, Poissonniere, France. Melbourne-McGill's Agency, 179 Elizabeth St., Australia. No. 307-Ditto 683 Dunedin-James Johnston, Ltd., New Zealand.

Operating Notes 684, 686

Test and illustrations of this magazine are copyright end must not be New Circuits in Modern Radio Receivers 44 -No. reproduced without permission of the copyright owners. F. L. Sprayberry 685

Servicing Record Players Willard Moody 688 *

642 MAILBAG

MORE ANENT HOMER BUCK'S would permit recognition of his ability and at the same time ensure his chances of "ALIGNING SUPERHETS" making a good livelihood. Action! (Continued from last month) Conclusive WILLARD MOODY, New York, N. Y. Dear Editor: Testing Continuing, "When alignment is attempted with an oscillator, there is no impedance A. C. SHANEY'S SOUND matching, the signal strength is not like ARTICLES that of a broadcast station and the oscilla- Dear Editor: tor is certainly not like the radio's own I have been a subscriber to your magazine antenna.. " Let's take the first state- for several years and have constructed sev- ment. No impedance matching. The genera- eral radio sets which were described from tor is connected, let's say, at point X. time to time with good results in all cases. Obviously, there is no impedance match However I am beginning to get disgusted between the generator and the L/C circuit with your section on amplifiers. It seems between 6SK7 grid and ground. But-and as though- rather than helping one con- this is the whole story-the circuits being struct a good amplifier you get one enthused aligned are separated from the generator in an article from'a description of a good by the tuba, a 6SK7, which stands between amplifier and about the time when all the generator and 6SQ7. Therefore, when align- good qualities of the amplifier are given and ment is properly done, there is impedance one is looking forward to the parts list and matching. the construction details, the article closes with "Courtesy of Amplifier Co. of Amer- ica." In other words your articles seem to be only advertisements for this company with no intentions of helping one build an amplifier for enough information is omitted from the diagram to discourage one from 1";;FVE;7 MODEL 1213 trying to build one without further construction details. I had been hoping that you would give D..l.r N.t Price construction details on about a 30 -watt high - $22.00 fidelity amplifier with volume expansion but in two articles -Oct. 1939 "All- Push -Pull Direct -Coupled Amplifier" and July 1940 Exclusive Handy Hinged Post- issue -you have described such an amplifier card Reference Tube Chart but in the end it is only advertisement for Let's take the 2nd part of the above. a factory -built job with no intention of help- Filament Voltages from 1.1 to mentioned statement by Buck, namely, "sig- ing your readers build it. This can be easily nal strength is not like that of a broadcast shown by the fact that I asked you for the 110 align not station." Buck, himself, tells you to parts list of both amplifiers and I did RED with a weak signal image, which is, in char- receive even a reply. DOT Lifetime Guaran- acter, like that of the signal generator, Do you intend to publish anything in the teed Instrument since the generator is always properly set near future on how to build a good amplifier for low output. The difference between the or are they all going to continue as in the Neon Shorts Test signal generator and the station is one of past -only advertisements for a factory -built modulation (generator, generally 400 cycles; job? I think that if you will investigate you Model 1213 contains all sockets to test all pres- station, of whatever audio happens to be will find that a good many of your readers including Pilot Lamps and momentarily impressed on the originating think as I do about your articles on am- ent day tubes microphone in the studio). At any rate, this plifiers. As to your sections on radio, etc.. Gaseous Rectifiers. Ballast Tube Continuity statement is meaningless and not connected no complaints to make for one can I have Test . . . Has separate line voltage control with alignment at all. "The oscillator is generally start building one with confidence certainly not like the radio's own an- for enough information is given, and I hope meter ... Housed in snappy appearing metal tenna . . that you will soon have all the necessary case with brown suede baked enamel finish. But, why go on? The more I try to keep construction details on the amplifiers which Three -tone panel is brown, tan and red with my temper, the more I burn up, and you. are described from time to time. I shall also Mr. Editor, surprise me that you publish appreciate the construction details and parts silver markings. such tripe. For Gbd's sake when are you list for the above -mentioned amplifiers if Dealer Net Price $22.00 going to start slanting your magazine policy you have them or can get them for me or towards the intelligent and technically- the parts list and construction details for trained section of the radio trade? Isn't it a similar amplifier. MODEL 1620 This is a type, plain that the men with the money and ELLSWORTH DODRILL, counter ability to buy are going to be the men with O'Neill, Nebr. lever switch operated education, at least a high school education, tube tester. Four "quick and that the others have no place in Mr. Shaney's articles in Radio -Craft may change" non -obsolescent radio? What we should have is an organiza- be broadly classified in two groups, one be- features, including RED tion comparable to that of the radio engi- ing of an educational nature, wherein the DOT Lifetime Guaranteed neering or other professions, and build up newest developments in commercial labora- Instrument. Speed Roll public esteem for the Serviceman, driving tories are made available to readers of Chart, New socket panel out all of the incompetents. As a require- Radio -Craft. and Switching Section can ment for admission to the Radio Service all be replaced should un- Guild, a man would have to pass a stiff Although it is not the intention of many anticipated changes make examination in fundamentals and service sound companies to always enable the con- it necessary. struction of these new items by its readers, technique. He would then receive a certifi- Dealer Net Price $37.84 cate of merit and be entitled to use the those who have sufficient technical knowl- letters, R.S.G. after his name. How about edge and training should have no difficulty you and the editors of all the other nation- in constructing some of the advanced de- MODEL 1621 -Similar to Model 1621, but con- ally -known magazines collaborating in a velopments. tained in smaller, more portable concerted, erious effort to better the status On the other hand, all articles which are case ... Dealer Net Price $34.84 of the professional radio Serviceman having written with the idea of being constructed necessary education and experience? Be- by readers, are usually accompanied by a the Write for Catalog -Section 165 Harmon Avenue ginners, apprentices, could receive a junior complete parts list, or in many cases, suffi- membership. Such a plan would ultimately cient information is given in the circuit better the entire industry, since a better diagram to enable an average technician to TRIPLETT ELECTRICAL INSTRUMENT CO. grade of man would be attracted to Radio select the correct parts. Bluffton, Ohio and carve out a career for himself that The disadvantage in supplying a parts

RADIO -CRAFT for MAY, I 941 643 MAILBAG

list furnished by a sound company, is that To the great gang of readers in the actual company names are specified, and States may I say -"Howdy, fellows, if you this is really unfair both to the readers and can spare a minute from Radio -Craft the RADIO to the suppliers, because a Od -mf. condenser address for mail is still that given at the TRAINING need not necessarily be of any specific manu- end of this letter (send your letters care of facture in order to work correctly in a dr- Radio- Craft, we will forward.-Editor) so cuit involving its use. It is therefore obvious let's be hearin'." (Continental visitors to that if a condenser is indicated in the circuit England please DON'T note.) diagram, an itemized parts list is really My respects to you, Mr. Editor. , . 1 f . , If; l¡it unnecessary, THOMAS CLARKE, It is to be further stressed that readers Sheffield, England should not attempt to construct items which DORT ARTHUR COLLEGE-not privately exceed their abilities and facilities. V. -T. VOLTMETER PROBLEM 2- owned, not operated for profit, a college The specific circuit diagram which Mr. Dear Editor: built and endowed by the late capitalist - Dodrill refers to, namely the 30 -Watt Direct - As your philanthropist, John W. Gates -offers the Coupled Amplifier with Expander and Sup- a regular reader of magazine, I most thorough Radio training in am submitting to you free of charge (thanks practical pressor, was completely indicated in the America. P. A. C. owns Radio Station November, -Ed.) the solution of a problem that is baf- KPAC, which is eanipr.ed with the very 1939 issue of Radio -Craft, Page many Servicemen who have built or 268. Every value has fling latest type 1000 -Watt high fidelity RCA been carefully indi- are building V.-T. Vm.s employing a so- transmitter, operating on 1220 kc. with di- cated, and any radio technician should be called bucking circuit in the plate circuit of rectional antenna system. The college is able to construct this unit without a List authorized to teach RCA texts. of Parts. the final stage, to balance -out plate current that might flow through these meters at no The Radio training covers thoroughly Air- There are occasions, where sound com- signal input at the grid. ways, Press, Announcing. Teletype, Type- panies can not give any technical informa- writing. Laboratory and practical experience tion because of extensive laboratory work Quite often, low-range microammeters are at KPAC transmitter, control room and in obtaining this data, but this, as far as used and the erroneous impression seems studios. Announcing is an optional part of we can recall, has come up very rarely. The to have gotten about (even in Terman's this training; a nevertheless number of stu- only specific instance that we know of has "Radio Engineering" textbook [1st Edition, dents annually make successful announcers. 8th impression, 1932]) among many that at Port Arthur College pioneered been in the design of the Amplifier Com- the teaching balance condition, 2 currents, equal in of radio with its first classes in 1909. and pany of America "Audio Spectrum" unit, this for thirty -one years has maintained an ac- which we are sure no reader can condemn magnitude, but opposing, give a zero meter tive Employment Bureau that is successful them for not giving complete detailed data reading, but that the coil of the meter is in placing graduates in airways, broadcast on.-The Editors carrying the arithmetical sum of these cur- and marine radio industries. rents which may reach a very high value and as a consequence the meter may be if interested in details about the Radio FROM A SPUNKY ENGLISH burned -out although registering zero cur- Course, write for Bulletin. R -4I READER rent! Dear Editor: However, a simple example will show that PORT ARTHUR COLLEGE As a newsstand reader of your journal the potential across the meter coil is zero, and a radio technician since early 1929, I as well as the reading of the current, and PORT ARTHUR (World -Known Porn hope a few words from me will be welcome. the current through the coil, when balance Since the "streamlining" is attained. A simple bucking circuit is TEXAS of your paper it has been a pleasure to refer to any particu- shown, together with an accompanying sim- lar section; also, it has made the indexing plification diagram. of subjects much quicker. Thanks for the innovation. Since our national preoccupation with a continental affair (still in progress) radio RADIO has been a little bit difficult to keep pace with and the arrival of Radio -Craft even a little late is a Godsend. DIAGRAMS Some 4 years ago I started a reference library of American textbooks. This has re- MOST - OFTEN - NEEDED ceived the marked attention of one of the recent continental visitors and is now blown to bits. ALL MAKES 1926 -1938 I am now negotiating with the authorities MOST here to In this n n e low- trivI manual obtain an import license for a small you love all the circuits you but necessary part of the destroyed texts.* OFTEN - really need. cis most-oftent -Often serviced radios the There are other and more weighty texts want for easier. foster. help you not so MICROAMMETER NEEDED nd better repairs. too will find but they are essential to me, and I 4 out of 5 [Mere, 595 can defer to some later date the ordering RADIO needed In this manual. of them. Rp 244 pages. large 8t'T11 (ruse) In. size. Special price Some American components are still ob- -90y. rb 110fsCH.1:ls- tainable here though the manufacturers are 4 VOLTAGE ACROSS HOW TO SAVE TIME not giving us all the help in publicising ab t-tO -IDO Service hints, diagrams, align- that ment data, test charts. and they could. Radio -Craft has been of great "EQUIVALENT pans ++90V. _ SIMPLIFIED 5 lists Included will prove time - value in this connection. OF4 savers and money makers for DIAGRAM -- CIRCU ITS you. Get these handy on -the- May I make a plea to Hammarlund, Su- YOU wowork blind- fo lded. The time preme, Readrite, Raytheon, and all the oth- saved in two days will more ers?: "Send us the dope on your products, NEED than Day for these manuals. The arrows show current flow, and that at we want to know about them and use them." correct choice of R the voltage across a -b is few personal 1939 -1940 DIAGRAMS Now for a notes. zero and that the main battery supply volt- Thanks, Shaney, is This single manual will give you Mr. yours one of the age appears at c-d. Any increase in current over 80% of all 1930 -10 circuits best sections in an excellent assembly. Keep rever needed, acquaint you with due to signal application on the grid (recti- ecent radio developments. and it up. Give us the dope on all the newer fied or otherwise) will then show as an up- teach you how to service quickly variations of direct -coupled amplifiers. millions of sets sold last yea.. ward reading on a current meter connected Models of 43 manefar- s SO Thanks, Mr. Sprayberry. The circuit across a -b. lurera. 212 large pages. analyses are great and keep us in touch SVexil inches. Limited I believe this exposition will bring out to quantity at with the latest phases in radio. No journal here does it your readers the following: (1) Correction of certain erroneous ideas FREE EXAMINATION Thanks, Mr. Editor & Co., for the new Radio -Craft. It will always find a welcome widespread about this connection, found SUPREME PULICATI9NS, Room SSS Chicane. knelt and honoured place on the bench and in the even in textbooks and periodicals. Ship the 17tÁ lls it must be satisfied or you will rafundamy money In full. bookshelf. (2) The theory of the bucking circuit, and Most Often- Needed Diagram.. 19284938, Di $1.95 why it increases the sensitivity and usable D Moat Popular 1940 Radio Diagrams, (di $1.50 D i am enclosing $ d postpaid A tube data -book, and the Amplifier range of the meter. D Send I will pay the mailman. Handbook and Public Address Guide.- (3) That current flowing from positive to NAME (write -daress below and sand this comer) Editor negative in a battery is considered a posi- 644 RADIO -CRAFT for MAY, 1941 MAILBAG tive voltage, one from negative to positive is a negative voltage, etc., through resistors. FRANK P. DEFINA, Ifazelton, Pa.

MR. RUSSELL RETORTS Dear Editor: I hope this will clear up some of the con- fusion in the "Regarding Hearing- Aids" in the Mailbag, which must be something to any interested bystanders, who have read both Mr. Cisin's letter and mine. About 16 letters have been received by me from hard -of- hearing people, requesting more information and all have the sane complaint; not sufficient power in the devices they have tried. I didn't reply, that they were stone deaf and could not hear if they put their ear in a 10 -watt speaker. But tried to explain that so many designers did not have a proper understanding of the requirements of the hard -of- hearing and Oat the more noise we made about it the better. In fairness to Mr. Cisin, let me say that the amplifier he described in "R.-C." was not primarily designed for a hearing -aid, but was in a class by itself for that work HALLICRAFTERS "TWENTY- THREE" and quote from the article: "The subject of the test was a person who depended upon of lip reading, due to the inability ordinary We have a Very Limited Number relief. Without the in- PRICES cut to the bone on Nationally Known hearing -aids to give of National 1940 Model NC -80X strument, one could get behind this near - Radio Supplies during this Spring Clear- and NC -81 X Receivers priced low deaf person and shout as loudly as possible ance of Demonstrators, guaranteed 1940 without being heard. With this amplifier, for immediate clearance. however, the hard -of- hearing person was models and brand new surplus stock of able to hear and understand every word Nationally known short wave receivers, ham spoken behind his back . in a normal gear and radio service instruments. Your chance tone of voice." Would not that lead a person who could hear shouting behind his to get highest quality radio needs at practically back, to believe that the instrument was your own price. worth constructing? Imagine this famous Hallicrafter 23 that Yes, I knew the 1S4 tube is rated at 0.065 -watt, but there was the author's state- formerly sold for $115.50 less speaker -now ment: "It was in a class by itself." The copy only $76.95! Current model -guaranteed! I made of the instrument had all of the and pick -up or sensitivity that was claimed for Saving of one -third! Complete with tubes it. A person with normal hearing could hear crystal- Speaker $12.00 extra. Compensated a watch tick a foot away and more. I have frequency stability, eliminating drift. Accurate no quarrel with Mr. Cisin and no doubt he a: ill be out soon with a real power amplifier reset band spread system. No bandset dial to that will help us all. fuss with. Stations always come in at same Me-? I am just faintly amusing and very place on dial. Shipping weight, 56 lbs. ignorant, qualified only to write gems of stupidity. However I do know that if you 57TC08816 -Cash Price $76.95 depend entirely upon voltage amplification All tbe. a bargains at ailable on Easy Terms -write for derails. to make people bear, who cannot get re- Mony items of popular and cur- merchandise and equip- sults with ordinary '-aids, it will take so Write for our folder showing rent model Test Instruments also in much that they will look like the fellow ment included in this sale. Ask for Folder No. RC6578M. included in this Bargain Festival. electrical show displaying the Tesla the Also get our new Ham and Service Supply Catalog No. Take advantage of this once -in -a- coil. How about a little of both, enough to give fair pick -up and a power output that RC717M and our complete PA Catalog No. RC732M. lifetime opportunity -ACT NOW! will really tickle the old mastoid bone on full volume? Then there will be enough output for these days of head colds, and for SEARS, ROEBUCK AND CO., CHICAGO, ILL. the future if the deafness is progressive. One of that power brings up the question much better for nie with the amplifier I had dope in the future. Come on, Mr. Cisin, of size. I stated in my first letter that lots reference to, where it is mounted on one end don't sulk -give us a real power job soon. of people could get a small device, hide it, of the box close to the hot metal tubes, and EARL RUSSELL, and nobody would ever know they were due to its high level the volume control need Celfax, In. deaf. "Easy to conceal" is a standard state- not be turned up much. Somehow, these ment with the manufacturers. This to my amplifiers just worked better that way. Oh, moronic mind is silly. Even "stone" deaf- I knew there would be a squawk there. For V. -T. VOLTMETER TROUBLE ness is not as dishonorable as Mr. Cisin most other work crystal mikes have "it." Dear Editor: would have us believe and it is not always About earphones -I have had several I have built the Vacuum -Tube Voltmeter accompanied by stupidity as in my case. If crystals. They just won't stand up. And if described by R. C. Turner in the June, 1940, you are hard -of- hearing the sooner those you leave them in the window where bright issue of Radio -Craft. you contact know it the better. They will filters heating up, sunlight through, them The instrument is very accurate on D.C. soon detect your infirmity anyway. The it is goodbye $12 (if of the ear-insert type). smaller the instrument the better, of course; measurements. However, on A.C., the meter In closing, let me say that in designing sloes not measure peak voltages correctly. my point was not to sacrifice efficiency and an amplifier for hearing -aid work the object power for size. For instance, when an accurate A.C. volt- is to hear, and then establish whatever voltmeter indicates 112 volts r.m.s., the vacuum - As to the medical profession, I still say, age amplification is required to give the tube voltmeter shows a peak voltage of 1.8 that if you want to get wired for sound let necessary pick -up; and NEVER the other which should read about 3.6 or 157.92 volts the radio men do it. It won't cost you a way around, as so many think. on the 0 -500 V. scale. On the 0 -100 V. scale, cent if they can't make you hear. Radio -Craft has done us fellows a real the reading is 8.1, whereas the needle should As to crystals -a carbon mike worked service and I hope to see more hearing -aid go beyond the "100" on the scale indefinitely. RADIO -CRAFT for MAY, 1941 645 MAILBAG

What would you suggest I do to make the firm telling his convened salesmen that: meter read peak volts correctly? "You cannot talk to the hard -of- hearing as LEARN I like Radio -Craft very much and it is a you would to a normal person, because the fine magazine for the radio Serviceman. I hard -of- hearing are not normal mentally. am hoping that the March, 1941, issue will They are just like children; you have to give some valuable information on the give them a line that will impress them, but RADIO don't EVER answer l acuum -tube voltmeter. (5) any of their questions. SERVICING WEEKS They really don't COURSE-8 STANTON N. DRUMMOND, want to know the truth, but just ask questions to annoy you." {AI OPERATING North Western, N. Y. 5MONTHS Phooey from nie to all such. Among the e. 4.4 hard -of- hearing when he wrote, had in I know of are not only I'LL FINANCE YOUR TRAINING Mr. Drummond, radio men but also electrical engineers, Mail coupon today for free book mind a notice, which had appeared in the several of biggest showing how you can learn Radio preceding issue, on "Vacuum -Tube Volt- the men in electrical and Operating, Servicing, Sound and radio history, ear doctors, educators, and Television in a few short weeks in meters- Theory and Practice." Coyne Shops and also how you can the heads of several of the most progressive pay tuition in easy monthly This letter was forwarded to the author payments after hearing -aid firms. The qualified deafened graduation. whose reply follows: man would LEARN BY DOING be my choice to lead me any On real Radio receiving and Dear Mr. Drummond: day in preference to a hearing man. transmitting equipment at You have had in Coyne. Simple, easy method of state that you difficulty The plain fact is that the hard -of- hearing practical shop tra...ing. You don't measuring the peak value of a 112 -volt person needs much need previous experience. Prepare greater -sometimes for Radio Operators' Government r.m.s. potential with the vacuum -tube volt- greater volume than the normal person, license Exam. and for good jobs in meter I described in the June, 1940 issue - Broadcast. Police Radio Aircraft and whether you call it voltage amplifica- Radio, Ships. Commercial Radio of Radio -Craft. It is presumed that you tion or power amplification is immaterial to stations, or in service shops and radio factories. "Learn by mean by this that you have attempted to the average person who doesn't know either doing ", the quick, modern way. measure the voltage of the same A.C. line from Adam. I know of at least one hearing - at Coyne Shops in Chicago. that supplies the instrument. EARN WHILE LEARNING aid firm that failed because of not realizing Jeb Nobs Aft.,.. Graduation The vacuum -tube voltmeter shown in my how much greater volume most hard -of- We assist you in getting part-time work to help pay article cannot be used for such a purpose hearing persons need. Some technicians give living expenses and give you real employment service after you graduate. because of the two 0.1-mf. line bypass con- too much attention to non -essentials and N. C. LEWIS Pres., COYNE RADIO SCHOOL. densers, C6 and C7. These condensers place too little to what is very much essential. SOO SO. Paulina St.. Dapl. S1 -8H, Chicago, M. the negative return (chassis) effectively at Once a hearing -aid firm's newly -employed Pleitee send me your free took and details of e.n, Pay- After. Graduation Plan. the center of the line voltage, giving rise to engineer took a look at the firm's old ' -aid NAME the error you have noticed. If 112 volts from and said, "The sound goes in front there, ADDltES. -. a separate source, such as a transformer but where does it come out ?" And so he de- STATE secondary, be applied to the A.C. meter ter- signed a new microphone case for an old J minals, you will find the correct peak value microphone they had been using for half -a- approximately 157 volts -indicated. dozen years, and put openings all around Please do not hesitate to call upon nie for the sides of it- The sound, Mr. Engineer, POWER AND LIGHT any further information. comes out exactly where it comes out of ELECTRIC supply your ear. rraanntfour RSdlos..Tr ,e .I .`,d TRee r. P. A. RUFUS P. TURNER, WIAY ereteme. Movie Projectors. Pound Trick APP.- calm. Tools, Licht,. AIIApptienees llsedaaStand. For Mr. Cisin's information, the crystal microphone has too low an output to en- bPublic Buildings fo cam of P oqe Lite Failure HEARING -AIDS AGAIN! doe to Storms. Flood., ote. Available in 110 or 220 YEP.... able a very seriously Volt A.C. -12. 32 and 110 Volt D.C. -Also Dual deafened person to Voltage AC-DC Ty pee- Manual. Full -Autumn tir A Dear Editor: get enough volume (yes, we &5I- Slutinr. tas and up snip wd_ RendsteRare. are talking of D. W. OKAPI ik SONS I've been afraid of making the enclosed volume, Mr. Cisin) from as conveniently - 721 Ronda= Omar. Mk,neapells. Miss. (below-Ed.) too long for you, but hope sized amplifiers as the carbon microphone you may find some of it interesting enough does. And crystal earphones cannot tolerate, to get at least part of it in. I belong to a and from some amplifiers cannot even de- discussion group of hard -of- hearing people liver, the high volume that magnetic ear- RADIO TECHNOLOGY who have been trying to talk out some of IICA Institutes °der an Intensive course of phones can. I've proved this to my sorrow high standard embracing all phases of Radio these hearing -aid problems by mail, and I by having 3 pairs and 2 single crystal and Television. Practical training with modern to Radio equipment at New York and Chicago schools. subscribed -Craft purely because of phones break down under the load I have Also specialized courses in Aviation Communi- the hearing -aid articles you occasionally had to put on them. cations, Radio Servicing and Commercial Oper- publish. (Italics ours I hope you will ating. Catalog Dept. RC -41. -Ed.) But persons whose continue to publish them in as much space for hearing is not too RCA INSTITUTES, Inc. seriously impaired Mr. Cisin is right that A Radio Corporation of America Service as you feel you can afford to give over to crystal mikes and are Varlek New York. 1154 Merchandise Mart, Chicago earpieces sometimes, 75 St.. such a topic. but not always, more satisfactory than The hearing -aid people have done won- carbon mikes and magnetic earpieces. But ders, really, considering the difficulties and his amplifier's enabling a person to hear R11D10TELEY151011 what they have done it with, yet some of the ticking of a watch at over one foot is the facts known in the laboratories, but no satisfactory test, and no test he could re- Oldest, largest Redio.Television school in West trains you for nod pay job. Complete inntrurtlon including Arab, little known outside, prove that even now port would be worth a hill of beans unless Construction Service, Broadcast Operating. Sound. Talking Pictures, Television, Public Address, etc. Flexible they are not always following the best meth- at the same time he reported the number of plan to meet . eeific need, of those w without Robe.be. ods, but the ones most profitable at the Transportation allowed to L.A. Earn room and board while decibels of hearing the subject had lost. learning. Request Free Catalog. moment. Selling -aids by hearing present My numerous and widely- spread deaf- sales methods is a very expensive proposi- ened acquaintances, and I, know quite a NATIONAL SCHOOLS, Los AngLes tion, and this has to be taken into consideration. number of doctors who don't know the first point about a hearing -aid and many M. B. K. of them don't know as much about the Job? Are You The Man For The Dear Editor: physics of hearing as some of us who Every radioman has the opportunity today to have studied the problem from a profound advance himself to a better radio job. With wide- This hearing -aid argument is getting in- interest born of our affliction. And those spread expansion programs there comes a need teresting! After about 20 years' study of who do think they know something and for men with modern technical training. CREI I a could home study training is the proven way to more what little as layman learn about are testing hearing and prescribing '-aids money -"6.000 other radiomen can't be wrong "! hearing -aids and the physical aspects of are failing in many cases and don't realize WRITE FOR FREE BOOKLET. CAPITOL deafness, I'd call both Mr. Cisin and Mr. that the reason is simply because the RADIO ENGINEERING INSTITUTE, Dept. Russell right, but in different ways. RC-5, 3224 16th Bt N.W., Washington, D. C. present method of testing is not scien- Mr. Cisin's argument sounds like the dope tifically applicable to prescribing even a of some of the hearing-aid men -men who hearing -aid of perfectly -controlled charac- Correspondence Courses In know a lot about technicalities and theory teristics (and none of them yet have per- RADIO and ELECTRICAL ENGINEERING but have been blundering around in the dark fectly-controlled characteristics). trying to find out about practical hearing. Tests are made of the audibility threshold ELECTRICAL ENGINEERINGGo;e There's a wide cal field. Prepare yourself, at Cost. w^for still gulf sometimes between of hearing, is lost, when what is Low or what future. Modem. Imnlifled. you can nodentantl quií u-kly. theory and practice. Extre rine

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648 RADIO -CRAFT for MAY, 1941 "RADIO'S GREATEST MAGAZINE"

. . . . Eternal Vigilance must be our watchword against radio subversion RADIOBOTAGE By the Editor - HUGO GERNSBACK

IT is a well -known fact that history tends to repeat itself in more are several methods, which need not be discussed here, to effec- ways than one. Right now we find ourselves again in a similar tively stop messages of this type. position to that in 1915 when we were not at war with Germany There are other means whereby Fifth Columnists, and foreign -yet German agents were active to the detriment of the agents. can disguise information in more subtle ways which are country then, as they are now. not immediately apparent. There are a number of foreign -lan- Prior to World War I, the then German Imperial Government guage stations in this country, which to the writer's mind. should, had planted on these shores a number of radio stations which at the present such were time, not be allowed to broadcast in languages. ostensibly for commercial purposes only, but were later I know I will be assailed vigorously on this point by those who proven to be violating the laws of this country by committing operate such stations, but I insist that while they might have a unneutral acts against the United States. purpose in peace times, they have no purpose at the present dan- The United States, at that time, took over the German wireless gerous period through which we are passing. stations at Tuckerton and Sayville, Long Island, to prevent un- You do not have to be a master of ingenuity to invent a number neutral messages from being sent to Germany. of simple and easily arranged codes which can be sent out over In an Editorial, entitled, "Sayville," in my former magazine, The these -and for that matter any other broadcast station -by foreign Electrical Experimenter for August 1915, I took pains to show agents who pose as entertainers. Popular songs, even ordinary how masked and disguised messages could be sent out, violat- music, can be arranged in such a way as to convey certain meaning our then neutrality. My reasoning was violently objected to by ings. You have often witnessed the so- called "mind reading" per- the head of the Sayville wireless station, the well -known Dr. K. G. formances where simple words or questions are asked by one Frank. For the record, I reprint elsewhere in this issue (), the performer, while the "mind reader" immediately tells you what exchange of letters that took place at that time. The arguments set object his partner has touched. Inflection of the voice, certain forth by Dr. K. G. Frank are exactly those used by present -day word combinations, etc., all can be made into effective codes. The German officials; and the tone and attitude was the same in 1915, same thing is true of music, whether it is a piano or violin solo. as it is in 1941. Dr. Frank was later proven to be the active United It can be done so effectively that even the owners of the station States head of the German Information Bureau (Secret Service) - would not be aware of what is going on. What good you say, are now the Gestapo. such masked codes? Of tremendous importance, for the simple Through the vigilance of some self- appointed American inves- reason that a number of listeners can take down and decipher the tigators -old -time wireless operators -who made phonograph rec- code and then pass it on by radio, or by other means, over the ords of the Sayville transmissions at that time, the United States border and across the oceans. You might say, "Why go to the Government became convinced that the two stations were used for trouble of doing all this ?" "Why not use the cable or similar illegal purposes and promptly took them over. methods ?" The point is that it is more difficult to do it by cable, In those days there was no broadcasting. All traffic had to go because certain word combinations arouse suspicion, whereas radio out via dots and dashes. The spoken word was not then broadcast. broadcasts by voice, singing or instrumental music appear harm- It should be noted that present subversive activities of the Axis less and the perpetrators run less chance of apprehension. Powers are much more thorough, less crude and infinitely more We note, therefore, with great satisfaction that our Govern- subtle. It is of paramount importance today that information ment, through the Federal Communications Commission, has set regarding much of our defense activity should not reach the Axis aside recently, $1,600,000 to aid in national defense. Plans have Powers; the more delay that can be interposed to important news been made for the establishment of listening posts to record, reaching the Axis, the better it will be for our welfare. Information translate and analyze foreign shortwave broadcasts in this coun- if it is to be of use at all, must be transmitted with reasonable try. While records have been made before of foreign shortwave speed. As far as the United States and its geographical position is broadcasts for various Government agencies-Departments of concerned, this can be accomplished nowadays only by means of State, Agriculture and Justice-the Government never had under- cable, radio or telegraph. Technically, this country at this writing taken any study of them on a large scale. Now, for this purpose, is not at war with the Axis, yet our defense activities are of the field service is adding about 40 trained men to its staffs at the terrific importance to the Axis partners. Eternal vigilance to keep stations from which the work will be carried on. These men, with information from going out, either by radio, cable or telegraph, the cooperation of the existing staffs, will now conduct a 24 -hour must be the watchword from now on. Since the advent of radio watch to find and record programs which might be of interest. The broadcasting, conditions have changed and the "radioboteur" -the records will be sent to Washington, where the headquarters and Fifth Columnist -who is active, will use the utmost ingenuity to main staff of the new division will be located. Here a force of get important information through. It should not take an Edgar more than 300 technicians, translators, clerks, propaganda analysts Atlan Poe to devise effective schemes as to how our radio broadcast and other experts will translate them into English and study stations can be misused to transmit such information, which on them to determine their importance. the surface looks innocent enough. It should always be borne in Up to the present time, various studies have been made chiefly mind that the United States, at the present time, is wide open and by private propaganda -analysts and patriotic organizations; and it is comparatively easy to get vital information out of the country. broadcasters as well as private investigation groups will be asked To the North of us the chances are slim, because Canada being at to cooperate. The Commission plans to supervise radio broadcasts war, probably stops most subversive intelligence. Our eastern and more carefully in order to tighten control. Thus, for instance, western coasts are not quite so safe, because radio transmitters additional men are working on monitor, or listening stations which placed at isolated points on our coasts can easily transmit short have been tracking down an increasing number of illegal stations. wave radio messages to Europe, or Asia, with outfits that can be The Federal Communications Commission has already a number of placed in automobiles or trucks and moved about whenever neces- mobile radio listening posts and their number will be greatly sary. That, of course, is the obvious thing, hence, the smart "radio - increased in the near future. boteur" will not use it, except when no other avenue is open. It But, what is needed desperately right now is actual record- is dangerous and will become more dangerous to foreign agents ings of every United States station of every broadcast emitted. as time goes on. This will be made clearer below. This in itself is an enormous undertaking, as can readily be The hot spot in the United States, at the present time, is our imagined, but in view of what I have said above, it will be found' southern border. Information can get across this border easily. It necessary, without a doubt. While the danger of "radiobotage" is can even be telephoned or telegraphed to Mexico, thence cabled, or not so great from network programs -due to the fact that these radioed, across the oceans if necessary. It is here where our Govern- usually are rehearsed and would, consequently, be stale by the ment will have to be more than careful and more than watchful, time they are actually broadcast -it is the smaller broadcast sta- and it will be a task of the first magnitude to prevent messages tions which are dangerous at this moment. They are chiefly dan- from leaking out in this direction. While on the surface it looks gerous because subversive activity may go on, and no doubt has hopeless, it is not quite as difficult as it might appear, because there gone on already, without the knowledge of the owners of the stations. The quicker we close this, now wide -open door, the more Pl See Page GK. effective our Defense efforts will become.

RADIO -CRAFT for MAY, I 9 4 I 649 THE RADIO MONTH IN REVIEW

The busy radio men. effect in Great Britain against unlicensed "radio news" paper for An radio transmitters. The war induced these illustrated digest of the important happenings governmental restrictions. of the month in every branch of the radio field. Berlin's shortwave propagandist Lord Haw -Haw, last month rebroadcast to Ameri- ca a recording of a portion of a transcrip- tion, broadcast a short time before over the N.B.C. net. The airing had featured Senator Nye during an N.B.C.-Chicago University Round Table Discussion program, the trade newspaper "Radio Daily" reported. RADIO DETECTIVE Here's what the interior According to "PM," it was news- looks like of an F.C.C. this "traveling radio detec- paper's "rather impish" suggestion that tor" for tracking -down Americans take Hitler at his word, when unlicensed radio send- he a Ameri- ing stations. Hallicraft- recently broadcast request that ers receivers are used. cans send 25 -word radiograms offering sug- This innocent -appearing gestions for improving the shortwave pro- car provides for recording the unlawful broad- grams to the United States. Until the news- casts, as well as deter- paper called attention to the potentialities of mining their source by the offer, 10 days after it was first made means of its direction - finding equipment. only 1 radiogram for Berlin had been received, it was reported; but before the 10th day was out, 4 telephone operators had been added to RCA's staff to handle the resulting "witskrieg" (as another newspaper put it). Total number of "sugges- tions" was over 50,000, which cost Hitler about $100,000, it was estimated. ABROAD F.M. Aß1Y officers and civilian ob- NOW that Frequency Modula- servers recently returned tion has received the bless- from Britain report that de- ings of the Federal Communi- velopment is progressing satisfactorily on cations Commission, in the form of its OK airplane detectors which utilize photoelectric to "go commercial," we look for meteoric cells or "electric eyes." Designed to coordi- progress in this field. Station No. 1 on the nate the aim of searchlight beams and anti- Honor Roll of commercial F.M. stations is aircraft gun batteries, they represent a new W47NV, owned and operated by WSM of hazard to night air- raiders, according to the National Life & Accident Insurance Co., AP reports last month. Nashville, Tenn.

Secret agents pf the Philippine constabu- A number of commercial F.M. stations, lary and Manila police last month arrested by special permission, will continue to utilize one Japanese and held several others for their experimental transmitters for a time questioning, in a sudden raid, for possessing pending the arrival of higher -powered equip- an unlicensed radio transmitter and receiver. ment that have been delayed by manufacturers' Defense orders. AERO SIGNAL BLOCK An "automatic block signal" system for airplanes A "target squadron" of 13 radio -controlled in flight has been perfected by radio engineers of airplanes soon will be in use, in Honolulu, Westinghouse Radio Stations, Inc., re- Transcontinental & Western Air. Inc. Miniature towing targets for U.S. anti-aircraft gun planes (arrow) proceed along a track at a rate ceived construction permits for F.M. batteries, UP reported last month. 4 trans- proportionate to keep pace with a plane aloft. When mitters, last month (no more than 6 are per- the miniature plane arrives at check points the Dis- patch Office radio man contacts the flight plane mitted under common ownership or control) by radio and then "clears" it into the next block. Radio -diathermy instruments come with- movie makers have moved into the The miniature plane rings an alarm if it arrives at in the province of bans recently put into F.M. field with the application by Metro- a check point before its airplane aloft.

SCOUTING BY TELEVISION] Television made its debut as a new and powerful defense weapon last month Headquarters and a dozen other points! The television camera was connected when 15 technicians of Allen B. DuMont Laboratories, in cooperation with U. S. by coaxial cable to a 25 watt transmitter in an army truck. The picture at right Army personnel, demonstrated its military uses. The photo at left shows how shows the mobile 25 -watt relay television transmitter out in a field, I the portable equipment was set up to view scenes of troop movements from operating directly from the television camera and flashing a signal to the ambush and transmit them over an ultra- shortwave channel to receivers at main transmitter miles away. The operator is monitoring the video signals. 650 RADIO -CRAFT for MAY, 1941 THE RADIO MONTH IN REVIEW

IISV.

IMMERSION PULLEYS HEATER

NICKEL THERMOMETER TUBE VIBRATES AT SUPERSONIC FREQUENCIES

SKI BROADCAST! 4, Ski- record breaker Torger D. Tokle is here shown in a preliminary jump prior to setting a new record last month, through a snow -storm at Lake Placid, N. Y.-at the same time broadcasting over WGY and the N.B.C. net.

SUPERSONICS CHEMICAL The machine at right demonstrates chemical reactions under supersonic fre- j quencies at up to 300,000 vibrations per second reported BEING "Science Observer" VIBRATED + magazine. M.I.T.'s Dr. W. C. Schumb and E. S. Ritter use a vibrating nickel ISV.D.C. 2000v,oc. tube, A, which is actuated by driving and polarizing coils D and E, respectively.

Goldwyn -Mayer for a construction permit. the Aise to director R. W. Richmond. The . The cost of F.M. transmitters at pres- SOUND reverse side of the disc was blank for ent lies between approximately $5000 and AMICROPHONE made of "reply." $100,000.... Zenith Radio Corp. has signed frozen rubber is the subject a 9 -year lease with Chicago's 600 -ft. Field of Patent No. 2,231.159. A KDKA shortwave broadcast to England, Building, which is to house this set -maker's granted here to Erwin Gerlash of Berlin. of a program which featured 25 British new 50 kw. station W51C.. . The "New Germany. Electrodes on either side of the refugee children, was recorded; it was then York Daily News" last month entered radio thin insulating plate pick up the electrical sent to Boston, and there again broadcast to via its application for a permit to construct voltages it generates, much like the soft - England by shortwaves. an F.M. station. . estimates place the rubber, taut- diaphragm static- electricity mi- totql of F.M. receivers now in use in the crophone described in Radio- Craft, several The actual sounds of aerial blitzkrieg, re- Metropolitan (New York) area at about years ago, in which a membrane of stretched 3,000. corded in England during the worst air rubber was shown to afford articulate speech raids last Fall, were dubbed into an Ameri- when arranged as a microphone. can radio drama, last month, in the "Hospi- A unique feature of F.M. is the fact that tal on the Thames" broadcast over the all commercial stations in a given area are "Uncle Don" wanted a vacation, so Uncle 1VOR-Mutual net. The sounds included the required, by regulations, to provide a signal Don obtained his respite from the usual chattering of machine guns in an aerial dog of equal strength. WOR stint for children by the simple ex- fight over Dover, the din of "yam poms" pedient of leaving a week's -work of tran- during anti-aircraft fire, the frightening Here's how the Big Cities stack up in F.M. scribed programs to carry -on in his absence. sound of "screecher" bombs, the sound of activity, by number of construction permits air raid sirens, the welcome "all clear" sig- requested (R) and granted (G): At Rutgers University, New Brunswick, nal. City R G City R G N. J., sound recordings are made of speech. Philadelphia, Los Angeles, as part of the technique in a new course The demands of Uncle Sam's Defense pro- Pa. 6 4 Cal. 5 1 designed to improve students' voices for gram are beginning to pinch the toes of Schenectady, New York City 18 7 dictating, telephoning, etc. An interconsmu- manufacturers of sound -on -disc blanks. No N. Y. 2 2 Chicago 6 5 nicator between 2 rooms enables class moni- material to equal aluminum as a backing has Providence, R. I. 2 0 Detroit 4 2 toring, from loudspeakers, of study conver- as yet been found, manufacturers say. The Hartford, Conn. 2 2 Boston 3 1 sations between students. present recording demand for aluminum Pittsburgh, Pa. 3 2 St. Louis 3 o totals about 500 tons annually. WHK- WCLE's program director last month received something new in the way of a fan "letter." A fellow with an idea, thought enough of his suggestion to make a sound -on -disc recording of it, and to send

"AIRING" THE CANARY 4 In a recent broadcast over the N.B.C. Blue network Dr. Orestes H. Caldwell (left) utilized the special birdcage arrangement shown here to transmit the heartbeat of a canary bird. The heart pulsa- tions of the canary were picked up through its feet, by a erch especially built by Brush Laboratories to include a Rochelle -salt crystal pickup, and broadcast world -wide over N.B.C.'s 50 kw. International shortwave station WNBI. o

WIRELESS "PA" A cable -free microphone* last month made its i 1Iii11li1fiH I!!; official debut as an important adjunct to public P-iawltnu! address work, when the Federal Communications Commission granted a construction permit to Guy S. ...lit: .1n11 Cornish for his Station WBXWI, shown at right, on 310 megacycles. It is proving an ideal solution to the problem of announcing winners at sporting events, etc., where use of a mike with attached cable would be impossible or unsafe. Also see "'wireless' Public Address." RadioCraft, Jan. '40.

RADIO -CRAFT for MAY 194I 651 SOUND

Front view of the P.A. - Radio - Recording Console as it looked in the process of construction (also note photo on cover). Only the playback turntable has been installed at this stage of the construction; and, the cover that encloses this equipment on top of the console has not yet been put in position.

#ow to Build a Modern 301/5 - Watt P.A. - RADIO - RECORDING CONSOLE Described here is a sound system incorporating the latest advances in circuits and equipment for high -fidelity Public Address, radio reception, and sound recording and playback. Complete construction details, including a specification list of components, will be given in this multi -part article so that anyone with a reasonable amount of technical ability may build it. This semi-portable P.A. - Radio - Recording Console incorporates exceptional versatility in a comparatively small amount of space.

R. J. BERGEMANN, JR. PART I

MANY Servicemen and sound tech- A.V.C., and a tuning "eye "; a volume in- FEATURES nicians are beginning to notice the dicator meter with calibrations for read- One 30 -watt high -fidelity Program ampli- fruitful field of semi -professional re- ing in db.; a self -contained 8 -inch moni- fier, 30. 10,000 c.p.s., ±1_5 db. cording. While it is often possible to tor speaker; crystal earphones, for moni- add a basic recording mechanism to an ex- toring when the recorder is used in the One 15 -watt high -fidelity ampli- Auxiliary isting amplifier, the results are never so same room that serves the function of the 30. 10,000 fier, c.p.s., ±1.5 db. gratifying as when the constructor starts studio, and the loudspeaker cannot be used Independent electronic tone controls for from scratch. The instrument described without feedback; completely variable tone each amplifier. here will satisfy the requirements of vir- controls for both Program and Auxiliary Dual -speed recording and playback. tually all recording engagements regardless amplifiers; and, a new "poly -directionai" Extra turntable for use in "dubbing." of location, and can be duplicated by a dynamic microphone, adjustable to meet all careful experimenter at a cost that is very recording difficulties, yet compact enough -in Built monitor speaker. low in comparison to the merit and versa- to be carried in the compartment which is Provisions for crystal headphones. tility of the unit. in the rear of the speaker. Built -in T.R.F. tuner with A.V.C., diode Plans will also be included for construct- detection and visual tuning "eye." EQUIPMENT ing portable baffles for 12 -in. speakers to be Talkback key for studio work. The P.A. - Recording - Console, shown here used in P.A. work. These baffles are especial- and on the cover of this issue of Radio - ly designed for the P.M. dynamic loudspeak- Versatile switching system (permitting Craft, comprises one 30 -watt Program am- ers which have been selected for this Sound of of the combinations different portions plifier; one 15 -watt Auxiliary amplifier, System. Two of these speakers in the baf- amplifiers for unusual effects, added whose function will be described later; a fles mentioned above, will handle the full power, or measurement work). dual -speed recording mechanism, and an ad- output of the Program amplifier for almost DU. meter also reads volts, ohms. justable -speed playback turntable, with all P.A. jobs. lightweight pickups on both tables; a Provisions for talkback system (with utility WHY 2 AMPLIFIERS? operation from points). broadcast -band tuner with diode detection, remote There are many arguments in favor of including 2 amplifiers in the unit. NORMAL (1) Recordings can be made from one CURVE amplifier while the other is used to amplify the sound for an auditorium or hall. Since each amplifier has its individual tone -- controls, response of amplifier ai 1,11'lí`'_ the the sup- .1Iu,',. Frequency response plying the hall may be adjusted to reduce °_ta 111111111fiMMIN curves were run using a feedback, without changing the tone of the 1111 frequency record in con- recording at the same time. UÚ11! í junction with a crystal 7iIIIIIí =i\ pickup. The built -In (2) The use of 2 amplifiers permits a sim- /IIII \11111 meter was used to read ple talkback system for use when recording IIIIIt the power level. from 11011 remote locations or studios. D1111í UUIIIIIí (3) Sound effects from a record source 211/11111 uiiuuuiuIIIIC may be fed to the studio over the Auxiliary 20 40 60 loo 200 400 600 1000 2Ó0O 4,000 100 amplifier so that the performers being re- FREQUENCY IN CYCLES PER SECOND corded may hear their cues. ® (BOTH AMPLIFIERS) (4) In the event of failure of one am- 652 RADIO -CRAFT for MAY, 1941 SOUND

Top and (rignt) underside views of the chassis assembly of the P.A. - Radio - Recording Console. Drawings that make it easy to properly spot these components will appear in subsequent Parts of this article.

plifier, the other can be substituted by Degeneration in the cathode circuit is used (the (lb. scale reading) plus 15.2 db., or 35.2 simple front -panel switching. to reduce the gain of the stage to unity db. above 0.006 -watt reference level. In- (5) With 2 amplifiers, it is possible to plot (1). Due to the very large amount of feed- cidentally, full -scale deflection on the 100 - a frequency response curve on a loudspeaker back necessary to reduce the 6F8G with a volt scale across 500 ohms, indicates 20 or microphone. To run a curve on a mike, gain of 14 to a stage gain of 1, the gain of watts, and so this scale will probably be connect it to the Program amplifier and the stage becomes almost independent of the used most in P.A. work. place it in front of the monitor speaker, the tube gain, and largely dependent on the A response curve of which is known. Feed the ratio of the plate and cathode resistors. STUDIO eDNMS 10w.) monitor speaker with the Auxiliary ampli- change in gain of 50% in the tube, would STUDIO SEIIR. MIKE B fier to which is connected the high- fidelity cause only a 5 ',4 change in the gain of the pickup. Play any commercial 30- 10,000 c.p.s. stage! Thus, the inverter tube maintains its ,% PRO- CONNS OD__ GRAM frequency record, and record the meter original balance. The degeneration would AMP. readings from the Program also iron -out any distor- LIFIER amplifier. By subtracting the COVER FEATURE tion that might be present ILI I speaker's curve, the curve in the inverter stage. AUX- s1--¡-:IIII.. i III... rrr---I ,RnD of the mike's response will be obtained. Since there is degeneration in the phase Eo ILIARV i V-oKNs 1 2 inverter stage, the inverse feedback is placed LIFIER LOCAL only on the power stage. The feedback is MIKE BLOCK DIAGRAM LEVER TYPE accomplished the 10'/r, special feed- MONITOR ® through SPKR.Y DOUBLE-POLE A block diagram of the entire unit is back winding on the Thordarson output ® TRIPLE ut given in Fig. L A switching scheme allows SW POS l MONITOR SPKR. CONNECTED TO PROGRAM AMELIFIER transformers. " " 2 MONITOR SPKR OUT STUDIO SP. .ONAUEIIIART AMP, the various channels to be connected to j- " 3 MONITOR SEER CONNECTED TO AUIIILIART AMPLIFIER either amplifier. By connecting the ampli- MONITORING {YT -TALKEACK CIRCUIT^ fiers before the master gain controls, it is Crystal phones are used for monitoring ,6.15 o.l-MF - possible to use both tone control channels when a speaker would cause feedback. The .1.MF. 20.000 on the same amplifier. Thus, a mike may be response of the phones is comparable to ;- / DA Óa06fA+~ tone -altered for unusual sound effects that of the more expensive phones used in while another is used straight at the same broadcast stations, and it is therefore pos- 0.5- liálwtN time. A total of 8 channels is provided, 4 sible to get excellent recordings even when MEG which may be mixed. The of lever switch a loudspeaker cannot be used for monitor- 2.000 above each channel gain control switches ing. ONMSy / Á WV that control into either the Program or The meter used for monitor level reading 2ó005 / - 2iÑT 0.5-ME(EACH) Auxiliary amplifier. is an inexpensive O-1. ma. "foundation" me- Fl .4 -TONE-CONTROL CIRCUIT - The ter connected as shown in Fig. 6 D.C. Both talkback switch connections are 6FBG 6L66-' shown in Fig. 2. In the upper position, the Volts and Ohms scales are included. If the 2 50034 monitor speaker is connected to the Pro- builder incorporates these, he will need no

other instruments to check and test fin- ASTER I . MF gram amplifier- In the bottom position, the the / ---s Ó ished recorder. To accurately calibrate EóiSDt IOW L6G monitor speaker is connected to the Aux- the , 5 tir"E6 iliary amplifier. In the middle position, the meter for reading relative as well as actual do0 ) db. is not very difficult. monitor speaker is disconnected, and the -. i is Using the 10 -volt A.C. scale and arbitrari- studio speaker connected to the Auxiliary 2.500 ro1.5) 'Ba ti50.0.0 1 EG5- at amplifier. As shown in Fig. 2, the local mike ly calling volt "0 db.," every reading will 000.45 250V oNMS rEK ) remains connected to the Auxiliary ampli- be a multiple of the reference level. Now MIMI -PHASE INVERTER CIRCUIT- fier, so to talk back to the studio; it is only watts equal E' /R, and for a constant re- + eATT AÑO,SiTÖwIi1FGOn necessary to sistance, watts varies directly as E'. There- .¡5,ou,-N2,000 EES. T ro 12 .NC. throw the switch from upper - EIrF "'-""- oNMS to middle position, thus disconnecting the fore, the reading of the meter, squared, . / 0.5MEG. --WAMA-/ 0.5-NEO I will be the change from the reference level O/ 25LM1O monitor speaker, and connecting the studio o.l NA f O.l_MEG in terms of power. The log to 10 FDIwEieñN speaker to the Auxiliary amplifier. the base vvvTT /o.I REGI of the power increase, times 10, will be the OS-MEO The tone control circuit is conventional, L . wAiwL _aaneA- increase in db. - ,D,'- b using degeneration in the cathode circuit. 6H6 EG 3 /°ait For example: suppose that the meter ISKTION) .1 -WA,-A _ Special hum -backing chokes and special reads full -scale. 10 That will be V., or 10 / P tapered potentiometers are used. The bass times the reference chosen. The power in- : +_ boost is 15 50 db. at c.p.s., the bass attenua- crease is therefore 100. Ten log,,, >?E times of gg°ló4 gó > tion, 25 db. at 50 c.p.s. The "highs" control, 100 is 20, and the gain is therefore 20 db. b boosts 10,000 c.p.s. 18 db., or cuts 10,000 This is / a an arbitrary scale used for making I/ ELO T0N LO000 XE /) W c.p.s. 45 db. Frequency response curves are frequency runs. Using the 100 -volt range, ' given in Fig. 3. The tone control circuit across the 500 -ohm line, add 15.2 db. to all } APPROA. may be seen in Fig. 4. I-11 SW.1. readings if you want the db. above 0.006 - CLOSE FTIRLOW The phase inverter circuit (Fig. 5) is watt reference level. Thus, full -scale deflec- 400V 1 + O S ANDSWITCN,oN.I (S) quite different from that generally used. tion on the 100 -volt range would be 20 db. IliFini -METER CIRCUIT -

RADIO -CRAFT for MAY, I 9 4 I 653 SOUND

coming issue of Radio -Craft, complete con- CRYSTAL r ÉYE G -' details and on re- 11111 6u5& RECORDER struction instructions cording will be given. CRYSTAL , ANT PKKLIP NE MIXER TONE CONTROL MASTER 6F8ß POWER OUT PUT nf 6SCS ruin. 6JS GAIN TUBES TRANSEORMER GENERAL COMMENTS MIR! N3I The advantage of the electronic tone con- 25o ' trol lies in the fact that either end of the MIKE NTt audio spectrum may be accentuated instead of merely being attenuated, as the majority MINE Kt3 30.w of circuits do. With the use of reAS tone control 65J7 .y. ' TREBLE AMPLIFIER this control it is possible to emphasize bass il GAIN in CONTROLS OUTPUT and /or brass recording and reproducing. *ól.écéMiáóii T TA EOXTUióúrL LEVER SWITCHES SFOL.E17 "Dubbing," or the adding of material to w GAIN CoNISIOLS MI TONE MASTER 6FBGOwR TUBES CRYL +'O EITHER AMPLIFIER 6SF5 6J5 an existing record, is accomplished by plac- N92 / I ing the existing record on the play -back METERt . turntable and copying to the recording turn- 5Ú4G table while blending the microphone with nSY I the pickup. Since both turntables are dual wi' speed (one is adjustable), it is possible to Lso.. 8AS5\ vac PIc AUXILIARY 5Ú4G AMPLIFIER test at the slow speed, thus saving on record cost. It is also possible to copy and dub IVDIDI 5,AKER from one speed to the other. The T.A.F. receiver incorporated in the ' PAP'. V ¡RgNES unit was expressly designed for "off the air" L-w SPEPEAO PLAYBACK SPEAKER RECORDING recordings. Although used primarily for MOTOR MOTOR - 6LocM DIAGRAM OF PA.-RADIO-RECORDING CONSOLE -- local stations, the R.F. gain is sufficient to Wars receive most of the smaller stations. Auto- At the present time, both amplifiers are response characteristics. We are waiting matic volume control was included to pre- completed, and curves are shown in Fig. 8. for a new dual -speed mechanism to arrive vent fading during recording. The tuning Recordings have been made with excellent before completing the cabinet. In a forth- eye insures accurate tuning.

CASE HISTORIES OF P.A. SALES WHEN RADIO WAS A BABY the Recollections of the No. 13 -Fi Sound For Old Sets Being Old Days -Hi of Broadcasting IOPERATE a small radio service shop, and This amplifier was chosen because it was recently have started public address work. reasonably priced and had sufficient gain. Broadcasting was a rough and tumble On August 7th, one of my customers I could then allow more money for a good business around the small radio stations a dropped in to say "Hello" and the subject of speaker. As it turned out, he went over the decade or so ago, with the spirit of the fun - high -fidelity sets came up. He seemed inter- original price, when he requested the Jensen loving Rover boys riding high, recalls WOR ested in the possibilities of improving his old speaker. announcer George Hogan. George remem- set, and building an amplifier for his record Now came the moment I waited for the bers the days back in the late 20's when he player, which I told him I could do. I quoted first test. The radio set played fine through worked at a small Midwest station, before him a price of $50 for the complete job the amplifier, but when I switched -in the broadcasting became a big business... . which he OK -ed. record player there was a decided hum. I When trying to break up an announcer I next purchased a 10 -watt amplifier kit suspected the player, so I remounted the while he was on the air was the favorite from a local manufacturer, Phelps - Dodge. motor on a suitable piece of Masonite which indoor sport, i.e., shooting a water pistol The amplifier employed the following tubes: gave a nice appearance to the player and, into an announcer's mouth while he was 6J7, 6C5, 6A5s in push -pull, and an 83 recti- with the placing of a 0.1 -mf. tubular con- talking, turning on the electric fan and fier. The amplifier had a resistance -capacity denser of 400 w.v. from ground to one scattering his script . . pouring water on coupled 1st stage which fed into a trans- side of the line feeding the player 110 V. his head . . undressing an announcer be- former- coupled push -pull output stage. The A.C. line this eliminated the trouble. Micro- fore the mike. amplifier had a gain of 85 db. and a fre- phone gave no problems and worked fine. . When programs used to run eighteen quency range of 40 to 15,000 cycles within or twenty -five or thirty -three minutes. 2 db. It also had a volume control in the The complete installation cost as follows: input circuit of the 6J7 (which is self -biased Phelps -Dodge type HF-11 When small town sponsors were so with Mallory cells), and a tone control in foundation amplifier kit $17.60 thrilled at the idea of being radio sponsors that they came up to the studios every night the plate circuit of this same tube. Tubes, condensers, resistors, jacks, I was busy thumbing catalogs to find a to watch their broadcasts and took the plugs, Yaxley rotary switch, etc 23.00 whole cast out to dinner afterwards. suitable speaker when my customer phoned Bade board 10.00 to tell me that he had decided on the new . When every little one -lung station was Jensen speaker, Hi Fidelity model PM -15, Jensen type PM -15 speaker 42.00 a training ground for talent and developed which the day before we had discussed using. Thordarson type 58F72 output trans- everything from hillbillies to classical or- I used a special matching transfoímer, for former 4.00 ganists. the output of the amplifier. The speaker, Inca D -25 filter choke 3.00 . When listeners called the station to a 15 -in. permanent -magnet type, I mounted Universal crystal mike type 15 -MM, ask what color eyes a singer had, or on a 5 x 6 ft. baffle made of % -in. Celotex. stand, cable, plugs 32.00 phoned -in to request a number. As the original amplifier called for a . When listeners called up to say they dynamic -type speaker I had to substitute a Total $131.60 enjoyed the show, and to tell whosis that filter choke in place of the field. The pur- he "sang especially well today." pose of the installation was to provide a plus $5 for labor. means of obtaining finer quality from an ... When every movie and stage actor, big - old AK -60 radio set; and also, to afford a $131.60 list price or small -time, who passed through town was 78.96 net brought up to the mike to say "Hello, I'm means of operating a record player and of with you." using a crystal microphone. so thrilled to be here In order to save time in making connec- $52.64 profit for 3 I wired tions these services the input Well that's the business, not bad for 3 circuit to 3 closed, single- circuit jacks and a SUPER -CYCLOTRON days' work. Sure wish I'd entered public The of California last month 3 -point single-deck rotary switch. The arm address work sooner. University was wired to the input of the amplifier, and announced it had received a grant of one side of the jacks to each of the taps on FREDERIC URLAN DILLICN, $1,150,000 from the Rockefeller Foundation which, with an additional needed $250,000, the switch. The other side of the input and Hollywood, Calif. the jacks went to ground. will make it possible to build a 100,000,000 - To remodel the receiver I disconnected volt "super- cyclotron" for the investigation the lead from the plate of the detector tube Mr. Dillion's prize was 5th in the 4th Sec- of atomic physics. The completed machine's and connected a single shielded wire which tion of the $4,000 P.A. contest "R.-C." spon- ion beam will melt a building brick like a plugged into the proper jack. sored at the beginning of last year. blow -torch melts a pound of butter. 654 RADIO -CRAFT for MAY, 1941 TAP - IN SHAFTS

PUT This isn't our verdict ... it's the verdict of thousands of servicemen who have found the Tap -in Shafts of IRC Type D and here's why! Controls the answer to countless problems. They save your time IRC Tap -in Shafts won't slip -they save you money. Tap -in shafts make it possible to handle or vibrate loose. The shaft the big majority of all replacements with only a small stock of socket is carefully gaged to controls. They frequently enable you to use a standard Type D a standard Brown & Shorp Control instead of a more costly, hard -to -get "special." Flats taper and the shaft taper is are easily located in any desired position. Shafts can be inserted accurately machined to such after the control has been installed. It's unnecessary to remove a smooth, close fit that the sur- other parts when making an IRC replacement in a crowded faces are practically bonded chassis. Once inserted, and tapped solidly in with a hammer, together when engaged by a the shaft is there to stay. You don't have to fiddle with lock few taps of a hammer. This washers to hold it in place. taper- socket principle is Don't fail to examine Type D the time you visit standard practice in machine Controls next to IRC fool work -from screw ma- your distributor ... and be sure ask him about the chines to giant lathes -where Master Radiotrician's Control Cabinet with its stock of only cuffing tools for high speed 18 Type D Controls, 6 switches and or rugged work must be held 5 special Tap -in Shafts that handle securely in from 60% to 75% of all replacements!

NEW DOUBLE- FLATTED TYPE "A" SHAFT Shaft position Shaft position' in knob requir- in knob requin ELIMINATES INSERTS and FILING ing 3!32" flat. ing 1/32" flat.

Here's the answer to simplified fitting

for all knobs. No more , bothersome metal inserts necessary .... shaft filing eliminated for all !,,_" or ;';,:" standard How shaft is Only edges of flats . . . easier -to -cut to length, too. positioned for shaft ore filed set -screw knob. for odd types.

IRC TYPE D UNIVERSAL VOLUME CONTROLS INTERNATIONAL RESISTANCE CO., 401 N. BROAD ST., PHILA., PA.

RADIO -CRAFT for MAY, 1941 655 SOUND

'max 4Imm) -In Zf

ADDLiEL IdNE _ _ -__ _- ______- Imo.

'ma +Imin. M

C2 -)i Ilp 2 `' ------HR3 LOW-RANGE 1 I. OW -RANGE [( LLIAMMETER _ DC '^ im;n. VOLTMETER "B+ - C I - . R4 -- SIMPLE AMPLIF ER STAGE WITH METERS------Mini Fora DETECTING OISTORTICN- FIG. 2 FIG.5 - WIEN BRIDGE - MEASURING DISTORTION IN AUDIO- FREQUENCY AMPLIFIERS In the following article (reproduced from "The Aerovox Research Worker" by courtesy of the publishers) the elements of qualitative analyses of A.F.-amplifier frequency distortion, as commonly applied by sound technicians, are mentioned. The more precise methods of making quantitative analyses are then described at length.

THE simplest qualitative test for distor- be evident from the fundamental relations scope having horizontal and vertical am- tion in a class A audio -frequency ampli- of this combination that the average value plifiers with substantially flat frequency re- fier stage may be made, as shown in of plate current, as indicated by the plate - sponses. The units are connected in the Fig. 1, by applying a signal voltage of circuit milliammeter, will be constant in the order shown. proper level to the input and inspecting the company of the alternating component un- It is the purpose of the oscillator to supply circuit for one or all of the following abnor- der distertionless operating conditions. a signal of as pure waveform as practicable mal conditions: Figure 2 is a graphical representation of to the amplifier, and that of the oscilloscope signal plate current. Here, I,,,,,x. is the to reproduce the wave -shape of the signal (a) Presence of D.C. grid current. maximum value reached by the fluctuating after it has passed through the amplifier. (b) Fluctuation of the D.C. plate cur- plate current; la the zero-signal value; In order that as little distortion as possible rent. I ,,,,,,, the minimum value. From these values, be introduced by the instruments them- (c) Fluctuation of the D.C. cathode Mayay be shown that the per cent 2nd - selves, the oscillator used for such a test voltage, if the circuit employs harmonic content (often the most trouble- must be of exceptionally high quality and cathode resistor bias. some distortion factor) is equal to: the amplifiers in the oscilloscope must possess an excellent frequency characteris- Each of these indications generally oc- % (Im,N,. + Imm.) -In. tic. Likewise, the oscilloscope sweep circuit curs in a positive direction, and each will X 100 must be uncompromisingly linear in its disappear upon removal of the signal. It Imax. - Imin. characteristic. must be borne in mind, however, that this If the amplifier had no distortion at all, method is purely rudimentary in nature and Quantitative methods of checking distortion the signal it delivered to the oscilloscope serves only to detect the presence of distor- are harmonic analyses, and are concerned would be an exact reproduction of the tion. One or two of the indications may be with measurement of the actual amount of input signal waveform. This is never en- absent, depending upon the main cause of energy present in each separate harmonic countered in practice, however, the most the trouble. of the signal frequency (or in the total har- efficient amplifier arrangement being beset monic content) and establishment of per- with the distortion characteristics of its QUALITATIVE TESTS FOR DISTORTION centages with respect to the fundamental tubes and other components. The 3 simple indications are well known frequency. The most representative meth- For observations, a perfect sine wave (or, and frequently used by Servicemen and ods employed in wave analysis and the better still, a tracing of a single cycle from P.A. testers who have no equipment suit- apparatus necessary thereto will be de- the test oscillator) might be inscribed on the able for making quantitative distortion scribed presently. transparent viewing screen of the oscillo- measurements, but must, in the course of scope, and signals from the amplifier routine testing, localize distortion without OSCILLOSCOPIC METHOD matched to this pattern to discover varia- reference to the actual per cent harmonic The cathode -ray oscilloscope is notably tions from the original shape due to am- energy present. useful in the observation of wave shapes. plifier distortion. In making such a test, it The cathode circuit effects noted above When the horizontal plates of the ray tube would of course be necessary to adjust both are due to fluctuations in the voltage drop are energized by a sawtooth -wave sweep - oscilloscope amplifier gain controls in such across the cathode resistor, occasioned by oscillator- amplifier circuit to furnish the manner that the maximum amplitude and variations in the D.C. component of plate linear time base, and a signal voltage which width of the signal trace coincided with current. The current indicated by the plate - it is desired to observe is applied to the those dimensions of the inscribed pattern. circuit milliammeter is the average value of vertical plates through a substantially flat - With the low percentages encountered the fluctuating "signal" plate current, is response amplifier, the cathode -ray trace with most well- designed amplifying equip- identical with the D.C. component, and is will be an exact reproduction of the wave- ment, it will be difficult to estimate the per- the current that produces the cathode re- form of the applied signal voltage. centage of harmonic content from the resistor drop. An audio- frequency amplifier may be produced wave- shape, in the oscilloscopic These facts may be better comprehended checked for distortion with the oscilloscope method, unless the operator makes use of when it is remembered that the fluctuating yin the manner illustrated in Fig. 3. At A is the transparent screens furnished by some signal plate current (Fig. 2) is an alter- an audio oscillator possessing an output oscilloscope manufacturers for the purpose. nating current, corresponding to the signal, voltage waveform of known purity, B is These screens carry printed patterns of superimposed uposl a direct current. It will the amplifier under test, and C is an oscillo- single cycles corresponding to the shapes 656 RADIO -CRAFT for MAY, 1941 SOUND obtained (variations from true sinusoidal) with various low percentages of distortion. Severe cases would result in images similar 15 WATT AMPLIFIERat1O15 CI VES to Fig. 4 which is an exaggerated representation of pronounced 3rd- harmonic content. 3100 NOURSCón tin /to /LISERVICE FREQUENCY BRIDGES Certain bridge circuits, notably the Wien bridge (see Fig. 5) can be used for the AND STILL GOING STRONG! identification of frequencies in the audio- frequency spectrum. If an alternating voltage is delivered to the bridge circuit, the latter may be adjusted for a null at that particular signal frequency. The null point would not hold for the same voltage of another frequency. Thus, the adjustable element of the bridge might be calibrated to read directly in cycles /second. Aff The Wien bridge in its most useful forni for this purpose would have its constants so chosen that the ratio arm, R. is twice the ohms value of R the condensers C, oate and C, are equal in capacity, and the 2 simultaneously- resistance legs, R. adjustable I and R, are at all positions equal. Under NEIL these conditions, the frequency of the im- 0ó'M engMpqNy pressed voltage at null would be equal to: .N a'aN14 ala S Y0K'Clrr03' Co. 4ot}yn 2*RC CHECK THESE FEATURES EXTRA HEAVY CHASSIS ryót:élátla' r Where: Inaá °v: Frequency is in cycles /second, COMPLETELY SHIELDED r ,ntié' »,eitO::`4'óbieetiñatr,,: ALL ."e:,n::;.é' . R is the resistance of R, or R, in ohms, OVERSIZED COMPONENTS ,,riaa rL'aíiéa¡ rr. - SIX TUBE `° a4 p+ ia a wó C is of C, or Ce in farads. CLASS A PUSH -PULL .,',e r. the capacity - Is n. ; FIFTEEN ú WATT OUTPUT nain,«eo,^ .a n Since the bridge may be balanced for only THREE INPUT POSITIONS ié.' m3` ' 2 Microphone, I Phono Input t`n`,.,el. w a..., one frequency at a time, it would appear . ¡ cir. in`¡a M , 'tr ,R4e that any residual voltage indicated by the 2 -4 -S -500 OHM OUTPUTS vacuum -tube voltmeter, M, at null would be HIGH GAIN, WIDE FREQUENCY RESPONSI sa,pt¡j[Ééreytl,r S,r,h.e: due to some other frequency or frequencies CONTINUOUS. VARIABLE TONE CONTROL. 3: ro FOUR STAGES neéWe>o"iot,°"'r (such as harmonics of the fundamental). OF AMPLIFICATION nl.,'e:eV nó:'.úrye¡i TUBE LINEUP 1 -6537. 2 -SC7. 2 -6N6. 1 -53V " And this harmonic voltage would be due to .L¡tóllr¡;::t`a a, OPERATION ON 110 -120 Volts. 50-60 CYCLE the total of harmonic voltages present. As a.,:,, such, the bridge might be connected, as LIST PRICE $21.50 -YOUR COST ONLY "o $10.75 (less tubes) shown in Fig. 5, to the output circuit of an 94y audio -frequency amplifier which is passing awrr rti° J: 4. BUY THE BEST FOR LESS! . { a signal from a high- quality audio oscillator. Don't let our low price fool you! i While the device might be used as shown When you buy a TokFone Am- 75 as such a harmonic totalizer, the percentage plifier you're buying QUALITY. Mr. O'Neil's letter and many FOR AMPLIFIER total harmonic content with respect to the others like it prove that you need Less T"h.' F O -a.. N. readings of the meter before and after null not pay high prices to get High I Ki, would not be reliable, nor would its error be Quality. uniform for all frequencies. These facts Regal Amplifier Mfg. Co. A COMPLETE ALL- PURPOSE 53250 16 West 17th St. are due the peculiar nature of the bridge N:w York to SOUND SYSTEM FOR ONLY City to attenuate various harmonics unequally. u.l i 13 watt anuditlit al $1!' - The amplifier described above may be purchased alone Another popular type of bridge harmonic I Ives (ube,) or as part of the Complete TokFone Sound System. (due to U.T.C.) is shown in Fig. 6. Here's what you get: PRICE send1 me 15 watt sound systems - totalizer LIST \ $32.50 Iles tubes) Here, 3 legs of bridge. and 15 Watt Amplifier as Described $24.50 the R R R 2 12' Speakers 4f 87.50 nub 15.00 send me kits of matched tube, i contain pure resistance, while the 9th leg 2 Wooden Speaker Baffles 49 $10.00 each 20.00 $1.50 I Crystal Microphone with Desk Stand 12.50 send me screen corers 5 $222.223 contains the shielded parallel resonant cir- I Kit of 6 Matched Tubes 9.30 L -C, is send me complete Illustrated catalog and price 11 cuit, which resonant at the test fre- TOTAL tyi quency. The transforrAer, T, like the one For ampli i, ever add $2. ].ime shown in the bridge previously described, must have an excellent frequency characteristic. MAIL COUPON TODAY: r At resonant frequency of L -C, the inductive reactance of the tuned circuit equals the capacitative reactance, the former is can- cent of total harmonics from this operation: amplifier output is connected to a high -pass celed by the latter, and the bridge balances R. filter which removes the test frequency but as if all 4 legs were pure resistance. Any leaves all of its harmonics. voltage applied by the circuit to the vac- %H= \ 100 uum -tube voltmeter is then due to har- R, -- R The actùal voltage due to the harmonics monics of the test frequency (and it is or: is then measured by means of an attenuator and vacuum -tube output voltmeter. This assumed that these harmonics have been A the potenti- dial indicator attached to measurement is one of total harmonic dis- delivered to the bridge by the amplifier un- ometer R, may be calibrated directly in these der measurement). tortion, but it is entirely possible to ar- percentages. range additional flat- response amplification In operation, the double -pole, double - with various high -pass filters to remove throw switch, S, is thrown to position 2 and FILTER -METER the various harmonics singly along with the bridge balanced with the assistance of A very efficient method of measuring the fundamental. the vacuum-tube voltmeter, M, as a null total harmonic content in the signal de- indicator. The reading at null (due to har- livered by an audio -frequency amplifier monics) is recorded. The switch is then makes use of the arrangement employed in WAVE ANALYZER thrown to position 1 and R, is adjusted un- the distortion and noise meters found in The ware analyzer, a highly -developed til the meter gives the sane reading (as broadcast stations. (See Fig. 7.) and refined form of heterodyne vacuum - before at null). The following calculation In this arrangement, the signal from a tube voltmeter, provides the most ad- may be performed to determine the per high- quality sine -wave audio test oscilla- vanced, accurate, and complete means of l'nited Tran,fnrmer t'orp. tor is fell into the amplifier under test. The measuring amplifier distortion by determin- RADIO -CRAFT for MAY. 1941 657 SOUND

OSCILLOSCOPE A B C TRACE PROOF LOW - DISTORTION AUDIO AMPLIFIER `I COMPARISON OSCILLATOR UNDER TEST - RN vows.. in0itl.lf 'S tu0844 (TRUE SINE WAVE) wttem. 9et -- ItCP. 'OSCILLOSCOPE FIG .3 TEST EQUIPMENT TO AMPLIFIER M 1 PRONOUNCED OUTPUT 3rd- HARMONIC RCP CONTENT RI ______.- T f l 2 MULTITESTER IIIII .-- AC -DC FIG. 4 Model 446

$995 50wC SO eC IND I F CHANNEL LIFIER R Five range DC voltmeter 0 5/50/250/500/2500 Four range DC milliammeter 0 /1 /10 /100 /1000 DC Ammeter 0 /10 y Four range AC voltmeter 0 /10 /100 /500 /1000 Three range ohmmeter 0 /500 /100,000 /1 Meg. Four decibel ranges -8 to 15/12 to 35,26 to OSCILLATOROSCILL RIOT '4).1fr'*sllIlliiiaa 49 32 to 55 INDICATOR Output meter-same as AC voltmeter Wan FIG.6 Actually, it's the equivalent of 25 different instruments in a single case! Professional appear- A F TEST AMPLIFIER HIGH -PASS ATTENUATOR ance. quality and performance put Model 446 in - UNDER TEST FILTER class with testers selling for twice its cost. OSCILLATOR You'll find BIG VALUE -LOW PRICE a feature VT.VM of all RCP Test Equipment. FIG 7 RCP'S COMPLETE LINE is described in Catalog No. 124. Send o postcard for your free The dial copy today. ing the various harmonic voltage magni- quency of 50 kc. may be produced. tudes. The instrument is tunable to the controlling this oscillator is graduated in fundamental and any of a series of its the frequencies admitted by the input harmonics separately, so that waveforms of channel. considerable complexity may be investigated. The fixed -frequency 50 kc. I.F. channel At the same time, measurements of hum is extremely sharp, containing 3 quartz RADIO CITY and noise amplitude are made available. In crystals and is preceded by a balanced CO., INC. effect it is a highly selective electronic volt- modulator, the output of which contains the PRODUCTS meter. upper and lower sidebands obtained from 88 PARK PLACE NEW YORK, N.Y. The representative wave analyzer (Gen- the heterodyning process. The carrier is eral Radio) receives the signal to be in- suppressed. The superselective I.F. channel spected through an input channel embrac- is followed by a 50 kc. amplifier and the BASIC ing an input multiplier, amplifier, and pad indicating instrument. (see the functional block diagram of Fig. In operation, the wave analyzer is tuned RECORDER 8). The frequencies accepted by the input to the fundamental test frequency and AND PLAYBACK channel lie in the range 20 to 16,000 cycles/ then to the successive harmonics to an ex- second -the entire common audio- frequency tent determined by the amount of frequency spectrum. A local heterodyne oscillator tuning range between the fundamental and stage supplies heterodyning voltage of such the 16 -kc. limit of the dial. The harmonic frequency variation that throughout the amplitudes are indicated directly by the signal input range, an intermediate fre- meter.

BROADCASTING Made by nationally known manufacturer. Has Now that station WLW has been booted A suburbanite solved the problem of how Astatic L-26 -W crystal playback pickup. Shure of first place among high -power stations to get the numerous birds in his local 9614D cutting head. 4.2 ohms impedance at 400 out cycles. Cuts up to 10 ". Plays 12" records. Mounted of the world, by being denied the right to woods to sing, mornings. Turning oh, full on steel plate 15" long and 10" wide. 115 volts put 500 kw. on the air, Mexican station blast, a portable radio set tuned to WOR's 60 cycles. XEW probably will move in with an an- Sunday morning airing of the "Singing 495 tenna power rating of 250 kw., according to Canaries" did the trick. 1 F.O.B. Broadcasting magazine reports. This Mex- AUTOMATIC RECORD CHANGER $12.95 ico City station will lay down a very strong 115 volts 60 cycles signal over a wide territory on 900 lie. UNCUT sometimes takes time COMBINATION AN CONSOLE (after shifting from 890 kc.). The question Radio broadcasting B of super -power apparently will not take out to play the part of a Good Samaritan. WRITE FOR LATEST FOLDER "no" for an answer, what with F.C.Com- Last month, for example, KDKA learned missioner Fly admitting that power in- the latest word on a situation which had creases have been considered from the developed from a broadcast of several LAKE RADIO SALES CO. all started when Mrs. ILL standpoint of affording better national months previously. It 618 W. RANDOLPH ST.. CHICAGO. coverage as aid in the defense program. In Jean Bichier, a French refugee living in this connection it is interesting to note that Rock Ridge Baths, Virginia, wrote to Mr. RADIO SERVICE EXPERT N.B.C. figures that the beam operation of Ralph W. Harbison, president of the Na- LEARN AT HOME IN WARE TIME 50,000 -watt international broadcast stations tional Council of Y.M.C.A. whose program WNBI and WRCA, both in Bound Brook, had included a detailed description of life The demand for radio experts grown 500 -kw. war in Germany. Mrs. every day. Learn this paying prMessIOf N. J., makes them equivalent to among prisoners of Under personal guidance of qualified en- non -directional broadcasters. Bichier asked Mr. Harbison whether be gineer and educator. Clear, fascinating instruction and experimental kits make learning cosy. could find out anything as to the where- Leaders In the radio industry endorse R.T.A. methods. could you spare one of abouts of her husband, her inquiries in Spare time soon for training. Mr. Winchell, profits pay your orchids for Dave Elman's priceless other directions having been fruitless. NOW IS A FINE TIME TO START He really does bring Well, what with the combined help of the New radofatr "Contact" program? ilcoiese a n everyrncl branch o f U. S. Service.ire together business people, friends and fami- Y.M.C.A., and the International Red Cross, OpIn for advancement almost limitless. Mrs. Bichier's husband has been located in VrIIe uMaT for romTIleIC Infnnnatlan d radio lies that have made great efforts and even FREE Mnk 10)0011 what R.T.A. trnin logea,, doforyou. spent fortunes in unsuccessful attempts at a German military camp, and letters are RADIO TRAINING ASSN. OF AMERICA now being exchanged. 2559 D Ave. Dept. RC111 Chicago, III. coalition. 658 RADIO -CRAFT for MAY, 1941 A BATTERY MERCHANDISER EASY THEN.U. t JUST SELECT WAY? HOW ThE EQUIPMENT DOES THAT 10OwANT-ANThIND Sells More Batteries WORN? ,RN`E -PUT UP m3.8EPOS{T-AND ,py FOP IT BY BUYING \.1 PRODUCTS Lets Your Customer See Battery's Condition

Set Switch foe Voltage Read condition of Battery tcin trade on percentage of useful life meter from drug stores Made by TRIUMPH and For Counter use or Can filling stations Be Mounted on Wall Ohl AKE MORE MONEY A $.00Delivery made $800 deposit and at once on order for $10.00 worth of batteries, dealer prices. Deposit refunded on purchase of 430 point.. All purchases of N.U. tubes, with N.U. BATTERIES condensera and batteries count u points. sold exclusively to Radio Service Dealers N.U. brings you a line of replacement batteries on which you can make your full radio service profit. It is not The N.U. line of radio replacement batteries has been necessary now for you to test sets and install batteries with- developed exclusively for radio service specialists. All out adequate compensation for your time and knowledge. popular types are included. Batteries are attractively pack- N.U. has been identified with the aged in rugged boxes incorporating the standard N.U. radio service dealer and his problems since the beginning. You can definitely color scheme -black and two shades of green. make more money handling N.U. products. N.U. Batteries are carefully manufactured from the finest materials obtainable and are fully guaranteed against defects NATIONAL UNION RADIO CORPORATION 67 State Street, Newark, N. in workmanship and materials. All types equipped with J. RC Gl standard plugs for quick and easy replacement installation. I am interested In your new Battery Meow chandiser calling for only $8.00 dpod% Please send moro Information. Thoroughly moisture proof for satisfactory and reliable Please have salesman call. service under all climatic conditions. Name Install N.U. replacement batteries for more hours of reliable service and satisfied customers. Street Address

RADIO -CRAFT for MAY, 1941 659 SOUND

HOW TO MAKE DYNAMIC TESTS ON AUDIO AMPLIFIERS This article answers all the elementary questions of how to make gain (amplification) measurements, derive fre- THE principle of "Dynamic Servicing" quency- response curves, and check for distortion and its is as important to sound amplifier prac- to tice as it is to receiver testing. Only a location, and quickly detect resonant points (due loud- true dynamic test will indicate func- speakers, loose components, or housings) in audio -frequency tional disorders in an audio system. When amplifiers. an amplifier is checked under actual operating conditions, the quantitative data obtained give a true picture of performance TED LADD and bares defects in design and components that no other type of inspection could reveal as quickly and definitely. A voltage indicator with exceptionally good preferably in a range that is continu- A full dynamic amplifier test, such as will frequency response is employed in tracing ously variable. The output voltage char- be described in this article, is in every re- the signal through the various circuits and acteristic of the oscillator must be Lao - spect identical with the system of engineer- circuit branches, and the system closely re- stantially flat. Most commercial ia tru- ing measurements performed by the audio sembles the familiar signal- tracing channel ments will deliver an output voltage design engineers and production checkers analysis which is the foundation- of dynamic (determined by the setting of the output who create amplifying equipment. A com- receiver servicing. attenuator) which does not vary more plete inspection of any power amplifier, Each channel of the amplifier system is than 1 db. or so between 100 c.p.s. and whether operating normally or below par, carefully checked to determine if the signal 10 kc. Above 10 kc. and below 100 c.p.s. will embrace each of the painstaking meas- is undergoing the proper amplification and there is usually a drop amounting to urements ordinarily made only in the fac- if it is being transferred efficiently from approximately 2 db. tory laboratories. The P.A. Serviceman who one stage tó another. Whether the signal (2) A good vacuum -tube voltmeter with learns and applies the methods of dynamic is present in circuit branches where it high and low ranges. This type of instru- audio testing will be able to offer bona fide should be, or detoured into branches where ment is recommended instead of the engineering service and will profit accord- it has no business, is thus definitely estab- more familiar copper -oxide rectifier type ingly. Various amplifier ills that baffle his lished. A common static test, such as the of A.C. voltmeter because measurements colleagues will no longer present a mystery measurement of operating voltages and cur- will be made throughout the A.F. range, to him. rents, cannot permit any such definite con- and the exploring voltmeter must, there- The man who masters dynamic testing clusions, although the static test may be fore, have excellent frequency response. will be able to offer a routine inspection used as a follow -up process to detect defec- Any frequency error possessed by the service to owners of every type of ampli- tive components after the dynamic test has usual V.-T. voltmeter is negligible in the fying equipment, a profitable service which isolated and identified the circuit disorder. audio spectrum. If the instrument has is invaluable in that it detects the breaking Aside from diagnosing trouble, the dy- not been provided with an isolated input down of tubes and circuit components some- namic test, of course, has value in the circuit, a mica fixed condenser of 0.01 - time ahead of actual failure. At the same checking of performance of amplifiers. And mf. capacity should be connected in series time, he will be equipped to inspect new while remaining unchanged in nature, it with the "high" input lead to isolate the audio equipment for prospective customers, becomes in this latter application the rec- instrument from D.C. voltage componerts appraising the worth of competitive units. ognized procedure for checking the tech- in the various high -voltage amplifier cir- His advice will be invaluable because it nical performance of an audio system. cuits. will be based . upon engineering measure- (3) A variable load resistor which may ments. REQUIRED INSTRUMENTS be adjusted in ohmic value to the rated In order to perform the tests, the sound load impedance of the output tubes or NATURE OF THE TESTS Serviceman will need the following items: output impedance of the secondary wind- Dynamic audio testing is easily under- (1) A good audio oscillator. This may ing of the output transformer. This re- stood. It is based upon the simple principle be either a beat -frequency oscillator or sistor will be connected to the output of supplying an audio -frequency voltage of one of the lately- introduced resistance - circuit in place of the speaker for most known level and frequency to an amplifier, capacity units, as the operator desires. measurements, unless otherwise indicated or any portion of an amplifier and following This instrument must be capable of cover- by the instructions. the progress of that voltage through the en- ing the entire A.F. spectrum; i.e., from (4) A cathode -ray oscilloscope with in- tire system to the output speaker circuit. 20 cycles /second to 20,000 cycles /second, ternal sweep oscillator and axis ampli- 660 RADIO -CRAFT for MAY, 1941 SOUND

fiers, or in its stead a distortion per cent meter. Most up -to -date service shops will have the equipment just listed included in their regular test gear. If each of these items is AMPLIFIER R1 not already owned, all should be purchased, AUDIO since no man advertising his skill as a OSCILLATOR P.A. Serviceman is stating facts if he Fig. 2. Block diagram of component arrangement RI C EZ claims to be able to give complete amplifier for making gain and fre- / / inspections without them. quency-response tests of A.F. amplifiers. s. R3 TESTS PERFORMED In the dynamic inspection of any amplifier, the following tests are made: e, METER (1) The audio signal is traced through CHANGE- OVER the entire circuit, observing its presence, SWITCH loss, or character as it passes through FIG 2 V.T. V M successive audio- frequency stages. (2) The gain of each stage is indi- vidually checked and recorded in voltage on the 1 -volt range when the oscillator is With the test probe at 7, the condition or decibel units. set to 400 cycles. It may be necessary to of the coupling condenser between the plate (3) The gain of the entire amplifier is insert a calibrated potentiometer between of V2 and the primary winding of T1 will measured and similarly recorded. the oscillator output and terminals 1 and be checked. Here the voltage should not (4) The distortion percentage of the 2 in order to reduce the input voltage differ greatly from that observed at 6. entire amplifier or individual stages is to a low value to prevent blocking of The voltage is noted carefully at 7. The checked, or (if the operator's equipment the amplifier stages. The V.-T.V.M. is low probe is then removed from chassis does not permit taking the actual per- then connected across the entire poten- ground, and connected to 9, while the high centage figure) the presence of distor- tiometer and the oscillator output adjusted probe is touched to 8. Here, the voltage tion is detected qualitatively. to 1 volt. The voltage selected at this point indication should be somewhat larger than (5) The frequency response (fidelity) for application to 1 and 2 will be a millivolt that obtained at 7, the difference indicating of the entire amplifier, any of its chan- or less. the turns ratio of the transformer, Tl. nels, or speaker is determined. Allow the "low" V.-T.V.M. terminal to Voltage readings taken between 8- and -12 (6) The condition of each transformer remain connected to chassis ground, such and 9- and -12 should be identical if the is determined as a routine matter. as at terminal 2, and explore the circuit center -tap of the T1 secondary winding is (7) Resonant points are localized in with the "high" lead, which should be shield- correctly placed and there are no shorted speaker, chassis, cabinet, or attachments; ed along with the test probe for this oper- turns in the secondary winding. ation. and - The meter probes are now transferred Touch the exploring probe to 1 -A. (8) Actual power output is measured If points 10 and 11 where a much increased the input condenser is not open, to with minimum signal voltage necessary a reading voltage should be observed if tube V3 is will be obtained on the meter. If, however, to maintain this output. amplifying. The same check may then be the input signal is very low (a millivolt or made on tube \'4 by connecting the meter The manner in which the various tests less) there is apt to be no reading. and are made will be considered and explained between 11 and 13. Slightly dissimilar read- the attenuator in the oscillator may be tem- ings obtained here may indicate unmatched separately in the paragraphs which follow. porarily advanced sufficiently to cause a tubes or a displacement of the T2 center - readable deflection. If there is still no read- tap. SIGNAL TRACING ing, the input condenser is open. The at- With meter probes at 14 and 15, one -half Figure 1 illustrates a conventional am- tenuator in the oscillator should then be winding returned to original of the secondary of transformer plifier circuit which will be used in our the test position. T2 is inspected. The ratio of voltages ob- The probe is then moved to point 3 which various explanations. This is an arrange- tained at 10 -11 (or 11 -13) and 14 -15 in- ment commonly found in the field and it is the plate terminal of the first tube, \'l; dicates ti-primary to ti-secondary and if is the embraces a high -gain pentode input suit- this tube operating, the signal will ratio. The half of the secondary be Moreover, since turns other able for high -quality, low- output micro- present at this point. winding is checked with probes amplification has place within the meter phones, a triode voltage amplifier which is taken the 15 and 16. the voltage should be considerably at of the shunt -fed plate variety, a push -pull tube, There is not a great deal of voltage gain triode "driver" stage, and a push -pull power higher than at 1 or l -A. in a power amplifier stage, however the A check 4 will output stage employing high -quality triodes, at indicate whether the probes may be transferred successively to coupling condenser between the plate of such as the 2A3. Class A operation will be 17 -18 and 18 -19 to determine if the output V1 and control -grid of V2 is operating. assumed throughout. Degeneration is intro- triodes, V5 and V6, are operating. duced through the expedient of a special "No signal" will indicate that this con- winding on the output transformer which denser is open. If the signal is present, its The meter probes are then transferred connects back to the return lead of the voltage level should not be decidedly differ- to 20 and 21, these terminals being con- primary winding of the first interstage ent from that observed at 3. nected to the load resistor. The voltage transformer, Tl. For purposes of simplicity, Transfer the probe to 5 and advance the reading here will be considerable and may the heater and power -supply circuit wiring gain control in the control -grid circuit of be used in a calculation of the amplifier has been omitted. V2 to maximum. Here the signal voltage power output. The resistor has previously The audio oscillator will be connected to should be the same as at 4. Now, slowly been measured and this value together with the input terminals of the amplifier, tuned reduce the setting of the gain control, the V.-T.V.M. reading may be substituted to a suitable frequency, its voltage adjusted observing that the meter reading falls pro- in the power formula to determine the out- to a reasonable level by means of the portionately according to the taper of the put watts: vacuum -tube voltmeter, and the various control. If the meter should cease to read P =E' /R numbered points throughout the amplifier at any point at which the control is set, Where P is the audio watts output, checked with the meter for presence and an open in this resistor is indicated. Should E the V.-T.V.M. reading in volts, strength of the signal. there be no reading at all when the control R the load resistance in ohms. Connect the output terminals (high - is at maximum and the probe is at the impedance posts) of the audio oscillator to control -grid of the tube, then the lead RESPONSE amplifier terminals 1 and 2. Turn the at- from control arm to grid is disconnected, An amplifier's response is a measure of tenuator in the oscillator to zero. Discon- the arm is not making contact with the its fidelity or ability to reproduce faithfully nect the speaker terminals 25 and 26 and resistance unit, or the resistance terminal the music and speech delivered to it by connect the load resistor to these posts, at the "high" end has become disconnected. the microphone. The response test is very having preset the resistance to match the When the meter probe is placed at point important, therefore, in establishing the impedance of the winding connecting to 6, a larger signal will be observed if the quality of an audio system or in determin- terminals 20 and 21. The watts rating of triode V2 is amplifying. No signal, a signal ing the effect of defective or poorly designed this load resistor must be at least twice voltage of the same level as that observed parts and circuits. There is neither mystery the power output expected from the output at 5, or a signal weaker than that observed nor magic to a good response test. Lack stage. at 5 indicate faulty tube action. This may of information, more often than lack of Switch -on both oscillator and amplifier be traced by a static test either to a faulty apparatus, prevents the more frequent tak- and set the control attenuator so that the tube or plate or cathode resistors or con- ing of such data by P.A. Servicemen. vacuum -tube voltmeter temporarily connect- densers, loss of plate or heater voltage, Figure 2 illustrates the connections em- ed between 1 and 2 is only slightly deflected or defective socket. ployed when the amplifier response is in-

RADIO -CRAFT for MAY, 1 9 4 I 661 SOUND undistorted power output is obtained from the amplifier. The meter is switched back to the input circuit, the oscillator returned to zero frequency, and the stage is set for the re- CUSTOM -BUILT sponse test. The amplifier gain control is at maximum. /vew PROFESSIONAL The oscillator frequency is increased from approximately 20 cycles through the range to 20,000 cycles in small steps (the smaller the better). The input voltage to the amplifier is kept constant by readjust- RECORDING AMPLIFIERS ing the oscillator attenuator to restore the original V.-T.V.M. reading when the meter Designed by is on the input side. 1 A. C. Shaney At each new frequency, the meter is switched to the output circuit, its reading noted and recorded, whereupon it is switched back again to the input circuit. e. Now you can make records to equal After the entire frequency range has a , or exceed the performance of com- been covered, the operator should have large list of frequency readings and the mercial pressings with these new cus- corresponding output voltages. And if these tom-built recording amplifiers. Avail- two sets of values are plotted in a graph, able features include; response ± I db the result will be a frequency-vs.-output 20 to 20,000 cps, expander- compres- voltage curve, which is a response curve. sot, high and low frequency equalizers Ideally, this curve should be flat and hori- a and dialogue filter for each input zontal. The more closely it resembles straight line, the higher is the fidelity of the channel, separate monitor channel, di- amplifier. Pronounced dips or peaks at cer- rect current heater supply, plate cir- tain frequencies indicate accordingly high cuit meter, vacuum tube VU meter, or low response at those frequencies, condi- universal output for crystal or mag- tions which disclose accentuation or attenua- netic cutter, choice of number and tion of certain notes by the amplifier. Poor be type of input channels for microphone, frequency response may traced directly to bad tubes, poor components, improper oper- play -back, radio, dubbing, and talk - ating voltages, etc., by checking the fidelity back. Send your rough specifications of separate stages and transformers. or requirements for quotations. The foregoing inspection is one of the response of the entire amplifier. Individual stage response may be similarly checked L It 14 4 ;E [ - S Ligi by connecting the high -impedance output 17 W. 20TH STREET, NEW YORK, N. Y. of the oscillator to the unconnected grid - input circuit of the stage under test, connecting the vacuum -tube voltmeter so that vestigated. While the oscillator might be it may be switched between the input and connected to the amplifier under test output circuits, and proceeding as explained through an ordinary potentiometer, which in the immediately foregoing paragraphs. FREE! will be satisfactory in most cases, the at- If there is a plate -circuit transformer in tenuator shown will be much more desirable the stage, it must be included in the output HAMMARLUND for exact work. circuit by connecting the meter across its NEW "41" The vacuum -tube voltmeter is connected secondary. RADIO CATALOG to show the voltage across the entire atten- Individual transformers may be checked uator, which is to say El. Z1 is numerically separately by feeding the oscillator voltage The latest Hammerlund catalog with equal to the amplifier's input impedance. into the unconnected transformer primary complete data, illustrations, draw- The voltage delivered to the amplifier input winding, and arranging the meter to be ings and curves on the entire Ham - marlund line. Address Department terminals may be determined from the switched from primary to secondary. The RÚ5-1 for your free copy. equation: secondary voltages for a constant input (primary) voltage are then recorded for HAMMARLUND MFG. CO., INC. E,R, e, a large number of frequency settings, as 424 -438 Wert 33rd Street, New York City - 2(R1 ß-R2) already described. Where poor response is traced to 1 or 2 Since the vacuum -tube voltmeter must be stages, a static test may be applied to employed alternately to measure input determine the efficiency of their tubes and AMAZING VALUES voltage and output power, a single -pole, operating voltages, condensers and re- single -throw switch has been included to Model 14o. 90 Velocity Micro- sistors. phone epresents the finest transfer the meter from one circuit to the type of r microphone for all sand caublig other. The actual output voltage, from orehealrae gbnoaad which the power output is determined, is: GAIN address work. With all connections made as explained Output level Mini. 52 Da. In- E2(R3+R4) Fre- under signal tracing, it will be remembered put - direct to grid. e_ quency response 40 to 12.000 C. - that increased voltages were observed as P. 8. Completo ith 26 ft. R3 cable detachable plug. the meter probe was moved from point tu ut of circuit. is thebeu do of R3 plus R4 are equal in D.C. ohms to point through the amplifier It pao., unit., trum- voltages pela baffle., le. the impedance of the output transformer the ratio between these successive ...hick..hice ,offer winding across which they are connected that expresses amplification or gain. direct t und m derea amateurs. etc. et in series. For example, when the signal is 'fed into tow prices, Bend The oscillator is set at zero frequency terminals 1 and 2 (Fig. 1), a certain volt- lteueef ofd oiratr ur other item.. and the connections made as shown in age is read at 3, but a considerably higher LIFETIME CORP. uxs ar,a:es ,.sit, TOLE00,OHIO Fig. 2. The vacuum -tube voltmeter is then reading is obtained at 6, indicating that switched to the input position and the amplification has been provided by the LIGHT BEAM TRANSMITTERS oscillator attenuator set to deliver voltage tube V2 and its coupling components. The tai Musing. ratio of these two voltages (or their you chum. Kit at some convenient setting of the oscillator I photo Cell frequency (e.g., 400 cycles) at the same quotient) is an expression of the voltage for ..tss Nï, dr nia all diacre for `iniúáia ' 'i-essarry mplifiera, complete for 15.00. level used in signal tracing. The meter is gain, or amplification of the V2 stage. For Complete amplifier and lamp for receiving. $10.00. Dicta et amplifier with PF. n for Iving. o1s.00. then switched to the output position and, example if the point 3 reading is 1 volt un to e. he crud. ebea ranha, microphone, or radioi can he played over the beam. We with the oscillator delivering 1,000 cycles, and the point 6 reading 10 volts, the gain 'n$alto 0O poly wireless communicatorsiralkra at 60.00 per unit. These talk of the input voltage is adjusted until maximum of the V2 stage is 10. The gain expressed block or morero If the wire common. good only hile t Iv lasts. ELECTRONIC PRODUCTS CO., St, Charls,11L 662 RADIO -CRAFT for MAY, 1941 SOUND by the readings taken at 6 and 14 -16, however, is an expression of the gain of tubes V3 and V4 and also transformers T1 and XTENDED RANGE T2. Transformer gain is equivalent to the turns ratio. ar Starting at the output circuit of the COAXIAL SPEAKERS input of an stage amplifier, the gain per ALL WITH THE SUPERIOR PERFORMANCE stage may be taken throughout, finally reading the gain of the entire amplifier as INHERENT IN DUAL SPEAKER OPERATION the ratio of the actual volts (or millivolts) signal input and the volts output across RANGE CONTROL the load resistor. I 7HF SWITCHICM This new feature gives you finger -tip control of response, pro- DISTORTION vides instant adjustment of high We have assumed that all tubes in the frequency range for best results on every record, every program amplifier shown in Fig. 1 are biased to FM or AM. operate as class A amplifiers. This would indicate that all direct plate and cathode currents are constant even during actual signal operating conditions. Distortion may be detected, although its percentage may not be measured thereby, by increasing the signal input delivered by the oscillator and watching for a shift in plate current as indicated by increase in JAP -60. Professional extended -range high- the voltage drop across cathode resistors. efficiency model for broadcast and transcrip- The vacuum -tube voltmeter, set to read tion use. Includes Range Control Switch and D.C., is used in this operation and is con- 2- channel network. 15" size. List price, $70.00 . JHP -52. For high quality extended -range nected successively across the various Phono, FM /AM receivers. Includes Range cathode resistors by connecting between Control Switch and 2-channel network. 15' chassis ground and the points 22, 23, 24 size. List price. $46.50... JHP-50. For general extended -range application. Bridging and 27 (filament center -tap). As long as network. Accessory Range Control may be the input voltage does not exceed the value added. 15" size. List price, $38.50... JCP -40. necessary to establish full output at the New 12" size gives coaxial performance at low cost) Excellent replacement unit. Bridg- speaker or load resistor, there should be ing network. List price, $29.50 ... Write no increase in plate current or cathode for Catalog 120. voltage. For more accurate measurements of distortion and obtaining of the actual percentage, it is customary to employ a cathode -ray oscilloscope or Percentage Dis- tortion meter. The latter device is connected in the output circuit of the amplifier or stage under test, a sine -wave voltage fed into the input circuit, and percentage reading obtained directly. (The construction of the speaker or certain chassis attachments an inexpensive meter for such measure- tend to vibrate at their natural frequencies to influence Such ments may be described in a forthcoming under the of sound. resonant "LAB" issue of Radio -Craft.-Editor) conditions may be detected quickly by re- A storing the speaker connections (25 and pocket The oscilloscope is similarly connected in 26, Fig. 1), feeding the oscillator into the fit ,your the output circuits and a pattern of the amplifier input terminals (1 -2), carrying actual signal wave as it is delivered by the the oscillator slowly through the entire amplifier (or amplifier stage) is observed audio range, and listening carefully for on its screen. The percentage distortion pronounced intensifications at certain fre- may then be determined by studying the quencies. Speaker resonant points will gen- variation of the pattern from the true sine erally be of 500 cycles or lower, headphone wave. diaphragms at approximately 1,000 cycles, chassis parts and loose tube shields at RESONANT POINTS 2,000 cycles or higher, and cabinets of An otherwise faithful amplifier might heavy wood or composition materials at VCiS -' emphasize certain notes of the scale because 100 cycles and lower.

MODEL OILCAN FOR "INACCESSIBLE" PLACES 739 FOR those hard -to- get -at- places that need oil a hypodermic syringe answers the purpose. The needles can be had in any S9.99 length and if a fine one is used the oil may be squeezed out in any quantity. Contrary to general belief there is no restriction on the sale of these instruments. Watchmakers can use the instrument by A.C. D.C. attaching the needle to a glass tube only, without the syringe, and depending on VOLT -OHM- MILLIAMMETER capillary attraction to deliver a spot of oil Pocket Volt - Ohm - Mlliammeter with Selector wherever the end of the needle touches. Switch. Molded Cese. . Precision 3 -Inch Meter with 2 Genuine Sapphire Jewel Bearings. CHAS. W. GREEN AC and DC Volts 0.15.150.750.1500; DC MA. 0- 1.15.15 -I50; High and Low Ohm Scales. . Dealer Net Pries Including all accessories. $9.90 MODEL 738. . DC Paket Volt- OhmMilli ammeter. Dealer . Net Pries $7.50 . NEXT MONTH .. WRITE FOR CATALOG from A commercial laboratory, speaking the angle of Crystal Microphone design and construc- SECTION 516 COLLEGE DRIVE tion, comments on the article, "Modern Microphone Technique." which appeared in the February. March and April, 1941. issues of Radio-Craft. Don't miss this article! READRITE METER WORKS, Bluffton, Ohio

RADIO -CRAFT for MAY, I941 663 SOUND

Mr. Longley's mobile sound apparatus, preparatory to being set up h In 10 minutes two men can set up these sound projecors, as the author here describes. ULTRA- MOBILE P.A. SYSTEM Mobile sound systems incorporate features peculiar to this type of operation, as author Longley here discloses. Also described in this article on a gas- engine powered sound system are the means which may be employed to obtain more mobility than is usual for equipment of this type.

EDWIN F. LONGLEY

MANY times during the 3 years the high -gain circuits. This feature is absolutely 6 -volt electric fan was placed so as to force author was selecting, building, test- essential in a mobile outfit because of road a stream of air directly onto the engine ing and operating the Ultra- Mobile shock and minute vibration set up by the cylinder and thence out a 21 -in. long x 16 -in. Public Address System here described, A.('. power plant usually installed within high opening. he hnd hoped to find a really comprehensive a soundproof compartment built into the Insulating the truck frame from vibra- nrtscle, on an entire sound system, in which truck body. tion set up by the A.C. plant was accom- the selection, design and construction with Amplifiers without vibration -protected plished by floating the plant on 4 double some of the difficulties encountered and preamplifier tubes give forth all kinds of sets of springs. their solution were clearly explained. strange sounds when using the microphone An efficient, while ultra- mobile sound system is the truck is in motion. SOUND REPRODUCERS & PROJECTORS made up of 6 will principal units which In public address work delayed automatic In the selection of sound reproducers be taken up in order and the reason of (Indio volume control is also obligatory as the P.A. specialist has 2 selection with a few of the difficulties of so many speakers insist general types to pick on crowding the from, i.e., (1) the dynamic cone operation discussed. microphone and raising their voices to speaker; and, (2) the 6 -foot exponential shouting while become horn with a levels others excited driving unit composed of a THE AMPLIFIER and from side side metal diaphragm step to causing the sound with voice coil attached. The amplifier is the very heart of the output to vary greatly. We have had occa- We found sions when A.A.V.C. in combination with that although the exponential entire system; on its reliability, versatility driving unit gave fine, crisp reproduction and output fidelity depend the success or manual control could not keep the output of voice frequencies level constant. it was woefully lacking failure of the whole venture. in the low frequencies so essential to fine From my experience in the field certain In some locations, the output level can not readily musical rendition; thus in order to fully minimum requirements are necessary, such be heard by the control operator, utilize our amplifier's fine frequency curve, so some visual volume indicator must be as 2 independently -controlled high- imped- 4 Jensen A -12PM used. We type reproducers were ance microphone inputs, 1 phonograph in- prefer a milliammeter in the plate selected, facing forward by the pair equipped put, delayed automatic audio volume con- supply to the output tubes for this means with waterproof cones. trol, and a floating rubber set plate assem- a constant check may be kept on both In sound volume level and power tube condition. projectors there are 3 types to bly to protect the voltage amplifier tubes select from, namely: (1) wall baffles; .(2) from external vibration. 6 -foot exponential trumpets with driving The ability to mix the microphone input A.C. GENERATING PLANT units; and, (3) giant parabolic projectors with that of the phonograph while not The static A.C. load of both amplifier and with dynamic speakers. absolutely necessary adds to the pleasing record turntable total about 275 watts so a operation The point to take into consideration here of the system on some occasions 350 -watt, self- starting, 60 -cycle plant is is and is well worth the extra control. used. that a given amount of sound energy will fill only a number of cubic The fidelity curve should be This certain feet the finest single piece of apparatus is the of air and that the length of projection obtainable, i.e., ± 1 db. from 30 make work properly, as we found to 20,000 hardest to varies in inverse to the width of cycles at 50 to 60 with when a A.C. ratio the watts distortion of that fully -loaded generating angle of diffusion; in we have 216%, or less. This specification necessitated plant is operated in a confined space, such short, the a specially -built amplifier within question: what type of projector will best but the extra ex- as a soundproof compartment a cover pense is worthwhile especially when book- truck body, proper engine cooling is a very the average crowd? ing repeat business. difficult problem. Wall baffles which have a wide angle of In our search for an amplifier we were In order to obtain reliable plant operation diffusion, a little under 180 °, have a limited very much surprised to note how few am- a length of 6 -in. stovepipe was riveted to projection and are only suited to cover plifier manufacturers made any provision the engine flywheel shroud and extended small sidewalk gatherings. for eliminating external vibration which is to a port in the truck back. This pipe sup- Exponential trumpets on the other hand the chief cause of tube microphonism in plies cool air to the flywheel fan. Lastly, a project a highly concentrated beam of sound 664 RADIO -CRAFT for MAY. 1941 SOUND

an angle of diffusion less than wall baffles ONE 2 INCH SHORT COUPLING CUT IN but greater than exponential trumpets with FOR For Ideal HALF LOCHS a length of projection in proportion (about 600 to 800 feet). Two -10 -s rX XS. COLO`` ' When 4 ROLLED FOR HANDLES these projectors are set in pairs on 2 tripods placed either side of a speak- HOME RECORDING er's platform, their area of coverage nicely fits that of most assemblies, large or small. PIPE 44- For public speaking, 2 velocity micro- Y I phones wired in parallel and spaced 71 ins. on centers give fine results, as this arrangement allows the orator to turn dL from side to side thus adding to the natural- ( 1 -L ness of the program.

TRREE1 /2 "X 1 YR. A velocity model equipped with an "acous- o BOLTS, AND SIX -1/2" SQUARE NUTS tic compensator" is ideal for distant pick -up or for close -talking conditions. LOCKS MADE BY FILING A '/a I^ SLOT AND BRAZING At track and field events where an- I Np 9NÑ nouncements made STEEL PLATE THE RIPE T-ATTACHED TO HORN are from the truck plat- 3/16 "X IC" TAP IS MACHINED TO SLIDE OVER form a dynamic type is used as this type CENTER FOR 2^ PIPE THE ZINC., PIPE THREAD is much less susceptible to wind conditions than the velocity type. ULTRA -MOBILITY Ir lo' .9" 1 '{ it Four swivel horn mountings 3 LENGTHS OF 1 V2. PIPE." consisting, .. THE POINTS TURNED ON A LATHE ANO DRIVEN of a flange and a length of 2 -inch pipe fitted INTO PIPE ENDS with a quick -change winged lock, are bolted to the truck platform. Each horn has a Details of the method of locking together the horn assemblies. 2 -inch pipe T attached which allows the horn to be rotated in any direction desired, easily heard over a mile away but their and locked in position, or the lock may be angle of diffusion released and the horn lifted off. is so limited that a In bank of several is needed to evenly cover a order to properly handle rallies, dedi- small crowd. cations, band concerts, etc., the horns are set up in pairs on two 13 -ft. tripods. The Insist On The exponential trumpet although the tripods fold up and are carried from loca- most generally used is also the most ill - tion to location strapped to the platform in suited to ordinary P.A. is use as it totally between the horn mounts interfering in no Meissner Quality! unnecessary to throw sound over a distance way with the operation of the sound system greater than that over which the eye can for street advertising. Engineered for superior performance- utmost readily distinguish the speakers (orators, This feature of ultra -mobility (the tripods fidelity in recording - lifelike quality in repro- etc.). can be set up, and the horns transferred duction- designed for those who want noth- The final selection was 4 giant parabolic from truck top to tripods, by 2 men in less ing but the best! projectors equipped with well- damped backs than 10 minutes) will enable the wide - Outstanding appearance- motor -board and (necessary in order to use the microphone awake P.A. specialist to obtain many jobs arms in brown crystal finish -panel plates of on the truck platform). These horns have over competitors not so equipped. two -toned etched brass- modern, styled case with bronze fittings- sturdy leather handle- a professionally artistic ensemble! Complete -ready for use -5 -tube built -In INDEX TO "RADIO- CRAFT" ARTICLES ON SOUND amplifier -may be used as Public Address Modern Microphone Technique -Part I. The Photoelectric Phono Pickup (Sept. '40). system! Includes crystal microphone -mike Microphone (Feb. '411; Part II, The Micro- Constant Groove -Speed Recording (Aug. '40) and power cord pack into space beneath name- phone and Its Relation to the Amplifier 20-Watt Booster Amplifier -for Radio Receivers. plate. Uses powerful magnetic cutting head (March '41) : Part Ill, The Microphone in Low -Power P.A. Systems. Etc. and highest grade crystal pickup. Ample rec- Action (April '41). Resistors and Condensers for Use in Audio - ord storage space in removable cover. Home Recording-Theory and Practice of Frequency Amplifiers (July '40). Sound-on-Disc (March '41). Specialized A.F. Tubes (July '40). No. 9 -1045 Phono- Recorder, List. $59.50 A New A.F. -Drift Correcting, Direct -Coupled .P.A. System for Ball Games, Etc. (July '40). F.M. 24 -Watt Audio Amplifier, Part I (Dec. *Church Sound Set -up (July '40). '40) ; Part II (Jan. '41) ; Part Ill (Feb. '41). "Transitional" P.A. System (July '40). Smallest Radio Tubes! (Article includes hearing - A Triode Direct -Coupled 15 -Watt Amplifier - aid circuits.) (Feb. '41). with Non -Frequency -Discriminating Scratch "Fantasia" Introduces "Fantasound" (Feb. '41). Suppressor (July '40). Sound Engineering (Selected items from this "Enhanced" Stereophonic Sound (July '40). monthly department) : The Use of Dual Loudspeakers (June '40). 4 -Watt A.C. -D.C. Amplifier .(April '41). Speaker Matching Technique (June '40). 4 -Tube Parallel Push -Pull Beam -Power Out- Universal 25 -Watt Amplifier Operates from 6- put Stage (Feb. '41). Volt Battery or 110 /220 -Volt A.C. /D.C. Line Design Data for Bass Reflex Enclosures (or (June '40). "Infinite Barnes") (Sept. '40). Profits in Recording -Part I (May '40) ; Part Mixer Circuits (July '40). II (June '40). Hearing-Aid Battery Amplifier (April '40). 'Selling and Installing a Call System (June 25 -Watt High -Fidelity Amplifier, with 4- '40). Channel Electronic Mixing (March '40). The Latest Miniature Hearing -Aid Amplifier MOTOR TURNTABLE 8-Watt Direct -Coupled Amplifier (Jan. '40). (May '40). UNIT "Wireless" 3 -Tube Phono -Oscillator (Jan. '41). Designing a 60-Watt Direct Complete dual -arm mechanism for any installa -Coupled Beam -Power lion; similar to unit used on above Recorder; Recording Pointers (Jan. '41). Amplifier (May '40). powerful, constant -speed motor for 110 -volt, 60- 4 Thumb -size Funeral -Parlor Sound System (Dec. '40). Pentodes for Hearing -Aids (Ar- cycle operation. Heavy steel base; brown crys- ticle includes hearing -aid circuits) (May '40). talline finish. Recording - Playback Amplifier Has Direct - How One Sound Man Drums -up Business (May Low-impedance magnetic cutting head connects Coupled Output Circuit and Equalized 30 -Watt '40) . to voice -coil secondary of output transformer; Output (Nov. '40). Radio- Sound Studio in College (May '40). high- impedance crystal pickup. Furnished with Dynamic Phono Pickup for Both Lateral and Convention Sound System (Article includes de- complete instructions. Vertical Recordings! (Nov. '40). tails for constructing an infinite baffle.) No. 9 -1039 Turntable Unit, List . Calculating Impedance at a Given Frequency (May '40). $31.50 '40). (Nov. The ABC of Db., VU, Mu, Va. Cm and Sm How to Make a Versatile Miniature- Battery -Tube (April '40). Write Today for Free General Catalog Audio Amplifier (Oct. '40). How to Select and Place Sound Equipment- Address Dept. New %-Volt Tubes (Article includes hearing - C -5 Part I. Microphones (Jan. '40) ; aid circuit) (Oct. '40). Part II, Loudspeakers (Feb. '40) ; Part III, Place- "Balanced" Audio Spectrums (Sept. '40). ment of Equipment (March '40). Articles on Sound in Past Issues (An index to Selected from the series, "Case Histories of articles in issues prior to the issue of May P.A. Sales." '40) . "PRECISION -BUILT PRODUCTS"

RADIO -CRAFT I 941 for MAY, 665 SOUND

OUTPUT

G TRANSFORMER kw C. R1 R2 f et e+" c' PLUG-IN A.0 0.5- i volr- MEG. R- METER CI OUTPUT OHMS .05 TRANE- 'a+' / C2 MF. f FORMLR O R I -, MF AC 0.5- OUTLET MEG. OUTPUT 1MA ^ 2 TRANSFORMER METER VÁLTRa; DROP MEG.if MEGS. R2 05- '¿,' =-TO'EYE'TUBE OI- MF W.- X LINE OR V:T.V.M. 10 CONTROL MEG. VOLTAS FIG.1 FIG.2 FIG.3 FIG.4

ADDING AN "EYE" TO A. F. AMPLIFIERS - and *that Practical ideas The following article travels along an almost unbeaten path in that it describes a series of circuits designed to achieve additional useful functions from existing radio equipment with minimum additional apparatus.

HAROLD DAVIS

ALTHOUGH not generally known, an The usual practice is to tie these diodes nected across the resistor so that a copper - electron "eye" indicator can be added together when only one function is neces- oxide rectifier type of A.C. meter can be to any audio amplifier without adding sary. In this case they may be separated plugged into them to read the voltage drop a rectifier tube. This is because the and the extra element used to rectify audio across the resistor. The watts power con- audio signal is pulsating D.C. and not A.C., fed back from the following stage or even sumed by the load connected to the outlet and all that is needed to make it work an from the plate of the triode section. The is the voltmeter reading divided by 10 and "eye" is a little filtering. ratio of R1 to R2 as shown in Fig. 2 will multiplied by the line voltage. Referring to Fig. 1, if a 1- or 2 -meg. control the amount of signal applied to the Example: The meter reads 7 volts when control is used as a voltage divider, the eye rectifier. Signals from an entirely different a certain set is plugged in. Seven divided by can be adjusted to where it will just close circuit may be fed to this extra diode for 10 is .7, or simply one place pointed off to on the amount of volume needed to do a rectification, and will often eliminate the in- the left. Hence, .7 times 110 is 77 (watts). certain job (recording, for instance). Then stallation of a special tube for this purpose. If an extra A.C. meter is available, it may the volume control on the amplifier can be Like the preceding and following circuits, be wired -in permanently. If a low- resistance advanced to give this required output. Con- the diagram shown in Fig. 2 is a "current A.C. meter is used an error will result due denser Cl is the blocking and coupling con- saver" inasmuch as it affords additional to the parallel resistance offered by the denser and C2 is a filter. The cathode re- services without the expense of additional meter. sistor and condenser can be omitted, and tubes. By making the resistors variable they If no A.C. meter is available, a 6.3 volt the cathode may be grounded, although may be adjusted until best results are ob- pilot light can be interpreted with a little generally used in Tadio sets to control the tained, their values then may be noted by experience gained by plugging -in electric sensitivity of the eye. The arrangement an ohmmeter check and fixed units of the light bulbs of known watts rating and makes a handy output meter. indicated values substituted for the variable noting the brightness of the pilot lamp. The writer strongly recommends the use units. A set pulling 100 watts will cause a 10- of such an "eye" attachment as it makes volt drop across the resistor, which will tax possible much more precise adjustment of ADDING A METER INDICATOR the pilot lamp filament resistance, and oc- the equipment than is possible when employ- TO add a meter indicator to an audio am- casionally one may be lost due to burn -outs. ing merely audible monitoring. For example, plifier requires adding a vacuum -tube or If heavy power consumption is encountered, in audio circuits designed for reproduction some other type of rectifier. The arrange- it may be found advisable to install a 5-ohm of more than usual fidelity, the need for ment shown in Fig. 3 employs a duodiode resistor alongside the 10 -ohm unit and us^ such visual monitoring becomes increasingly with any old 0 -1. ma. meter in the cathode. a double -throw switch to select the desired apparent. We believe the circuit suggested The size of RI controls the amount of value. The watts reading, using the 5-ohm here will appeal to many experimenters and signal applied to the diode and should be resistor, can be found by dividing the meter sound specialists. kept as large as possible because the diode reading by 5 and multiplying the result Of course in connection with this circuit has very low impedance and will load the by the line voltage. as with the other circuits shown in this circuit. If too much loading is noticed, use The lamp or meter must be watched care- article, it is necessary to give due considera- only one of the diodes and ground the other. fully when the set is first plugged -in and is tion to the characteristics of the available Resistance may be added to the cathode in warming up, as a shorted condition in the power supply equipment and its ability to series with the meter to control the meter set will cause an excessive voltage to be im- deliver the requisite additional power. In action if needed. Condenser Cl is made as pressed on the meter or lamp. view of the fact that the circuit suggested large as possible without "bass -ing" the Note that in Fig. 4 a 10 -ohm resistor here requires an almost inappreciably -small amplifier. Resistor R2 may also be varied to would be connected across the line in the additional total watts power, it is to be give proper deflection. event of a shorted device being plugged into recommended for use where the associated the A.C. outlet. This suggests that both apparatus is being operated to close toler- SIMPLE WATTMETER sides of the lightline should be fused, to ances and where an additional rectifier SIGNAL TRACERS incorporating watt- guard against all eventualities. This is a under these conditions might result in un- meters have made that instrument popu- wise precaution to take in connection with stabilizing the voltages throughout the lar. Wattmeters will show up shorted power any test circuit that connects to a power system. transformers, filter condensers, chokes and line. rectifiers. In fact anything that causes ex- (Experimenters are also recommended to USING "EXTRA" DIODE IN DUODIODE- cessive drain on the power supply. A very the simpler add -on circuits described in the TRIODE efficient and simple wattmeter is construct- 2 -part article, "Servicing 'Orphans' and THE extra diode, when not otherwise used, ed as shown in Fig. 4. Private -Brand Radio Sets," in the March may be connected to perform an entirely A 10 -ohm resistor, with a capacity of at and April, 1940, issues of Radio -Craft.) different function than that of the other least 10 watts, is installed in series with the Subject to pourer factor correction, but close enough in a duodiode- triode. 110 -V. line and outlet. Tip -jacks are con- for comparison measurements. 666 RADIO -CRAFT for MAY, 1941 MODEL S -19R The Sky Buddy-4 bands, covers everything on the air, from 44 mc. to 545 kc., including the 10, 20, 40. 80 and 160 meter amateur bands. Electrical bandspread. Built -in 5" Dynamic Speaker. 6 tubes. Beat frequency oscillator. Built -in line filter. AVC. Send - Receive and band switches. The Sky Buddy gives superior performance with amazing ease of operation. 529.50

MODEL S -20R The Sky Champion represents the best value ever offered in the communications field. 9 tubes. 4 bands. Tunes from 540 kc. to 44 mc. Automatic noise limiter. AVC switch. Stand- by switch. inertia bandspread tuning. Sep- arate electrical bandspread. Beat frequency oscillator. Battery-vibrapack, DC operation socket. Incorporates all the essential control. for good amateur reception. $49.50

MODEL SX -24 An outstanding communications receiver value. 9 tubes. 4 bands with a frequency range of from 540 kc. to 43.5 mc. Band 1 -540 kc. to 1.730 kc.; Band 2 -1.7 mc. to 5.1 mc.: Band 3-5.0 mc. to 15.7 mc.: Band 4 -15.2 mc. to 43.5 mc. Frequency stability throughout a wide range of line -voltage humidity and temperature variations. DC operation socket- battery or Vibrapack. $69.50

MODEL SX -25 (not illustrated/ Embodies every worthwhile advancement that has been made in the communications field. Tunes from 540 kc. to 42 mc. in 4 bands. 12 tubes. 2 stages of preselection. Separate calibrated bandspread dial for the 10. 20, 40 and 80 meter amateur bands. 10" heavy duty PM Dynamic Speaker. 110 volt 50 -60 cycle AC MODEL EC -1 tion, opera- DC operation Rocket provided for battery or vibra -pack. Complete with speaker. All these important communications crystal and tubes $99.50 fea- ... tures at this sensationally low price. 3 bands covering from 545 kc. to 30.5 mc. -(550 to 9.45 meters). Electrical bandspread on all bands. Beat frequency oscillator. Self -contained PM Dynamic Speaker. 6 tubes. AC /DC operation -115/125 volts. Every necessary feature to give the shortwave listener and the amateur an up-to-the- minute communications receiver for only 519.95

MAIN STORE: CORNER 7th AND ARCH STREETS, PHILADELPHIA, PA. 5133 Market Street Philadelphia. 811 Federal Street 101 W. 6th Street Pa. Camden. New Jersey Wilmington. Oct. 1042 Hamilton Street 3145 N. Broad Street 9 N. 2nd St. Allentown, Pa. North Philadelphia. Pa. Easton. Pa. RADIO -CRAFT for MAY, 1941 667 SOUND

FROMq 125,1 Dt. Da E a ONIC 10 5, +ER AIXEMTON C MEG pMF- /\Se MF. ACLNryrun M t Tp p _._-_ u.l. _ 0 RECORDINGÑIi1p AiiExWiior SLOPE M RANGE . FITWAAWON/ R2 EG M .10 10000 L000 33,000 MF OHMS 50 1.000 1000010 FIG .6 -PASSIVE EOwIIZER NETWORK,. 05- - ONMS ONM5 p11A5 Oy FREOVENCY MEG 02 5. MEG. E MEG. 'Bi 250V Cf WEAL ygvErwE FIG 3 EowuzcR cx 1 /1 t M1- MF ' EG. \I-. I\Ir COMPRESSOR ió W '\/ \ tO y cG. 10 fo '-i* NORMAL ppp 0 0 0.5- EXPANDER EG. ' x6 +. MEGS FI j 7 --ACTIVE EOLVLL"ER NETWORK-. FIG 10 ...OPTIONAL EXPANDER-COMPRESSOR CIRCUIT-. 10 W TOW SWPLV BLOTMGHT TO 50 COOS 1.000 6000 104A OG 117 65J7 6v6G ONr 2SI. PRE- 2 FREQUENCY ANN4FIERr /MECS. FIG4WLRESPoNSEcFRF30NWrEpNL¢ER- o2- aw a F ,,,r,S. "5öò I 1 ió..t c. I/`,1// ü0 1+ 000 FG. ^. (L.1..) a. \ M1 MF / N 16

nOiEG. 1 , M[GS IIII e MF II T MEG. 5V.A

I -. 35V.AGqA M F 005oHN OHMS COM. t, OHMS ASOUARE WAVE WED FOR TEST - - FIG 5 BAFrtn Ir PASSES A RESONANT NETWORK FIG 11 N-1-HE D C'A' 5uPP25-2 FIG IS -.T}IE MONITOR CHANNEL--

/Xoc%tn eiteuit ríTeatutes 6 a Semi-/btotassional /O- Watt RECORDING AND PLAYBACK AMPLIFIER A technical discussion of an amplifier specifically designed to fit the needs of the serious- minded recordist, particularly one who has stepped out of the amateur class. Variation in output amplifier provides for choice of 2A3 or 6L6G output tubes.

A. C. SHANEY

NE OF THE most popular questions system capable of producing results equiva- received from the field is, "What type lent to the much more expensive studio O of amplifier do you recommend for equipment. recording, playback and Public Ad- It is to be understood, however, that this dress applications ?" Strange as it may amplifier is not intended to be constructed seem, no one amplifier can be a perfect re- from junk -box parts. The nicety of its over- cording, playback and P.A. unit because all performance depends upon straightfor- each one of the applications requires a series ward amplifier engineering and clean-cut of specific performance characteristics which mechanical design and workmanship. do not coincide with the other application. To expect one amplifier to operate perfectly for all 3 applications, is like expecting a THE AMPLIFIER IN GENERAL man to be a jack of all trades and a master Figure 1 shows a photograph of the am- of all. plifier mounted in a conventional rack. The In order to attain results comparable to system is composed of 3 essential parts. The commercial studios, it is obviously necessary top panel has the combined "B" supply, bias to use comparable equipment. There is no supply and D.C. "A" supply. The center short cut or skimpy route to accredited com- panel houses the amplifier, VU meter, gain mercial performance. To obtain real good control, plate current measuring switch and records there is no getting away from the VU range switch. The lower panel contains fact that a good amplifier is of prime im- 3 microphone channels, 1 radio channel, and portance. Naturally, all of the other inter- 1 playback channel. Each of these channels connecting components are equally impor- has its independent, slope -type high -fre- equalizer, low- frequency equalizer, Fig. I. The completed Semi -Professional Record - tant. A good cutter, microphone, turntable, quency ng - Playback Amplifier. Identification of the equip- pickup and tuner are also essential. and dialogue filter. ment on the 3 panels (reading from left to right): in addition, is a master gain Top Panel: "B" supply pilot and switch' "A" supply It is obvious that the cost of professional Included switch and pilot. Middle Panel: off-Ìp of triodes equipment would be considerably beyond the control, an individual monitor amplifier with Nos. I and 2; power level VU meter off-plus 30 reach of the graduate amateur recordist. A independent gain control, and expander - VU and 40 VU; (center knob) amplifier gain con- control with its associated trol. Bottom Panel: consider these controls in vertical careful study, however, of a number of compressor rows, the first 5 of which are channel controls, bot- professional recording amplifiers, which switch. A vacuum -tube VU meter in con- tom to top, as follows: channel volume control, have been built in our laboratories, clearly junction with a range selector, provides LF. control, H.F. control, dialogue filter switch. for accurate visual monitoring. For sim- Row 6: master volume control, monitor volume con- indicates the feasibility of eliminating some trol, expressor control, expander - compressor switch. of the non -essentials from the mechanical plicity of playback, pre -recording, and mon- The meter is a vacuum -tube VU type; underneath construction and thereby enabling the pro- itoring, a Recording - Playback - Monitor is the VU range selector switch; the 3 buttons under - switch is incorporated. Death read: Monitor, Recording and Playback. duction of a reasonably -priced amplifier MAY, 668 RADIO -CRAFT for I941 SOUND

INDEPENDENT vaTFGe rcE c,..N.,rL CHANNEL EQUALIZATION w,uMVMFrta To understand the elements and arrange- AMVLIFIER IE A ,. R 5 AM`r,ER LONS'áúL CI%jrEiel ments of the preamplifier circuit, a block .®. LON,RLL C ® ® m ELCROmF l ER diagram has been indicated in Fig. 2. It 0FLC will be noted that each channel has its own set of high- and low- frequency equalizers. C `L IMI ® 12'. ® I79 <,4á f RELT, This arrangement enables ideal equalization for any type of microphone, pickup and c NNEL IC= w.Liáä CEI ® ® m ._ EóMA..T°soR R radio program. It also provides for special --CO. 3. r sound effects equalization and unlimited AN flexibility, particularly when mixing voice CwÚn2L ® n21 ® m nmvÓl while dubbing. Slope equalization is differentiated from RADIO ® CA...l iaoL the more common, resonant -type equalizers, ® m ® m in that it provides for a degree of accentuation or attenuation proportional to fre- rOR Mów `;aR MON ,TA° W pÁIVfR quency at the high- frequency end of the SpwPEAKER GE audio spectrum, and inversely proportional FIG 2 "BLOCK DIAGRAM OF PREAMPLIFIER AND MONITOR AMPLIFIER K. to frequency at the low- frequency end of 1620 I:SJT '.'3- 12SJT DFSw Mpy the spectrum. A typical slope equalizer re- i ' M-F' J sponse characteristic is indicated in Fig. 3. IFIáuROPNONE I. ,0ni 1 M-5is It will be noted ' f6CBG o00 - that all settings of equaliza- /y y 5ao _-.....+ - / tion within its operating range provide for I li t i , , 1 a linear slope response. A resonant equalizer, however (which is indicated in Fig. 4), usually provides for broad or sharply -tuned resonant response E1 G I':R9Í curves. The latter arrangement is subject 32- , to what is known as shock excitation where- Mo reé o5MS . T. in sustained oscillations occur, when a tone, os-

RADIO -CRAFT for MAY, 1941 669 SOUND

DC. AC NOTE TO MOVABLE ARM ON OHM RECTIFIER BLANK CONTACT BUTTON METER 'Phi 'RA 3.000 1.000 LINRp2pp BM OHMS OHMS ÓIMS EFFICIENCY is the Magic Word MI=E111111M... The Reason Whv a Shot. r - - 1 UNIVERSITY REFLEX t HIGH EFFICIENCY 1 7 i40V I SPEAKER 1 I does the work of three 1 O I ordinary low efficiency _I r --J speakers. Ch! Rh TO I OHM CENTER - UNIVERSITY TAP Of ram RI 2A3 5000 Reflex Speakers have FILAMENT OHMS highest possible effi- WINDINGS ciency, far greater than any other type of loud FI G.14 -POWER LEVEL VU METERLACROSS CuTTER- OURING TESTS ONLr)' speaker. UNN for nest. eatatoea. sheet. a very- high -powered cutting head (which UNIVERSITY LABORATORIES where P = Power output after equaliza- 195 Chrystie Street tion at equalized frequency should handle up to 100 watts). New York City A = Average output power before equalization OTHER CIRCUIT FEATURES db.B = Decibel boost at equalized The Input Channels- Figure 8 indicates frequency circuit diagram of one of the 3 microphone As an example, if the average level fed to input channels. It will be noted that elec- a cutter is 2 watts and a 12 db. boost is set tronic mixing is used for mixing 5 channels. at 10,000 cycles, then The Vacuum -Tube VU Meter-In order 12 to monitor the output of the voltage ampli- P = 22- = 224 = 2.16 = 32 watts fier without introducing any distortion, a AUDIOGRAPH 3 special type of vacuum-tube VU meter has Sound Catalog Under this seemingly possible boost of been developed (Fig. 9). This VU meter has 12 db. 32 watts will actually be fed into the as by AUDIOG ballaatic characteristics standardized APH Sound Systems repre- cutter. Under this condition, the average N.B.C., C.B.S., and Bell Labs. Its essential sent the atest in Sound Engineering recording amplifier usually overloads or the difference, however, is that it has a rela- and Desig -a complete line plus pre- cutter is damaged. A high boost at the tively high impedance input and does not tested accesso high- frequency end of the spectrum for the introduce any distortion upon the line to inside of 83 1/3 r.p.m. records is usually which it is connected. It furthermore con- NEW LOWE °RICES requested only because the "highs" seem sumes no power. A careful analysis of Outstanding quality and ,. erform- to fall off excessively. Ex- the recording processes employed will dis- The Automatic and Complimentary once of prices that help you eet any - Compressor some types of re- close why these highs are lost excessively pander -For competition. frequency boost cordings, it is desirable to have an auto- and also why excessive high- a con- may not alter the detrimental condition. matic compressor so as to prevent WRITE FOR FREE OPY tinual "riding of the gain" control. As such JOHN MECK INDU RIES a compressor is used, it is obviously de- 1313 W. Randolph St., hicago, ILL RECORD SPEED sirable to be able to play the record back The surface speed of a record may be with a complimentary degree of expansion calculated from the following formula so that the final program will be a close r.p.m. duplicate of the original material. Such an DATAPRINTS Ss - 2iR expander - compressor is illustrated in Fig. -60 10. A switch is provided to cut these fea- TESLA -OUDIN COILS s ifk where Ss = surface speed per second tures out of the circuit for all normal re- 20e Ea. in order for 10 r.p.m. = revolutions per minute cording. (Data and Drawings only.) R = radial distance from center of The Preamplifier "A" Supply -In order 86" Sp'k Tesla -Oudin Coil 40e needle. (1 KW. Exc. Trf. Data fuel.) turntable to cutter to keep hum well below the threshold of 8" Sp'k Tesla -Ondin Coil 40e At the outermost edge of a 16 -in. record, hiss, a special "A" supply (Fig. 11) is in- _"I Exc. Data incl.) -- (l4 K.W. Trf. the surface speed is approximately 28 ins. corporated into the amplifier so that all the 8" Sp'k Ondin; 110 Vt. per second. When a 10,000 -cycle note is = -- 'Kick Coil" type 40e preamplifier, voltage amplifier, equalizer and - 8" Tesla Works on Ford impressed, there is approximately 2.8 thou- electronic mixer tubes have pure D.C. ap- V Coil 40e sandths of an inch between peaks. With a fea- 1' Sp'k Vibrator Ili -Freq. Coil 40e plied to their respective heaters. This 2 -in. radial distance, however, the surface ture reduces hum by more than 12 db. and NEW ! 5 ft. Sp'k. Oudin Coil & Exciter Data 75e speed is approximately 7 ins. per second, is essential for obtaining humless records. a 10,000 has '7/10,000 and -cycle tone nearly is a minimum by careful PIPE & Hiss kept at Induction of an inch between peaks. selection of plate loads and grid resistors ORE LOCATOR be obvious that unusual pre- It should so a maximum gain -to -noise ratio is cautions must be taken in cutting- needle that Induction Type. Data 401 maintained in each stage. It should be noted design in order to prevent successive wave - Radio TYPO 40e that the non -microphonic 1620 triple -grid fronts from cutting into each other. It stage higher amplifier is used in the preamplifier More DATAPRINTS 40c each! should be borne in mind that the of the microphone channels. 6 Meter Superhet. Electric Refrigerator the frequency the steeper is the wave -front, 1h Meter Tr. & Rec. Wheatstone Bridge and the slower the surface speed, the steeper The Amplifier- Triode Versus Beam Power 20 A.C. Probe. & Ans. Weld. Transf. 2 H.W. is the wave -front. -Recording technicians can broadly be 20 Telephone Hook -ups Rewinding Armatures which 100 Mech. Movements To add to the complexities, at high fre- classified into 2 general groups, one of 20 Motor Hook -ups String Galvanometer quencies, the cutting needle usually pushes believes in triode output tubes only and Television Hook -up 20 Simple Bell Circuits the other favors the beam power type. In 20 Elec. Party Tricks Steel Wire Recorder! the material from side to side instead of Solenoids and Magnets Water Wheels cutting it. Naturally, after the resilient order to satisfy everybody it was decided to -get list. Water Turbines material has been pushed, it will tend to flow make available 2 types of recording ampli- Fry Eggs on Ice! Photo Cell and Relay fiers so that either triode or a beam power Experimental Photo- back into its original position. This in turn Ring 4 bells: 2 Wires an high- frequency loss. unit could be selected! The characteristics Phone 20 Tesla Tricks causes additional Radio Control for This latter loss, however, is independent of of both amplifiers are as close as possible Models Polarized Relay so that one may be substituted in place Diathermy Apparatus Induction Balance radial distance and can therefore easily be Inductor Organ Thawer compensated -for by one fixed setting. of the other without making any changes in Electric Pipe the rest of the equipment. The discrepancies caused by variation of Special Prices: 4 prints $1.00; 10 for $2.00: distances can only be compensated - Single. 40e each. Get New Catalog 100 A. radial for when unusual design features are in- COMMON FEATURES OF TRIODE AND The DATAPRINT Co. corporated. For example, a very -high- BEAM POWER AMPLIFIERS are equipped with a VU Lock Box 322D, Ramsey, N.. J. powered recording amplifier should be used Both amplifiers (possibly up to 100 watts) together with meter so as to read the actual level being 670 RADIO -CRAFT for MAY, 1941 SOUND

delivered to the cutter (Figs. 9 and 14). Suitable switches are provided to change the YOURS LOOKING IN T+IE WRONG range of the VU readings. An additional pushbutton switch is provided to read the PLACE, DOC. IT H URTS plate current of the output tubes, so that balance could be easily maintained through RIGHT HERE/ suitable adjustment within the amplifier. Both amplifiers are equipped with a universal output transformer to match any magnetic or crystal cutter. An auxiliary thermal relay is incorporated in the plate supply circuit to avoid the application of plate voltage before correct bias has been attained. The 2A3 Amplifier- Fixed -bias has been used with these tubes so as to provide the lowest output impedance. A 6F8G acts as a push -pull driver which in turn receives its signal voltage from a 6C8G equalized inverter. (See Fig. 12.) The Beam -Power Amplifier -This particular unit incorporates a conventional 2 -stage direct- coupled push -pull circuit utilizing two 6SJ7s as balanced drivers which in turn is fed by an equalized 6F8G inverter. (See pg. 352, December, 1940, Radio -Craft.) Here is a definite and direct The Monitor Amplifier -A separate mon- procedure for servicing a itor amplifier (Fig. 15) is provided for mon- Information which every Serv- itoring purposes. One of the most common errors made by recordists is to use a speaker iceman has needed but which or headphones across one of the windings has not been published until of the transformer feeding the cutter. This now. Clearly written, easily invariably introduces additional discrimina- understood, with many dia- tion because the impedance of the headphones or speaker does not remain constant grams and illustrations, it throughout the frequency range. In addi- proves this new system of tion to this, a true aural picture is not analysis the natural way to do presented to the operator because the cutter a good job. impedance changes with frequency so that low frequencies may seem to be lacking in Now you can make a radio the monitor speaker. For apparent reasons, talk to you, just as a patient it is best to use an entirely independent am- talks to his doctor, "You're plifier to drive any type of speaker desired. looking in the wrong place. This, too, is standard practice followed in the design of commercial professional equip- Doc -it hurts right here." ment. The Supreme Instruments Corporation has been manufacturing good MECHANICAL FEATURES instruments for the Servicemen for the last fourteen years-they The amplifier is completely housed in a are, and, we believe, they will sturdy standard rack, and is designed for always be your best investment. But desk type operation. Under these conditions, here is information YOU NEED-and need badly! The quickest all master controls are easily accessible way we can get this to you is in booklet form.... It is yours for and may be operated without undue fatigue. only 35c to cover cost of printing and mailing. Separation of the power supply from the amplifier and preamplifier further decreases Send coupon today, or buy from your favorite parts jobber the possibilities of induced hum. 1 11MIN MM 111M INZUMZEMME1111 MAI YOUR The recording system may be extended ORDER SUPREME INSTRUMENTS CORP., Dept. RC., Greenwood, Miss. by adding additional recording amplifiers Gentlemen: ..ok s should it be desired to make 2 or more I enclose 35c. Please send me your new book described above MI records at once. NAME While this particular amplifier has been ADDRESS specifically designed for recording applica- TODAY el tions (which involve the most stringent re- CITY _.__._...._,.._.._...STATE MI quirements) it is naturally admirably adapt- el ed for exceptional playback. Its application if- . X111 111.111111111111tMIIII to P.A. work is, of course, limited by its power output (10 watts) but it is recom- in overseas reception by putting into prac- Amazing New "WAL5CO" mended where unusual results are desired. tice the tips that are given despite the fact that the descriptive material is built around STAPLE DRIVER This article has been prepared from data a particular product of one manufacturer. supplied through for red :. g .g. M end the courtesy of Amplifier n At the same time, bookleti cleverly cable ,.11e, .ahn saw. ball M. this Company of America. Wien, can b. ,tpkd .n etymon, beb..d p.pa :Me shows the necessity for certain controls " bl. .eh . Femme. and erdlnry , .plr,. Dd.o d.pM, which considers into lh. brick. m .. M.g.0 wails a.,emahcally and Hallicrafters essential in hloldholds p, of ,,. ,l., .14M cwn. in ...rnl colon. Ad year ¡abbe, their newest shortwave receivers. for d.meedratim and whbg.eI -Roi.1 WALSCO PRODUCTS "A SHORT STORY ON SHORT- or write WALTER L. SCHOTT CO. 5264 W. Pko Boulevard, Los Angele., California WAVE RADIO RECEIVERS" Meissner I.F. Transformer Replacement Under this title, and the subtitle "Things Manual. -This 254 -page, paper -cover book You Never Knew 'Til Now," Hallicrafters of course is intended primarily to help sell For WIRED MUSIC OPERATORS presents 12 "Dispatcher" Leaves in a -page booklet released last the company's products. It is a handstpfreee. so dielectricL pleasant co- Arj. fabric neck -band. Choice of month considerable information of interest incidence therefore that is also single or double high low it inform- impedance. Available total f 12 die to the shortwave listener. ative reading matter, as compared to merely typa and imp. Incl. 7u ft. flex. rd. The Wt. a oc. Most attractive. l!ghtweioht majority of radio listeners who own a catalog, in that it includes a virtual course, rugged. chest mike yet de- receivers vised for of race" capable of shortwave reception in brief, on the design and installation of At your dealer jobber. will find after reading interesting this book- I.F. coils of all types; detail circuits eluci- UNIVERSAL MICROPHONE CO., LTO. let that they are able to get better results date the text. INGLEWOOD, CAL U. S. A. RADIO -CRAFT for MAY, I 9 4 I 671 Order your copy NOW the only

P. A. HAND B 00K A Resume of the Contents of the AMPLIFIER HANDBOOK AND PUBLIC ADDRESS GUIDE FOREWORD INTRODUCTION Definitions -decibels, frequency, input, output, impedance, etc. SECTION 1- SOURCE Carbon microphones (single- button and double- button) Condenser microphones Velocity (ribbon) microphones Dynamic microphones Crystal microphones (sound -cell types, crystal diaphragm types) Cardioid microphones Contact microphones Phonograph pickups (magnetic types, crystal types) SECTION I1- AMPLIFIERS Voltage Amplification Design of resistance -coupled voltage amplifiers Commercial voltage amplifier The Power Stage Class A amplifiers Class AB amplifiers Class AB,. amplifiers Class ABU amplifiers Class B amplifiers When to apply class A, AB, and B amplification Power Supplies Half -wave rectification Full -wave rectification Voltage doublers Filter Circuits Power supply regulation, etc. Practical Hints on Amplifier Con- struction ONLV Microphonism Placement of components Tone compensation Inverse feedback Remote control methods SECTION III- DISTRIBUTION The Loudspeaker Dynamic speakers Speaker performance (frequency 25 response, efficiency) High -fidelity speakers Speaker Baffles and Housings CENTS Outdoor speaker installations MOST COMPLETE Power cone speakers A COPY Radial (360° distribution) speaker AND AUTHENTIC baffles SECTION IV- COORDINATION P.A. BOOK PUBLISHED Input impedance matching P.A. installa- HAN book has yet been published which covers A MATCHLESS VOLUME Matching speakers to ...amplifiers and systems ( Lems As complete as you would expect to find any engineer- tions ic volumeu is almost ing handbook -this Is how the radio or P. A. an believable.eYet. It Is a fact. there Is no book In finds the AMPLIFIER HANDBOOK AND PUBLIC AD- Phasing speakers print which covers Puhlie Address from A to Z. SSSSS GUIDE. technical fdata compiled To bridge this wide -sead gap. RADIO-CRAFT ill sources. Effect of mismatching speakers to complete. Public from With essential publiree: of volume ova nearly hundred different ject, amplifier output magnitude -so complete and uthorua- every conceivable branch or sub -division tive -that every man engaged In radio can have both A typical P.A. installation (in a a theoretical andd practical knowleÉ off the function ocoordinatingf Public Address. and ope fion sound Systems. The pages THE CO skating rink) are so filled with Instruction and r fete with To actually show the scope andd gnitude of the that the volume fully justifies ltt. title AMPLIFIER HANDBOOK AND PUBLIC ADDRESS SECTION V- USEFUL PUBLIC ! HMO AMPLIFIER HANDBOOK AND PUBLIC AD- analysis the is the ADDRESS DATA AND IN- DRESS GUIDE. This Wrest HANDBOOK on PuhlicrAd- right. showing bre own of tÌe alnfeaturre those who n within each particular section. A thorough reading of FORMATION fbtFantiy build. eseevi e andsand selldied .ndu equipment. ° the contents shows the completeness of this book. Speaker matching technique RADCRAFT The ABC of Db., VU, Mu, Gm and PUBLICATIONS . 20 VESEY STREET NEW YORK, N.Y. Sm Charts and formulas useful to the practical P.A. sound man r RADCRAFT PUBLICATIONS. INC. * 20 VESEY STREET * NEW YORK. N. Y. RC-541 Handy index to important articles centirmrir Enclosed tini my remittanre of 25e for e bleb send me POSTPAID, one copy of your N LIV- on public address and sound 1940 AMPLIFIER HANDBOOK AND PUBLIC ADDRESS GUIDE.

send me others, for friends, also POSTPAID (4 25e each. I W^me tddreff Order Your Copy NOW-

T Cliy elate eiipet'a an and Remit check m money order; reglfter letter 1 If you fend cash or unused l'. P. Portage Stamps. /Hail Today

672 RADIO -CRAFT for MAY, 1941 SOUND

Eo E0 o / -

50 i M C 9 M 1 N - rd C -- -

- (5 1 I FIG.1 FIG.2 FIG.3 FIG.4 FIG.5 FIG.6 MICROPHONES --and *Iv They Woth An explanation is given of the different ways in which sound THE name microphone was originally vibrations may be converted in a microphone into vibrations used by C. Wheatstone to denote an instrument which served to make very of a membrane, ribbon or plate, which possesses one degree weak sounds audible to the human ear, of freedom, and how these vibrations can in turn excite an and which was therefore what we now call alternating E.M.F. in an electrical circuit. The considera- a "stethoscope." Now, however, the name microphone is used exclusively for instru- tions provide a basis for the division of microphones into ments which convert air vibrations into elec- different types. trical A.C. voltages. The early development of microphones took place during the years 1876 and 1877. J. de BOER At that time Alexander Graham Bell (1876) was laying the foundations of modern te- (3) Finally, the different useful combina- denser, Fig. 3. The motion lephony. In his work he used a microphone of the membrane, tions of electrical and acoustic characteris- M, causes changes in the capacity of the which worked on the electromagnetic prin- tics and several different types of micro- condenser. When ciple. opposite chargea have One year later (1877) both Thomas phones will be described. been put on Edison and were the plates of the condenser Berliner granted patents over a resistance R by means for a much more of an electric sensitive microphone, ELECTRICAL CLASSIFICATION OF battery, the changes in capacity whose action depended upon the variable lead to MICROPHONES changes in voltage between the 2 plates, resistance of powdered carbon. Since that which in this case also have the same rela- time the further development of the micro- E.M.F. is dependent upon the deviation of the membrane. tion for all frequencies with respect to the phone has gone hand in hand with the de- deviations of the membrane. velopment of reproduction recording Carbon Microphone. -In the classic car- and For these 3 types of of sound. bon microphones, Fig. 1, a deviation xf of microphones the membrane M causes a change in the transi- E.M.F. generated is proportional to the tion resistance between grains of carbon deviation of the membrane, no matter how FROM SOUND TO VOLTAGE which are enclosed in a container K. For high the frequency. For satisfactory func- The various kinds of microphones have sufficiently small deviations of the mem- tioning of these microphones, therefore, it one feature in common, namely they all first brane the change in resistance AR is pro- is only required that this deviation is pro- convert the motion of the air particles into portional to the deviation: portional to the sound pressure for all fre- motion of a membrane, ribbon or plate. AR = Cxj (1) quencies. We shall discuss in the following This motion leads to the excitation of an where the proportionality factor C does not the degree to which this requirement can electrical A.C. voltage, so that the following depend upon the frequency. The constant be satisfied. sequence is always involved: E.M.F. E applied to the microphone cir- E.M.F. depends upon the velocity of the motion of air motion of mem- cuit gives a current i, = R when the membrane. E/ Dynamic (so- called brane E.M.F. total resistance of the circuit is R,. The Electrodynamic) Mi- small change in resistance AR of the micro- c1ophone. -In the case of an electrodynamic In a sound field the total air pressure microphone, a membrane, ribbon or plate can be divided into the normal pressure P phone is therefore equivalent to an alternating E.M.F. in the microphone circuit: which is set in motion by the air vibrations of the atmosphere, which is constant, and is either itself electrically conducting or a varying term pf due to the sound vibra- E, bears a conductor upon it. This conductor tions, the so- called sound pressure. Ef = i3OR =C -xf . . (2) is placed in a constant magnetic field, Fig. 4, The following requirement is made of a R. and included in an electric circuit. Due to good microphone. The amplitude of the The relation between the E.M.F. Ef and the motion of the conductor (B, a vibrating E.M.F. excited by the sound pressure must the deviation xj of the membrance is thus metal ribbon) in the magnetic field the be directly proportional to the amplitude of independent of the frequency. The A.C. out- number of lines of force enclosed within the sound pressure for every frequency put of the microphone is made available the electric circuit changes: wan in the frequency range to be repro- in external circuits by means of coupling N= N, +Cxf= N, cos tot . (3) duced and the proportionality factor must transformer T. +Cx be independent of the frequency. The am- Crystal Microphone. -When a piezoelec- where C is a constant and w /2n the fre- plitudes occurring are usually small enough tric crystal plate is stretched or compressed, quency. According to the law of induction, to satisfy the first condition. In this article electrical charges occur upon the side sur- an E.M.F. is hereby excited in this circuit, it will be explained how it is possible to faces. If such a crystal, C, Fig. 2, is intro- which is equal to the decrease per unit of make the proportionality factor independent duced between a rigid wall and a membrane, time in the number of lines of force en- of the frequency. M, which is set in motion by sound vibra- closed: (1) In the first place we shall discuss the tions, an E.M.F. Ef is excited in the crystal dN conversion of the motion of the membrane which is proportional to the deviation xf of Ef =-- =- Cx'= Cwx, rift wt, (4) into an E.M.F., which involves the classifi- the membrane. dt cation of microphones according to their This alternating E.M.F. in turn can be where xf represents the derivative of the electrical behaviour. made to excite an alternating current in an deviation with respect to time, i.e., the (2) We shall then deal with the different electric circuit. The generated A.C. voltage velocity of the membrane. In the diagram, devices for the conversion of the air vibra- may be taken from across resistor R. N is the North pole and S is the South pole tions into motion of the membrane. This Condenser Microphone.-In the case of of a permanent magnet; the 2 leads from involves an acoustic classification of micro- the condenser microphone the moving mem- ribbon B deliver the generated A.C. voltage phones. brane forms one of the plates of a con- to external equipment.

RADIO -CRAFT for MAY, I 9 4 I 673 SOUND

Magnetic (or so- called Electromagnetic) region to be reproduced falls below it. If log Microphone. -A small piece of soft iron the resonance frequency should still lie in which completes the magnetic circuit of a the region of the highest tones to be repro- permanent magnet is fastened to the vi- duced, a disturbing resonance will occur \ brating membrane of an electromagnetic unless care is taken to make the damping -°- 1: w Microphone, Fig. 5. Due to the motion of sufficiently severe. FIG.7 the membrane, M, a varying magnetic field In the frequency region above the log x occurs which generates an E.M.F. in a sta- resonance frequency the ratio between the tionary. electric coil, Sp, surrounding the acceleration (x "f = w'xf) and the sound magnet, P, and forming part of an elec- pressure is constant, according to equa- trical circuit. tion (8). For the 3 types of microphone -r109w The situation in the case of such an mentioned the amplitude of the E.M.F. gen- FIG.8 electromagnetic microphone is quite simi- erated in the microphone circuit varies for lar to that in the case of an electro- high frequencies according to po /w'. The dynamic microphone, and in both cases an sensitivity thus decreases rapidly for high - - --.- -.- .--M. - E.M.F. occurs which is proportional to the frequencies, as may clearly be seen from velocity, the proportionality factor being in- the resonance curve given in Fig. 7. In this FIG. 9 dependent of the frequency. In order to illustration the resonance curve of a mem- obtain satisfactory reproduction for all fre- brane with one degree of freedom is indi- I ,.._cost A quencies with microphones which function cated; and, the log. of the deviation x is 1 on the electrodynamic or electromagnetic a function of the log. of the frequency. 1 principle, the ratio between the velocity of A microphone working on the electro- _y_ 1 the moving system and the sound pressure dynamic or electromagnetic principle will,

1 should therefore be independent of the fre- according to equation (4), reproduce sound I j1 quency. satisfactorily if the relation between the velocity of the membrane and the pressure FIG.10 AY ACOUSTIC CLASSIFICATION OF does not depend upon the frequency. . MICROPHONES A pressure microphone may therefore qc Microphones. one only only be used in these 2 types in the fre- Pressure -If side a of the membrane M of the microphone is ex- quency region for which equation (9) is _ A posed to the variable sound pressure pf, valid, i.e., in the vicinity of the resonance Fig. 6, one speaks of a pressure microphone. frequency. By making the mechanical damp- In force Kf which ing large, provision may indeed be made this case the the air frequency in which re- vibrations (shown here as a plane sound that the region the FIG.tt production may be considered satisfactory wave) exert upon the surface S of the fairly membrane is equal to the product of this becomes wide, but the sensitivity then usually becomes too low, so that such a area and the sound pressure: solution remains inadequate. Kf = Spf = Spo oos wt . . . (5) If for certain reasons it is nevertheless When the mass of the membrane is m, the desired to construct a pressure microphone iht4% frictional resistance r and the stiffness s, the according to the electrodynamic principle, equation of motion of this membrane as- it is advisable (in order to improve the sumes the following form: reproduction of tones above and below this resonance frequency), to pass from the sys- mxf + rx'f + axf = Kf = Spo cos wt (6) 12 NIP FIG where x' represents the acceleration. tem with a single degree of freedom, which has only 1 resonance frequency to a system In such a moving system with one degree with several degrees of freedom which give zo of freedom a db. the greatest deviation for rise to several successive resonance fre- to given amplitude of the external force oc- quencies, Fig. 8. Provision may indeed be curs when the frequency of the force is o , made in this way that the sensitivity of Io= z 5 IO2 2 , 1 ,'J equal to the resonance frequency: the microphone remains practically constant FIG 14 JCYCLES/SEC. w 1 t r over a wider frequency range, but the (7) reproduction, especially of speech, is of :o d 2>t 2:r m poorer quality due to the maxima which indicate the position db. Io b The way in which the amplitude of the vi- of the resonance fre- system varies with frequency quencies in the sensitivity curve of Fig. 8. brating the We shall not differs very much according to whether the therefore consider such microphones further in this article, and consider 0 frequency in question is - eo° -90° 90° ioo'a sufficiently far only microphones with one degree of 0° above or below the resonance frequency or freedom. FIG.15 in its vicinity. In these 3 different fre- The sound pressure at a point does not + + + + + + t + + quency regions each of the 3 terms in the depend upon position left -hand member of equation (6) in turn the of the surface element upon which one wishes to measure A 1 1---- plays one the most important part, so that its effect. may use the following approximations of Therefore the action of a pressure microphone will not depend upon the equation (6) for the 3 regions: ++++++++ position of the vibrating membrane with i _ i w > wo: mx "f = Spo cos wt; . . (8) respect to the direction of propagation of w = wo: rx'f = Spo cos wt; . . (9) the sound waves. A pressure microphone is e t,++ +++++P) w < wo: 8x/ =Spo cos wt; . . (10) therefore equally sensitive from all direc- In the neighborhood of the resonance fre- tions, so that the polar linear direction dia- 4. + ' +++++ quency, therefore, the sound pressure de- gram of the sensitivity of this microphone termines the velocity of the motion of the is a circle. If the dimensions of micro- the C membrane; for higher and lower frequencies phone are not small with respect to the /I the sound pressure determines the accelera- wavelength of the sound, the microphone tion, viz., the "deviation." itself distorts the sound so that deviations In the case of carbon, piezoelectric (crys- from this simple picture of the situation tal), and condenser microphones, as we have occur ('). Pressure- gradient Microphones.-If both already mentioned, the E.M.F. depends upon o the deviation of the membrane. As to the sides of a membrane or a vibrating plate Ili can be reached by the sound waves, Fig. 9, i acoustic part of these microphones, care must therefore be taken that the ratio the latter will arrive at the front and rear between the deviation and the sound pres- sides with a certain phase difference. If sure is independent of the frequency, i.e., the sound pressure on the front of the equation (10) must be valid. These types of membrane, M, amounts, for instance, to ao PI Po wt (11) microphones must therefore be constructed = db. to as pressure microphones, and care must be and if the waves must cover a distance '-'),/1/ taken that the resonance frequency for which is I cm. longer in order to reach the o these systems with a single degree of free- other side, then for a wavelength ), the 5 IO' 2 5 10' z s 10J dom is made so high that the frequency (') Cf.: "Phillip Teclmtcal Review." 4, 144, 1939. FIG.18 0- CYCLES/SEC 674 RADIO -CRAFT for MAY, 1941 SOUND sound pressure on the rear side of the mem- If the membrane is perpendicular to its brane becomes position of greatest sensitivity, no E.K.F. at all is induced, since the sound waves = Po cob wt - 2:t 6/), = Po cob (Wt- reach both sides of the membrane simul- kl) (12) taneously and no difference in pressure oc- where k = 2r/1. = w /c, c representing the curs. Microphones with such a direction velocity of sound. If the membrane has an diagram may very well be used in rooms in area S, a force Kf acts upon it which is which there is much noise and reverberation the product of this surface and the differ- (') since this undesired sound in general ence in pressure p, - p,: arrives at the membrane with the same Kf = S (p, - p,) = Spo [cos wt - cos (wt- intensity from all direction while the sound kl)] (13) to be amplified comes from a definite direction. Since this force is proportional to the "pressure gradient," i.e., to the fall in pressure Combination of Pressure and Pressure - per unit of length, such devices are called gradient Microphone -If a pressure and a pressure- gradient microphones. pressure- gradient microphone are connected If the dimensions of the microphones are electrically in series, a microphone is ob- small compared to the wavelength, i.e., if tained whose E.M.F. becomes equal to the 1 < X or kl V 2.u, the force becomes: sum of the E.M.F.s of the microphones. If is in Kf = -Spo kl ein wt . . . .' (14) for instance the E.M.F.E, generated the pressure microphone and E, in the pres- At a certain value po of the sound pressure- gradient microphone at its optimum sure the amplitude of the force acting on position, then at any given position of the the membrane of a pressure -gradient micro- latter (at an angle a) the total E.M.F. is: phone increases proportionally with k, i.e., El (1 . . . . (19) proportionally with the frequency. The E = + cos a) equation of motion for the membrane be- The polar direction diagram then becomes a comes: so- called eardioid, Fig. 12. For a = O' the m "f + rx'f + 8xf = Kf = - Cpo w sin sensitivity, is greatest, while in the opposite wt (15) direction (a = 180 °) the sensitivity would zero and - which may be simplified in the following be since the pressure pressure way for the frequency regions indicated: gradient microphones give exactly equal ánd opposite E.M.F.s with such orientation. Ad- e vantage may be taken of this characteristic W > Wo =V : mx" Cpo sin (16) = - w wt; when it is desired to reproduce the sound 1941 EDITION . from a given direction while suppressing .. w wo : rx' - Cpo w sin wt; (17) = _ that from another direction. < Wo Cpo tin (18) w : sxf = - w wt; including By integration of (16 and 17) the follow- TECHNICAL MODELS ing expressions are obtained: After this general discussion we shall now the first collection of mxf = Cpo cos wt (16') deal briefly with 4 types of manufactured rxf = Cpo cos wt, (17') microphones (originally pictured as Figs. 13, 16, 20 and 22, but not reproduced here, which are valid for w > w° and for w = wa since the following discussion is taken to PANEL LAMP NUMBERS respectively. From formula (16 and 16') it relate to general rather than specific types may be seen that for frequencies sufficiently -Editor). ever far above the resonance frequency the ratio attempted! of the velocity of the membrane to the Microphone.- Carbon micro- becomes independent of the Carbon 272 Pages -16,730 shown sound pressure phones may very well be used for the record- Radio Models - frequency. This is exactly what is required including data on '41 receivers. Tube re- ing of the spoken word. While it is true that placement of a good electrodynamic (or electromag- with very high sound pressure the mag- information for 100,380 Tubes or netic) microphone, since in that case the nitude of contact surfaces between the Sockets. 586 Trade Names of receivers. E.M.F. excited, and the the Names and Business Addresses of 190 Re- ratio between the grains is not proportional to the pressure of membrane, is independent of ceiver Manufacturers. PATENTED, HOLD -TITE, velocity the exerted, so that the sound will be distorted, the frequency. When a pressure-gradient nevertheless carbon microphones have HIRE-0 BINDING. The First and Only Compi- on the of Lamp microphone acting the electrodynamic produce a much larger lation Panel Numbers. principle is used, very good can advantage that they reproduction kinds of microphones. A be obtained all frequencies E.M.F. than other Every Radio Serviceman should own one therefore for amplification is above the region of resonance. much smaller therefore of these Sylvania Tube Complement needed with a carbon microphone, which Books. All the information you'll need If one desired to construct a pressure - suitable for simple appli- - makes it peculiarly from the oldest set to the latest '4 1 model gradient microphone on the principle of the cations. -is packed into those 272 pages. And carbon, piezoelectric or condenser micro - The of carbon microphones is by phones, it could only be used for frequencies sensitivity the book is bound the WIRE -O proc- resonance, higher, the larger the grains of carbon ess. It opens flat and there's no danger in the neighborhood of the since which it The quality of sound, only in region is the deviation propor- contains. the of it falling apart or of the pages pull- that however, diminishes at the same time, since ing out. tional to the sound pressure. the noise also increases with the size of the The gradient of the sound pressure de- grains. Furthermore the carbon packs to- This valuable compilation, the only book pends upon the direction in which it is con- gether somewhat in use, so that the sen- of its kind in the field, is one of the 125 sidered. Therefore in the case of a pressure - sitivity decreases with time. This can be silent salesmen Sylvania has created to gradient microphone the induced E.M.F. will prevented by tapping the microphone help you build a sounder, more profitable depend upon the position of the membrane lightly. business. Write today for Sylvaniá s Tube with respect direction of propagation to the An interesting use of the carbon micro- Complement Book, enclosing 35e in of the sound waves. If in Fig. 10 we indicate stamps or coins. and for a full list of the of membrane by phone is the reporter's or lapel microphone, the front and rear sides the which can be worn in buttonhole. The sure -fire, tested, dealer aids. Some are V and A, for which points the the points the relative sensitivity expressed as sound en- free, some cost you just what they cost largest possible difference of path 1 for the ergy in db., is shown in Fig. 14 as a function us -all of them are designed to help you sound exists, then the difference of path sell more, and make more! when membrane is of the frequency, from which it may be seen the turned through an frequency lies angle a with respect to the direction of that the resonance at about propagation 3,000 cycles -per- second (or c. /sec.); the sen- of the waves becomes equal to sitivity, meanwhile, decreases rapidly with l cos a. In formulas (12) and (14) 1 must increasing frequency above 5,000 c. /sec. then be replaced by I cos a so that the induced E.M.F. will also be proportional to In Fig. 15, the dependence of the relative SYLVANIA cos a. If this E.M.F. is plotted in a polar sensitivity in db. on the direction is plotted Set -Tested Radio Tubes diagram as a function of the angle a, a in horizontal coordinates for frequencies of directional diagram is obtained which con- 1,000 and 5,000 c. /sec., respectively. It may Aleo makers of Hygrade Lamp Bulbs, Hygrade sists of 2 circles touching each other, be seen (curve a) that the sensitivity at Fluorescent Lamps and 5firalumc Fluorescent Light 1istar, s Fig. 11. 1') Cf.: "Philips Technical Review," 3, 221. 1938. RADIO -CRAFT for MAY, 1941 675 SOUND

1,000 c. /sec., varies very little with the responsible for the piezoelectric properties. direction, so that the ideal pressure micro- When the temperature is lowered the water phone is very closely approached. At higher of crystallization does not recombine. The frequencies, however, a much greater de- limit of 55° C., is fortunately so high that pendence on direction appears, as may it is never reached under normal conditions. clearly be seen in Fig. 15b. This is a result of the fact that for frequencies of about Ribbon Microphone. -In the ribbon or 5,000 c. /sec., the dimensions of the micro- velocity microphone, a corrugated aluminum phone are no longer small compared with ribbon several microns thick is stretched the wavelength. between the pole pieces of a permanent lull] IJ4IiVAILTa s1411iihli7 magnet. The air vibrations exert their pres- Learn How with this! Crystal Microphone. -In the diagram of sure on both sides of the ribbon which thus Here's the world's most highly TO 2 as a raied course practical radio Fig. the air vibrations are transmitted by acts pressure -gradient microphone. Due service elins TUD Y means of a membrane to a piezoelectric to the motion of the ribbon in a magnetic 1300 -page 1 homes stuhandsomedy book. First. a big 420 -page Rochelle 4H5O) field an E.M.F. is excited section a plains an types of UP crystal. salt (NaKC,0g, in the circuit in icing test instruments-how they which is how use for example. In crystal microphones con- the ribbon included, so that the how to uild your own. Then a90Ó structed in this way care must be taken microphone is of the electrodynamic type. give you a thorough. snor-A Y-steppages procedure course in trouble -shooting. testing, aligning that the sensitivity for high frequencies According to the foregoing considerations, and repairing all makes of radio does not vary irregularly with the fre- for a pressure -gradient microphone of the Ytalatest scientific "factory meth- quency due resonances in od.." Learn radio servicing to the driving electrodynamic type the reproduction will this quick, easy. economical Learn mechanism. The following point must, how- be practically independent of the frequency HOW it. Send f ryourcopd S4 ever, be in 1a3d000. kept mind in connection with when operating above the resonance fre- PP.. 708 ills... 722a0arreevview guest. this type of microphone, even though due quency; formula (16'). In the case of this care has been taken in this connection. ribbon microphone the resonance frequency RADIO 74471614-5:444,i HANDBOOK If a thin crystal plate of Rochelle salt is lies at about 50 c. /sec., which is sufficiently Save Time with this! stretched in the direction of its length (see low to ensure a practically constant sen- Here's real help for servicemen- TO 518 j Fig. 17A) the 2 sides of the plate take on sitivity for the frequencies of speech and big es of valuable. .na n g.slafud -checked SPEED music. rvicing data right at your Anger opposite charges, so that when it is used as tips to help you do every ic- a membrane 17B) and so It may be seen from Fig. 21 In Job quickly and accurately. it's UP (Fig. thus bent that the 10 SECTIONS contain 275 2 relative ribbon 2large pages of time- saving "Case that the sides are stretched and com- sensitivity of the micro- 3,300 eiv. pressed respectively, they will then assume phone changes only slightly for the fre- ers Alilignment data for over . ; Servicing the same polarity. If 2 crystal plates are quencies from 100 to 5,000 c. /sec., while d installation data for Auto 0 at nachos . . Trouble Shooting Charts stuck together in such a way that their 5,000 c. /sec., it begins to decrease. This may Resistor and Condenser harbpbe, Mwith! Speed opposite 17C), be explained as follows. D your service work polarities are (Fig. a plate vital servicing data NOW ,S K is obtained which assumes opposite charges inen this one big book. v The sensitivity of the microphone is given 518 po.. t 134 illus.. Ii/pa11 sl -e. on its 2 free sides upon bending ('). Two by the approximation formula (14). If, such "bimorphous" are CLIP OUT! MAIL NOW! crystal plates however, the product kl = 2;trl /c becomes Radio Tech. mounted together in the manner shown in of order of magnitude of (14) no i Publ. Co.. Dept. RC -111 hare I the 2t, 45 Astor Pl., New York cor FE:F.R g Fig. 17D. Each of these plates experiences find payment for: circular.. longer holds; suceeding terms of the series EnclosedO MODERN RADIO TROY- p BOTHbooks at I the effect of the sound pressure on the outer in the precise formula (13) must also be ELF. SHOOT- $0.50 Money- I RADIO [NG FA'S HAND. Saving Corn' side only, so that the result is a pressure taken into account. From further considera- BOOK !situation !'rice i microphone. NAME tions (') it is found that the sensitivity of I 18 19 ADDRESS I In Figs. and it is shown how the the microphone will be practically inde- tarn IAT relative sensitivity in db. depends, respec- pendent of the frequency up to that fre- L..MONEY-RACK IF NOT SFATIIFIED1... tively, upon the frequency and the direction quency for which k1 = it, i.e., where the INVALUABLE! from which the sound strikes the micro- product of k and the width a = l /:1 of the BRAND phone. For frequencies up to 5,000 c. /sec., pole piece is about unity. Now k = 2nr /c = ONLY the sensitivity is found to be practically 2;ír/34,000, so that with a width of the NEW! independent of the direction, and above that pole pieces of about 1 cm. we may expect copy frequency only slightly dependent on the that the sensitivity will change little up to 25¢s direction. This is because of the small a frequency of 5,000 c. /sec. For higher fre- dimension of the piezoelectric crystal. The quencies the sensitivity then decreases quite resonance frequency lies in the vicinity of rapidly as may be seen in Fig. 21. If it The greatest, most 4,000 to 5,000 c. /sec., and presents no diffi- were desired that the sensitivity should de- compact formula hook In culties. In order to provide that not only the crease only at a higher frequency, this may twrllyL 10Ó0t trade secrets, recipes and E.M.F.Ef generated, but also the terminals be accomplished by making the pole pieces rocesses for home. factory and work. voltage Vf shall depend only slightly upon smaller, but under otherwise similar con- ßRECIPES -foil of the frequency, the external resistance R ditions this takes place, according to equa- pßOCESSES newest ideas. which completes the electrical tion (14) at cost of ¡pIEO BTEStO Hass circuit of the the the magnitude of rad tested formulas microphone must be made larger the sensitivity. PRICTFOßM=LIflY1F1E0 for making Glues. than the Cements d Ad. internal heslaes: Cleaning impedance of the microphone. Since In Fig. 22 may be seen the manner in mppoo de Paint FOßbw ORT Co; the crystal behaves as a capacity C, the ORKSNOP which the relative sensitivity of the ribbon WWood Stains: Var. fishes. Disinfect. terminals' voltage becomes therefore: microphone at 5,000 c. /sec., depends upon ants and Deodorhell Ef the direction from which the sound comes; andExt Innguui VI . s (20) for lower frequencies the dependence is Inks:In ecticides - La V 1+1 /w1C'R° practically the same. As must be expected i ; Metal l Work: aEtching in the case of a pressure -gradient micro- This expression is only independent Paper Making: Photographic Pros sses: Preserving of the phone, the sensitivity to sound from direc- Essences: Extracts and i-lavors: Plastics; Plating Solu- frequency when WWCR > 1, i.e., for sufficiently tions: Polishes; Preservnlives: Fireworks: Soaps; tions perpendicular to that of greatest sen- Creams: Dy s. Toilet Preparations. Dentifrices d high frequencies, and increases linearly with Cosmetic.: Hair Preparations: Manicure Preparations; sitivity is practically zero. Sun Burn Creams and Lotions; Waterproofing: Waxes the frequency when 0,CR >1. If the micro- and countless others. A Wonder Reek of Useful Knowledge! Worth its phone capacity is, for instance, 0.002 -mf. weight in gold. C= Coil (Dynamic) Microphone. -In conclu- Book Is up-to-date. Newest methods-latest discov- and the external resistance R megohm so- called eries. Remember-1000 clearly described SIMPLIFIED =1 at sion we shall describe the dynamic formulas. processes and recipes-MANY NEVER BE. a frequency of about 80 c. FORE DISCLOSED. /sec., the abso- microphone, a second type of pressure - 68 Pages. printed on good paper. lute value of WCR will be approximately gradient microphone of the electrodynamic for Remember, great for. tutBend y u c copy r °ODAY. unity, so that V= 0.72 -E. Above 100 c. /sec., type. In order to obtain a larger E.M.F. the 25e a copy, sent postpaid anywhere Send money order, cheek. unused V.S. stamps or the terminals' voltage does not change very conductor which moves in a magnetic field emu much more with the frequency. consists of an electric coil in this case. In NATIONAL PLANS INSTITUTE 24$.R Fifth Avenue New York. N. V. In the technical construction of this type the diagram of the cross -section, Fig. 24, it of microphone, it is usual to take care that is shown how the coil (Sp) is wound upon the microphone is well protected against the a paper cap fastened to the membrane. The entry of moisture since the. Rochelle salt coil moves in a ring -shaped magnetic field. DUMONT crystals are not unaffected by water. Furthermore, the magnetic circuit is not Precision Built Condensers Furthermore the microphone may not be- closed over the entire circumference of the at your nearest ,obber or write to come too warm. If the Rochelle salt reaches ring, but only over parts by means of pieces DUMONT ELECTRIC CO., INC. a temperature above 55° C. (131° F.) it of magnet steel M. The outer ring and the 311.516 BROADWAY NEW YORI(. N. Y. loses its water of crystallization which is inner ring which form the North and South PAPER - MICA ELECTROLYTIC - (a) According to the Brush Crystal Co. (4) 7. de Boer. "Physic.," 5, 545. 1938. 676 RADIO -CRAFT for MAY, 1941 SOUND poles are made of soft iron. If the magnet steel had the form of a complete cylindrical shell, as is the case in the magnetic system of a loudspeaker, the total number of lines of force cut by the vibrating coil, and thus also the E.M.F. excited in the latter, would be larger. In spite of this that form is not chosen, in order to prevent the formation oot by the magnetic system of a resonating cylindrical space behind the membrane (open organ pipe) which leads to irregular- ities in the dependence of the sensitivity upon the frequency. BECAUSE IT By making the fastening of the membrane very flexible care is taken that the resonance frequency will lie below 100 STANDS UP! c. /sec. From this frequency, according to Fig. 25, the relative sensitivity is found to Here's the instrument that has been leading depend little upon the frequency, up to about 5,000 c. /sec. The magnitude of the the field for five years ... and still is "tops" pres- sensitivity is such that with a sound for radio servicing. That's because it meas- sure of 1 dyne/cm.' and an output impedance of the microphone of 500 ohms, a volt- ures in fundamental quantities ... no trick age of 0.9 -mV. is obtained. circuits or gadgets to cause early obsoles- The manner in which the relative sen- WESTON Model 772 sitivity depends upon the direction from cence ... and because it is designed and which the sound comes is shown in Fig. 26. built to high WESTON standards. It is an Super- Sensitive Analyzer At a frequency of 500 c. /sec., the sensitivity for directions perpendicular to that instrument that, with usual instrument care, RANGES of greatest sensitivity is indeed approxi- will remain serviceable indefinitely. Be sure volts... DC-2.5/10/50/250/1000 ... mately zero (Fig. 26A) as is to be expected to get full information on this complete, in- sensitivity 20,000 ohms per volt from a pressure -gradient microphone; for AC- 2.5/10/250/1000... sensitivity higher frequencies the minima are less expensive test instrument. Write for bulletin 1000 ohms per volt. 26B). pronounced (Fig. describing Model 772 and other WESTON Current... DC- .1/1/10/50/250 milli- amperes 1 /10 amperes instruments for radio and television servic- Ofuns...0- 3000; 0-30,000 ohms . , . megohms This article is another of the valuable con- ing. Weston Electrical Instrument Corp., 0 -3 megohms; 0-30 Decibels... -14 +2/ -2 +14/ +12 + tributions by the Research Laboratory of 599 Frelinghuysen Avenue, Newark, N. J. 28/ +26 +42/ +38 +54 N. V. Philips' Glowlampworks to the field of radio literature. It is here reprinted from the magazine "Philips Technical Review" by special permission to Radio- Craft. WE S TON Radio Instruments

7D O db. ANY RADIO db. lo Amazing New Invention 0 0 -180° -90° O° 90° ,GO°a 5 IOC 2 5 101 2 5 IOS,) of hours. Revolu- Complete tionary , different FIG 21 JCreLE5/sec. Comparison tech. flume. Its Soo to do e 20 work immediate. $1 I ' IY. Most repairs Training M sp 5p M db without t st lo ' 1 d enuipm t. Sim- L ple. point-tn.point, erose Terenre. circuit tgteestions locare the fault 0 Insta tlly. Plan copyrighted Covers -ISO. -90° O° 90" IF.O"a I FIG 22 FIG 24 In Manual form. 8th it 1 I IiSsudSslSKlke beatified In., 112 pages. Over 1.000 The complete simplified ractical service hint.. 18 training is yours for only 20 20 b large trouble -shooting blue- 11.00. postpaid. Will y 114 different testa for Itself with time saved Prints.using a Se Ialor. Invent on the first Jot,. Limited db d b lo . a ed by M. N.lieltman, ted quantity. Money -hack guar" `i"-' and radio teacher. teen. Write today. \ engineer SUPREME PUBLICATIONS. Room 857 O . 0 3727 West 13th Street Chicago, Illinois IOa 2 S 103 2 s iO4 ÿ -160° -90° O 9o° 1a0a FIG 25 JCYCLEs/sec FIG 26

UNIQUE CALL LETTERS tion of the station as being in the Metropoli- WIZEN WOR's new commercial F.M. tan area. station -a construction permit for The first letter, W, indicates a location which has been granted by the F.C.C. -be- east of the Mississippi River. This is the gins operation, the station will have the only similarity to the method of assigning new inaugurated call letters to the regular, or longwave, type call letters recently ex- by the F.C.C. broadcasting stations, although some If the assignment of these letters runs ceptions exist among the regular stations. according to indicated pattern the station Pending the construction of the new sta- Write For Complete Derails will bear the letters-W71NY. The "71" will tion, WOR will continue to operate its ex- AMPLIFIER COMPANY of AMERICA indicate the station's location on the new perimental F.M. transmitter, W2 %OR, from 17 W. 20TH STREET NEW YORK. N. Y. F.M. radio dial, which corresponds with 444 Madison Avenue, New York City. The the frequency used by the station of 47,100 new station will be at the same address. kilocycles (47.1 mc.). li}:1)-CAPS The new method of identifying all F.M. Smaller Dlameler Dry Eiectrolytics stations recently announced by the F.C.C. NEWS SHORT provides that the 2nd and 3rd numerals of The cow juice output dropped, last month, SPEF the station's frequency be used, since all when someone atole a radio set from the F.M. stations will be alike in having a fre- dairy barn of a Chillicothe, Mo., farmer; -6.11111011r Wt,,, lo. Co,aia9 // quency beginning with 4 and ending with and went right back up, when a new set was 00. The letters "NY" will identify the loca- installed! SOLAR MEG. CORP. Bayonne, New Jersey,

RADIO -CRAFT for MAY, I 94, 677 THE FINEST RADIO BOOK WE EVER OFFERED :- the 1940 -41 Radio- Television Reference Annual With $1.00 iuóscrirtion to FREE RADIO - CRAFT

WITH our compliments. w a want to send a copy of the 1940 RADIO -TELEVISION REFERENCE ANNUAL to you FREE. if you will simply lake advantage or RADIO -CRAFT magazines special subscription offer NOW. This offer is being made fora limited time only. Read the summary of contents The 1940 RADIO- TELEVISION REFERENCE ANNUAL has 68 pages, large size BSr x II' /, with over 170 Illustrations. The contents of this book has never appeared before in handy book form. Its pages in this FREE BOOK! cover practically every branch of radio sound. public address. servicing, television, construction articles for advanced radio men and technicians, time and money- saving kinks, wrinkles, useful circuit Informs: Bon, "ham" transmitters and receivers, and a host of other data THE 1940 RADIO -TELEVISION REFERENCE ANNUAL contains a collection of the best and for every branch of radio The Annuals have always been regarded as a standard reference work practical moat important articles. as they do operation and service. This 1940 edition ably sustains this reputation. Every radio man wants a copy Covering of this valuable book. lust as this book will be of unquestionable value to you. so. too will every nearly every branch of radio, they form a handy monthly issue of RADIO- CRAFT. This magazine brings you big value every month. It keeps you reference works. In addition, many time and labor- Intelligently Informed about new developments in radio and television- You want the news, want It saving kinks, circuits and wrinkles, tried and tested fully but concisely. want it drat -that is why you should read RADIO -CRAFT regularly. by practicing Servicemen, experimenters and radio This very special offer is made for lust one purpose -we want you as a regular subscriber. The Annual. fans have been included. This book cannot be whose contents appears at the right, is not told, but a CODY is FREE to you If you subscribe now. bought anywhere at any price. Yet It is yours by merely subscribing. Use the convenient coupon SAME SIZE AS below. RADIO -CRAFT BEGINNER'S SIMPLE INEXPENSIVE CONSTRUCTION ARTICLES Beginner'. Breadboard Special - a 1 -Tube High -Oain Ail Wave Receiver- R'tring Pointer. for Radio Beginners -A Watch Charm Size 1 -Tube Set-Beginner's Simple Volt - Milllammeter- Making a 1 -Tube Broadcast Loop R -A.C. -D.C. Power Supply for Battery Portables -A 1 -Tube Short -{Waver with Band Coil Switching. MORE ADVANCED SET CONSTRUCTION The "High -Seas 4" Broadcast Lamp Radio -How to Build a 6 -Tube 1.4 -Volt Short -Wave Superhet for the "Ham" or Short -Wave Fan -Build the "Lunch Box 5" Super Set a Broadcast Battery Portable -How to Build a Plug - Together 8 Tube Broadcast Set -The '5-in -4" All -Ware Radio for A.C. Operation -An Easily-Built 3 -Tube Midget Broadcast Superheterodyne Receiver. THE SERVICEMEN'S SECTION Baca Tone Control--Simplified Variable Selectivity-Prac- tical Servicing Pointers -Servicing Universal A.C. -D.C. Re- ceivers- KI1Hng the "Intermittent" Bug -A Service Shop A.C. -to D.C. Power Supply -Sideline stoney for Serrlq- men- Adding A.V.C. to any Screen -Grid T.B.F. Receiver -Iron Particles In Speaker Air Gap. TEST INSTRUMENTS A l'eoful Neon Lamp Teeter-An Inexpensive Output Meter -Slaking Milliammeter Multipliers- Home -Made Frequency Slalulator -Tlw Busy Servicemen's V.T. Volt-Meter. PUBLIC ADDRESS AND AMPLIFIERS Build this Combination A.C. -D.C. Radio and Inter -Cbm- munlcator- Speaker Placement in P.A. Work-The Design and Construction of an Inexpensive All -Pugh -Pull 10 -Watt Amplifier -Ohmura Source. of Hum in High -Gain Ampli- fiers-How to Build a High- Fidelity 5 -Watt Versatile Amplifier. "HAM" SECTION Ultra -High Frequency Antennas -The Beginner'. LowCost Xmitter- Modulator Meter -Phone Monitor-The Begin- ner's Ham" Receiver-2% Meter Acorn Trenseelver. TELEVISION How to Build 441 Line T.R.F. Television Receiver-flew ful Notes on Television Antennae. MISCELLANEOUS Simple Photo -Cell Relay Bet Up-Making Burglar Alarm -flow to Build A.C. -D.C. Capacity Betsy -How to Make a Modern Radio Treasure Locator. USEFUL KINKS, CIRCUITS AND WRINKLES Making a Flexible Coupler-Two -Timing Chime -A Simple Portable Aerial-An Improvised Non -Slip Screw Driver. NOTE: The book contains numerous other useful Kink Circuits and Wrinkles. not listed here. (approximately) 45 ARTICLES (approximately) 170 ILLUSTRATIONS THIS COUPON BRINGS YOU THE ANNUAL 68 BIG PAGES RADIO -CRAFT 20 VESEY STREET NEW YORK, N. Y. Gentlemen: Enclosed you will find One Dollar for which enter my subscription to RADIO -CRAFT Magazine for Eight Months. Send me ABSOLUTELY FREE "N and POSTPAID, my copy of 1940 RADIO -TELEVISION REFERENCE ANNUAL. This is a new order O Extend My Present Subscription RADIO -CRAFT NAME ADDRESS 20 VESEY STREET CITY STATE NEW YORK, N. Y. DON'T DELAY - MAIL TODAY! RC -541 678 RADIO -CRAFT for MAY, 194 SOUND SOUND ENGINEERING `Ttee l7esiyn and ldvisoty Setviee 7ot Xadio- CJtatt .Sáhcti d ets

Conducted by A. C. SHANEY

This department is being conducted for the benefit of RADIO -CRAFT subscribers. All design, engineering, or theoretical questions relative to P.A. installations, sound equipment, audio amplifier design, etc., will be answered in this section. (Note: when questions refer to circuit diagrams published in past issues of technical literature, the original, been intentionally drawn as a resistor, or a copy of the circuit should be supplied in order to facilitate reply.) which is what it actually is insofar as its A.C. function is concerned. No. 17 In the analytical diagram of Fig. 2, ItEg represents the input voltage to the control - grid of the 6J7 multiplied by its amplifica- LOW- FREQUENCY ATTENUA- have inverse feedback and they drive a tion factor. Item rp represents the plate 12 Sounds FOR RECORDERS -inch speaker in a reflex cabinet. resistance of the tube; rl, its load resist- TION like a mess, but it works beautifully, except ance; Xc, the coupling condenser, and rg, The Question . . . for the recording. I have a preference for the control -grid resistor in the succeeding I have built an amplifier for radio and low "bassy" sound, so I did not mind the 6F6 grid circuit. recording but I have some difficulty with too radio set playing the way it did, as it was The voltage Eg2, which appears at the much bass response. The tube lineup I have perfect as far as I could see, and others control -grid of the 6F6, will depend upon is a triode 6J7 to a triode 6F6 to a pair of thought so too, but it is too bassy for re- the value of Xc, assuming that rg, rp and 6F6 triodes to a pair of 6L6s. The last two cording. Thank you for any aid you may be rl remain constant, which is the usual con- stages are transformer- coupled but the 6.17 able to give me. dition in an ordinary amplifier. If Xc is and 6F6 are resistance- capacity coupled, At. Rvore, made large, it is obvious that only a small using a 0.05 -mf. coupling condenser while Maywood, Ill. portion of the voltage that appears across the 6F6 to pair of 6F6s is transformer - rl will appear as Eg2. On the other hand, coupled; the primary is not connected direct- The Answer . . . if Xc is made small, then most of the voltage ly to the driver tube, but through a 0.25 -mf. As you did not include a circuit diagram which appears across rl will appear at Eg2. condenser. with your question, I presume that the Now, if you will imagine that a condenser I have read and also been told that these basic coupling circuits of the 6J7 triode to is a resistor, which has the property of coupling condensers will control the fre- the 6F6 triode, and its associated input varying its value inversely proportional to quencies that are passed, but I do not un- transformer is similar to the assumed dia- frequency, you will have a pretty good pic- derstand how this is accomplished or what gram indicated in Fig. 1. ture of what actually happens in this cir- the technical reason is. Could you tell me You can attenuate the low frequencies by cuit. For example, if you were to calculate how I may cut these low frequencies so decreasing the 0.25 -mf. coupling condenser the capacitative reactance of a 1 -mf. con- they will help me in getting a better re- from the plate of the 6F6 into the push - denser at 100 cycles, you would find it to be cording? The radio receiver sounds all pull input transformer, or by decreasing the approximately 1,600 ohms. A 0.5 -mf. will right, but when it comes to recording, it is 0.05 -mf. coupling condenser between the be approximately 3,200 ohms. A 0.05 -mf. not right. As you perhaps know in record- 6J7 triode plate and the 6F6 grid. will be approximately 32,000 ohms. At. 50 ing, the amplifier can be in the higher regis- To understand how decreasing this cycles, however, a 0.05 -mf. is 64,000 alms, ter but still make a "fidelity" recording, as coupling condenser will decrease the low - 0.01 -mf. condenser at 50 cycles is approxi- the recorder will build up the low frequencies frequency response, it is necessary to re- mately 320,000 ohms, and a 0.001 -mf. unit (this I have been told). Could you explain draw the coupling circuit between the 6,17 is 3.2 megohms. a few of these bugs to me? and 6F6 of Fig. 1 into the circuit indicated If rl (in Fig. 2): is small compared to Well, to complete the story, the 6L6s in Fig. 2. The coupling condenser Xc has rg - Xc at any frequency, then the voltage

(PgAOT) 6F6TRIODE TO GRID OF 85 DEM., AMP O.2 TO 6F6 -' MF TO 42 42 611 TE TORE 'MF CONTROL IF. 0.2- / MF.

SPEAKER 9 TO GRID OF 6F6

-. ASSUMED COUPLING CIRCDITS-

10.000 -0 I- OHMS MEL, A r MF 2.000 MF DH0MS MF OHMS

P T. ANALYTICAL COUPLING CIRCUIT"- 4

-3 Amer.. ww yFIILIIj 310v Mram lllA wni 11A 61í1 ti o 4l 5 250V. In 6 AVAlirlrl> 15CIMOSO O 661 (A.V C) I NOT SHOWN .sE A,. INCLUDED 111Í1ii IN CIRCUIT ai O V. WgIIMlf, -OMS O IO AIMI v----mil. : ¡Z 1000 r-IAf>20 50 100 200 500 1127 RECT FREQUENCY CPS. 52 3, OHMS CHART SHOWING LOSS INCURRED AT LOW FREQUENCIES BY DECREASING CAPACITY ^BOR GINAL CIRCUIT -STROMBERG- CARLSON MODEL 68", FIG.3 OF COUPLING CONDCNSERS (SEE TEXT) FIG.4 RADIO -CRAFT for MAY, 1941 679 SOUND

PREFERRED o BY

U. S. GOVERNMENT 100 " IN THE NATIONAL STORM- DEFENSE PROGRAM PROOF SUPER- POWER, COMPACT DR -72 SIX FOOT TRUMPET ®.utv, >CO oa.ur. vw4> S29.40 *ATLAS SOUND NET by ens b preferred which appears as Eg2 will be equal to possibility, however, that you are losing by the U. S. Navy. Arm. high frequencies in your input push-pull for dalÌaonceitable applications under the severest rg pe ntiná Billons. Such colt loÿ Is tuas idence f and coupling transformers. This high -fre- high qu ISO tandards. ATLAS SOUND PREFERRED bye SOUND ENGINEERS fomicrophonewe greatest selection quency loss will subjectively produce the of speakers, projectors. bellies. tando, and Rc +rg connectorslors °lor all P. a. punisses. ATLAS SOUND 'PRE. same results as excessive bass boost. The . . by RADIO J for datent 1 we of Sound If designate Xcl as a smaller condenser, way to check for excessive bass or excessive ith assurance of depenJobbersiiy and aaticf ction. which we describes intend to substitute in place of high- frequency attenuation is to make a re- * FREE CATALOG F-40 and accessoriess '0Iles for'meekeb cry Xc, then the ratio of voltage which appears p. Send for your copy sponse run and compare the output with a at Eg2 using first Xc and then Xcl is cycles. ATLAS SOUND mid -frequency of 600 CORPORATION equal to 1146 89th STREET BROOKLYN. N. Y. For a more detailed discussion on this rg phase of the subject, please refer to "Bal- anced Audio Spectrums," which appeared Xc+rg in the Sept. 1940 issue of Radio -Craft, pg. 164. rg IMPROVEMENT OF RADIO Xcl rg RECEIVER AMPLIFIER which may be simplified to The Question .. . rg Xcl In the March 1940 copy of Radio -Craft, pg. 531, you published a circuit including Xc rg a simple self -balancing phase inverter work- As this is a voltage ratio, it may be ex- ing into a pair of 6L6s. pressed as loss in db. by multiplying its (1) Is this circuit applicable and suitable log by 20 so that our final formula becomes WOODSTOCK for replacing the audio end of the Strom - Xcl rg berg- Carlson model 68 shown in the accom- TYPEWRITERS loss in db. = 20 log panying diagram, Fig. 4, for home use at Re + rg moderate ta high output? In actuality, the impedance of rg and Xcl (2) What can I expect in the way of fre- ELECTRIC LANTERNS is equal to the square root of the sum of quency response from control -grid of 85 to their squares, so that the correct technical $3.19 secondary of output transformdr? No 400. Price formula for the loss in db. when condensers (3) What is the probable output? Eß$3.19. 1500 foot are substituted, becomes Power Beam. Two re- (4) Can you suggest any further changes, flectors. Steel con- VXc1+rg or additions in this circuit? struction. Finished ebony baked enamel. loss in db. = 20 log The lineup of the present circuit is:- Takes standard 6 v. Bat- \/Xc +rg 6D6 R.F. 6B7 A.V.C. 6D6 I.F. tery. To simplify the application of this formula, 76 Osc. 42 1st Audio 85 Dem & 1st the chart of Fig. 3 is given. A casual ob- 6A7 Mod. 42 P: P.Output Audio servation will disclose that a 10.4 db. drop 5Z3 Rectifier will be encountered at 50 cycles when the (5) Is there a simple explanation and cure 0.05 -mf. condenser is changed to a value of for the type of distortion, accompanied by -411. 0.001 -mf. This chart has been prepared by steady whistle or hiss. which is sometimes No. 500. Price $1.48. assuming rg equals 1 ntegohm. It is also stopped by grounding the speaker frame, 1000 foot Power Beam. assumed that the plate resistor and load or bypassing the grid- return of the output Size: Height 7 ". Steel resistor, in parallel, of the preceding stage stage with up to 8 mf. (oscillation ?). construction. Finished are negligibly small in comparison with rg. CHARLES D. Hess, M.D., baked lacquer. Takes When this does not hold, the attenuation Hainilton, Canada standard 6 v. Railroad is by The type Battery. less than that shown the curves. exact amount of attenuation can be cal- The Answer . . . culated, but it is felt that this extreme (1) The audio circuit (Fig. 1, March 52.69 accuracy is not required for purposes of 1940 Radio will considerably im- discussion. -Craft) this prove the present radio receiver audio am- No. 600. Price From your letter, I presume that you are plifier. Eß$2.69. 1500 foot positive that you have excessive bass. I (2) The frequency response of the am- Power Beam. Steel can therefore safely guess that you are using plifier circuit should be substantially flat construction. drawn a magnetic cutter. Magnetic cutters have a between 30 and 15,000 cycles. The 85 de- Unbreakable plastic tendency to build up low frequencies, be- Takes standard modulator will probably limit the high - lens. cause the amplitude of the cutting stylus 6 v. Battery. frequency response to approximately 8,000 depends upon the amount of current passing or 9,000 cycles. Shipping weight for any through the cutting head coil. As the cutter (3) The 6L6 amplifier will produce ap- of the above Models impedance decreases at low frequencies (this proximately 25 watts with less than 5% 3 lbs. Furnished with- is characteristic in all inductive devices), out Batteries. distortion. higher currents will pass through the head, (4) Your present amplifier can be im- which in turn will provide greater amplitude proved by changing the type 42 push -pull excursions. This All merchandise in original packages -never used. will naturally cause exces- output tubes to 6L6s and removing all inter - Money -back guarantee. sive bass. This phenomenon is also char- stage coupling transformers. acteristic of constant -velocity recordings. (5) The steady whistle or hiss that you All Shipments will be forwarded by Express In commercial work low frequencies are mention is undoubtedly a high- frequency Collect if not sufficient postage included. attenuated in order to avoid this condition. oscillation, which is caused by high -fre- WELLWORTH TRADING CO. It can easily be done by following the quency feedback (regeneration), and is suggested previously. There is a stopped by grounding core 1915 So. State St. Dept. RC.541 Chicago. III. method usually the of 680 RADIO -CRAFT for MAY, 1941 SOUND the output transformer, or one side of the secondary, or both. If the high potential plate or screen -grid leads of the power output tubes run close to any of the input circuits, they should SPRAGUE and NATIONAL DEFENSE be shielded. National defense needs first! DETECTIVE AMPLIFIER come Sprague is putting them first -fully and whole- . The Question .. heartedly. One of our two big factories in North As a subscriber to Radio- Craft, I am Adams, Mass., is now devoted almost solely to presenting my problem: to build an ampli- defense work. fier for detective work, for which I have ATOM Meanwhile, our other factory is turning out more frequent calls. The amplifier should be light- CONDENSERS weight and portable (frequently have to Sprague Condensers, Koolohm Resistors and Test Equipment for the Radio trade than carry entire equipment by hand 4 or 5 ever before. blocks after leaving car). Deliveries are prompt. Full Sprague quality is being rigidly 10 watts output for operating loudspeaker maintained. Development work continues as and recording. in the past. Gain 130 db., or better, so as to pick up KOOLOHN Thus, thanks to Sprague's size and unique facili- HLSISTORS weakest voices. ties, we are able to put defense first -and still not Mike should have high output and be neglect our longtime friends of the Radio profession. sensitive to weakest sounds. What type of microphone shall I use? TC TUBULARS Sprague Products Company Microphone, of course should be 200 ohms, North Adams, Mass. so as to run long lines. Dual frequency control, without cutting gain, so as to get different voices to best TEST EQUIPMENT advantage. Greatest amount of A.V.C. without distortion (and without increasing size and weight of amplifier too much), so as to eliminate blasting and at same time, pick up faint voices. Hum and tube noise at minimum (way down) so voices can be understood, as in detective work it is frequently required to snake a typewritten transcript which means that every word spoken must be understood without chance of error or guessing what was said. Appreciate very much if you can give us circuit diagram of amplifier to meet above requirements. C. I. SCHUPP, Redondo Beach, Calif. These Easy -to -Sell- Giant -Sire The Answer .. . a usable gain of 130 db. throughout the Your question is typical of many received entire audio spectrum. which we can not intelligently answer be- To introduce an A.V.C. circuit, which AIR CIRCULATOR FANS would cause of the indeterminate specifications provide a usable control range, WILL NET YOU given. For example, you state the amplifier would add considerably to both the cost should be lightweight and portable. It is and complexity of the circuit. To design $15 to $35 PROFIT virtually impossible to design an amplifier an amplifier in which hum and tube noise ON EACH SALE is in accordance with your requirements, and "way down ", you would have to define Make Sig Money -sell these "how is flashy- chrome. giant-atze Cirri- have it lightweight in the true sense of low down ". ' : Jailor Ions to the stores, restau- ts, beauty parlors. taverns,rns, the word. I wonder whether you realize how I hope you don't mind this constructive officesVices and factorles In gain 130 In much db. actually is. the pres- criticism, for if it helps other readers state Cheapest summerr cooling. ent state of the art, it is extremely costly their problems more definitely, then it will cools and lets people bette . shoopp m to design a really quiet amplifier providing have leisurely. and rest or la in Corns accomplished its intended purpose. fort. No sales realstancel We sell you these fans at rock - bottom prices. 812.80 buys the powered CounterhModel: for $4.25 additional you get the $60.00. Technical Review of Catalogs N d F.0.11. Chicago, shihipped K.D. Fans come with cadmium ppaaetd guards. columns and Ohmite Catalog No. 40.- Contains not and Matched Power Supplies. and 12-ft.throme cord with on- a switch. Assembled In 5 minutes. 300ro only the usual illustrations and descriptions Sears, Roebuck & Co., "Silvertone Sound profit per fan. of company products, but also contains such Systems" Catalog.-Contains a useful Cet started now and you can grata tech- $$2.350 ruer like Wilson; useful information as how to select a re- nical page on 1.020, like Whitlow: $3,0011. like the selection and use of micro- lioell. Order your demonMmtion sistor, characteristics of resistors at high phones, amplifiers, and loudspeakers for fm, n w na copy of our whole- wSall, yawl.. today -it's Jam-Packed frequencies, how a tapered winding func- various services. ith electrical merchandise at bar- goin i -s. tent to DEALERS oil tions, selecting proportional and tapered Birnbach General Catalog No. 41. -Con- RESALE MANUFACTURERS ONLY. rheostats, circuits and data on the control tains a useful article on determining the of motor speed, selection of rheostats for lengths of the elements of ultra- high -fre- y4 H.P. SPLIT PHASE MOTOR BARGAIN Our purchasehate is lamp dimming and for remote indicators, quency antennas for transmission and your gate. selection of generator power controls, Ohm's reception. famous maker. Serves x4.99 1ÌO1 V.. (In C. A.C. Law in chart form and application data on General Catalog No. 41. -Just issued by 1750 a.P.M. ','' shaft, 3' 41110i base to shaft center. 3- g 4,.' Ohm's Law for A.C. and D.C., definitions of Atlas Presa Co., this publication is of ex- bolt hule,. Buy now and as,. y over 25, -,. resistor terms, parallel resistor chart, and ceptional interest to shop men who own 357 ca. useful reference data as for example, wire power tools. In addition to catalog informa- $4.99 net f.o.b. tables, etc. Write on company letterhead. tion on the regular line of Atlas lathes, Thordarson Transmitter Guide, No. 344 -E. Catalog No. 41 also illustrates and concisely W. W. Grainger, Inc. -This catalog includes circuits, photos 821 W. Congress St. and describes an exceptionally wide range of Chicago. III. construction on fixed data and mobile phone power equipment including lathe attach- Send WHOLESALE code CATALOG and transmitters, with outputs from 12 ments and accessories, milling machines, Ship 20. Counter Fan @ $12.69 net to 1,000 watts at various wavelengths. Of shapers, drill presses, vises, and grinders, D Ship 20' Floor Fan @ $10.94 net particular interest are the articles on Class and arbor, hydraulic and general presses. o Ship ilk HP. Motor @ $4.99 net B Output Calculations. Driver Transformer Of particular interest to many radio shop Same Ratios, Matching Class C Loads of Modu- owners and garage radio- mechanics, are the Address lators, Matching Universal Transformers, descriptions of armature servicing outfits. (Paste Coupon on la Post Card and Mall Today)

RADIO -CRAFT for MAY. I 94 AA1 SERVICING

RCA VICTOR MODEL VHR -307 HOME RECORDING - PHONO - RADIO COMBINATION (Chassis No. RC -555)

12 -Tube Superhet.; Automatic Volume Control; "Magic Eye" Indicator; Microphone Preamplifier; Power Output 18 watts; 15 -inch Electrodynamic Speaker; 105-125 volts, 60 -cycle operation; Crystal Pickup; Crystal Cutting Head. Ranges: 540 -1,600 kc.; 5.8 -18 mc.; and, a Spread Band of 9.34 -9.86 mc. (See Data Sheet 307 for additional information.)

682 RADIO -CRAFT for MAY, 1941 SERVICING

307 Radio Service Data Sheet

RCA VICTOR MODEL VHR -307 HOME RECORDING - PHONO - RADIO COMBINATION (Chassis No. RC-555) 12 -Tube Superhet.; Automatic Volume Control; "Magic Eye" Indicator; Microphone Preamplifier; Power Output 18 wafts; 15 -inch Electrodynamic Speaker; 105 -125 volts, 60 -cycle operation; Crystal Pickup; Crystal Cutting Head. Ranges: 540 -1,600 kc.; 5.8 -I8 mc.; and, a Spread Band of 9.34 -9.86 mc. (See Data Sheet 306 for additional information, and main diagram.)

SUa-G CAT MODE CURRENTS 1.113 OID - ' (^O° `a**) REGT. (11) 6F63G _ - _'21.5 K4. 1400,.1 1 uaP4O7.ge4NRM4 117)0i66 --- 25.5 Mk CMAN.LV$T. eco/ (15) SUG ---132.0mA. 6F6G 1'LL VOLTAGES SHOULD HOLD OUTPUT _ t'= alA O N * 20% WITH 117V. AC. , ea SUPPLY. IIP úl si G. BOTTOM v1Ew C2 TUBE SOCKETS^" ó TO'rín.L T2 R0-Val. 1-1:77_,C2 RSA ml _-4rL7 Q s KL mat 34oA BULL'S Rao CIAMP pRO+.N REO eacwH ;;Pr°" LK.óe L17 sorrow Ç13-01C.500 Mr. 65 KC 425 KC. 4fS KC. -22 lócK[i

OOw eR 5w. Y WOKS. PK.va BASS TONI CONTROL) SLACK USE ONLY JACK METAL - 15E pro PILL. A s TYPE TUBE DUPLICATE / eL'K- 02'N MNO AC 125es SOCKETS Leff, chassis view showing locations of tubes, main components, and trimmers. Right, schematic diagram of the power supply and output stage. (Continued from Data Sheet 306) 40TO 1O TO I YO TO Connect the high side 1 Tune test- I Turn Radio 'Adjust the following for . Steps I KC I aO KC I iO KC 10 KC of the test-osc. to- osc. to- dial to- maximum peak output

1 I Turn "Treble counter-clockwise so that I.F. is in "Sharp" position. Tone Control" 0 0 0 0 0 0 l e 0:Kiws 2 2nd I.F. control -grid L17 and L18* coR in series with 0.01 mf. (3rd I.F. Trans.) o e e e : 0 e e RODS T 3 I 4 3 1st I.F. control -grid "A" Band Quiet L15 and L16* in series with 0.01 mf. 455 kc Point at HF end (2nd I.F. Trans.) 4 1st -Det. control -grid L12 and L13* in series with 0.01 mf. (1st I.F. Trans.) GG Turn Treble Tone Control full clockwise to "Broad" position. Response on CRO 5 should be the conventional double-humped type. If necessary, retouch 3rd I.F. transformer slightly (so as not to disturb the "Sharp" curve appreciably). Leave control in sharp position for the following steps. 1=1 "C" Band C21 (osc.)es LG 6 15.2 mc. mc. C17 (det.) *** essa Ant. terminal, in series 15.2 C8 (ant.),** with 47 nand. (link C28 (osc.) closed)f. "31M" Band 0.5 mc. 9.5 C14 (det.) mc. C5 (ant.)*** 1!1 "B" Band C22 (osc.) e!I 8 2.44 mc. 2.44 mc. C15 (dei.) Rear stator "A" Band L10 (osc.) 9 of gang, 600 kc. 600 kc. Rock in in series with 0.01 mf. A" Band C23 (osc.) 10 1,500 ke. 1,500 kc. C16 (det.) mewN

11 I Repeat steps 0 and 10. Install and connect chassis in cabinet with antenna link closed. Tune -in a 12 radiated oscillator signal at 1,500 kc. and peak the "A" band twnmer Cl (on t=1 loop). Rock-in L10 for peak output at 600 ke. Ns Ad'uat for coincidental curves and maximum gain. efi "Use minimum -capacity peak if 2 peaks can be obtained. (Check for correct peak on "C' band by tuning receiver to 14.29 sac., where a weaker signal should be received.) PO * **Rock in.

Calibration Scale. -The glass tuning dial (3) Refer to calibration scale printed in may be easily removed from the cabinet and this Data Sheet. This is a reduced reproduc- e temporarily attached to the chassie for quick tion of the dial with an inch -scale drawn PO reference during alignment. In the event at top and bottom. To find the correct n1 that only the chassis is returned for service, pointer position in inches for any desired and the cabinet. with its tuning dial is left frequency, draw a vertical line through this in the customer's home, the calibration scale frequency on the calibration scale. printed in this Data Sheet can be used. Each method is described below. ó 1=1 Using Tuning Dial. - LG (1) Slide not the flat spring clamp at each end of the dial, and remove the glass from the cabinet. ó (2) With gang in full -mesh, movethe dial pointer to the reference mark at the left -hand end of the dial backing plate. 13) Place the glass dial under the pointer so that the extreme left scale graduations coincide with the pointer. Use scotch tape to hold the glass dial in this position. 1:1R I I=

Using Calibration Scale. - Top, adjustments for the teflon tuning pu hbuttons. (1) With gang in full -mesh, move the Above, the calibration chart used in aligning the dial pointer to the reference mark at the receiver see text). left -hand end of the dial backing plate. (2) Place a flat, 12 -inch ruler on the dial 4 RCA Victor model VHR -307 receiver. This com backing plate so the left-end of ruler is bination instrument ncorporates the facilities at the reference mark at left -end of backing Of home recording, playback (and record phono- plate. Temporarily fasten the ruler with graph), and radio receiver. The latter incorp scotch tape to the backing plate. a 'spread" band.

RADIO -CRAFT for MAY, 1 941 683 SERVICING

of receiver, due to voltage surge and also NEW FM AUDIO AMPLIFIERS sometimes due to the breaking -down of the electrolytic condenser. Designed by A. C. Shaney To overcome this trouble once and for all, FREQUENCY RANGE: 13- 30.000 c.p.t. ±I db place in series with plate lead of the POWER RANGE: 0.003 to 30.000 Milli/AD, the MUSICAL RANGE: I I Octaves 25Z5 rectifier (note -lead nearest the speak - DYNAMIC RANGE: 80 db et' field) a 2 -watt, 50 -ohm resistor. This will Outstanding in their performance,e, co truction, Cod appearance. These protect against such reoccurrences again. frequency modulationwe

. . . FERRODYNE R -137 AND R -I38 OPERATING NOTES Oscillation or lack of sensitivity on the low- frequency band, and /or failure to align ltGUi1Q in . . . properly at the low- frequency end of the broadcast band (600 kc.), in this set, may .... MOTOROLA 65 be caused by a defective, 11 -mmf. mica -and- in bakelite condenser which is connected across Speaker tattle on low frequencies, this the oscillator shunt padding trimmer. model, frequently may be cured through the use of a piece of 5/16 -inch rubber cushion, such as used around car doors, windows, .... SILVERTONE 1640 etc., long enough to go around the speaker Complaints: delayed A.V.C.; static; re- rim. Mount it between the speaker rim and ception interrupted and weak. This trouble baffle. frequently is caused by a defective 0.1 -mf. condenser in the A.V.C. circuit. (Note-It .... CHEVROLET 985538 is sometimes necessary to replace this unit Intermittent operation, and reception on with one of 0.01 -mf. Or better still, if this only the upper end of dial, in this model, does not cure it, try different values as this sometimes may be corrected by replacing the circuit seems to be critical.) Also change the antenna coil, part No. 1213859. (This assem- type 283 tube and at the same time, insert All the con- bly also contains the A.V.C. load resistor an R.F. choke in the plate lead of same. / venient operating and blocking condenser.) features of mod- .... SPARTON 931 . . . . CHEVROLET 985537 Usually, distortion, or failure to properly ` em engineering, This set sometimes exhibits high noise handle the output power, may be caused by all the economies level. a defective cathode bypass condenser. L of modern manu- A 0.5 -mf. metal -clad generator condenser facturing, all the skill acquired placed from spark plate to chassis housing EDISON RI AND R2 The ears .... in building many thousands of will remove 90% of the noise. on Complaints: hum; and, weak station re- the condenser can be pushed down between ception. lathes is incorporated in the held by the the chassis and the housing, and Change the detector and 1st audio tubes. screw already in place. 1941 Atlas 10" and 6" Lathes. self- tapping Replace with type 56 tubes in place of the present ones. It will be necessary to remove ATLAS PRESS COMPANY .. . CHEVROLET 985537 the gridleak and condenser, changing the 545 N. PITCHER ST., KALAMAZOO, MICH. Intermittent high hum level in this set detector bias by using a 1 -watt, 45,000 -ohm has been traced to the filter condenser, part resistor and bypassing same with a 0.1 -mf. No. 7239053, which develops a high- resis- condenser. Also change the 1st audio fre- tance short between sections. This is a quency cathode resistor resistance to 2,500 triple- section condenser having two 10 -mf., ohms, and bypass it. PATENTS -TRADE MARKS 400 -V. sections for illB" voltage and a 20 -mf., Booklet concerning Inventions & Patents V. 25 section for output bypass. . . BRUNSWICK 22 Form "Evidence of Conception" with instructions for use and "Schedule of Complaints: periodic reception; and Government and Attorneys Feae" -Free .... CROSLEY 555 noise. LANCASTER. ALLWINE & ROMMEL, A report of "intermittent; and, reception To cure, replace the A.F.-circuit coupling Registered Patent Attorneys. on part of the band," in a Crosley 555, may condenser. Its value is 0.02 -mf. 436 Bowen Bldg. Washington. D. C. be due to a shorted 0.01 -mf. condenser from GEORGE F. BAPTISTE, wave -switch to oscillator plate (No. 14 in Howard, R. I. diagram). ELECTRIC EYES . . . RCA 46X3 . . . RCA 9T2, 97K, 99K Special 'e.c fpr a limited tun` only. on elerinr ere Hum, after heating up, burglar al ras. These ro -lxt of: One taan tube how that is not due One light source with lens. transformer and b lb. An intermittent drop in volume may be to a defective 50L6GT output tube, or to a Orelay.ne alarm it itL lay. ben and batteries. Compplete with all tube., bums. cella, ball, and batteries. for only due to defective A.F. coupling condensers. shot filter system, may be caused by the $10.00 fob St. Charles. Sent COD rash with order. It o. satisfied return to In good condition, in Ave rectifier tube's wiring days and money will refunded. heater coming in con- finish We supply thesheavybakelite . . . PUSHBUTTON SETS tact with the control -grid of the 12SA7 cases tiling radios for Ìn bed room and hotel .A two piece it having self contained busses. Noisy pushbuttons, in radio sets using mixer. This will have the effect of hum - Dtué cot, rent. to egreuUnitsante ins maatierrial cost. Ohéel prices may be modulating the oscillator m good only until supply condenser substitution methods, frequency and a present is easily repaired by the following process. tunable hum will be present when ELECTRONIC the re- PRODUCTS COMPANY, St. Chifles, IRMIWO Clean both bottom and top sliding con- ceiver is tuned to a station. Off resonance, tacts with carbon tetrachloride, and bend the the hum will diminish. Separation of the stationary contact fingers down until a leads cures the trouble. FACTORY -TO -YOB/ bright surface is obtained where the fingers Nf /tIS Aft/1944 touch. .. FADA 5F60 -T 15TUBE °"11e EARL GARNER, Intermittent howling, which stops if the RADIO-PHONO Knightetown, Ind. wn dans N COM set is touched, may be caused by the output YOUR MOOR aim RADIO.P110110 tube's plate condenser opening up or coming CRAM .;d . SPARTON 61 AND 62 Ad SPIRO .. loose, causing audio regeneration. pr125¡r.ADg.IMi0 ea Meal Medal I"_ Flashing of the 25Z5 rectifier is quite WILLARD MOODY, iRE[ M.Iry,od.y. wae Dept. 12-11 ulowtsr tapio cóuiourlor.. cb.x.ad, ow raGG common, in the early models of this make New York, N. Y. 684 RADIO -CRAFT for MAY, 1941 SERVICING

BANO'B 1251(7 TO TO I F 5.25 MC 1852 I.F. 5.25 Mc- 6H6, 2Na-DETEC7OR A ANTENNA R.F 1ST.I.F 2NO.I.F IIMITER.FM. y Ra ol------MF BAND'A' 5G000 OHMS

O.1- MEG. TO A. F 0.25 - 01- MF (EACH) MF o MEG . NEW CIRCUITS I MODERN RADIO RECEIVERS In this series, a well -known technician analyzes each new improvement in radio receiver circuits. A veritable compendium of modern radio engineering developments.

F. L. SPRAYBERRY No. 44

(FIG. I.) SIMPLIFIED INPUT SWITCHING Somewhat more A.V.C. voltage is required, new. In this circuit we find the customary PROVIDED BY LOOP CIRCUIT for A.M., and additional filtering is needed equalizing resistors in each output grid and in the unfiltered A.V.C. unequal series grid resistors to assist in RCA MODELS 16X -11 AND 16X- 13. -In- as audio changes stead of using the loop alone for broadcast voltage are inherent in A.M. systems. As permitting more nearly equal signals from reception, the tuned circuit for the high - shown in Fig. 2, the A.V.C. line from the the direct and inverted circuits to be ap- frequency band remains in series with it. A.M. 2nd -detector uses a 0.5 -meg. filter re- plied to the output grids. Though commonly done in the case of 2 sistor. Moreover, the limiter and last I.F. or more coils, the association of a loop grid are supplied in parallel, and the 2nd - grid (FIG. 4.) PHASE INVERTER TRIODE COM- coil in this manner as in Fig. 1, detector which follows this last I.F. stage and a grid BINED IN OUTPUT -TUBE ENVELOPE represents a new and simplified circuit. The has a much greater output than the limiter would not contribution of the high- frequency tuned input. Moreover the A.M.-A.V.C. SIL%ERTONE (SEARS, ROEBUCK & Co.) circuit to the tuning of the loop circuit is be suitable for the F.M. circuit control be- 7330. -A triode using a cathode in common largely inductive and constant in value. cause its activity is confined to a very with the regular output pentode section of When high- frequency reception is desired, narrow band of frequencies. A small stabi- the tube is used as a phase inverter. voltage over a very broad frequency the loop is shorted. The high- frequency lizing In this circuit, Fig. 4, we find tubes of for F.M., from the limiter - trimmer will affect the loop tuning adjust- band is obtained, different types used in a push-pull circuit. grid return with less filtering needed. ment only slightly but it is best in this case A 6AD7G is used in one side of the circuit to make the high- frequency adjustments while a 6F6G is used in the other. The pen- no first. Virtually losses are introduced into PHASE INVER- tode section of the 6AD7G, however, has the loop as it is not (FIG. 3.) SELF -BALANCING the circuit by shorting the TUBES IN PAR- same characteristics as those of the 6F6G. magnetically coupled to the high- frequency TOR SUPPLIES OUTPUT ALLEL Apart from the triode connections the cir- coil except very. indirectly through the an- cuit is conventional even to the common tenna circuit. For high- frequency reception, GENERAL ELECTRIC MODELS J.1106 AND cathode resistor for self -bias of the output however, the coupling coil to the antenna is J- 1108. -Output tubes in a parallel push -pull tubes. The triode grid is fed roughly 1 /4th essentially shorted. arrangement may be driven quite as well of the total signal applied to the 6AD7G from a high- impedance self -balancing phase pentode grid in the same phase. The familiar (FIG. 2.) SWITCHING CIRCUIT ADAPTS inverter as an ordinary push -pull output phase reversal takes place from the grid A.V.C. TO A.M. OR F.M. circuit. to plate, the gain of the triode amplifier as HALLICRAFTERS MODEL S -27. -The require- Figure 3 shows the circuit connections. used being about 4. Thus by being coupled ments for A.V.C. in A.M. and F.M. circuits Neither the self- balancing circuit nor the to this triode plate, the 6F6 control -grid differ somewhat so that when the same I.F. output circuit is new by itself but their receives an equal and opposite -phase vol- amplifier is used for both A.M. and F.M., combined application with degeneration in- tage from the pentode control -grid of the a change in A.V.C. wiring must be made. troduced in the phase inverter cathode is 6AD7G.

FROM AF l

Áb MF. 0.33 MEG.

RADIO -CRAFT for MAY, 1941 685 SERVICING

(FIG. 5.) "WIRELESS" RECORD PLAYER NOTES USES SUPPRESSOR MODULATION OPERATING MODEL 22A. ordinary re- MOTOROLA -An izoUble in . (SCREENS) mote -cutoff pentode is used as a modulated 15 with cathode volume control. Or- 15 oscillator RCA R7A dinary plate coil feedback is used and the .... suppressor -grid can be biased to approach The oscillator of an RCA R7A which did plate current cutoff by means of the cathode not align properly, and which oscillated control. badly, after complete check (with all voltages correct) was found to have a defective A.V.C. At low bias of the suppressor -grid the volume control. It wasn't until I had re- is while the m intensity of oscillation large moved and replaced this unit several times, FLL.GIIN? percentage of modulation is low. As the each time giving it a careful examination, bias is increased the oscillation intensity is that I found the control was set with an ev. -ZENITH 48.106,131 WV- reduced but the percentage modulation is arm soldered to the back of the control by BEFORE increased. Since the oscillator intensity re- a drop of solder. Being very small, it was duces by a much greater factor than the easily overlooked. This solder had been loos- modulation percentage increases the effect ened by heat and the arm stop slid around is one of a gradual control of the output. to the wide -open position. The output may be adjusted to minimum To correct this trouble, replace the arm distortion using the receiver's volume con- and solder it down, when the resistance trol for actual control of volume. The usu- reading corresponds with that specified on al "wireless" record player has no means of the diagram of the set. The set will then adjusting modulation percentage or of cor- operate perfectly. rection of distortion inherent in some modu- C. L. MCGINNIR, lated oscillator circuits. Adel, la.

aEfilLDF1L.GaCNTZENIT1413 . . . ZENITH 4B, 106, 131 AND 132 106 When receivers come in with the 131d 13L- Mr. Radio Serviceman: these + AFTER of reception very weak, and only ieV.+ Tell us about the Operating Notes -faults complaint on high- frequency end of dial at all, that occur repeatedly, in the same set model, the the is due to the type 15 1st- - oscil- 'n the revamped circuit, I use a new, small, and their repair -which you have found. We it detector properly. By making unshielded Meissner oscillator coil. pay for all those we publish. lator not oscillating the changes shown in my diagram (shown here) this receiver works better than new. .... ADMIRAL A.C.-D.C. MIDGET In the revamped receiver, no changes are These sets sometimes blow out one 85Z5 Catalog F40 of Atlas Sound Corporation -This made in the I.F. stage except in the filament tube after another (also the pilot light) and piece of literature will be of especial interest to circuit (the type 39/44 being put in the show no shorts. The cure: connect a 36 -ohm Sound men because it includes 2 pages of photographic illustrations of practical sound installa- socket originally occupied by a type 15). Of resistor between the plate and the filament tions which may serve as guides in the selec- course, a new socket must be installed for of the 3626. (Originally connected together tion and placement of loudspeakers. the 6M. The changes shown give you A.V.C. direct.) Any A.C.-D.C. receiver using this This catalog describes over 100 types of P.A. on both the 6A7 and 39/44. Although the tube and having these symptoms, and having speakers and other sound parts accessories. original oscillator coil might be OK to use no resistance between the plate and filament

we're Not Tadiny, Mt. Setviceman NATIONAL DUES IN THE R S A ARE ACTUALLY ONLY $1.00 A YEAR

incredi- RADIO Yes-it seemed amazing ... the RSA Push -Button Changeover Plan ble ... but Servicemen found it was Get true. Now hundreds of applications are Are you ready to get your share of the Push- Button pouring in from all parts of the coun- Changeover business? Join the RSA now and get the think full year member- F. try! Just -a complete plan available to RSA members. tN Or ship in the National RSA for only $1.00 (that's less than 2c a week/. Protected territories will be established as rapidly a. local chapters are formed. Applicants in present chap- You get the RSA Membership Certificate and the RSA ter areas will be referred to the local chapter. House Organ. You have access to RSA Technical Helps Bureau, and you can participate in all the other func- It's your big opportunity. Fill out the coupon, attach tions and benefits RSA offers. a $1.00 bill and mail it in today. ei RADIO SERVICEMEN OF AMERICA, INC. RADIO SERVICEMEN 304 S. Dearborn Street, Chicago, Illinois 1 OF AMERICA, INC. $1.00 enclosed for 1941 National Dues in ESA. I "Reliable Service Assured" Name 1 1 JOE MARTY, JR., EXECUTIVE SECRETARY Address I 304 S. DEARBORN ST., CHICAGO, ILLINOIS I City State 1 Radie -Craft -May

686 RADIO -CRAFT for MAY, 1941 SERVICING

should have a peak limiting resistor of at . ZENITH RECEIVERS least 25 ohms in series with the plate and Many of these receivers have incorrect filament. bias on the 1st A.F. tube, causing distortion and low, sometimes varying volume. HOW TO SAVE ON . , . . SENTINEL 86ATE A simple cure for incorrect bias whether A high -pitched squeal at all settings of overbiased or underbiased is: ground the volume control may be caused by incomplete cathode. Ground the low end of the grid EVERYTHING IN RADIO R.F. filtering. To eliminate this trouble, resistor. Insert a bias cell in series with connect a mica, 350 mmf. condenser from the grid lead. bias cell to chassis. Some models in the Zenith line operate either on 110 V. A.C. or 6 6V. D.C. by the WILCOX -GAY A -93 AND A -94 flip of a switch. The last A.F. stage is a 2-V. .... (a 1JG), all other tubes being 6.3V. V If recorded "wow" is encountered in dual - tube SEND speed recorder units of the automatic record types. This 2-V. tube receives the correct 2 V. as per As can readily be seen, changer type used in equipment bearing diagram. -out of either of the 2 pilot FOR numbers to 624010, a the burning serial prior correction lights causes filament voltage on the may usually be effected by increasing the the 1.16 1 V., causing ALLIED'S tension of the intermediate drive wheel tube to drop to about spring. Remove 12 turns at the hook end of distortion. M. F. CROWELL, JR., NEW the spring. Straighten out 3 turns of the Crowell, Texas. coiled spring, and form a new hook so that the bend in the hook is only Sft -in. from the NirAaCATALOG Do coiled spring. not make a sharp bend in Hey Fellers . , . SOS! forming the hook. Instead, form a 1 /16 -in. We're running low on op. notes for this radius. department. Let's hear from some of you men who have never contributed before. We pay 19 WILCOX -GAY, R D1D space rates for printed notes. fN Charlotte, Mich. ?Ntl'fG

(In connection with the following reprint, see Editorial in this issue, Page 649) ONCE MORE iv q4ee/ "SAYVILLE 180 PAGES -AN ATTACK ON THE ELECTRICAL EXPERIMENTER" SHOWING (Under the above title, the October 1915 edition sages pregnant with unneutral information. yet of The Electrical Experimenter carried the inter - harmless on their face, could not be sent in spite EVERYTHING eating correspondence reprinted below. History is of all censorship. The imaginary case of the NEW IN RADIO its own indisputable evidence that "history re- message from the "Adriatic" was cited as an Petits itself." We can only hope therefore that e illustration. Anyone by paying the usual tolls can review of past events, may in some measure even now send such a message. The management throw into sharp relief for alt to see, the coming or the operators at the Sayville station obviously events which thinking men foresee. Editor) need not necessarily have cognizance that the message is an unneutral one. The two letters reproduced herewith require That the writer's viewpoint was correct is no comment. The one illustrates the German best shown by the announcement of Secretary of viewpoint, the other the American. We leave it the Navy Daniels under date of August 18, "that to our readers to decide which is the correct one. as a result of the demonstration that unneutral Dr. K. G. Frank, as is well known, is the messages could be sent through the Sayville sta- present executive head of the Sayville wireless tion he had issued orders that in all cases where station. On August 17 the Providence Journal the Government experts were in doubt about any laid before the U. S. Neutrality Board in Wash- message presented for sending it should be re- ington eight formal charges. One of these charges ferred to Washington for judgment." A GREAT BOOK OF RADIO VALUES was that Dr. K. G. Frank is the head in the As to the second paragraph in your letter your United States of what is known in Berlin as an attention is directed to page 210, September issue Just off the press! ALLIED'S. new Spring Catalog- Information Bureau (secret service). of The Electrical Experimenter. It gives facts crammed with smashing values from cover to cover: ALLIED offers you everything new to Radio. real savings The letters follow: with which you are doubtless familiar. These and fast service. Dealer. Serviceman. Soundman, Bulli r. facts disclose one of the main reasons why Say- or Amateur-whatever your Interest In Radio, you'll Mel ATLANTIC COMMUNICATION CO. ville was taken over by our Government. the new ALLIED Catalog a necessity! Send for y "ur (Telefunken System of Wireless Telegraphy) Your assertion that "The Electrical Experi- FREE copy today. Keep It handy for speedy referee .,.! 47 -49 West Street menter is joining some of the daily newspapers The Experimenter Publishing Co., In the contemptible attempt to cast suspicion NEW RADIO SETS VALUES IN P. A. Attention, Mr. II. Gernsback, Editor. upon Sayville and, further, that no single in- see them all- 1041's Radio 25 New Sound Systems! For Dear Sir: -With regret and surprise I have stance of a dishonorable act can be proved, but lilts! Models for every pur- Publie Address ppli- editorial in No. 28 of The Electrical rose. Every one a Champion ea ban 7 to 75 watts. widest read your also that there has never been any charge of -new 'Personal" three-waY selection: finest features. Experimenter on "Sayville." such act made by any official of the United States Portables:a latest Recorders: Easy Payment Plan and 15 to my knowledge your is the Plastic Day Trial. Complete P. \. According paper Government," is as perverted as it is unfounded. Ste; Wläoriew l ß accessory listings for every only technical paper which joins some of the FM -A111 receivers: Auto purpose. Dollar foryoudollar. Its tone is also resented by the writer. The Elec- annewd Farm Radios: Amateur when daily newspapers in the contemptible attempt to trical Experimenter certainly never attempted to Stn -all lowest prices 1041 KNIGHT d mily that spell Real Savings and Solidend for your FREE. cast suspicion upon Sayville. One would at least cast suspicion upon Sayville, but it has shown Fronts for you! catalog now! expect that your paper would take cognizance of that the station can, and perhaps has been used fact that not only no single instance of an r- the to convey unneutral messages, though not neces- FOR THE BUILDER 15,000 RADIO PARTS] unneutral act can be proved, but also that there sarily with the knowledge of the management or has never been any charge of such act made by its operators. If you build you . you The biggest selection o any official of the United States Government. voice nul oil ALLIED for the quality parts In all ra dl ". At the same time the writer desires to latest low. eo.t kits and for Every leading make -coin The standard of the technical and scientific his opinion that there is sufficient circumstantial help in building y type listings for of radio circuit. Complete áút` and s -ir. press in this country is, fortunately, so high evidence at hand to lend anyone who wishes to listings of up- Ing requirement. Parts,`ntool that I am convinced your paper will remain the and equipment, view the matter in its true light to believe that plies. Neweperimenters' diagrams. Com- Yoi, Mix reliablesGuide only one which distinguishes itself in such the management of the Sayville station probably Ham v for the to Everything In Radin a Apletemateur. ALLIED Is Head- the lowest prices! Don't he manner. had some knowledge of the real purport of the quarters for Radio Builder,! without it! Don't w an! Very truly yours, many "irregular" messages sent over the Atlantic or Radio Amateurs! Mail coupon TODAY! (Signed) Dr. K. G. Frank, by Sayville before the station was finally taken New York, N. Y., August 17. 1915. over by the United States Government. The slur contained in your last paragraph is best met by bringing to your attention the fact Atlantic Communication Co., 833 W. Jackson Blvd., Dept. 2 -E -1 New York City, that The Electrical Experimenter to-day is con- Attention Dr. K. G. Frank sidered an authority on wireless matters in this Chicago, Illinois country. As such it is its duty to publish any Dear Sir: - matter of interest to the wireless world. It will Send ms your FBEB 180 -l'ago 1911 The writer was indeed surprised to receive distinguish itself in the future by continuing to Spring Catalog. your communication of August 17. He is at a do so. It will also continue voicing its opinion loss to understand how you could possibly mis- especially at times when the welfare of this construe the true meaning of his editorial in country is concerned. Name ... view of the fact that at the time it was published Very truly yours. Sayville had already been taken over by the Address Government. What the editorial meant to convey The Electrical Experimenter Co. was even the had (Signed) H. Gernsback, Editor that though Government taken L Clty State over Sayville. it was not at all certain that mes- New York, August 30, 1915. -J

RADIO -CRAFT for MAY, I 941 687 SERVICING

RECORDS RECORD-nOLDER SHELF Millions of "phono- radio" sets,

RE CORD -n OLDER and "record players," are now in SPINDLE POST use. Servicing the record -playing PICK UP - ARM systems -both manual and auto- PICKUP matic -in 9nany instances pre- INDEX ANO RECORD sents real problems . REJECT LEVER. for the tech- RECORDMOLDER nician. Mr. Moody tells how he POST PICKUP REST NEEDLE GAUGE PLAT& has licked many of these "posers." RECORD- HOLDER SHELF NEEDLE EJECTOR TAB The article concludes with detailed USED NEEDLE BOX information concerning a repre- TURNTABLE TURNTABLE sentative type of automatic WITH RECORD IN PLACE SWITCH REJECT record changer. NEEDLE SCREW It INCH IO INCH NEEDLE MOLE MANUAL. 4 Top view of the RCA Victor Automatic Record Changer. SERVICING RECORD PLAYERS WILLARD MOODY

THE record players used in RCA, Strom- in the Farnsworth automatic record changer justment. Tightening the screw increases berg-Carlson, Westinghouse, General a similar trouble very often evidences itself. the pressure of the collar, and conversely, Electric, Emerson, Philco and Farns- That is, the turntable makes a noise as it a loosening of the screw decreases the worth radio - phonographs are familiar revolves, and inspection will 9 times out of pressure. If too tight, the rejection may to the writer through actual and first -hand 10 show that the pin which evidently is in- occur in the middle of a record; if too loose, experience. In writing this article, then, we tended for a recording blank disc has it may take several seconds to reject -or shall try to be practical, leaving out some slipped down into the motor or else that it may fail utterly to reject. of the complications and sticking to the the spring which props up this pin has subject of what is likely to be the matter dropped and is hitting against a motor bolt. RECORD TROUBLE when the average unit of this kind gets out The remedy is to push the pin out com- Binding of order. and breakage of records may be pletely, with a long screwdriver; or, in an due to insufficient clearance between record emergency, so to speak, use may be had blades or knives. On the underside of of the REPULSION MOTOR a long, thin -bladed kitchen knife, to pry lowest knife in each set of two is an ad- First, the simple record the spring out of harm's way. The Farns- justing screw with player will have worth locknut that controls a relatively simple motor. The majority, automatic record changer known as the amount of blade separatiop. A record these days, are of the rim -drive variety, the P -2 is typical of this type. may be placed in position and this adjust- although some earlier and even present -day ment affected without great difficulty. Oc- RCA units use the induction -type motor, D.C. OPERATION casionally, a record knife will have a rough or rather, the repulsion -type motor, similar In many sets operated on direct current, edge which will need to be filed, followed by in its action to the ordinary wattmeter used a vibrator will be used for supplying A.C. a smoothing operation with fine sandpaper. on alternating current. to the small phono motor, with the result On all record players in general, of the This repulsion type needs a push to make that all of the troubles usually associated crystal pickup type, the A.C. voltage meas- it start. When, having been given the nec- with vibrators are present. Electrical noise ured with a vacuum -tube voltmeter, when essary primary impetus, it still refuses to is common, but may be reduced satisfactorily the pickup is "idling at the center of a turn, the chances are that the aluminum by a heavy condenser (0.5- or 1 mf.) across record," will be close to 2 volts. This may piece, circular in shape, which is attached the motor. Vibrators often have to be re- be used as a figure -of- merit, or criterion of to the turntable's underside, is hitting placed, se check before tearing the motor crystal sensitivity. since the voltage will against the rest of the motor. The RCA apart. drop off if there is anything wrong with VA -20 is typical of this type. Frequently, On the RCA record players, the common- the pickup. A fractured or broken crystal, too, the ball bearing on which the turntable est trouble is that of the tone arm not immediately broadcasts the fact that it is shaft rests may need oil or may need to be swinging back to repeat the cycle, or of the damaged, by evidencing a poor tone and replaced due to wear and tear. A steel buck- tone arm not hitting the edge of the record low output. In automatic record changers shot, such as is used for toy air guns, will as it should. Also quite common is the particularly, this is apt to occur, for the serve the purpose. Lead shot will not do. trouble which involves breakage of records tone arm will frequently be set down with Other troubles usually developed by this due to the binding of the mechanism when too severe a shock, due to carelessness on particular motor are: burned -out coils and a record becomes jammed in the record the part of the owner or to mechanical loose wires; or, the motor switch may be knives. failure in the mechanism. open- circuited. Sometimes, the turntable As for the first mentioned trouble, the will wobble too much due to loose rubber tone arm swing may be adjusted by loosen- LUBRICATION cups not holding the underside securely to ing or tightening a small screw at the end In many record changers, of whatever the revolving disc. Duco cement fixes that. of the tone arm lift cable. Very often the manufacturer, oil and grease are frequently adjustment is one to take up a certain needed to ensure proper operation. In cold RIM -DRIVE MOTOR amount of slack. when the screw will first weather especially this will be noticed, since As for the simple rim -drive motor, fric- be loosened, then turned to tighten -up on the oil or grease will tend to gum-up. In tion that is lacking where it should be, or is the cable, and locked in position by a nut cases where the record player is new and too much where it shouldn't exist at all, on the screw shaft. has just been taken out of the carton, it are the usual faults. Taking a rag and wip- In some cases the opposite action may be may be found that it is either dried out of ing spilled oil off the rubber friction drive required. If the tone arm is swung to the oil or that there was a slip -up at the fac- or rim will permit better traction. center, near the part of the record with no tory and some one forgot to oil the ma- Typical of this type is the RCA V -100, recording impressed, where the arm cus- chine. Those things do happen, strange as which also is sometimes afflicted very com- tomarily "idles," the cable may be adjusted it seems. In servicing in the home, a bit of monly with another trouble, that of me- so that the arm is raised just 1 in. above 3 -in -1 oil and some vaseline borrowed from chanical noise arising out of the condition a record on the 'turntable. Sometimes it is the owner's medicine cabinet will do the job. wherein the rubber sleeve or cover on the necessary to try 8 or 9 records and check Record player troubles that aren't really drive wheel has slipped from its proper po- the adjustment so that the underside of the troubles at all often crop up, such as, for sition, with perhaps a segment of the metal 8th or 9th record will not be hit as the tone example, the very common one of a loose exposed, so that there is an intermittent arm swings upwards and out. and pulled -out cable of the Philco photo- and puzzling sound that appears to be a With regard to repetition of the cycle, electric -cell (or lightbeam pickup) outfits. click, slick, click, as the motor spins there is a small collar underneath the turn- In this particular form of record player. around. table and motor "board." It is located near the phono mechanism is pulled in and out To digress from the subject for a moment, the record reject lever and is a screw ad- to operate it, with the record player con- 688 RADIO -CRAFT For MAY, 1941 SERVICIN cealed when not in use; the pickup lead will often come out of its jack and may be se- 2 IC IT 3 9 34 2- 3! cured by simply tying it into position with a piece of tape or wire. Other troubles in 24 this type center around the head, which may sometimes need a new pilotlight or 3.S oscillator adjustment. A high- frequency 15 A.C. voltage is supplied to the pilotlight in the head, and anything which affects the ROLLER oscillator will, source of course, affect the 20 pickup. CAM PLATE Recourse should be made, wherever possible, to manufacturers' published data which 22 are replete with full technical details needed in servicing record players. Write either di- as rectly to the manufacturer, specifying type of motor or radio set, for information re- PAPT quired, or to your local distributor of the T OF 6 radio receiver or record -playing equipment that you have. Typical of such manufactur- e ers' data, taken verbatim from the RCA Victor service bulletin on Phono -Radio lo models U -30 and U -129, follows:

GENERAL INFORMATION Before servicing the automatic record 12 13 21 25 27 changer, inspect assembly see the to that all Bottom view of the RCA Victor Automatic Changer. Numbers refer to parts; levers, parts, gears, springs, etc., are in letters, to adjustments. good order and are correctly assembled. A bind or jam in the mechanism can usu- point adjust lockouts "C" to obtain 1 inch justment. After setting screw "F" adjust ally be relieved by rotating the turntable spacing between needle point and turntable screw "G" so that when its tip is depressed in the reverse direction. top surface. flush with top of record shelf, the vertical The changer can be conveniently rotated (D & E) Needle Landing on Record. -The spacing between the knife, in its lowest ro- through its change cycle by pushing the in- relation of coupling between the tone arm tational position, and the shelf, is 0.072- dex lever to "Reject" and revolving the vertical shaft and lever "20" determines the 0.078 -inch. turntable by hand. A total of 6 turntable landing position of the needle on a 10 -inch (H) Record Support Shelf.-The record revolutions are required for 1 change cycle. record. Position of eccentric stud "E" gov- shelf revolves during the change cycle to The turntable, spindle, and pinion gear erns the landing of the needle on a 12 -inch allow the lower record to drop onto the are assembled by means of a 3/32 -inch record; this, however, is dependent on the turntable. Both posts are rotated simul- straight pin. This pin may be removed by proper 10 -inch adjustment. taneously by a gear and rack coupled to the gently driving with a standard pin -punch. To adjust for needle landing, place 10 -inch main lever "15," and it is necessary that ad- If the record changer or cabinet is not record on turntable; push index lever to justments be such /hat the record is re- perfectly level, normal operation is likely to reject position and return to the "10- inch" leased from both shelves at the same in- be affected. position; see that pickup locating lever stant. To adjust, place a 12 -inch record on The 10 and 12 inch records must be ab- "17" is tilted fully toward turntable; rotate the turntable, rotate mechanism into cycle solutely flat for smooth operation when mechanism through cycle until needle is just to the point where tone arm is at maximum using a mixture of the 2 sizes. ready to land on the record; then see that distance outward from turntable; lift record A shorting switch, located in the pickup pin "V" on lever "14" is in contact with upward until it is in contact with both head, operates due to pressure when the "Step T" on lever "17." The correct point separating knives, then loosen screws "H" pickup is placed on the pickup rest. of landing is 4 -11/16 inches from the near- and shift record shelves so that the curved est side of the turntable spindle; loosen the inner edges of the shelves are uniformly ADJUSTMENTS 2 screws "D" and adjust horizontal posi- spaced at least 1/16 -inch from record edge. (A) Main Lever. -This lever is basically tion of tone arm to proper dimension, being Tighten the blunt -nose screw "H," run important in that it interlinks the various careful not to disturb levers "14" and "17." mechanism through cycle several times to individual mechanisms which control needle Leave approximately 1/32 -inch end play be- check action,. then tighten cone -pointed landing, tripping, record separation, etc. tween hub of lever "20" and pickup base screw "H." One adjustment is provided for the main bearing, and tighten the blunt -nose screw If record shelves or knives are bent, or lever. Rotate the turntable until the changer "D "; run mechanism through several cycles not perfectly horizontal, improper operation is out -of- cycle; and adjust rubber bumper as a check, then tighten cone -pointed screw and jamming of mechanism will occur. bracket (A) so that the roller clears the "D." (J) Tonearm Rest Support (not shown). nose of the cam plate by 1/16 -inch. After adjusting for needle landing on a -When the changer is out -of- cycle, the (B) Friction Clutch. -The motion of the 10 -inch record, place 12 -inch record on turn- front lower edge of the pickup head shoull tone arm toward the center of the record is table; push index lever to reject and return be 5/16 -inch above surface of motor board. transmitted to the trip pawl "22" by the to 12-inch position; rotate mechanism This may be adjusted by bending the tone - trip lever "7" through a friction clutch through cycle until needle is just ready to arm support bracket, which is associated "5." If the motion of the pickup is abruptly land on the record; the correct point of with the tonearm mounting base, in the re- accelerated or becomes irregular due to landing is 5 -11.16 inches from nearest side quired direction. swinging in the eccentric groove, the trip of spindle. If the landing is incorrect. turn (K) Trip Patel Stop Pin. -The position finger "7" moves the trip pawl "22" into stud "E" until the eccentric end adjusts of the trip pawl stop pin "K" in relation to engagement with the pawl on the main gear, lever "14" to give correct needle landing. the main lever "15" governs the point at and the change cycle is started. Proper ad- The eccentric end of the stud must always which the roller enters the cam. By bending justment of the friction clutch "5" occurs be toward the rear of the motor board, the pin support either toward or away from when movement of the tone arm causes posi- otherwise incorrect landing may occur with trip pawl bearing stud, the roller can be tive movement of the trip pawl "22" with- 10 -inch records. made to enter the cant later or earlier, re- out tendency of the clutch to slip. The fric- (F & G) Record Separating Knife. -The spectively. This adjustment should be made tion should be just enough to prevent slip- upper plate (knife) "25" on each of the so that the roller definitely clears the cam page, and is adjustable by means of screw record posts serves to separate the lower outer guide as well as the nose of the cam "B." If adjustment is too tight, the needle record from tut stack and to support the plate. will repeat grooves; if too loose, tripping remaining records during the change cycle. Lubrication.- Petrolatum or petroleum will not occur at the end of the record. It is essential that the spacing between the jelly should be applied to cam, main gear, (C) Pickup Lift Cable Screw.- During knife and the rotating record shelf "27" be spindle pinion gear, and gears of record the record change cycle, lever "16" is actu- accurately maintained. The spacing for the posts. ated by the main lever "15" so as to raise 10 -inch record is nominally 0.058 -inch; and Light machine oil should be used in the the tone arm clear of the record by means for the 12 -inch record, is 0.075 -inch. tonearm vertical bearing, record post bear- of the pickup lift cable. To adjust pickup To adjust, rotate the knife to the point of ings, and all other bearings of various for proper elevation, stop the changer "in- minimum vertical separation from the rec- levers on underside of motor board. cycle" at the point where pickup is raised ord shelf and turn screw and locknut "F" to The felt washer between the turntable to the maximum height above turntable give 0.055 -0.061 -inch separation. Screw and spindle bearing should be soaked in plate, and has not moved outward; at this "G" must not be depressed during this ad- light engine oil whenever the turntable is

RADIO -CRAFT for MAY, I 9 4 I 689 SERVICING

e e .e.

SPEED ADJUSTMENT "° (SCREW) TO DECREASE SPEED TURN CLOCKWISE 5PEE0 ADJUSTMEN LOCK NUT TURN SPEED ADJUSTER SO THE PEI. WILL PROTRUDE lieWNEN ALL THE WAY IN. AD UST GOVERNOR 50 AS TO LEAVE sa' BETWEEN FELT AND DISC, THE NNN SECURE SYMEANS KEEP FILLED WITH OF GOVERNOR SCRE LICHT OIL TO INSURE SMOOTH OPERATION

ADJUST .50 TAT TO ITHOUT END PLAT 7 DO NOT CHANGE Further details of the THIS ADJUSTMENT REMOVE TO RCA Victor Automatic TAKE OPF GOVERNOR iI Record Changer 1 showing (top) data on the motor and its method of cou- Motor Data and Coupling pling to the changer txtta Omaqe mechanism; and (bot- 25 32 COVER 18 27 3 17 tom) details of the. record shelf posts and locating -lever assembly.

PINV' ON END 14 Fur the minal amountunt of 35e we will send ya OF the following alne 1. one duo -tone Imperishably etched mmlIze Name Plate. . a 3511/2 Inches with your Social Security Oumber' and name permanently stamped Ulan It. 2. ne Genuine Leather Billfold. size Nl,,x3 Inches in naturall brown color. with four compartments. This Pocke,bi000kralIs "oour free present to you. Abso- lutely no strings of any kind. We make you this gift because we expert you to do us a favor by showing your name plate to your efienda, to Interest them In ordering e for themselves. That's all. Help us and 28 you win plrunt yourself. nail the coupon :,el.w and enclose 35c In IO 23 26"W G- STEP -T' 34 ACT NOWt and Use Leather Inllt.li Shill to1lforwarded ncea to yomuaat ' Details of Record Shelf Posts, and Locating Lever Assemblies PREM'UN DISTR;BUTING SERVICE Wrigley Bldg.. Chicago. Dept. RC -541 Kuriosen and 35e for which mall to the address below, my Social Security Bronze Plate and the Genuine Leather Billfold without extra cost, as per r offer. removed, or as required for proper opera- table. -Adjust lift cable per adjust- My Social Security Number Is tion. ment "C." NAME Do not allow oil or grease to come in (6) Needle does not track after landing. ADDRESS contact with rubber mounting of tonearm -Friction clutch "5" adjustment CITY base, rubber bumper, or flexible coupling "B" may be too tight; bind in tone - of drive motor. arm vertical bearing; levers "7" and "12r" fouled; or phono pickup output MISCELLANEOUS SERVICE HINTS cable twisted. ELEMENTARY Incorrect adjustment of a particular (7) Cycle commences before record is mechanism of the changer is generally complete. -Record is defective, or ad- ex- justment "B" of friction clutch "5" MATHEMATICS hibited in a specific mode of improper opera- is tion. The following relations between effects too tight. on operation and the usual misadjustments (8) Wow in record reproduction. -Record EASY - SIMPLIFIED - PRACTICAL will enable ready adjustment in most cases. is defective; flexible coupling between motor and changer mechanism not HERE Is book for the business man, the tech. (1) For any irregularity of operation, stelae and craftsman explaining and answering correctly assembled; or instrument is eery operation and meaning with interpreting the adjustment of the main lever r'15" not operated normal room Illustrations and examples. being at It is the key to a simple understanding of many should be checked first as in "A." temperature (65° F.). perplexing problems In daily life. In clear, positive and definite language. the author (2) Needle does not land properly on both (9) Record knives strike edge of records. popularizes and clarifies every subieet and helps the 10- and 12 -inch records. -Make com- reader to overcome any apparent dimculty in the study -Records warped; record edges are el mathematics. plete adjustments 11D" and "E." rough; or knife adjustments "F" and A real home study. course In mathematics for the student or the man who wants to achieve proficiency (3) Needle does not land properly on "G" are incorrect. or desires to brush up on his knowledge. 12 -inch record but correctly on 10- (10) Record not released properly. -Ad- Entire Chapter on Special Math- inch. -Effect emetics for the Radio Technician its adjustment "E." just record shelf assemblies in re- CONTFNTS OF BOOK (4) Failure to trip at end of record. - spect to shaft by means of adjust- CHAPTER I. Arithmetic -Addition- Subtraction- Multipll Increase clutch "5" friction by means ment "H." lLon -Div Iso ton CHAPT' ER I1. Faetorlrg and Cancellation -Fractions -Deci of screw "B." Also, see that levers (11) Needle in 10 -inch position mals-Percentage-Ratio-and Prapnr ton. lands on CHAPTER III. "7" and "12" are free to move with- 12 -inch record or misses record when How g to Measure Surfaces and Cawelt GeRmelV. out touching each other. playing both types mixed.- Increase CHAPTER V. Pow ara and Involution -Roots and Boole. (5) Pickup strikes lower record of stack tension of pickup locating lever CHAPTMon.ER ' Mathematics Ior Ms Man- ual and 'conn 1 Craft or drags across top record on turn- spring "34." conversionsn- th Gs-Us Ce1ePlol tine-Logarithms-Usei ONLY the Slide Rulá f CHAPTER VII. Speclal Mathematics for the Radio Technician. CHAPTER VIII. Commercial Calculations - .31,C Ineresti - Discounts - Short Cut POSTPAID m NEXT MONTH a .. e CHAPTER ex. weights and M ore Useful Tables. ..Elementary Mathrmatiea b carried readily is your manpw Do not miss the article, "HOW TO BUILD AND USE A LEAK DETECTOR," which will Send SStamps, Cash or Money Order appear in the June issue of Radio- Craft. IT'S JUST THE STORY YOU'VE BEEN T E í:HNIFAX ASKING FOR! 1017 S. State St. RC -541 Chicago 111 690 RADIO -CRAFT for MAY, 1941 F. M.

1eb:. 1852 TO LIMITER CONVERTER GRID Several requirements in the PLATE 0- / MEG. 0.1- I.F. channel of a frequency (EACH)CH) (EACH) MEG. modulation receiver, that / oddly enough sometimes oppose each other in their effect in the circuit, must be made 20,, to balance. It is these consid- 50.000 01-0,11.- OHMS erations which Mr. Dezettel 5000Jí OHMS here describes. 1.000 O5-M F OHMS (EACH I 00 -35000 MF. 1.000 OHMS OHMS Figure 3- Schematic circuit of the I.F. channel of FIG.3 a representative Frequency Modulation recel er

F. M.- RECEIVER I. F. CHANNEL DESIGN- L. M. DEZETTEL, W9SFW EQUAL to any other consideration in is part of the I.F. channel, it does not add standard practice in the wirint of an F.M. proper design of a Frequency Modula- to the gain appreciably and is, therefore, I.F. channel. tion (F.M.) receiver is the I.F. chan- not considered in this article.) The use of This article has been prepared from data nel. Several requirements must be met the 6AC7/1852 type tube adds considerably supplied by courtesy of Allied Radio Corp. that are not common to Amplitude Modula- to the gain. This tube is particularly suited tion (A.M.) receivers. These requirements, for use in F.M. receivers. It was designed oddly enough, sometimes oppose each other especially for television amplifiers where in their effect in the circuit, and it becomes design considerations vre very similar to necessary to strike a balance between them. F.M.

BAND -WIDTH IMAGES Number 1 on the list of considerations is Since F.M. receivers must operate exclu- 1/51=i111.- band- width. sively at ultra -high frequencies, the reduc- You already know that extremely wide tion of images is another important con- band -width is required. Unlike the I.F. chan- sideration. nel in A.M. receivers, it is not frequency The I.F. channel is called upon to perform response that is affected by lack of sufficient this function by operating at a high inter- band -width -it is dynamic range. The fre- mediate frequency. The 4,300 kc. I.F. trans- quency deviation is directly related to vol- formers are fast becoming the standard in- ume of audio signal. It is the rapidity of termediate frequency. Thus, then, the :1111111r MN the frequency deviations that determines the "image" must be twice 4,300 kc. or 8,600 kc. audio frequency response. Insufficient band- above the incoming frequency before it could width, then, results in the music or speech pass the I.F. channel. It is not difficult for I MI being compressed. The difference between the input circuits to reject signals 8,600 Ice. loud and soft passages of music, for in- off -resonance. stance, will be less than it should be. High- frequency I.F. transformers do not 1 The original band -width recommended have the gain generally associated with 456 UM III for the I.F. channel was 200 kc. This width kc. transformers, for instance. But we must is more, however, than is actually being load our circuit with resistors anyway which 1111 used in practice. Commercial receivers 307. 11/MI to- forces ús to accept even less gain- per -stage 4.200 4,300 4400 day are being made with a band -width of than normally would be obtained. So we see PERCENT RESPONSE VS. FREQUENCY 150 kc. This amount is based on a frequency that the use of higher- frequency I.F. stages ORDINATES ADJuSTED 1 NO LOADING III - O.2-M1EG. deviation of 75 kc., the maximum used in make worse. I . does not the matter - 0.5- MEG. O.1 -MEG. F.M. broadcasting today. n IY RESISTORS IN PARALLEL WITH How do we accomplish such band- width? OTHER CONSIDERATIONS PRIMARY AND SECONDARY. By simple of increasing FIG.1 the expedient the Many other considerations must be kept coupling between windings; and, loading of both and secondary with in mind. They concern mostly detail con- primary resistors. struction and would not be very interesting Curves, Fig. 1, are shown picturing the re- of a for various reading matter. sponse single transformer Some manufacturers of I.F. transformers degrees of coupling and loading. These for F.M. receivers have kept the home con- curves show the effect on the shape of the envelope and not resulting gain. Actual- structor in mind in their design considera- the tions. One manufacturer, at least, has de- ly we have reduced the gain by lowering the signed his so selec- effective "Q" of the windings by the addi- transformers that the tivity curve shows a slightly rounded nose. w tion of the shunt resistors; and since great- to aid home con- N er band -width requirements necessitate in- This is done purposely the z creased coupling, an I.F. characteristic free structor in lining -up his I.F. channel. A 0 -1 o from "double hump" response will require a ma. meter is inserted in the grid circuit of the limiter stage, and the preceding I.F. lower value of resistance in shunt with the CC windings. transformers are adjusted for maximum meter reading with an unmodulated signal LLI from the signal generator fed into the re- GAIN -PER -STAGE ceiver. The limiter stage will flatten out the z Consideration No. 2 in the design of the curve; so it is not necessary to worry about V I.F. channel is gain- per -stage. a perfectly flat nose in the I.F. channel. It follows that with coil "Q" reduced, re- This is shown in Fig. 2, in which the over- sulting in loss of gain, more I.F. stages all response action of the limiter is shown are required. Whereas the average gain - in dotted lines. per -stage in A.M. receivers with a 456 kc. The double peak or "stagger" method of band -width is about 80, the figure for the aligning an I.F. channel should be avoided. average F.M. receiver is only 20. This method of tuning tends to produce extra I.F. stage and one extra R.F. which will affect the quality of CURVE SHOWING ACTION OF LIMITER One transients UNDER IDEAL CONDITIONS IN WHICH IT - stage are usually employed to make up for reproduction. ACTS TO REMOVE VARIATIONS IN THE the loss. (Note: Although the limiter stage The diagram, Fig. 3, is representative of FIG RESPONSE OFTNE LF,AMPLIFIER.

RADIO -CRAFT for MAY, I 941 69I LATEST RADIO APPARATUS

FLUSH WALL RADIO ficient sensitivity. Its filter circuits provide ment for educational institutions, particu- more than 60 db. of attenuation of the fund. larly in voice -training schools. The per- Flush Wall Radio Co. amen tal .-Radio -Cro formance is extremely simple, the only ad- 57 State St., Newark, N. J. justment being made by the user is the play- NOW every home can have built -in radio back volume. Starting and stopping are ef- in every room at very low cost. Each COMPACT BATTERY -TYPE fected by ordinary pushbuttons.-Radio- radio is complete and does not depend upon TRANSMITTER - RECEIVER Craft with a interconnections "master receiver." TayBern Equipment Co. To install it, it is merely necessary to cut a 135 Liberty St., New York, N. Y. RADIO hole in the wall between two adjacent studs COMPASS AND and insert the receiver which is contained RECEIVER in an approved fireproof steel box. The re- The Hallicrafters, Inc. ceiver is a 5 -tube broadcast unit of conven- 2611 S. Indiana, Chicago, Ill. tional design. Front panels can be had to match color schemes of individual rooms.- Radio -Craft

DUAL -SPEED RECORDING MECHANISM Speak -O -Phone Recording Co. 23 W. 60th St., New York, N. Y.

MODEL 9MA is a 78 and 33 1, 3 r.p.m. recording mechanism characterizing onstant speed of operation. Its turn - table is 12 ins. in diameter, weighs 7% lbs., is made of cast aluminum, and is rim driven. MODEL TR -10 is a compact, portable, The drive consists of sa positive clutch as- Ultra -High Frequency Transmitter MODEL S30 is a compact combination sembly employing 4 precision- machined and Receiver in a single housing. It is Radio Compass and Receiver, designed gears. The pickup is of`the tangent, crystal designed especially for military radio use. for use on small boats. Uses signals from type. The cutting head has a spring adjust- Also applicable for forestry, marine and regular beacon stations for direction find- ment for regulating pressure; and, a cut- other non -military services. ing. ting -angle adjuslinent which permits the It is available in 2 types; one with 4 -tube Its 3 bands include the 220 -540 kc. bea- use of variou types of needles. The entire superhet. receiver using fixed tuning at any con band, 535 -1,340 kc. broadcast band, assembly mots on a 15 x 15 inch steel frequency from 1,500 kc. to 6.5 megacycles, and 1,200 -3,000 kc. marine band. Accurate panel s -in. thick. May be used with any and the other a 3 -tube superregenerative re- null indications are provided both visually good A.F. amplifier or radio set.- Radio- ceiver with fixed tuning at any frequency by a tuning eye and aurally by phones. Craft from 28 to 80 megacycles. Both types have a Other features include a "sharpness" con- 4 to 5 watt (R.F.) transmitter, complete trol and a switch -controlled static filter. All with modulator stage, for radiophone trans- power is drawn from a 6 -volt battery DISTORTION ANALYZER mission. The range is 10 to 100 miles de- through the medium of a vibrator power pending upon altitude of transmitter or re- pack. The entire unit measures 11x10x7%4 Hewlett- Packard Co. ceiving station. Fixed -tuned ins. high. With the loop, the overall height Palo Alto, California transmitter and receiver circuits require only off -on opera- is 23% ins.-Radio-Craft tion of the power switch to put the instrument into instant operation on a pre- select- ALL -METAL MERCURY SWITCH ed frequency. All batteries are self-con. Durakool, Inc. tamed .-Radio-C raft 1010 N. Main St., Elkhart, Ind.

"FOREIGN CORRESPONDENT" RADIO SET MODEL 320A distortion Analyzer is a RCA Mfg. Co. convenient instrument for measuring Camden, N. J. harmonic distortion in audio frequency apparatus. It is ideal for production work APPROPRIATELY named, this table - model well - since 'it is easy to operate, and provides a dual -band receiver uses the rapid and accurate check for normal opera- known "Overseas Dial" (first introduced on tion. consoles) which spreads out the popular 25 and 31 wave bands many Essentially, it is a comparison device used short to times their to compare the distortion content of the normal space on the dial. Aside from the wave with the fundamental voltage. When standard broadcast band, it has a foreign this comparison is made with a cathode -ray band coverage of from 8,600 to 12,000 kc. It is a 5 a P.M. oscilloscope, the type of distortion can be -tube set with dynamic speaker conveniently obazrved. The various order and designed for A.C.-D.C. operation. - harmonics will show up on the oscilloscope Radio -Craft pattern, the'hum voltage and noise voltage THIS inexpensive, all -metal mercury switch will appear, and a great deal of information "SOUNDMIROR" called the "Tiny Tipit" has a capacity of about the voltage being analyzed can be %- ampere at 24 volts to 4 ampere at 6 volts. obtained. It is designed for measurement at The Brush Development Co. Has no moving parts mer- Cleveland, Ohio (excepting the 400 and 5,000 c.p.s. Its input impedance is at cury pool). It operates on a tilt of 20° or least 20,000 ohms. A bridging transformer THIS equipment records sound magnetical- more. Although designed for use on automo- 7 may be used to increase the input im- ly on an endless steel tape. The sound may biles to turn the trunk light on when the pedance. Concerning its distortion range, then be "erased" at will and new messages lid is open, it may be used for many other vplues as low as 0.1 5o of the fundamental recorded. Recordings may be played over purposes Measures only 7 /16x% -in. dia.- 5nay be measured with a detector of suf- and over again indefinitely. Excellent equip- Radio -Cra f t 692 RADIO -CRAFT for MAY, 1941 LATEST RADIO APPARATUS

HANDY PRODUCTION TOOL Other features include a filter switch which cuts down volume of the beam signal Independent Pneumatic Tool Co. to permit audible reception of weather re- 600 W. Jackson Blvd., Chicago, Ill. ports and other communications; R.F. stage on all bands; headphone jack for noise -free i- reception in places where outside noises in- FREE terfere with loudspeaker use; operation from self -contained batteries as well as the RfAI A.C. or D.C. electric line. In addition to all this, the instrument can receive standard broadcast stations.-Radio-Craft fLfC1ROPl1T/NG

PLUG -IN TUBE -TYPE RESISTORS OUTF/T aerostat Mfg. Co., Inc. 285 N. 6th St., Brooklyn, N. Y. FO R extra- heavy- duty service such as in sets employing 150 and 300 milliam- pere tubes, served by a single voltage re- KNOWN as the Pix -Up Finder, this port- drop able electric machine which sorts, picks sistor, this new glass - in sul ated- element plug - up and holds screws for driving, eliminates is much of ordinarily in- in tube -type resistor the hand operation The re- cidental in screw -driving assembly opera- just the thing. tions. Other Thor production speedup tools sistance element is available are drills, nut setters, grinders, wound on a fibre -glass sanders, polishers, saws, and a new sheet core instead of mica metal tool known as the Portable and is then covered cutting a fibre -Cra t with -glass Now Nibbler.-Radio f braiding. The element is then able to handle 3 ELECTROPLATE HIGH -SPEED LEVEL RECORDER times the watts pow- You Can er of the usual bare Sound Apparatus Company winding.-Radio-Craft A BRUSH 150 W. 46 St., New York, N. Y. EASILY WITH HE model PS Recorder is an instrument I designed to make a permanent, continuous TRIANGULAR SPEAKER SOMETHING new for radio men - record of the variation of intensity of any CABINET something which gives you the log- opportunity to make additional prof - electrical signal, on either a linear or Atlas Sound Corp. is to improve your type of serv- arithmic scale. It especially designed to 1443 39th St., Brooklyn, N. Y. its-or record rapid changes of intensities over a ice. Here's an ELECTROPLATING wide range, can be used for: transmission KIT amazingly simple to operate - measurements; radio measurements; acous- you just Electroplate with a Brush! tic measurements; noise and vibration measurements; and, speech and music. This model possesses 1 writing and I paper speed. Other models are available pos- NOT A TOY! sessing more than one speed. Model PS has a writing speed of 0.0188 -mm. per second. Electroplate for profit, hundreds of things In 'the h.,u,ehnl.l- asbtravo, fixtures. water faucets. worn The apparatus weighs 20 lbs., and measures knobs. musical tel .1 koIlverware and It's s' et.res. 8x10x12 ins. -- Radin -Craf same niece of articles In lute's. office buildings. medical ami dental of. th es`mfactories. schools, Ir)ontorIn. etc. Exnrt'v outfit (but larger) la used nrofeesl'e electricians, radio service men. mobl!a re- palrpair shops. etc. Rerate. one single alley cell Its AIRCRAFT PORTABLE RADIO volt battery to Optiate. You rn receiver yens. Sentinel Radio Corp. none-Mons.setct worn ners accessories,, and dlanlav ehsss. Put this REAL ELEC- 2020 Ridge Ave., Evanston, III. TROPLATING KIT to use immediately -make It the useful article In your hoe or laboratory. A ^d. ymostou can get it absolutely FREE (except for slight mailing cost). Subscribe today to RADIO-CRAFT for Oh Year 112 Issues) and receive absolutely FREE this ELEC. TROPI.ATINO KIT. New subscriben are accepted n extend present subscription twelve nths. Mall remittance of $2.00 Mims 10e for shonbc, hone nn kit) to the ntmllshers. Canada and foreign 52.5%). You will cive your FREE. REAI. EI.Elrnni'l. \TINO OUTFIT by return mall. U ne ...0 Pon 1.a Too t.i (,.lot t

ONE of n new line of "Tri-Angle" acoustic enclosures. Excellent for corner mount- RADIO-CRAFT ing, side wall hanging and cluster arrange- 20 Vesey Street New York, N. Y. ment in groups of 2, 3 and 4. Excellent as extension loudspeaker for coin phonographs. I The construction is such as said to give in- RADIO -CRAFT RC -511 finite -baffle and bass -reflex characteristics. 20 Vesey Street, New York. N. Y. Illustrated is model TR -12 for 12 -inch speak- Gentlemen: find remittance 1vr for enter my subscriptionscrription ers. Measures 19 ins. wide x 10 ins. deep. fur Otte Yer 12 Issues). Send me FREE. EI.EC- OUTFIT and foreign Has overall height of 22 ins. The cabinet is Int ULS.r l dd IOtC i dditional snipping made of natural grain walnut and has an rharpea on kit. a attractive grille with a musical motif.- New Subscriber O Extend Present Subscription Radio-Craft Name ADUAL -PURPOSE portable radio receiv- 8 -WATT PORTABLE P.A. SYSTEM Address er designed for use by airplane pilots Erwood Sound Equipment Company The instrument is capable and boat owners. 223 W. Erie St., Chicago, III. city Sat. of receiving beam signals on the high -wave dCena remittance by heck. money order band (140 -410 kc.). The instrument can also DESIGNED especially for small installa- Register letter if you receive weather signals, navigation reports, tions in churches, schools, clubs, lodges, u U.a1lS.o Pomes.aps. and radio compass signals. etc., this 8 -watt portable system includes

RADIO -CRAFT for MAY, I 9 4 1 693 LATEST RADIO APPARATUS

amplifier, desk -type crystal microphone and PRESSURE -OPERATED DYNAMIC OPPORTUNITY AD-LETS a 10 -inch P.M. dynamic speaker. Provision Advertiscments in this sort ,m . »t Ib ten, a ain-.1 is made MICROPHONE for each insertion. Name, address and initials must for attachment of a record playing be included at the above rate. Cash should accom- mechanism. Tone control allows tone to be RCA Mfg. Co. pany all classified advertisements unless placed by Camden, N. an accredited advertising agency. No advertisement adjusted to suit acoustical conditions. En- J. for less than ten words accepted. Ten percent dis- tire assembly contained in a single carry- count six issues, twenty percent for twelve issues Objectionable or misleading advertisements not ac- ing case measuring 14x11x12 ins. high, and cepted. Advertisements for June. 1941 issue must weighs less than 28 Radio -Craft reach us not later than April lth. lbs.- Radio -Craft 20 Vesey St. New York, N. Y. F.M. STATION MONITOR BOOKS AND MAGAZINES General Electric Company ASSURE YOURSELF OF GREATER PROFITS BY doing radio service jobs more quickly. Authentic service Schenectady, N. Y. show locate and troubles in any dioy re iher, OmcialcRadio ice Manuals show you how to complete more repair Jobs In less time -how to earn more money by faster servicing. itadcraft Publications, 20 I esey St., New York City.

WE HAVE A FEW HUNDRED RADIO ENCYCLO- podiu, by 8. Gernsback, second edition. originally sod at $3.98. Book has 352 pages, weight 3 lbs.. six. 9 a If inches. Red morocco-keratol flexible binding. Bend 22.99 in stamps, rah or money order and book will be forwarded express collect. TecMlfax, 1915 So. State Street. Chicago, Illinois. WHAT DO YOU KNOW ABOUT AMPLIFIERS AND Sound Systems? The Amplifier Handbook and Public Address Guido covers P.A. from A to Z. Most complete and authentic hook published on the subject. Contains 80 pages of vital Information on Amplifiers, P.A. Systems. MODEL MI -6206 is a new, lightweight Speakers. Accessories, Pickups, Microphones, etc. Printed DESIGNED exclusively for frequency on fine coated stock, with numerous photographic illustra- modulation work, this new unit per- "Aeropressure" microphone especially tions and explanatory diagrams. and only 2 1c. See page 672. forms the functions of a Center -Frequency designed for announcing and remote pick- Monitor, Modulation Monitor, High -Fidelity ups. Ideally suited for outdoor use. A "para - EDUCATIONAL COURSES Audio Monitor, and Modulation -Limit Indi- coustic reflector" adjusts the pick -up pat- CORRESPONDENCE COURSES AND EDUCATIONAL cator of the flasher type. tern to suit individual requirements. The books, slightly used. Sold. Rented. Exchanged. All sub. facts. Satisfaction guaranteed. Cash paid for used course,. Center frequency deviation and percent- mike has a styrol (plastic) diaphragm and Complete details and bargain catalog FREE. \\'rite Nelson a moving coil element. Its frequency range Company. 500 Sherman, Dept. E -242, Chicago. age of modulation are read on illuminated meters. The self- contained crystal standard is 60 to 10,000 cycles; output impedance, FLUORESCENT LIGHTING provides for instant calibration of the cen- 250 ohms; output level (10 bars) 69 db. (0.006 - 240 FLUORESCENT BENCH LIGHT $1.49, LESS TUBE. ter-frequency deviation instrument at a flip -watt) open circuit. Available in low Harold Davis, Jackson, Mississippi. of a switch. The frequency response of the impedance (260 ohms) and high -impedance (40,000 MANUSCRIPTS WANTED audio monitor is better than 4í db., from 30 ohms) types. -Radio -Craft 15,000 cycles. The WANTED -MANUSCRIPTS, POEMS. SONGS FOR to modulation limit flasher publication. Fortuny's. 87 Fifth Ave., New York. is adjustable to show whenever modulation value MOTION PICTURES exceeds the for which the panel con- NEW DIAL FOR SELF- CALIBRA- trol is set -the range being from 50 to GENERA FILMS FOR MOVIE CAMERAS. SAVE 50%. 120%. TION Two films "The End," t0c. (Size?) Bargain circulars. Other features: built -in voltage -regu- Frumadera, Davenport, Iowa. lated power supply, and a de- emphasis cir- National Company on Malden, Mass. PHOTOGRAPHY cuit the audio monitor.-Radio-Craft

1 /100,000 OF A SECOND SPEED FLASH UNIT FOR Stroboscopic Photography, $200.00. Apt. 83, 782 west WI Are., Nov York City. CONDENSER ANALYZER RADIO Solar Manufacturing Corp. SACRIFICE! I2 -TUBE LINCOLN SUPER -DELUXE Bayonne, N. J. .ill -that Receiver, complete with 12 -inch electrodynamic .peaker d separate power pack. Cost originally $145. Will seer flee for $25.00. Set is 1933 model, rarely used and in good working condition. Excellent sensitivity and selectivity. A resonance tuning meter, ware -band switch. photograph input terminals, an ideal set for the DX fan. Excellent tonal fidelity. All components mounted on a beautiful chromium- flnlshed chassis. Rush) First tame. first served! D. Krelseman, 1930 Ocean Ave., Brooklyn,

WANTED - ELECTROLYTIC DETECTOR, TUSKA "Superdyrte" Adams -Morgan woolen -case, braa- plate variable condenser; Bunnell 1 sparkcol, Hare radio sets. meters, eta -or what do you want? B. Bernard, 90 Manning Awe.. No- Plainfield, N. J. type ACN dial here illustrated should THEprove to be a boon to all individuals who RADIO KITS construct their own radio apparatus such RADIO KITS --$3,95 UP. SINGLE BAND; ALL WAVE. as oscillators, receivers, transmitters, etc., 5.10 tube.. Fluorescent lighting. Save 5040. McGee Radio. l' -2035, K.C,. Mo. since its 5 range scales can be calibrated by hand. To facilitate calibration, a 0 to 100 TECHNICAL ART SERVICE division linear range is provided. The trans- DRAFTING AND ART SERVICE -A COMPLETE parent lucite pointer has a black etched Service for the Industrial and Commercial Marketers. THIS new "quick -check" condenser analyz- hairline A bezel holds If your business or plant can't afford a designing or for accurate reading. drafting staff, don't let this stop you from going ahead 1 er affords both qualitative and quantita- the dial scale and its transparent lucite win- with that new product you Intend marketing. or that tive tests. Known as model "QCA," it indi- catalog of mechanical or electrical items which needs dow in place. Two extra dial scales are sup- fine detailed drawings and highly retouched photographs. cates leakage, insulation resistance, R.F. im- plied with each dial. The tuning ratio is 5 Any mechanical, electrical or radio problem can be solved pedance, power for you by our associated staff of designers, draftsmen factor, capacities and in ad- to 1 and is designed to be used with a 180° and artists. and technical copywriters. write for any dition, affords a dynamic check for shorts, additional Information at to methods and price for this rotation condenser having a 144 -inch diam- ervlco. Tee -Art Drafting Service, 228 Charlotte Terrace, opens, and intermittents. -Radio -Craft eter shaft. The dial frame measures 5z7'á Roselle )'ark, N. J. ins. -Radio -Craft DRAFTING SCHEMATIC DIAGRAMS SEND US A rough sketch of your circuit. Estimates by return mall. Our prices aro moderate: our work guaranteed. No Jobs ROD ANTENNA COMMUNICA- too big or small. %'m. Kadlecek. Jr.. 440 East 85th St.. Now York, N. Y. TIONS PRIVATE -PLANE RECEIVER Verfrod Mfg. Co. RCA Mfg. Co. 132 Nassau St., New York Camden, N. J. RADIO RESEARCH BUREAU ALTHOUGH this is said to be an all - ACUSTOM -BUILT receiver has been de- Technical questions, engineering data, wave rod antenna, it is so designed as veloped which covers both the standard write for quotation. Radio receiver design, to be peaked for maximum efficiency on the broadcast band and the weather band of electronic, mathmatics, physics, statistical 10 -, 20- and 40 -meter bands. Both amateurs the C.A.A. In addition, it can receive the surveys at economical rates. and owners of all -wave receivers will wel- radio range signals for navigation purposes. come this conveniently -installed, non- direc- Designed especially for the 1941 Piper Coupe Address;Willard Moody, 403 West 205 St., M.Y.C. tional rod antenna. -Radio -Craft airplane, the receiver has a range of 250 694 RADIO -CRAFT for MAY 1941 LATEST RADIO APPARATUS miles and gives excellent performance without the necessity of shielding the ignition system. The set is completely self- contained A NEW BOOK TO ADD TO YOUR TECHNICAL LIBRARY and is operated by a vibrator powered from the plane's battery, which in turn is charged by a wind- driven generator. Another fea- AUTOMOBILE RADIO- Principles & Practice ture includes dual phone jacks for pilot and passenger. -Radio -Craf t by B. Baker Bryant OIL- FILLED CONDENSERS Aerovox Corporation New Bedford, Mass. lti\l l'LEI i; cemvaatlen ,.r P lIII how to installt and service uw II n mobile radio r cci,er. All of the Ial details which have [crept Into the pea, been weeded out. Earl, topic is trot', contain a precise MalemetlL or .,1,1-1 princdpie involved. too e the I deritandlug of titis principle, VI 11./111 dlaln, 11114 Ins attention by the discussion of a multitude of tietalls and mathematical expressions. which are primarily for the engineer. and terni to contuse rather than Clarify a statement for the auto radio technician. A practical treatise based on practical experience by practical radio people for the practical radio - technician. Brief Outline of Contents- Introduction -The Auto -Radio Art. Features of the Modern Automobile Receiver. Installations of Automobile Radios and "ntepba. The Automobile High and Low Tension Electrical systems. Automobile Electrical Disturbances. MIDGET, oil- impregnated, oil -filled con- Vibrator Converters and Motor Generators. Service Hints, Classified Automobile In- densers in handy tubular form, are stallation Notes, and Conclusion. finding many uses in radio assemblies, particularly for vibrator applications, coupling Send 50c check. money - order. unused U. S. stamps, or coin for your functions, low -power transmitters and 50c ropy of "Automobile Radio- Prhicl- receivers, television receivers, ampli- ples A Pra 'ICe " -it will he sent to trans- POSTPAID upon receipt or your in many laboratory applications. repostpaidmittance. fiers, and Sire -6 zO New units in an inexpensive " -89" series Inches are housed in cadmium -plated brass cans, 64 Panes RADCRAFT hermetically sealed. They are available. in atinoveesible Numerous 400, 600, 1,000 and 2,000 volt ratings and in illustrations PUBLICATIONS, Inc. and capacities from 0.006 -mf. to 0.5-mf.-Radio- dianram. 20 Vesey Street, New York. N. Y. Craf t STATION LOCATOR Allied Radio Corp. 833 W. Jackson Blvd., Chicago, Ill. DEMAMDS for readjustment of pushbuttons, due to frequency reallocation, GEOPHYSICAL will continue for quite some time after PROSPECTING March 29th. This new Station I:ocator will aid considerably in cutting the time neces- PRINTS sary to readjust the pushbuttons. No direct connection to the set is necessary. A drift- free oscillator generates either a modulated 50¢c. or unmodulated signal at the flip of a switch. The range covers the entire new broadcast band and the frequency is easily 7 FOR read on a simply- calibrated dial. Operates $3.- on self -contained batteries. Measures 3 x 4 x 5 ins. high.- Radio -Craft With any one of the modern geophysical methods described, instruments can be Sivils, an elaborate and unique drive -in constructed to locate metal restaurant located on the Ft. Worth pike, and ore veins, .ubterranean water and oil deposits, buried near Dallas, Texas, has glamourgals, ham- pipes, tools, or other metallic burgers, and an extensive sound system, objects sunken in water, etc. Mr. Hix Smith advises Radio -Craft. The Folder contains blueprints and detailed data sound installation utilizes a lookout in a covering each instrument. control tower atop the restaurant. He is Folder No. 1. Badloflecux Prot. Construction and use of 2 tube transmitter and 3 tube equipped with field glasses, and a micro- receiver. Reflected wave principle. Visual and phone feeds a public system. aural signals. that address Folder No. 2. Harmonic Frequency Locator. When a car arrives on the lot, the spotter Radiates low frequency wave to receiver. Aural signals. assigns a girl car -hop to the vehicle via P.A. Folder No. 3. Beat Note Indicator. 2 oscil- She places on the windshield a menu which THE PICTORIAL lator.. Visual and aural signals. Folder No. 4. Radio Balance Surveyor. Bal- carries the instruction to "flash headlights FLYING REVIEW anced loop principle. :Modulated transmitter. for service." This method of silent summons- FLIGHT Visual and aural signals. and Folder No. 5. Vartahle Inductance Monitor. from tlrinr. :nl the phases of world aviation .rural signals. ing does not interfere with the music . , ,: n.teehnleal yet educational manner Inductance principle. xciii lk' Helen a hic catch your breath. yet Folder No. 6, Hughes Inductance Balance an electric phonograph. A 35 -watt amplifier, routa the fundamental. of flyina and ALL related givesub. Explorer. Bridge principle. Aural signals. 5 outside and 8 inside loudspeakers, 3 micro- Folder No. 7. Radio dyne Prospector. Balanced FLIGHT keeps you abreast of all the major develop- loop principle Very large field of penetra- phones, and an electric phonograph with enta as fast as they appear anywhere. tion. Aural signals. Now When planes and flying are of vital Importance automatic record changer constitute the en- to ll. you just can't do without this highly absorb. set of blueprints and instructions enclosed in publication. You must be "in the Each tire sound system. instructive heavy envelope ($t i' x 12''v "). Blueprints 22^ n 34'1 Inóaw.et eloht -nape Illustrated 8'' r' x II^ folder of Partial Contents of the May Instructions and construction data 50c Issue: When The ÌnYRRICATIE attractive /k Add 50 for postage It is estimated by N.B.C. that if every Attacks? Plane Catapults. The threeecol d. V "Slip-Wing" Flhe - Front cover, nit: The complete set of seven folders -motion picture house in the country is filled Cover Feature. The Tel Shippinv weight 2 lbs. (add 25e for ship- $3.00$ Plane -Hugo Gernsback. New lu.trat,ons. 5co Py every night there would be still 80,000,000 INVISIBLE Bomber - H. W. ping anywhere in U.S.A.) Secor. Bomber's New Send radio listeners outside the theatre. According 11á Car- ple Coo, a riers Wok. How Airplane En. to this arithmetic, "the theatre" need not are Started Subscript price T M:HMO:In H. P. rpe o Pláes $1 ern Cñ in Tactics. and foreign 42.00. RC.541 take too seriously the perennial bleat that Action. R.A.F. 1117 S. STATE ST. CHICAGO, ILL. radio is ruining theatre attendance. FLIGHT MAGAZINE. 20.Ií Vesey St.. New York. N.Y. RADIO -CRAFT for MAY, 1941 695 CLASSIFIED RADIO DIRECTORY

SECTION II (REVISED) Where tv Eu y St! CLASSIFIED RADIO DIRECTORY Handy Buying Guide, by Products and Manufacturers' Names and Addresses, for the Entire Radio Industry

This DIRECTORY is published in sections -1 section per month. This method of publica- There is no charge for regular Lion permits the DIRECTORY to be constantly up -to -date since necessary revisions and light -face listings in the Class-- corrections can be made monthly. All navies preceded by an. asterisk (S) indicate that fled Radio Directory.However, if they are trade names. dominant hold -face listings are If you cannot find any item or manufacturer in this section or in previously -published desired, we make a charge of $2 sections, just drop us a line for the information. for concern names and $1 for trade names for each bold -face Presented here in Section 11 of the completely revised Second Edition of the CLASSIFIED RADIO DIRECTORY. listing. Please write to the Ad- vertising Dept., Radio -Craft. 20 ElWhile precaution Vesey St., New York, N. Y., for every is taken to insure accuracy. Radio -Craft cannot guarantee against the possibility of occasional errors and omissions in the preparation of this Classified details. Directory. Manufacturers and readers are urged to report all errors and omissions at the earliest moment to insure corrections in the very next issue.

CRYSTALS I for detection BASTIAN BROS. CO., 1600 N. Clinton St., THE H. R. KIRKLAND CO., INC., 10 King St., Morris- - town, N. J., "Bulls -I- Units " -DL, PA receiving I is DETECTORS Rochester. N. Y.-CD, CR, ES, NP KNIGHT- Allied Radio Corp. BENDIX RADIO CORP., 920 E. Fort Ave.. Baltimore, KURZ -KASCH, INC., 1421 S. Broadway, Dayton, Ohio Md.-CR -K BIRNBACH- Birnbech Radio Co., Inc. LAFAYETTE RADIO CORP., 100 6th Ave., New York, BIRNBACH RADIO CO., INC., 145 Hudson St., New N. Y. CD, CR, DL, ES, K, KWD, NP York, N. Y., "Birnbach" -DCC, K LEOTONE RADIO CO., 63 Dey St., New York, N. Y. BOND PRODUCTS CO., 13139 Hamilton Ave., De- -CD, FS troit, Mich. -CD LIBERTY ENGRAVING & MFG. CO., 2911 S. Cen- BROWNING LABORATORIES, INC., 750 Main St., tral Ave., Los Angeles, Calif. -CR, DP, ES, F5, NP Winchester, Mass. -CD M & H SPORTING GOODS CO., 512 Market St., Crystals (mounted) . . . CM BUD RADIO, INC., 5205 Cedar Ave., Cleveland, Phila., Pa. -CD, DL, K KWD, NP Crystals (unmounted) . CU Ohio -CD, DP, K, NP MAJESTIC RADIO & TELEVISIN CORP., 2600 W. Detectors (fixed) D BULLS.I.UNITS -The H. R. Kirkland Co., Inc. 50th St., Chicago, I11. -CD, CR CANADIAN MARCONI 211 St. Sacrament P. Detector stands DS CO. St., R. MALLORY & CO., INC., 3029 E. Washington Montreal, Can. -CD, DL, ES, FS, K, KWD, NP St., Indianapolis, Ind. -CL, K CANADIAN RADIO Fleet W., MEISSNER THE CARBORUNDUM COMPANY, Globar CORP., LTD., 622 St., MANUFACTURING CO., Mt. Carmel, Div, Toronto, Ont., Can. -DL, DP, K, KWD 111.-CD, ES, K Niagara Falls, N. Y. -CU, D JAMES MILLEN MFG. CO., INC., 150 MELOMITE COMPANY, Allen Chevrolet Co., CARLTON LAMP CORP., 730 S. 13th St., Exchange St., Malden, Mass. DL, DP. ES, Clinton, Mo.-CM, CU, r D, DS -CD, FS, K, PA Newark, N. J. -DL J. W. MILLER CO., 5917 S. Main St., Los Angeles, CHICAGO MOLDED PRODUCTS CORP., 1020 N. Calif. -CD, DP, ES, FS, K PARTS Kolmar Ave., Chicago, III. -K ELMER E. MILLS CORP., 812 W. Van Buren St., DIALS & CINCH MANUFACTURING CORP., 2335 W. Van Chicago, III. -CR, DP S, FS, K, NP Buren St., Chicago, 111. -DS MONTGOMERY WARD 8, CO., 619 W. Chicago Ave., CLAROSTAT MFG. CO., INC., 285 N. 6th St., Chicago, III., "Airline "CD DL, DP, ES, K, NP Brooklyn, N. Y. -K NATIONAL COMPANY, INC., 61 Sherman St., CONSOLIDATED WIRE & ASSOCIATED CORP., 512 Malden, Mass. -CD, CR, K S. Peoria St., Chicago, III. -K NATIONAL UNION RADIO CORP., 57 State CONTINENTAL -DIAMOND. FIBRE CO., Newark, Dela. -NP St., Newark, N. J. -DL CROWE NAME PLATE & MFG. CO., 3701 Ravens- OFFENBACH ELECTRIC CO., 1452 Market St., San Francisco, Calif. -CD, Complete dials . CD wood Ave., Chicago, III. -CD, CR, DCC, DP, CR, DL, DP, ES, K, KWD, NP Crystals & bezels CR ES, FS, K, NP, PA CRUMPACKER DISTRIB. CORP., 1801 Fannin St., PARISIAN NOVELTY CO., 3510 S. Western Ave., Decalcomanias D Houston, Tex. -CD, DL, DP, ES, FS, K, KWD, NP Chicago, III. -CD, CR, DP, FS, NP PHILCO RADIO & Dial cables & cords . . . DCC THE DAVEN COMPANY, 158 Summit St., Newark, TELEVISION CORP., Tioga & "C" N. J. -ES, K Sts., Phila., Pa.-CD, D, DL, DP, ES, FS, K, KWD Dial lamps . . DL HAROLD DAVIS, INC., 428 W. Capital St., Jackson, PH1LMORE MANUFACTURING CO., INC., 113 Dial -lamp sockets DS Miss.-CD, CR, D, DL, DP, ES, F5, K, KWD, NP University PI., New York, N. Y.-CR Dial pointers DP DOW RADIO SUPPLY CO., 1759 E. Colorado, Pasa- RADIO ELECTRIC SERVICE CO., INC., N. W. Escutcheons ES dena. Calif.-CD, CR, DL, DP, ES, F5, K, KWD, Cor. 7th & Arch Sts., Phila., Pa. NP -CD, CR. Faces or scales FS D, DL, DP, ES, FS, K, KWD, DRAKE MANUFACTURING CO., 1713 W. NP Faces or scales (machine en- RADIO EQUIPMENT CORP., 326 Elm St., Buffalo, Hubbard St., Chicago. 111.-PA N. Y.-DL DP ES, graved) FSM FS, K, KWD, NP HUGH H. EBY, INC., 4700 Stanton Ave., Phila., RADOLEK COMPANY, 601 W. Randolph St., Chi- Handwheels H Pa. -K cago 111.-CD, DCC, DL, K, KWD, NP, PA Knobs (molded) K EMPIRE NOTION CO., 105 E. 29th St., New York, RCA MANUFACTURING CO., INC., Camden, N. J. N. Y. -K, KWD -CD, CR, DP, ES, FS, Knobs (wood) KWD K. KWD ERIE RESISTOR CORP., 644 W. 12th St., Erie, Pa.- THE RICHARDSON COMPANY, 27th & Lake Sts., Knob springs KSP DP, ES, K, NP Melrose Park, I11. -ES K Nameplates NP FEDERAL SCREW PRODUCTS CO., 24 -26 S. Jefferson ROGERS- MAJESTIC CORP., LTD., 622 Fleet St., St., Chicago III. -PA Toronto, Can. -CD, ES Nameplates (machine en- K, KWD, NP FEDERATED PURCHASER, INC., 80 Park Place, New WALTER L. SCHOTT CO., 5266 W. Pico Blvd., Los graved) NM York, N. Y.-CD, CR, DCC, DL, DP, ES, K, KWD, Angeles, Calif., "Walsco" -DCC NP, PA MAURICE Pilotlight assemblies . . . . PA SCHWARTZ & SON, 710.712 Broadway, Vernier FISCHER DISTRIB. CORP., 222 Fulton St., New York, Schenectady, N. Y. -CD, CR, D, DL, DP, ES, FS, drives V N. Y. -CD, CR, D, DL, DP, ES, FS, K, KWD, NP K, KWD, NP FLOCK PROCESS CORP., 17 W. 31st St., New York, SEATTLE RADIO SUPPLY CO., INC., 2117 2nd Ave., AIRLINE -Montgomery Ward & Co. N. Y. -CR, ES, FS Seattle, Wash. -CD, CR, DCC, DL, ES, K, KWD, ALDEN PRODUCTS CO.. 715 Center St., Brockton, FONDA CORPORATION, 29 W. 57th St., New York, KSP, NP, PA Mass. -K, PA N. Y.-CD, ES, K, NP, PA SHELLEY RADIO CO., 1841 S. Flower St., Los An. ALLIED RADIO CORP., 833 W. Jackson Blvd.,Blvd. Chi- GEMLOID CORPORATION, 79.10 Albion Ave., Elm- geles, Calif. -CR DL, DP, ES, FS. K, KWD, NP cago, III., "Knight " -CD, DCC, DL, K, hurst, L. I., N. Y,-CDM, ESM NP F. W. STEWART NIFG. CORP., 340 W. Huron St., NP, PA GENERAL CEMENT MFG. CO., 919 Taylor Ave., Chicago, 111. -CD, DL, K, PA ALTO MANUFACTURING CO., 1647 Wolfram St., Rockford, III. -DCC, K, KWD, KSP SUN RADIO CO., 212 Fulton St., New York, N. Y.- Chicago, III. -DP, ES GENERAL RADIO CO., 30 State St., Cambridge, CD, CR, DL, DP. ES, F5, K, KWD, NP AMERICAN EMBLEM CO., INC., P. O. Box 116, Mass. -CD, K SYRACUSE ORNAMENTAL CO., Syracuse, N. Y. -ES, Utica, N. Y. -CD, CR, DP, ES, FS, NP GORDON SPECIALTIES CO., 1104 S. Wabash Ave., K. KWD AMERICAN PHENOLIC CORP., 1250 W. Van Buren Chicago, III. -CD, DP, FS, K, NP, V TRY -MO RADIO CO., INC., 85 Cortlandt St., New York, N. Y. -CD, St., Chicago, III. -CR CARL GORR PRINTING CO., 2615 N. Ash- CR, D, DCC, DL, DP, ES, FS, AMERICAN RADIO HARDWARE CO., INC., 476 K, KWD, KSP, NP, PA land Ave., III. Broadway, New York, N. Y.-CD, DCC, DL, DP, K, Chicago, -ES (paper) TUNG -SOL LAMP WORKS, INC., Radio Tube L. F. GRAMMES & SONS, INC., 364 Union St., KSP, NP, PA Div., 95 8th Ave., Newark, N. J. ANGELL MANUFACTURING CO., 1265 Broadway, Allentown, Pa. -DP, ES, FS, NP -DL New York, HARRISON RADIO CO., 12 W. Broadway, New VICTORY MANUFACTURING CO., INC., 1217 Wash. N. Y. -NP ington Blvd., Chicago, III.-K AUBURN BUTTON WORKS, INC., Auburn, N. Y. -K York, N. Y. -CD, DL, K NP THE D. L. AULD COMPANY, 5th Ave. & 5th St., INSULINE CORP. OF AMERICA, 30 -30 Northern °WALSCO- Walter L. Schott Co. Columbus, Ohio -ES, NP Blvd., Long Island City N. Y. -CD, CR, DP, ES, THE WATERBURY BUTTON CO., Waterbury, Conn. FS, KWD, NP -ES, K THE O. AUSTIN COMPANY, 42 Greene St., KAAR ENGINEERING CO., 619 Emerson St., Palo WESTINGHOUSE ELEC. & MFG. CO., E. Pittsburgh, New York, N. Y. -D, FS, NP Alto, Calif. -F, S, M, NM Pa. -K Please say you saw it in the Radio -Craft "Classified Radio Directory" 696 RADIO -CRAFT for MAY, 194I CLASSIFIED RADIO DIRECTORY

ELECTRIC FENCE ELECTRONIC CONTROL CORP., 2667 E. Grand THORDARSON ELECTRIC MFG. CO., 500 W. Huron CONTROLLERS Blvd., Detroit, Mich. -CR LS, M. PA, PS, P, R, RC St. Chicago, 111. -P ELECTRONIC PRODUCTS tO.. St. Charles, 111.-PA TIBBETTS LABORATORIES, Camden, Maine, "Mono - ' ELECTRO- PHONE, Communications, Inc. ba " -PI ,05 EYEMO Bell & Howell Co. TRANSFORMER CORP. OF AMERICA, 69 Wooster FARNSWORTH TELEVISION & RADIO CORP., 3700 St., New York, N. Y. 'Clarion " -PA, PR, P PSV, R E. Pontiac St., Ft. Wayne, Ind. -CR, EL, PA, UNITED CINEPHONE CORP., Torrington, Conn. - PM, PS, PV PA FEDERAL TELEGRAPH CO., 200 Mt. Pleasant Ave., VICTOR ANIMATOGRAPH CORP., Davenport, Newark N. J. -PR CR,Iowa -EL PS (for sound projection) Condensers C FEDERATED PURCHASER, INC., 80 Park PI., New WESTERN SOUND & ELECTRIC LABS., INC. 311 N. Units or kits UK York N. Y.-EL, PS, PV, PSV Kilbourn Ave., Milwaukee, Wis. -PA, P, PSV, R FILMRAPH, Miles Reproducer Co., Inc. WESTINGHOUSE ELEC. & MFG. CO., E. Pittsburgh, AEROVOX CORPORATION, New Bedford, Mass. FILMO, Bell & Howell Co. Pa. -HG, PS, PV, PR -C FILMOSOUND-Bell & Howell Co. WHEELCO INSTRUMENTS CO., 1933 S. Halsted ALLIED RADIO CORP., 833 W. Jackson Blvd., Chi - St., Chicago 111. -CR, PA, PV, PSV, R FINCH TELECOMMUNICATIONS, INC., 1819 cago, III., "Knight " -UK WILCOX ELECTRIC CO., INC., 40th & State Line, ARLAVOX MFG. CO., 430 S. Green St., Chicago, Broadway, New York N. Y. -LS, PA, P Kansas City Mo. PSV New -P, 111.-UK FISCHER DISTRIB. CORP., 222 Fulton St., WORNER PRODUCTS CORP., 1019 W. Lake St., York N. Y.-CR, EL, PA, PS, PV, RC RADOLEK COMPANY, 601 W. Randolph St., Chi- Chicago, 111. -M, PA, PV C FISCHER- SMITH, INC., 162 State St., W. Engle- cago, 111. -UK WEBSTERELECTRIC CO., Clark & DeKoven Ave., BOND PRODUCTS CO., 13139 Hamilton Ave., De- wood, N. J. -CR, PA, R, RC Racine, Wis.-R, RC troit, Mich. -UK GENERAL ELECTRIC CO., Schenectady, N. Y. & III. Bridgeport, Conn. -EL, PA, PS, PV, PR, R CONTINENTAL ELECTRIC CO., Geneva, G -M LABORATORIES, INC., 4326 N. Knox Ave., -UK Chicago 111.-PO, PS PV ELECTRONIC MUSICAL INSTRU- HARTMAN ELECTRICAL MFG. CO., 31 E. 5th St., GUARDIAN ELEC. MFG. CO., 1621 W. Walnut MENTS PARTS Mansfield, Ohio -UK St., Chicago, 111.-CR, R, RC & KNIGHT Allied Radio Corp. HAMMOND MFG. CO., 40 Wellington St. W., LAFAYETTE RADIO CORP., 100 6th Ave., New York, Guelph, Ont., Canada -P N. Y.-UK HARRISON RADIO CO., 12 W. Broadway, New SORENG- MANEGOLD CO., 1901 Clybourn Ave., York N. Y. -CR, LS, M, PA, PM, PC, PO, PS, Chicago, 111. -UK PV 1'R P, PSV R, RC THORDARSON ELECTRIC MFG. CO., 500 W. Huron HARVEY -WELLS COMMUNICATIONS, INC., North St., Chicago, 111.-UK St., Southbridge, Mass.-PA, R INCRIMINATOR, Communications, Inc. INTERNATIONAL TRANSFORMER CO., 17 W. 20th Contact microphones CM ELECTRONIC EQUIPMENT St., New York, N. Y.-PR, P, PSV Electronic organs EO CHARLES JACK MFG. CORP., 420.22 Lehigh 5f., Electronic percussion instruments Allentown, Pa.-P. R KAAR ENGINEERING CO., 619 Emerson St., Palo (chimes, etc.) EC Alto, Calif. -PSV Electronic pianos EP KNIGHT, Allied Radio Corp. Electronic reed instruments . ER LAFAYETTE RADIO CORP. 100 6th Ave., New York, Electronic string instruments . . ES N. Y.-CR, EL LS, M, P,A, PC, PS, P, R, RC LEKTRA LABORATORIES, INC., 30 E. 10th St., New Harmonicas H Capacity relays CR York, N. Y.-T "Musical amplifiers" (music- instru- THE LINCROPHONE CO., INC., 1661 Howard Ave., Exciter lamps EL ment type amplifires) . MA Utica, N. Y. -PA -frequency generators (heat - High LUMENITE ELECTRIC CO., 407 S. Dearborn St., Organ adapter OA treating, brazing, soldering, Chicago, 111.-CR, PA, PM, R, RC, T Pedal volume controls . . . . PV etc.) HG T. R. McELROY, 100 Brookline Ave., Boston, Mass. PA, PC, PO, PS, PV systems LS -EL, *AIRLINE, Montgomery Ward & Co., Inc. Lens P. R. MALLORY & CO., INC., 3029 E. Washington Micrometers MI ALLIED ENGINEERING INSTITUTE, 85 Warren St., St., Indianapolis Ind. -P battery- operated) New York, N. Y.-CM, MA MELLOPHONE CORPORATION, 65 Atlantic Ave., Mirrors M ALLIED RADIO CORP., 833 W. Jackson Blvd., Rochester, N. Y.-EL, PA, PC, PR, P Chicago, III., EC, MA PV Photocell amplifiers PA MILES CO., INC., 812 Broadway, "Knight"-CM, REPRODUCER ALOHA MUSICAL INSTRUMET& PUBLISHING Photocell multipliers PM New York, N. Y., "Filmgraph ", "Dictafilm "- CO., 807 W. 79th St., Chicago, 111. -CM, ES, PA PR, R Photocells (chemical) PC MA, PV MUSIC MASTER MFG. CO., 508 S. Dearborn St., Photocells (oxide) PO AMPERITE COMPANY, 561 Broadway, New York, Chicago, III. -PA, P N. Y,-CM PV Photocells (selenium) PS NATIONAL COMPANY, INC., 61 Sherman St., AMPLIFIER CO. OF AMERICA, 17 W. 20th St., Photocells (vacuum) PV Malden, Mass., "National ", "AC " -P, PSV NATIONAL, National Company, Inc. New York, N. Y. -EC, MA Power rectifiers PR ANSLEY RADIO CORP., 4377 Bronx Blvd., New N. Y., naphone Power supplies P NATIONAL UNION RADIO CORP., 57 State York, "D ", "Dynatone " -EP Perkins St., Newark, N. J. PS THE BRUSH DEVELOPMENT CO., 3311 Power supplies, vibrator PSV -EL, Ave., Cleveland, Ohio -CM Remote controls R NORTHERN ELECTRIC CO., LTD., 1261 Shearer St., CINEMA ENGINEERING CO., 1508 W. Verdugo Montreal, Que.. Can.-PA, PC, PO, PS, PV, P R Robot electronic controls (alarms, Ave., Burbank, Calif, -PV OFFENBACH ELECTRIC CO. 1452 Market Et., CLARION, Transformer Corp. of America level controls, etc.) . . . RC San Francisco, Calif. -PC, PO, PS, PV COMMUNICATIONS INC., 3215 Western Ave., MANUFACTURING CO., St. Charles, Stethophones (electric) . . S OPERADIO Seattle, Wash., "Commun -O one ", "Incrim- 111. -PA Timers (photo) T inator", " Electro- Phone " -EO, ßR, MA OHIO CRANKSHAFT CO., TOCCO DIV., 6600 COMMUN -O- PHONE, Communications, Inc. Vibration pickups (microphones) PI Clement Ave., Cleveland, Ohio -HG TELEVISION CORP., Tioga & 'CO- RECHT, Electronic Sound & Music Co. "Wireless" detective devices . W PHILCO RADIO 8 "C" Sts., Phila., Pa. -PS CRACRAFT, INC., 28 Grove St., New York, N. Y. Wire- tapping devices . . . . WD PHONOTONE LABORATORIES, INC., S. E. 15th & -CM, PV, EP, ES State Sts., Washington, Ind. -LS, M, PA, R CRUMPACKER DISTRIB. CORP., 1801 Fannin St., 'AC, National Company, Inc. PHOTOBELL CORPORATION, 123 Liberty St., New Houston, Tex. -CM ADVANCE ELECTRIC CO., 1260 W. 2nd St-, Los York N. Y.-CR, LS, M, PA, PO, PS, PV, P, R, RC DAVITT & HAUSER, 304 Main St., Cincinnati, Ohio Angeles Calif.-R PHOTOSWITCH INC., 21 Chestnut St., Cambridge, -ES ENGINEERING INSTITUTE, 85 Warren St., ALLIED Mass. -CR, LS, M, PA, PV, P, R DYNAPHONE, Ansley Radio Corp. New York N. Y. -R, RC, W DYNATONE Ansley Radio Corp. ALLIED RADIO CORP., 833 W. Jackson Blvd., Chi - RADIO ELECTRIC SERVICE CO., INC., N.W. ELECTRONIC' SOUND & MUSIC CO., IO Stuyvesant cago, III., "Knight"-CR, M PA, PC, PS, P, RC Cor. 7th & Arch Sts., Phila., Pa. -CR, EL St., New York, N. Y., ""Co-Recht"-EC, EO, ER, AMERICAN TELEVISION& RADIO CO., 300 E. 4th RADIO EQUIPMENT CORP., 326 Elm St., Buffalo, ES, MA St., St. Paul, Minn. -PR, PSV ELECTRO- PHONE, Communications, Inc. AMPLIFIER CO. OF AMERICA 17 W. 20th St., RADOLE-K 601 W. Randolph St., Chi - EPIPHONE INC., 142 W. 14th St., New York, N. Y. New York N. Y.-CR, PA PSV, PR, P, R COMPANY, 111. PA PS -CM ES MA, PV BELL & HOWELL, Bell Nowell Co. cago, -EL, RAYTHEON MFG. CO., 190 Willow St., Waltham, EVERETt PIANO CO., South Haven, Mich, -EO, EP BELL & HOWELL CO. 1801 Larchmont Ave., Chi- EVER -NU PRODUCTS, 919 N. Latrobe Ave., Chicago, cago, III., r, ", Mess. -PR "Filmó "Filmosound ", "Eyemo INC., 111.-CM "Bell & Howell " -LS RCA MANUFACTURING CO., Camden, PA, PS, PV, P, RC FEDERATED PURCHASER. INC., 80 Park PI., New CANADIAN MARCONI CO., 211 St. Sacrament N. J. -EL, REGAL MFG. CORP., 14 W. 17th St., York, N. Y.-CM, MA, PV St., Montreal Can -PR P, PSV AMPLIFIER New York, N. Y. -WD FISCHER DISTRIBUTING CORP., 222 Fulton St., THE BENWOOIS -LINZE CO., 1838 Washington Ave., New York N. Y, -CM St. Louis Mo. -P ROWE RADIO RESEARCH- LAB. CO., 4201 Irving Park 111. P R, RC FONDA CORPORATION, 29 W. 57th St., New CINEMA ENGINEERING CO., 1508 W. Verdugo Blvd., Chicago, -PA, MAURICE SCHWART & SON, 710 -712 Broadway, York, N. Y,-CM, PV Ave., Burbank, Calif. -PR, P, PSV, R FRETTED INSTRUMENT MANUFACTURERS, INC., "CLARION, Transformer Corp. of America. Schenectady, N. Y. -CR, EL, LS, M, PA, PC, 127 S. 15th St- Newark, N. J. -ES 3215 Western Ave., PO PS, PV, P. R, RC COMMUNICATIONS INC., GIBSON, INC., '219 Parsons St., Kalamazoo, Mich. Seattle, Wash., Commun-O- Phone" "Incrimina- SEATTLE RADIO SUPPLY CO., INC., 2117 2nd Ave., " PS, PV, PR PSV ES tor", " Electro- Phone " -PA, PSV, R, RC, WD Seattle, Wash.-PA, - COMMUN-O- PHONE, Communications, Inc. SHURE BROTHERS, 225 W. Huron Et., Chicago, III., GULBRANSEN COMPANY, 816 N. Kedzie Ave., Shure", "Un,dyne ", "Unipplee ", 'Rocket ", Chicago, III. -EO CONTINENTAL ELECTRIC CO., Geneva, Ill. "Stratoliner ", "Super -Level ", "Ultra-Wide Range ", HAMMOND INSTRUMENT CO., 2915 N. Western -PO, PV "Zephyr " -S Ave., Chicago, III.-EO, EP, OA CRUMPACKER DISTRIB. CORP., 1801 Fannin St., S.O.S. CINEMA SUPPLY CORP., 636 I 1th Ave., HARDMAN PECK & CO., 33 W. 57th St., New York, Houston Tex. -EL, PA, PV, R New York N. Y. -EL, LS, M, PA, PO, PV, P N. Y. -EP DeVRY CORPORATION, 1111 Armitage Ave., Chi - STARK ELECTRICAL INSTRUMENT CO., 161A King HARMONY COMPANY, 1742 N. Lawndale Ave., cago, 111. -EL, LS, PA, PS, P St., W. Toronto, Ont., Can. -MI Chicago, III. -ES DICTAFILM. Miles Reproducer Co., Inc. SUN RADIO CO.. 212 Fulton St., New York, N. Y. HARPTONE MANUFACTURING CORP., 127 S. HUGH H. EBY, INC., 4700 Stanton Ave., Phila., Pa. -CR, EL, PS, , RC 15th St. Newark N. J.-ES, MA -PS TAY BERN EQUIPMENT CO., INC., 135 Liberty St., HARRISON RADIO CO., 12 W. Broadway, New E. A. ECKSTEIN CO., LeRoy, Minn, -PA New York, N. Y.-RC York, N. Y.-CM

Please easy you saw it in the Radio -Craft "Classified Radio Directory" RADIO -CRAFT for MAY, 1941 697 SERVICEMEN SAY: -`doe ilyyee Pt 4iti mot 1?ttet SQtIr««y USE GERNSBACK MANUALS

Since 1931 more and more Servicemen have been buying GERNSBACK OFFICIAL RADIO SERVICE VOLUME 7 OFFICIAL RADIO MANUALS year after year. The authentic material, SERVICE MANUAL easily accessible diagrams and complete service data The newest and most outstanding GERNS- BACK Manual published since 1931. Con- make them invaluable to dealers and radio Service- tains many features never previously published in any service guide. OVER men. Without a Gernsback Manual with you on a 1,800 PAGES OVER 3,000 ILLUSTRA- repair job, there's time and lost. Your service TIONS OVER 1,600 SETS REPRE- profit SENTED Complete Service Data In- kit or laboratory is incomplete without all the GERNS- cludes 1938 Models Not Found in Other Manuals All New Material-No Reprint, BACK OFFICIAL RADIO SERVICE MANUALS. There or Rehash Master Index of 74 Pages are GERNSBACK MANUALS servicing Includes Sets Listed in All Other Gems- also for auto- back Manuals Stiff. Leatherette Loose- radios, refrigeration and air conditioning equipment. leaf Covers Size 9 x 12 Inches Net Weight 10% lbs. Price $10.00 1935 OFFICIAL AUTO -RADIO SERVICE 1936 OFFICIAL RADIO MANUAL SERVICE MANUAL If you are overlooking Just packed with service data of sets manufac- servicing auto radios, then tured during 1935 and 1936. Schematic diagrams you are missing a great show speaker connections, power -transformer con- i deal of business. The nections, R.F. and I.F. coil connections, phonograph great number of car connections and other information. OVER radios in use require con - 1.200 PAGES OVER 1.600 SETS SHOWN stant servicing and you OVER 2.500 ILLUSTRATIONS Index of can make it a profitable Trade Names Complete Tube Chart Stiff, sideline to your regular Leatherette, Looseleaf Covers Size 9 x 12 Inches servicing business. This Net Weight 8 lbs. complete Manual contains schematic diagrams, Price chassis layouts, mounting instructions and trouble $7.00 shooting hints. OVER 240 PAGES OVER 500 ILLUSTRATIONS Flexible, Leatherette Looseleaf Covers Size 9 x 12 Inches Net Weight 1% lbs. Price $2.50 OFFICIAL REFRIGERATION OFFICIAL L. SERVICE MANUAL 1934 OFFICIAL RADIO REFRIGERATION 2 SERVICE MANUAL SFPVICE MANUAL Volume This manual contains values of I.F. trans- This is the only complete refrigeration service manual ever published. Contains detailed formers on superheterodyne receivers, valua- information on servicing, outlines of theory ble tube data, complete list of American and methods of operation, instructions for broadcast stations, free question and answer the handling of refrigerants, charging systems, diagnosing troubles, charts, electrical service, service material and schematic dia- hook -ups and a wealth of other necessary grams. OVER 400 PAGES OVER 2.000 data. OVER 852 PAGES OVER 300 ILLUSTRATIONS Flexible, Leatherette, DIAGRAMS Flexible. Leatherette, Loose- leaf Covers Size 9 x 12 Inches Net Looseleaf Covers Shell x 12 Inches Net Weight 1%. lbs. Weight 2% lbs. Price $5.00 Price $3.50

OFFICIAL AIR CONDITIONING 1932 OFFICIAL RADIO SERVICE MANUAL SERVICE MANUAL This manual is edited by L. K. Wright. who is an expert and leading authority Ilere is the volume which shows a step -by- on air conditioning and refrigeration. step analysis of how to service any receiver. Nearly every page is illustrated, every It is one of the early editions and covers modern installation and individual OFFICIAL many old sets, thousands of which are still hart carefully explained, diagrams fur- i nished all known equipment, special AIR CONDITIONING in use throughout the country. OVER of care given to the servicing and in- , SERVICE MANUAL 1.000 PAGES OVER 2,000 ILLUSTRA- stallation end. Necessary tools are il- TIONS Flexible. Leatherette, Looseleaf lustrated and explained. etc. OVER Covers Size 9 x 12 Inches Net Weight 352 PAGES OVER 600 ILLUS- 4% lbs. TRATIONS Flexible. Leatherette, Price $5.00 Looseleaf Covers Size 9 x 12 Inches Net Weight 2% lbs. Price $5.00 FRADCRAFT PUBLICATIONS, Inc., 20 VESEY ST., NEW 'YORK, N. l'.1 ' Gentlemen. Ent I MI to) remlttuuce of for hielt :end me, POSTPAID, the Manuals Indli rued below a (st panel. by cross In the (1 Volume 7 4t $10.00 ( ) 1936 Manual 4D $7.00 ( ) 1934 Manual (d $3.50 ( ) 1935 Auto -Manual 4t $2.50 ( ) 1932 Manual 20 $5.00 ' To order these famous Manuals, see or write to ( ) Refrigeration Manual 2) ( ) Air ' (Vol. 4 $5.00 Conditioning Manual 44 $5.00 your jobber or favorite mail order house. l j more convenient, mail coupon directly to publishers. I Name Address

City state 1 RADCRAFT (Send remittance In form of cheek or money order; register your letter ' PUBLICATIONS 7 Inc. If you send cash or unused U. S. Postage Stamps.) RC -541 L J 20 V ES EY STREET N EW YO RK, N. Y. 698 RADIO -CRAFT for MAY, 1941 CLASSIFIED RADIO DIRECTORY

FRANK HOGAN COMPANY, Elkhorn, Wis. -EP FINCH TELECOMMUNICATIONS, INC., 1819 BIRNBACH RADIO CO., INC., 145 Hudson St., 'INCRIMINATOR Communications, Inc. Broadway, New York, N. Y. -R, T New York, N. Y., "Birnbach " -AR, AT, C, D CHARLES JACK MFG. CORP., 420.22 Lehigh St., FONDA CORPORATION, 29 W. 57th St., New York, BOND PRODUCTS CO., 13139 Hamilton Ave., De- Allentown Pa. -EC, EO N. Y. -R troit, Mich. -R KAY MUSICAL INSTRUMENT CO., 1640 Walnut St, RADOLEK COMPANY, 601 W. Randolph St., Chi - L. S. Chicago, III. -ES, MA cago, III.-K, R BRACH MFG. CORP., 55 Dickerson St., *KNIGHT, Allied Radio Corp. WILCOX ELETRIC CO., INC., 40th & State Line, Newark, N. J. -AR, AT KRAKAUR BROTHERS, 115 St., New York, Karsas City, Mo. R, T W. 57th -K, BROWNING LABORATORIES, INC., 750 Main St., N. Y. -EP Winchester, Mass. -A, CO, D, R LAFAYETTE RADIO CORP., 100 6th Ave., New CANADIAN MARCONI CO., 211 St. Sacrament York, N. Y, -CM PV FLUORESCENT & NEON LAMPS St., Montreal, Can. -A, AR, AT, CO, CA, CR, P, LEOTONE RADIO CO., 63 Dey St., New York, N. Y. EQUIPMENT) R, T -CM l& CARRON MANUFACTURING CO., 415 S. Aber- MAAS ORGAN CO., 3015 Casitas Ave., Los Angeles, deen St., Chicago, III. -CO FA, R Calif.-EC THE CLOUGH- BRENGLE CO., 5501 Broadway, MASON & RASCH, LTD., 642 King St., W., Toronto Chicago, 111.-FG 2, --EP Ont., Can. CONSOLIDATED WIRE & ASSOCIATED CORPS., JOHN MECK INDUSTRIES, 1313 Randolph, Chi - 512 S. Peoria St., Chicago, 111. -AR cago, 111.-CM, ES, MA CORNISH WIRE CO., INC., Park Row, Ne.. MIESSNER INVENTIONS, INC., 18 Main St., Mill- York N. Y.-AR AT, C, burn N. J. -E0, EC, EP, ER ES, PV THE CROSLEY CORPORATION, Cincinnati, Ohio MILLION TELEVISION, 1617 N. RADIO Damen Ballasts (lamp and, or line) B -R Ave., Chicago, 111. -MA Fixtures F CROWE NAME PLATE & MFG. CO., 3701 Raves s- MONOBAR- Tiboetts Laboratories wood Ave., Chicago, 111.-0 MONTGOMERY WARD & CO., INC., 619 W. Chi. Flashers (6 V,) for neon tubes . FL HAROLD DAVIS, INC., 428 W. Capitol St., Jac: - cago Ave. Chicago, Ill. "Airline " -CM Lamp holders LH son, M iss.-A MUSIC MASTER MFG. CO., 508 S. Dearborn St., Lamps L DELTA RADIO CORP., 115 Worth St., New York, Chicago, III. -MA N. Y.-CO 400 S. Peoria Power . . . NATIONAL DOBRO CORP., St., Chi. factor condensers PC DOOLITTLE RADIO, INC., 7421 Loomis Blvd., CF' cago, 111.-PV, ES Power packs (6 V. to high voltage) cago, III. -P R, T NORTHERN 1261 ELECTRIC CO., Shearer St., DOW RADIO SUPPLY CO., 1759 E. Colorado St., Montreal Que., EP for neon tubes PP Can. -CM, EO Pasadena, Calif. -A, AR, AT CO, D, R, T OFFENBACH ELECTRIC CO., 1451 Market St., San Starters S 'DYNAPHONE, Ansley Radio Corp. Francisco, -CM Calif. Starting compensators . . . SC OPERADIO MANUFACTURING CO., St. Charles, DYNATONE, Ansley Radio Corp. Switch SW I11. -MA controls ESPEY MFG. CO., INC., 305 E. 63rd St., New York, PACENT ENGINEERING CORP., 79 Madison Ave., N. Y. -A, CA, CR, R New York, N. Y.-MA ART SPECIALTY CO., Ins N. Franklin St., Chicago, FARNSWORTH TELEVISION & RADIO CORP., 3700 PHONOTONE LABORATORIES, INC., S. E. 15th & 111. -B, F, FL, LH, L, PP, S SC, SW E. Pontiac St., Fort Wayne, Ind. -FA State Sts., Was'ington, Ind. -MA *ATOMS, Sprague Products Co. FISCHER DISTRIB. CORP., 222 Fulton St., New RADIO ELECTRIC SERVICE CO., INC., N,W. CHICAGO TRANSFORMER CORP., 3501 Addison York, N. Y. -A, AR, AT CO, CA, CR, D, R, T St., Chicago, III. -8 FISHER RESEARCH LABOkATORY, 1961 University 7th & -ch Sts., Phila., Pa. PV Cor. A -CM, GENERAL TRANSFORMER CORP., 1250 W. Van Ave., Palo Alto, Calif.-P, R, T RADIO EQUIPMENT CORP., 326 Elm St., Buffalo, Buren St., Chicago, 111.-B, F, FL, LH, L, PP, S, FLERON, M. M. Fleron S. on, Inc. N. Y, -CM PV SC. SW M. M. FLERON & SON, INC., 113 N. Broad St., RADOLEK COMPANY, 601 W. Randolph St., Chi- KOOLOHMS, Sprague Products Co. Trenton, N. J., "Antennex' , 'Fleron ", "Saftest ", cago, 111.-CM MA RCA MANUFACTURING CO., INC., Camden, RCA MANUFACTURING CO., INC., Camden, "Signaler"-AR N. J. -R, T FREED RADIO CORP., 39 19th N. J. -EC, EP, ER W. St., New York, 600 LINE, Spag.e Products Co. N. Y. -CR REGAL AMPLIFIER, 14 W. 17th St., New York, N. Y. -MA SMITHCRAFT, A. L. Smith Iron Co. GALVIN MANUFACTURING CORP., 45-45 W. A. L. SMITH IRON CO., 217 Everett Ave., Chelsea, Augusta Blvd. Chicago, III., "Motorola " -P ROWE INDUSTRIES, INC., 3120 Monroe St., Toledo, Mass., "Smithcraft " -F GENERAL CO., Schenectady, Y. S. Ohio -CM, PV, ES ELECTRIC N. SPRAGUE PRODUCTS CO., N. Adams, Mass., "600 Bridgeport, Conn. AR, AT, FRANCIS J. RYBAK & CO., 4131 White Plains Rd., -A, C, CR, FA (for Line ", "Atoms ", "Koolohms ", Telohmike " -PC station), P, R, T Bronx N. Y. -EC TELOHM !KE Sc _á.á od.xts Co. RADIO EATTLE RADIO SUPPLY CO., INC., 2117 2nd Ave., GENERAL CO., 30 State St., Cambridge, Seattle, Wash.-CM Mass. -FG I. OSCAR SCHMIDT INTERNATIONAL, INC., 87 EDWIN GUTHMAN & CO., INC., 400 S. Peoria Ferry St., Jersey City, N. J. -ES FREQUENCY MODULATION St., Chicago, III. -CO D. H. HARRELL, 1527 E. 74th Pl., 111. MAURICE & SON, 710 Broadway, Chicago, -AR, SCHWARTZ -712 EQUIPMENT AT Schenectady N. Y, -CM, EC, EP, ER, ES, H, PV HARRISON RADIO CO. 12 W. Broadway, New MARK SIMPSON DISTRIBUTING CO., INC., 16 York, N. Y. AR, Al, C, CO, CR, FA, L, R Hudson St., New York, N. Y, -CM, PV -A, SHERLOCK MANNING CO., Clinton, Ont., Can. HARVEY -WELLS COMMUICATIONS INC., North -EO St., Southbridge, Mass. -AR, AT, C, L, P, R, T INSULINE CORP, OF AMERICA, 30 -30 Northern SLINGERLAND BANJO & DRUM CO., 1325 W. Blvd., Long Island City, N. Y. -AR Belden Ave., Chicago, 111.-ES J. F. D. MANUFACTURING CO., 4111 Ft. Hamilton SOUND PROJECTS CO., 3140 W. Walton St., Pkwayy Brooklyn, N. Y. -AR Chicago, III. -EC, ES Adapters, add -on A E. F. JOHNSON COMPANY, Maseca, Minn. -AR, STORY & CLARK 'IANO CO., 64 E. Jackson Blvd., AT Antennas (receiving) . , AR Chicago, 111. -EP `KNIGHT, Allied Radio Corp. Antennas (transmitting) AT SUNDT ENGINEERING CO., 4757 Ravens- KURZ- KASCH, INC., 1421 S. Broadway, Dayton, wood Ave., Chicago, III. -EC, MA Cable C Ohio -D LAFAYETTE RADIO 100 York, SUN RADIO CO., 212 Fulton St., New York, N. Y. Coils CO CORP., 6th Ave., New N. Y. -A, AR -CM PV Combination A.M.-F.M. adapters CA JOHN E. LINGO & SON, INC., 2nd St. & Buren TIBBETTS LABORATORIES, Camden, Maine, "Mono - bar"-CM Combination A.M. -F.M. receivers CR Ave., Camden, N. J. -AT FRED M. LINK, 125 17th New TRANSFORMER CORP. OF AMERICA, 69 Wooster Dials D W. St., York, N. Y. -P MAC St., New York, N. Y., "Clarion " -MA F.M. amplifiers (audio) . . . FA -ADAMS EQUIPMENT CO., INC., 507 W. THE TURNER CO., 909 17th St., Ceder Rapids, Iowa 56th York, CR F.M. signal generators . . . FG -CM MAJESTIC RADIO & TELEVISION CORP., 2600 W. THE VEGA COMPANY, 155 Columbus Ave., Boston, Lead -ins L 50th St., Chicago, 111. -CO, CA, CR, R Mass., 'Vega ", "Vitar " -CM, ES, MA, PV Police, emergency services, utility P MARINE RADIO CORP., 117 -19 168th St., Jamaica, "VEGA, The Vega Company Receivers R N. Y. -R, T VITAR The Vega Company MEISSNER MANUFACTURING CO., Mt. Carmel, VIVI TONE COMPANY, 6321 Gratiat Ave., Detroit, Test apparatus (see Test Equipment) 111. -A, CO, CA, CR, D, R Mich. -EC, EP ER, ES. MA, PV Transmitters T JAMES MILLEN MFG. CO., INC., 150 Exchange St., THE VOLU -TONE COMPANY 252 S. Broadway, Los Malden, Mass. -CO ES MA J. W. MILLER CO. 5917 S. Main St., Los Angeles, Angeles, Calif, -CM, 'AC, National Company, Inc. WESTERN SOUND & ELECTkIC LABS., INC., 311 W. Calif., Miller ality Products ", "Miller " -CO ALLIED RADIO - Kilbourn Ave., Milwaukee, Wis. -MA CORP., 833 W. Jackson Blvd., Chi *MILLER, J. W. Miller Co. cago, III., "Knight " -A, AR, CO, CA, CR, FG, R WESTERN STATES WHOLESALE MUSIC CORP., ALPHA WIRE CORP., 50 Howard St., New York, MILLER QUALITY PRODUCTS, J. W. Miller Co. 4333 Brooklyn, Seattle, Wash -ES N. Y.-C L MILLION RADIO & TELEVISION, 1617 N. Damen AMERICAN COMMUNICATIONS CORP., 123 Lib. Ave., Chicago, 111. -FG FACSIMILE erty St., New York, N. Y.-AR, AT, T MIMS RADIO CO., P. O. Box 504, Texarkana, Ark. AMERICAN EMBLEM CO., INC., P. O. Box 116, -AR, AT Utica, N. Y.-D MISSION BELL RADIO MFG. CO., INC., 831 AMERICAN PHENOLIC CORP., 1250 W. Van Buren Venice Blvd., Los Angeles, Calif.-R St., Chicago, 111.-C, L MOTOROLA, Galvin Manufacturing Corp. AMERICAN RADIO HARDWARE CO., INC., 476 NATIONAL COMPANY INC., 61 Sherman St., Broadway, New York, N. Y. -D Malden, Mass., "Nat)onal' , "AC "-CO, FA, R AMPLIFIER CO. OF AMERICA, 17 W. 20th St., New NATIONAL, National Company, Inc. York, N. Y.-FA VICTOR J. ANDREW CO., 6429 S. Lavergne Ave., NORTHERN ELECTRIC CO., LTD., 1261 Shearec St., Kits K Chicago, 111. -C (coaxial) Montreal, Que., Can. -T Receivers R ANSLEY RADIO CORP., 4377 Bronx Blvd., New OFFENBACH ELECTRIC CO., 1452 Market St., San N. Y. Francisco, Calif. -AR, D, R, T Transmitters T York. "Dynaphone ", "Dynatone " -CA ANTENNEX, M. M. Fleron & Son, Inc. PACENT ENGINEERING CORP., 79 Madison Ave., BARBER & HOWARD, INC., East Ave., Westerly, New York, N. Y. -A, CR, FA, R AMERICAN RADIO HARDWARE CO., INC., 476 R. 1.-CO RADEX CORPORATION, 1733 Milwaukee Ave., Chi- Broadway, New York, N. Y. -K BARKER & WILLIAMSON, Ardmore, Pa. -T cago, III. -CO J. H. BUNNELL & CO., 215 Fulton St., New York, RADIO ELECTRIC SERVICE CO., INC., N. Y. -R T BELDEN MANUFACTURING CO., 4647 W. N.W. 7th & RCA MANUFACTURING CO., INC., Camden, Van Buren St., Chicago, 111.-AR, AT Cor. Arch Sts., Phila., Pa.-A, AR, N. J. -R, T BIRNBACH, Birnbach Radio Co., Inc. AT, CO, CA, CR, D, R, T Please say you saw it in the Radio -Craft "Classified Radio Directory"

RADIO -CRAFT for MAY, I941 699 CLASSIFIED RADIO DIRECTORY

RADIO ENGINEERING LABS. INC., 35 -54 36th St., AMERICAN PHENOLIC CORP., 1250 W. Van Buren MANUFACTURERS SCREW PRODUCTS 214 W. Long Island City, N. Y. -AT P, R. T St., Chicago, I8.-CC, C, CF, GR, PL, SK, SO, Hubbard St., Chicago, 111. -BI, GR, SÓ, M RADIO RECEPTOR CO., INC., 251 W. 19th St., TS, M MEISSNER MANUFACTURING CO., Mt. Carmel, New York N. Y. HARDWARE 476 -AT AMERICAN RADIO CO., INC., 111. -C, CO, SK, TS RADOLEK COMPANY 601 W. Randolph St., Chi- Broadway, New York, N. Y. -BI, CC, CO, GR, JAMES MILLEN MFG. CO. INC. 150 Exchange cago, 111. -AR, C, CO, CA, CR, L, R JA, MO, PL, SK, SM, SO, TS TU, M St., Malden, Mass. CC`, C, CO, SK, TS, TU RCA MANUFACTURING CO., INC., Camden, ATLAS SOUND CORP., 1447 9th St., Brooklyn, -BI, N. J. -AR, AT, CR, R, T N. Y.-CC, GL, PL J. W. MILLER CO., 5917 S. Main St. Los Angeles, SAFTEST M. M. Fleron & Son, Inc. AUDIO DEVELOPMENT CO., 123 Bryant Ave. N., Calif., "Miller Quality Products", "Miller" - MAURICE` SCHWARTZ & SON, 710 -712 Broadway, Minneapolis, Minn. -JA BI, CO, MO, SK, TS, TU Schenectady. N. Y.-A, AR, AT, CO, CA, CR, D, BARKER & WILLIAMSON, Ardmore, Pa. -C, JA, PL MILLER, J. W. Miller Co. R, T 'BIRNBACH, Birnbach Radio Co., Inc. MILLER QUALITY PRODUCTS, J. W. Miller Co. SEATTLE RADIO SUPPLY CO., INC., 2117 2nd Ave., 145 Hudson St., Seattle, Wash. -AR, AT, C, CO, CRS L, P, R BIRNBACH RADIO CO., INC., MONTGOMERY WARD & CO., INC., 619 W. Chi- PL, SK, SETCHELL CARLSON, INC., 2233 University Ave., New York, N. Y., "Birnbach "-CC, JA, cago Ave., Chicago, Ill., "Airline " -CC, C, SO TS M St. Paul, Minn. -CR, R CO, GR, JA, MO, PL, SK, SO, TS, TU, M BOND PRODUCTS CO., 13139 Hamilton Ave., De- * SIGNALER, M. M. Fleron & Son Inc. MUELLER ELECTRIC CO., 1583 E. 31st St., Cleve- JA, MO, PL, SK, STROMBER- CARLSON TELEPHONE MFG. CO., troit, Mich. -BI, CC, CO, GR, land, Ohio -SP 100 Rd., CR SO, TS TU, M Carlson Rochester, N. Y.-CA, THE MUTER COMPANY, 1255 S. Michigan Ave., SUN RADIO CO., 212 Fulton St.. New York, N. Y. BUD RADIO, INC., 5205 Cedar Ave., Cleveland, R, JA, MO, PL, 5K, SO, TS, Chicago, 111. -SO -A, AR, AT, CO, CA, CR, D, R, T Ohio -CC, CO, TU, M NASH RADIO PRODUCTS CO., 6267 Gravois Ave., INSTRUMENTS CORP., Miss. SUPREME Greenwood, CANNON ELECTRIC DEVELOPMENT CO. 3209 St. Louis, Mo.-C, GL -FG Humboldt St., Los Angeles, Calif. -CC, SK k, NATIONAL COMPANY, INC., 61 Sherman St., TECHNICAL APPLIANCE CORP., 17 E. 16th St., CARRON MANUFACTURING CO., 415 S. Aber- New York, N. Y. -AR C Malden, Mass. "National ", "AC " -BI, C, CO, deen St., Chicago, 111.-C CC, JA, PL, SK, TS, TU TECHNICAL PRODUCTS INTERNATIONAL, 135 CINCH MANUFACTURING CORP., 2335 W. Van New York, Liberty St., N. Y. -T Buren St., Chicago, 111. -BI, CC, GR, JA, MO, 'NATIONAL, National Company, Inc. TELERADIO ENGINEERING CORP., 484 Broome PL, 5K, SO, TS, TU M NEW ART SPECIALTIES, INC., 816 W. Erie St., St. New York, N. Y.-CO CINEMA ENGINEERING CO., 1508 W. Verdugo Chicago 111.-MO SO M TILTON ELECTRIC CORP., 15 E. 26th St., New Ave., Burbank, Calif. -JA, MO, PL SO, TS OFFENBACH ELECTRIC CO., 1452 Market St., San Y. York, N. -AR CONSOLIDATED WIRE & ASSOCIATED CORPS., Francisco, Calif.-BI CC, C CO, GL, GR, JA, STATES TELEVISION MFG. CORP., 220 E. PL, TS TU UNITED 512 E. Peoria St., Chicago 111. -C, JA MO, SK, SO, York, td 51st St., New N. Y. -R CRESCENT TOOL & DIE CO., 4140 Belmont Ave., PARISIAN NOVELTY CO., 3510 S. Western Ave., VERTI -FLEX ILLINOIS SEATING CO., 2138 N. Chicago, III.-MO, M Chicago, 111. -SM Racine Ave., Chicago, 111.-AR CRUMPACKER DISTRIB. CORP., 1801 Fannin St., PARKER -KALON, Parker-Kalon Corporation PARKER -KALON CORPORATION, 200 Varick St., VERTROD 132 Houston, Tex. -CC CO, GL, GR, JA, MO, PL, MANUFACTURING CO., Nassau St., New York, N. Y., "Parker -Kalon " -M, ST New York, N. Y. -AR 5K, SO, TS TU, NI DOW RADIO SUPPLY CO., 1759 E. Colorado, PAUL & BEEKMAN Div. of Phila. Lawn Mower & WARD PRODUCTS CORP., 1523 E. 45th St., Cleve- Mfg. Co., 4250 Wissahickon Ave., Phila., Pa.- land Ohio -AR Pasadena, Calif. -81, CC, , CO, GL, GR, JA, MO, PL, SK SO, TS, TU M CO TS WILCOX ELECTRIC CO., 40th & State Line, Kansas Tioga & EAGLE ELECTRIC MFG. CO. INC., 59 Hall St., PHILCO RADIO & TELEVISION CORP., City, Mo. -FA, P, R T Sts., Phila., Pa. -BI, CC, CO, GL, GR, JA, WINCHARGER CORPORATION, Sioux City, Iowa Brooklyn, N. Y.-CC, PL SIL, TS "C" MIKE -STAND CO., 56 Christopher Ave., PL, SK, SO, TS, TU, M -AR, AT EASTERN 113 Brooklyn, N. Y. -PL PHILMORE MFG. CO., INC., University PI., PL, HUGH H. EBY, INC., 4700 Stanton Ave., Phila., New York, N. Y. -BI, JA, 5K Pa. -BI, CC, JA, PL, 5K. TS RADIO ELECTRIC SERVICE CO., INC., N.W. HARDWARE- CONNECTORS & EMPIRE NOTION CO., 105 E. 29th St., New York, Cor. 7th & Arch Sts., Phila., Pe. -BI, CC, Y. NO, TU, M MISC. PARTS & SUPPLIES N. -BI GR, 5K C, CO, GL, GR, JA, MO, PL, SK, SO, FAHNESTOCK ELECTRIC CO., 46 -44 I Ith St., Long Island City,. N. Y.-BI, CC, JA, SO, TS TS, TU, M FEDERAL SCREW PRODUCTS CO., 24 -26 S. Jeffer- RADIO EQUIPMENT CORP. 326 Elm St., Buffalo, son St., Chicago, 111. -BI, CC, GR, JA, MO, PL, N. Y. -BI CC C, CO, OL, GR, JA, MO, PL, SK, SO TS, M SO TS, .4 FEDERATED PURCHASER, INC., 80 Park PI., New RADOLEK CMPANY, 601 W. Randolph St. Chi- 14.146V) York N. Y. -BI, CC, C, CO, GL, GR, JA, MO, cago III. -BI, CC C, CO, GL, GR, JA, MO, PL, PL, SK, SO, TS, TU, M SK, SO, TS, TU, M FISCHER DISTRIB. CORP., 222 Fulton St., New York, ROGERS -MAJESTIC CORP., LTD., 622 Fleet St., Bakelite sheet, tubes & rods . B N. Y.-BI, CC, C, CO, GL, GR, JA, MO, PL, Toronto, Can. -BI CO MO, TS, M Binding posts BI 5K, SO TS, TU M WALTER L. SCHOTTCO., 5266 -70 W. Pico Blvd., "Welsco " Cable connectors CC FONDA CORPORATION 29 W. 57th St., New York, Los Angeles, Calif., -C N. Y.-GL GRK, JA, tU MAURICE SCHWARTZ & SON, 710 Broadway, Coil dope C GENERAL CEMENT MFG. CO., 919 Taylor Ave., Schenectady N. Y.-BI, CC, C, CO, GL, GR, JA, Coil shields CO Rockford, 111. -C, GL, GR, JA, PL, SK, SO, M MO, PL, SIS, SO, TS, TU M GOAT RADIO TUBE PARTS, INC., 314 Dean St., SEATTLE RADIO SUPPLY CO., INC., 2117 2nd Ave., Conduit fittings CF Seattle, Wash. -61, CC C, CO, GL, GR, JA, CU Brooklyn, N. Y. -TU Couplings GORDON SPECIALTIES CO., 1104 S. Wabash Ave., MO, PL, SK, SO, TS, TU M 15th New Engraving on bakelite panels . EB Chicago, 111.-GR SELECTAR MFG. CORP., 30 W. St., York, N. Y. -CC, JA, PL Flexible control shafts . . F L. F. GRAMMES & SONS, INC. 361 Union St., Allentown, Pa.-CC, 5K, SO, hi SHAKEPROOF LOCK WASHER CO., 2501 N. Fuses & mountings FM 111. EDWIN 1. GUTHMAN & CO., INC., 400 S. Peoria Keeler Ave., Chicago, -M Grid clips G St., Chicago, III. -CO SHELLEY RADIO CO.. 1841 S. Flower St., Los JA, PL SK, SO, TS, TU GL MFG. Angeles, Calif. -B1, GR Grille cloth HAMMARLUND CO., INC., 424 W. 33rd St., H. 8. SHERMAN MFG. CO., 22 Barney St., Battle Grommets GR New York, N. Y. -CO, TU Creek Mich. -CC, SO Jacks JA HARRISON RADIO CO., 12 W. Broadway, New SORENG- MANEGOLD CO., 1901 Clybourn Ave., York, N. Y. -BI, CC. C, CO, GR, JA, MO, PL, SK, 5K, TS brackets MO Chicago, III. -PL, Mounting SO, TS, TU, M STANDARD ELEC. PRODUCTS CO., 417 1st Ave., Plugs PL Minneapolis, Minn. -TS A. G. HINTZE, 300 W. Adams St., Chicago. N., Push -on knob springs PS STANLEY TOOLS, New Britain, Conn. -GR I11. -GL 621 E. 216th St., New Rivets RI STEWART STAMPING CORP., H. HORN, 1201 S. York, N. Y. -MO, SO RF HERBERT Olive St Los Angeles, Rubber bumper feet Calif. -BI CC, C, CO, GL, GR, JA, MO, PL, SUN RADIO CO. 212 Fulton St., New York N. Y. Screws SC SK, SO, t5, TU, M C, CO, GL, GR, JA, MO, PL, K, SO, -tapping screws ST TS,ITCC, Self INSULINE CORP. OF AMERICA, 30 -30 Northern TELERADIO ENGINEERING CORP., 484 Broome St. Blvd., Long Island City N. Y. Sockets SK -BI, CC, C, CO, New York N. Y. -CC C, CO, MO, SK SO TS GR, JA, MO, PL, SK, SO, TS, TU, M Soldering lugs SO TILTON ELECTRIC CORP., 15 E. 26th St., New J. F. D. MANUFACTURING CO., 4111 Ft. Hamilton York, N. Y. -TS Special washers SW Pkwy:, Brooklyn, N. Y.-CC. C, TU, M Springs S THE TRIPLETT ELEC. INSTRUMENT CO., Bluffton, E. F. JOHNSON CO., Waseca, Minn.-JA, PL, Ohio, "Triplett" -JA, SK insulated; and un- Spring clips, /or SK, SO TRIPLETT, The Triplett Elec. Instrument Co. insulated SP HOWARD B. JONES, 2300 Wabansia Ave., Chicago, *WALSCO- Walter L. Schott Co. Spring washers SR 111. -MO, PL, 5K, SO, TS Stampings SM KELLOGG SWITCHBOARD á SUPPLY CORP., 6650 X -L RADIO LABS., 420 W. Chicago Ave.. TS S. Cicero Ave., Chicago, 111. -81, CC, JA, PL, Chicago, III. -BI Terminal strips SK, SO, TS Tube shields TU ZIERICK MANUFACTURING CORP., 385 Gerard LAFAYETTE RADIO CORP. 100 6th Ave., New Ave., Bronx, N. Y.-SO Washers, felt W York N. Y. -BI CC, C, CO, GL, GR, JA, MO, fibre PL, 5K, SO, TS, TU, M THE BRUSH DEVELOPMENT CO. 3311-3405 Perkins Washers, WA Ave., Cleveland, Ohio-CH, HE, PP Washers, lock WS 'LAVA, American Lava Corp. Miscellaneous screws, washers, etc. M LEOTONE RADIO CO., 63 Dey Sf., New York, N. Y. -MO HEADPHONES 4757 AC, National Company, Inc. LITTELFUSE, INC., Ravenswood Ave., * AIRLINE, Montgomery Ward & Co., nc. Chicago, 111.-FM ALDEN PRODUCTS CO., 715 Center St., Brockton, M & H SPORTING GOODS CO. 512 Market St., Mass. -CC, C, GR, JA, PL SK, SO, TS, M Phila. CC, f CO, GL, GR, JA, MO, ALLIED RADIO CORP., 833 W. Jackson Blvd. Chi- PL, Pa. BI, cago 1,T ÚC, C, CO, GL, GR, JA, MO, PL, MAJESTIC RADIO & TELEVISION rQtka0- oSÖ1.- CORP., 2600 W. ALSIMAO American Lava Corp. 50th St., Chicago, 111. -MO Crystal CH AMERICAN LAVA CORP., Cherokee Blvd. & Manu- P. R. MALLORY & CO., INC., 3029 E. Washington DH facturers Rd., Chattanooga, Tenn., "'AISiMag ", St., Indianapolis, Ind.-CC, GR, JA, MO, PL, Dynamic "Lava " -GR SK, SO, TS, M Hearing -aid (bone conduction) HB Please say you saw it in the Radio -Craft "Classified Radio Directory" 700 RADIO -CRAFT for MAY, 1941 CLASSIFIED RADIO DIRECTORY

AMPLIFIER CO. OF AMERICA, 17 W. 20th St., New Hearing -aid (earphone) . . . HE (Complete) HEARING -AIDS York N. Y. -C Magnetic MH AUDIOGRAPH SOUND SYSTEMS, 1313 W. Ran - Pillow phones PP dolph St., Chicago, 111.0 CHICAGO SOUND SYSTEMS CO., 315 E. Grand *AIRLINE, Montgomery Ward & Co., Inc. Ave., Chicago, III. -C, CP ALLIED RADIO CORP., 833 W. Jackson Blvd., *COMMERCIAL, Trimm Manufacturing Co., Ltd. Chicago, III., "Knight " -CH, HE, MH CO- RECHT, Electronic Sound & Music Co. CANADIAN MARCONI CO., 211 St. Sacrament St., CRYSTAL EAR, INC., 2007 S. Michigan Ave., Chi- Montreal, Can. -CH, DH, MH Desk type (battery and/or cago, 111.-C, CP C. F. CANNON CO., Springwater, N. Y. -MH electric) C DEPENDABLE, Trimm Manufacturing Co., Ltd. CARDAK- Electro -Voice Mfg. Co.. Inc. Portable, personal CP DICTAFILM Miles Reproducer Co. Inc. ELECTRONIC SOUND & MUSIC CO., 10 Stuyvesant CARRON MANUFACTURING CO., 415 S. Aberdeen St., New York, N. Y., Recht"CP St. Chicago 111.-D1-1, MH "Co- ERWOOD SOUND EQUIPMENT CO., 224 W. Huron CHICAGO TELEPHONE SUPPLY CO., 1142 W. St. Chicago, 111. CP Beardsley Ave., Elkhart, Ind. -C, -MIf EXEdUTONE, INC., 415 Lexington Ave., New York, 'COMMERCIAL -Trimm Manufacturing Co., Ltd. N. Y.-C CRUMPACKER DISTRIB. CORP., 1801 Fannin St., Preceding Listings in RADIO- CRAFT'S FILMGRAPH, Miles Reproducer Co., Inc. Houston, Tex. MH -CH, GEM EAR PHONE CO., INC., 47 W. 34th St., New DEPENDABLE -Trimm Manufacturing Co., Ltd. CLASSIFIED RADIO DIRECTORY York N. Y.-C, CP DOW RADIO SUPPLY CO., 1759 E. Colorado, Pasa- GLOBE PHONE MFG. CORP., Reading, Mass. - dena, Calif.-CH, DH, HB, HE, MH Sec. I (Revised), April 1941: C, CP ELECTRO -VOICE MFG. CO., INC., 1239 S. Bend CHARLES JACK MFG. CORP., 27 E. Philadelphia Ave., S. Bend, Ind., " Electro- Voice", "Cardek" Sec. II, Nov. '40: St. York, Pe.0 -DH Amplifiers "KNIGHT, Allied Radio Corp. ELECTRO- VOICE-Electro -Voice Mfg. Co., Inc. Antennas & Accessories LAUREHK RADIO MFG. CO., 3918 Monroe Ave., FEDERATED PURCHASER, INC., 80 Park PI., New Automatic Tuners & Parts Wayne, Mich.-C, CP York, N. Y.-CH, DH, HB, IE, MH Auto -Radio Controls M. & G. HEARING AIDS CO., 30 N. Michigan Ave., Chicago 111.-C, CP FISCHER DISTRIB. CORP., 222 Fulton St., New Aircraft Radio (see Receiving Sets -including 83 S. 9th St., York N. Y.-CH DH HB, HE, MH Adapters and Converters) MAICO COMPANY INC., Minnea- PHONE MtG. CORP., Reading, Mass. -HB, Battery Chargers (6 Parts)) polis, Minn. -C dP GLOBE TELEVISION CORP., 2600 HE MH Batteries (& Cells) Dry & Wet (Storage) MAJESTIC RADIO & W. Manuals, Books & Magazines) 50th St. Chicago, 111.-C, CP HARIiISON RADIO CO., 12 W. Broadway, New Books (see Service MECK 1313 Randolph St., Chi - York N. Y.-CH, HB, HE, MN Cabinets Cases, Parts & Services JOHN INDUSTRIES, 111. HERBERT H. HORN, 1201 S. St., Los Angeles, Chemicals for Radio cago, -C Calif.-MH Coils & Transformers (R.F S. I.F.) & Accessories MILES REPRODUCER CO. INC., 812 Broadway, Records & Record -Playing York, N. Y., lmgraph "Dictafilm' INSULINE CORP. OF AMERICA, 30-30 Northern Coin Controls (see New "Fi, ", Long Island N. Y. -MH Equipment- also, Receiving Sets) -C, CP Blvd., City Tibbetts Laboratories SWITCHBOARb & SUPPLY CO., 6650 S. Condensers, kind MONOBAR, KELLOGG Condensers, Variable Cicero Ave., Chicago, 111. -MH MONTGOMERY WARD & CO., INC. 619 W. Chi- Crystals (Quartz) cago Ave., Chicago, Ill. Airline'C, CP & Parts *KNIGHT- Allied Radio Corp. Dials E. A. MYERS & SONS, 306 Beverly Rd., Mt. Lebanon, LAFAYETTE RADIO CORP., 100 6th Ave., New York, Electric Fence Controllers Pittsburgh, Pa. -C, CP N. Y.-CH, HE, MH Electric- Generating Machines NORTHERN ELECTRIC CO., LTD. 1261 Shearer LAUREHK RADIO MFG. CO., 3918 Monroe Ave., Electronics St., Montreal, Que., Can.-C, CP Wayne, Mich.-HB Electronic Musical Instruments & Parts PARAPHONE HEARING AID, INC., 4300 Euclid T. R. McELROY, 100 Brookline Ave., Boston, Mess.- Frequency Modulation Equipment Ave., Cleveland, Ohio-C, CP Hardware -Connectors & Misc. Parts MH Trimm Manufacturing Co., Ltd. M. & G. HEARING AIDS CO., 30 N. Michigan Ave., Headphones PROFESSIONAL, Chicago, III. -HB HE, MH Hearing -Aids RAY -LAB COMPANY, Grand Central & l Ith St., M & H SPORTINGG GOODS CO., 512 Market St., Hearing -Aid Parts Elmira Heights, N. Y.-C Phila., Pe., -CH, MN Insulation RAY -LAB, Ray -Lab Company Intercommunicating Systems MAICO COMPANY, INC., 83 S. 9th St., Minne- Sq., apolis, Minn.-DI-I, HB, HE, MH Line Filters ROTOPHONE CORPORATION, 26 Journal Next month: Section III, revised. Jersey City N. J.-C, CP MAJESTIC RADIO & TELEVISION CORP., 2600 W. TAY BERN EQUIPMENT CO., INC., 135 Liberty St., 50th St., Chicago, III. -NB, HE Sec. III, Dec. '40: New York, N. Y.-CP MEISSNER MANUFACTURING CO., Mt. Carmel, TELEX PRODUCTS CO. 1645 Hennepin Ave., Min- III.-MH Magnets neapolis, Minn. CP Metal I. Fittings (for Radio) C, MONOBAR Tibbetts Laboratories Special TIBBETTS LABORATORIES, Camden, Maine, "Mono - Metal, Ore & Oil Locators MONTGOMERY WARD & CO., . INC., 619 W. Chi- Microphones bar"-CP cago Ave., Chicago, III., "Airline" -CH, HE, MH CO., LTD., 1770 W. Noise Elimination Equipment TRIMM MANUFACTURING E. A. MYERS & SONS, 306 Beverly Rd., Mt. Chicago, Ill., "Trimm" "Commercial ", & Was Berteau, Lebanon, Pittsburgh Pa.CH HB, HE, MR Paint, Cement Products CP Plastics "Professional ", Dependable " -C, NORTHERN ELECTRIC` CO. LTD., 1261 Shearer St., TRIMM, Trimm Manufacturing Co., Ltd. Mortreal Que. Can.-HB, HE, MR Plastic Molders VACOLITE COMPANY, 3003 N. Henderson St., OFFENBACH ELECTRIC CO., 1452 Market St., San Radio Logs Maps Globes Tex.-C, CP Francisco, Calif.-CH, HB, HE MH Receiving Sets (including Adapters & Converters) Dallas, PARAPHONE HEARING AID, INC., 4300 Euclid Records & Record -Playing Equipment WEBSTER ELECTRIC CO., Clark & DeKanen Ave., Ave., Cleveland, Ohio -HE, MH Racine, Wis.-C, CP PHILCO RADIO & TELEVISION CORP., Tioga & Sec. IV, Jan. '41: WESTERN ELECTRIC CO., INC., 300 Central Ave., "C" Sts. Phila.. Pa. -HE MH Recording Equipment Kearny, N. J. -CP PHILMORE MANUFACTURING CO. INC., 113 Resistors & Volume Controls (Attenuators S. Net- University Pl., New York, N. Y. -Mit works) "PROFESSIONAL, Trimm Manufacturing Co., Ltd. Schools HEARING -AID PARTS RADIO ELECTRIC SERVICE CO., INC., N.W. Service Manuals, Books S. Magazines Servicing Equipment Cor. 7th & Arch Sts., Phila., Pa. -CH, DH, Sound Systems, Amplifiers & Accessories MB. HE. MH RADIO EQUIPMENT CORP., 326 Elm St., Buffalo, Sec. V, Feb. '41: N. Y.-CH, MH Speakers (& Parts) RADOLEK COMPANY, 601 W. Randolph St., Chi- Switches & Relays cago III.-CH MH Television RCA MANUFACTURING CO., INC., Camden, N. J. Test Equipment -Laboratory Z. Production -HB, HE, PP Tools Batteries, miniature . . . . BM ROTOPHONE CORPORATION, 26 Journal Sq., Jer- Transformers & Chokes Cases C sey City, N. J. HE -HB Transmitters (& Equipment) S MAURICE SCHWARTZ & SON, 710 -712 Broadway, Condensers, small -space variable Schenectady, N. Y. -CH, DH, HB, HE, MH Sec. VI, March '41: Earphone (and /or Bone SEATTLE RADIO SUPPLY CO., INC., 2117 2nd Ave., Tubes (& Parts) Conductors) E Seattle, Wash.-CH, HB, HE, MR Vibrators Microphones M SHELLEY RADIO CO. 1841 S. Flower St., Los An- Wire geles, Calif.-CH, DH, MH Literature Plugs & Sockets P SUN RADIO COMPANY, 212 Fulton St., New York, Resistors, special small -space N. Y.-CH, DH, MH The above back -issues of Radio -Craft are avail- variable R TAYLOR AIRPHONE PRODUCTS, INC., Long able at the regular price of 25c per copy. Testers TE Beach Municipal Long Beach, month: Section III of the Classified Radio Airport, Calif. -MH Next Transformers & chokes . . . . TC TELEX PRODUCTS CO., 1645 Hennepin Ave., Minne- Directory, revised, with new classifications, and Tubes (and /or sockets) . . . . T apolis, Minn. -CH, HE new names and addresses added; and with obsolete listings removed. Reserve your copy, today, from TIBBETTS LABORATORIES, Camden, Maine, "Mono - your regular newsdealer. ba."CH, HE AEROVOX CORPORATION, New Bedford, Mass. -S TRIMM MANUFACTURING CO., LTD., 1770 W. ALLIED RADIO CORP., 833 W. Jackson Blvd., Chi- Berteau, Chicago, Ill., "Trimm ", "Commercial ", cago, Ill., "Knight " -T "Professional ", "Dependable " -HE, MH AMERICAN MICROPHONE CO., INC., 1915 S. ' TRIMM, Trimm Manufacturing Co., Ltd. ACME HEARING AID CO., 516 5th Ave., New York, Western Ave., Los Angeles, Calif.-M TRY -MO RADIO CO., INC. 85 Cortlandt St., New N. Y.-CP AMPLIFIER CO. OF AMERICA, 17 W. 20th St., New York, N. Y.-CH, H, HB, HE, MH *AIRLINE, Montgomery Ward & Co., Inc. York, N. Y.-C, M, R, TC UNIVERSAL MICROPHONE CO., LTD., 424 Warren ALLIED ENGINEERING INSTITUTE, 85 Warren St., ASTATIC MICROPHONE LAB., INC., 830 Market St., Lane, Inglewood, Calif. -MH New York, N. Y.-C, CP Youngstown, Ohio -M VICTOR ANIMATOGRAPH CORP., Davenport, Iowa ALLIED RADIO CORP. 833 W. Jackson Blvd., Chi- AUDIO DEVELOPMENT CO., 123 Bryant Ave. N., -DH, HE cago, Ill., "'Knight'-CP Minneapolis, Minn. -TC Please say you saw it in the Radio -Craft "Classified Radio Directory" RADIO -CRAFT for MAY, 1941 701 CLASSIFIED RADIO DIRECTORY

BOND PRODUCTS CO., 13139 Hamilton Ave., De- toit Mich. -C THE BRUSH DEVELOPMENT CO., 3311 -3405 Perkins Ave., Cleveland, Ohio -E, M BURGESS BATTERY CO., Freeport, 111. -BM CANADIAN MARCONI CO., 211 St. Sacrament St., 10 1O.1¢ Montreal, Que., Can. -R, TC, T BEST Ma CARDAK- Electro -Voice Mfg. Co., Inc. RADIO FANS EVERYWHERE -these amazed at the wealth of information CHICAGO TELEPHONE SUPPLY CO., 1142 W. fine ten cent text books give you an contained in these handy books. Ex- Beardsley Ave., Elkhart, Ind. -R excellent foundation for the study of cellent for reference-ideal for every CLAROSTAT MFG. CO., INC., 265 N. 6th St., RADIO. They YOUR MONEY Brooklyn, N. Y. -R are clearly written, pro- technical library. 'COMMERCIAL- Trlmm Manufacturing Co., Ltd. fusely illustrated and contain over BACK if you are not satisfied. 15,000 SHIPMENTS CO- RECHT- Electronic Sound & Music Co. words in each book. You'll be PROMPT DEPENDABLE -Trimm Manufacturing Co., Ltd. DICTAFILM -Miles Reproducer Co., Inc. DOW RADIO SUPPLY CO., 1759 E. Colorado St., Pasadena, Calif. -E, M, R, TC, T No. I No. 2 ELECTRONIC APPLICATIONS. Brunswick, Maine - HOW TO BUILD FOUR HOW TO MAKE THE MOST TC DOERLE SHORT WAVE SETS POPULAR ALL WAVE I- anti ELECTRONIC SOUND & MUSIC CO., 10 Stuyvesant 2 -TUBE RECEIVERS Thousands of radio fans have St., New York, N. Y. "Co- Recht " -C 1239 This book contains a numberber of ELECTRO -VOICE MFG. CO., INC., S. Bend tageSgWaveRadio Receivers. So in. &late demand excellent set some of which Ave., S. Bend, Ind., Electro- Voice ", "Cardak 'bee re th. have appeared in past of " "- l as RADIO- CRAFT. These seta have M construction details. this been book pub engineered. They 'ELECTRO- VOICE -Electro -Voice Mfg. Co., Inc. fished. are not experiments.l ed. 'FILMGRAPH -Miles Reproducer Co., Inc. Contains that are these wÚ described In this been printed ong these book. but It ntains all of the GEM EAR PHONE CO. INC., 47 W. 34th St., famous receivers. Illustrations. hookups. etc. New York, N. Y. -E, M GLOBE PHONE MFG. CORP., Reading. Mass. - C E, M, TC, T EDWIN I. GUIHMAN & CO., INC., 400 S. Peoria No. 3 No. 4 St., Chicago, Ili. -TC HARRISON RADIO CO., 12 W. Broadway, New ALTERNATING CURRENT ALL ABOUT AERIALS FOR BEGINNERS This book variousthe York, N. Y. -R, T underlying the ypeéá HOLLYWOOD TRANSFORMER CO., 5334 Hollywood This the Inverted "L." the hold °in electricity ands Radio. Doublet. Doublet. Blvd., Hollywood Calif. -TC Electric ircuits are explained. xpnslaiDouble HYGRADE SYLVANIA CORP., 500 5th Ave., New (ohm's Law, o of the funds. Lion. how low-impedanceyse.f tnsp.a. York, N. Y. mental l laws of radio. Is ex mission trans- -T planted: the generation of al. posed cad.i. are .Itgives HYTRONIC LABORATORIES, 23 New Derby St., (emoting sine waves: in detail the construction of units-volts. 'rial' suitable for long -wave Salem, Mass.-T wttreauae li.i°deer, broadcast receivers. for short. INTERNATIONAL RESISTANCE CO., 401 N. Broad transformers. A.C. receivers and for allwave Pa. e instruments. receiverswave St., Phila. -R INTERNATIONAL TRANSFORMER CO., 17 W. 20th St., New York, N. Y. -TC KNIGHT- Allied Radio Corp. LAFAYETTE RADIO CORP., 100 6th Ave., New York, No. 6 N. Y. -E, M, R, TC T No. 5 LAUREHK RADIO M4G. CO., -3918 Monroe Ave., BEGINNERS' RADIO HOW TO HAVE FUN WITH Wayne, Mich. -C, E DICTIONARY RADIO LEKTRA LABORATORIES, INC., 30 E. 10th St., New Are by radio tan. Stun. iced York, N. Y. you puzzled Tetrode? Jokes.s. scientifparties.c eimentst -M qr1uu ncy? Kiloocqyycleáeñ amusementss which r o 1* M. & G. HEARING AIDS CO., 30 N. Michigan Ave., done t your radio are Screen grid? Baffle? Anode? Tri. t III. E, M, R TC, T Ionisation? Joule's In tills fascinating vol. Chicago, -C, m . It how to make Law?La Harmonyic?l rai ll alla MAICO CO., INC., 83 S. 9th let., Minneapolis, Minn. If you cannot define these very talk-how to p dure sinewspaperlent music for dances -how to -C, E, M, P, R of ther, do visible"ilent P. R. MALLORY & CO., INC., 3029 E. Washington a technical. dterms make uraddio" unit. in all radio magazines and St., Indianapolis, Ind. Iusednstruction books, you need this usable by the deafened -how -R book in your library. make toys which dance to radio MILES REPRODUER CO., INC., 812 Broadway, music. etc.. etc. New York, N. Y., "Filmgraph ", "Dictafilm " -M MONOBAR- Tibbetts Laboratories NORWALK TRANSFORMER CORP., 50 Day St., S. No. 7 NS. 8 Norwalk, Conn. -TC ELECTRIC CO., 1452 Market St., San HOW TO READ RADIO OFFENBACH RADIO FOR BEGINNERS Francisco Calif.-E, M, R, TC, T DIAGRAMS Hugo Cernsbark. the intonation. All of 'ao symbols commonly ally famous dio pioneer. author PROFESSIÒNAL -Trimm Manufacturing Co., Ltd. used in din diagrams are pre. and editor. hose famous g.. together sine.. RADIO AND TELEVISION RAYTHEON PRODUCTION CORP., Small wiit>hpictures of the and RADIO-CRAFT are by present ands nations million., another triumph Tube Division, 55 Chapel St., Newton, Mass. theyn ex l with this new hok. Any be -T ria them. Thi.book by Robert Eimer who ads it ill stet Eichberg, the well-known radio thorough ground work in radio ROCKET -Shure Brothers and memberbe of erry explained oittra staff f ssimple language. and through ROTOPHONE CORPORATION, 26 Journal Sq., Magasine. two the usel of many Illustrations Jersey City, N. J.-C, E, M pictur..wirinr diagrams are t.b radio eat that you an myAnalogiesst clear. SEATTLE RADIO SUPPLY CO., INC., 2117 2nd Ave., uild. Seattle, Wash.-E, R, TC, T SELECTAR MFG. CORP., 30 W. 15th St., New York, N. Y. -M SHURE BROTHERS, 225 W. Huron St., Chicago, III., No. 9 No. ID "Shure ", "Undyne ", "Uniplea ", 'Rocket ", "Super -Level Rangé', SIMPLE ELECTRICAL TELEVISION "Stratoliner ", ", "Ultrawide- EXPERIMENTS ''Zephyr"-14 the SHURE -Shure Brothers Over How does television work? This book explains all of the different STACKPOLE CO., Tannery St., St. Marys, electrical are systems of television from the CARBON described in this book. covering Pa. every branch of electricity-from to the most complex. -R simple experiments with ItIimpyles describes in A - style lust THE STERLING MFG. CO., 9205 Detroit Ave., Cleve. high frequencycy how All of de- the scene i. picked by th land, Ohio -TE can be carried with toi yoouryhome etc. VMoouusatype, STRATOLINER -Shure Brothers Simple apparatus. most of to of television systems are de. can be about here, scribed. SUPER LEVEL -Shure Brothers TAY BERN EQUIP. CO., INC., 135 liberty St., New York N. Y.-BM, E, M, TC, T TELERADIO ENG. CORP., 484 Broome St., New BOOKS ARE ALL UNIFORM York, N. Y. -TC TELEX PRODUCTS CO., 1645 Hennepin Ave., Minne- Every book In the GERNSBACK EDUCATIONAL LIBRARY bas 32 pages -with Illustrations varying from apolis, Minn. -C, E, M, R, TC 80 to 86 in number. Each title volume contains over 15,000 words. Positively radio's greatest book buys! If you MFG. Huron do not think these books worth the price asked, return them to 24 Hours and your money will be Instantly refunded. THORDARSON ELECTRIC CO., 500 W. St., Chicago, III. -TC TIBBETTS LABORATORIES, Camden, Maine, "Mono- bar"-E, M RADIO PUBLICATIONS 20 VESEY STREET NEW YORK, N. Y. TRIMM MANUFACTURING CO., LTD., 1770 W. Berteau, Chicago, III., "Trimm ", "Commercial ", r "Professional ", "Dependable " -TC RADIO PUBLICATIONS. Dept. RC5.4I TRIMM -Trimm Manufacturing Co., Ltd. 20 NEW YORK, N. Y. VESEY STREET, CO., 909 17th St., Cedar Rapids, Iowa, Gentlemen: Please send immediately, POSTPAID, the book numbers circled below, I am enclosing cents THE TURNER -each book being 10e. "Turner " -M 1 2 3 4 5 6 T 8 s 10 -The Turner Co. NEW NEW TURNER Bend FREE listing of 48 new loa publications. ULTRAWI DE- RANGE -Shure Brothers UNIDYNE -Shure Brothers Name Address UNIPLEX -Shure Brothers City State UNITED TRANSFORMER CORP., 150 Varick St., Beals by check or money order -register letter if you send calla!: unwed U. S. Do.i[9 clamp. New York N. Y. -TC MAIL COUPON TODAY!. J ZEPHYR -Shure Brothers 702 RADIO -CRAFT for MAY, 1941 SHOP NOTES -KINKS -CIRCUITS

240V SIMPLE PHONOGRAPH H F CRONE 8MF,450V. OSCILLATOR / 4000001MS ! RADOLEK'S WITH the recent interest in phonograph as FREE oscillators, many experimenters have .01-ME "A NEW 1941 RADIO FILTER + wanted to- find a really simple and inex- CHOKE GiiRYy. 11A001EK PROFIT GUIDE pensive unit. The one illustrated here fills .UtMey the bill entirely. ll j iwv. Bargains/ H.FCNN( S,o.0o0 PRI. SEC. 15stF. OMMS 150Y. Galore Like This L TO p PHONO. G PICKUP sw. PROFESSIONAL TYPE MACE ti IO" RECORDING DISCS'. y0-100 113V.,AC. INSERT MILLIAMMETER HERE ACME. TO CONTROL FILAMENT CURRENT. F G7I OUTPUT 190 ittle that it does not influence the working Made To n eforemostr uperlioratooanr +250v. MICROPHONE conditions of the resistor. In the same man- discs of ]his type regardless of cost These blanks have very slight I . (CARBON) ner, the filament resistance can be neglected PHONO perfections but guaranteed to make 1./. SPECIAL perlect recordings or your money OSCILLATOR COIL PRI. in comparison with the bleeder resistor. The ^SEC on both sides -heavy MOST back Coated OBTAINABLE AT unit, built in a metal case, with all connec- metal hase - non-mllammable Bag RADIO SUPPLY HOUSES tions to it except the line strip blinded (au- v ngsl 6" Discs -10e. B " -14r. 10" -19c; lc less in 25 lots Send thor's copy -presumably meaning protected no money - shipped COD plus postage The diagram is nearly self- explanatory, against accidental contact.-Editor), gives except the is an audio EVERYTHING NEW IN RADIO that transformer excellent results in voltage stability and A -- inter-stage unit, used back to front. If for there isn't any noticeable hum. Note that the AT BARGAIN PRICES! home broadcasting, you desire to use a mi- case is only connected to the ground at the Shows hundreds and hundreds of up.to.the. output gear.electric crophone, use a "tube to voice coil" point marked on the primary side of the appliances and supplies In transformer. A word of warning; tune this transformer and not to any point on the dispensable to every amateur. experiment. ex set builder. sound engineer and se to the high- frequency end of the broad- secondary side. man Keep up with radio and all its phxs cast band, and use no antenna. The F.C.C. is easy to modify unit for any other und for the big FREE Fr, I I. -L l'a na It the Guide now' DONT WAIT-:. i . h 'I ... doesn't like people who do otherwise! tube or tubes, calculating the bleeder re- AT RADOLER EUGENE SIMPSON, sistor by Ohm's law. For instance, if the Naperville, Ill. output voltage of the rectifier tube is supplied with 300 volts, and the filament cur- RADOLEK CO., Dept. C -54 IMPROVISED MERCURY rent needed is 0.120 -amp. (106 tube), the W. Randolph St., Chicago, III. value of the bleeder resistor has to be 601 SWITCH 5. FlEE Ca.dcy - 2,500 ohms, rated at 50 watts, as it has to A SWITCH may be improvised from the ID Snip_ Discs Sie ches C O D Plus Poupe support about 35 watts. In this case it is Name starter of a fluorescent light. I used a recommended to use a 5Z3 rectifier. burned -out starter (frequently obtainable Address Dealer? FRANZ PLACZEK, Experimenter? Serviceman? Amateur?.. free from electric shops). Porto, Portugal I removed the switch from its metal shell, broke off the small glass tip, and poured just enough mercury into the switch to close the contacts. Then by plugging the Dectt c Pry _haver hole with a match and pouring some glue gQt1hI on it, I had a dandy mercury tilt- switch. ABSOLUTELY FREE!

JUST THINK OF' 1T -you can get absolutely FREE, the useful DRY ELECTRIC SHAVER which is shown at the left. This ELECTRIC DRY SHAVER is sent to you by the publishers with a one -year subscription to RADIO- CRAFT. Here Are the Features of The ELECTRIC DRY SHAVER Constructed of metal with attractive red bronze finish. Scientifically constructed to give a perfectly A switch of this type may be used on lean shave. 5 -foot rubber insulated clod and plug, furnace controls, thermostatic controls, etc. Constructed to last for many year,. Operates from 110 -volt, 60 -cycle A.C. elec. Taped on a potentiometer shaft, it may be trie line. Carries a two -year manufacturer's used on tilting surfaces; and, on rotating guarantee. A floe quality, self- sharpening to shut off and turn on the current toilet necessity. I devices, Send your sub,crlpt ion to RADIO- t9tAFT at different intervals. It also may be used for One Year (12 Issues) and receive alit ,nlutely FREE one of these remarkable with a toggle switch by being taped to it. t :ltntric Dry Shavers. New subscriber[ OPERATES ON are accepted or you may extend your pr.'s- HOMER L. DAVIDSON, eilt subscription another twelve months. Fort Dodge, Iowa 110 -VOLT. 60 -CYCLE SIll your remittance of $2.00 (plea L,s Fur ,hipping charges on Shaver) to the A.C. LINE publishers. (Canada and foreign SUPPLY E2.75.1 Ton will receive your "A" £T'B1e DRY ELECTRIC SHAVER im- BATTERIES as the power source for sig- mediately by return mall. Use coupon below to enter your nal generators often give reason for subscription. trouble by poor contacts, loss of power, etc. On the other side, when substituting a bat- RADIO -CRAFT tery supply for a line power supply, be sure 20 Vesey Street. New York. N. Y. that it works "batterylike," with stability of voltage and as humless as possible. Here is the description of a unit which RADIO- CRAFT, 20 Vesey Street, New York, N. Y. Gentlemen: Enclosed find my remittance of S2.00 for which enter my corresponds to this condition, and which tuhsrriplion to RADIO-CRAFT for one year (12 Issues). Send me was built for use with a battery -operated immediately FREE. ELECTRIC DRY SHAVER (Canada and foreign $2.751. In U. S. add only 25e additlonsl to cover shipping charges on signal generator, using 2 type 30 tubes WE DRY Shaver. SHIP ELECTRIC Present Subscription with their filaments connected in series SHAVER THE SAME DAY O New Subscriber O Stead (Readrite "Ranger," model 557). YOUR SUBSCRIPTION Name As the figure shows, the drain of the ORDER IS RECEIVED. bleeder resistor, which is 0.06 -amp. is used AddresS for the filament current of the type 30 City state tubes. The plate current for the tubes, taken (Send renfnlnn,. by itiiik. money order or unused U.S. Postage SIaml= CLIP COUPON i RC .571 I t.±u - It, rari lrn.r II > o iuI 1a.Il p. from a tap of the same resistor, is so AND MAILI 703 RADIO -CRAFT for MAY, I 9 I BOOK REVIEWS*

O O 0.5 100 / I 00 ISOOOO 2 is OA. IMF. N. 9.rß a(LeA#yvti 100 5.000 l0.000 1, MF. 100 a i 4 4 4 4-11 on Mus 100 4 loo %SOo '6 --- i 1 OHMS 1 ELECTRONIC EQUIPMENT 3 ! 3 50 1 1500 0.1m3 .005MF q / IOO 100 .1 I500 2 2 2poo 2 .01-3.46 1 is loo 21 / Electronic Products Company 662, GS4 /I 3 200 3 5500 3 .O5MF So.. ._100 500.,. ' 4MF 4 300 4 5000 4 Ó.11.1F. PUBLISHERS 5 400 5 10.000 5 .25-MF. e..500 OHMS 1SMr. 5114F. 6 500 6 25,000 6 Amplifier Co. of America 647 12MF LO 141 7 600 7 50.000 7 Mr. Data Print Co. 670 1. 50 8 700 8 0.1-MEO. 8 MF. Miller, Wayne 647 OHMS 4. 9 800 9 025 9 National Plans Inst. 676 13. MF. n 900 100.5 10 Radcraft Publications, Inc.....672, 695. 69, 12./aF. ti SW. r 11 moo 11 1.1450 II 16. MF Radio & Technical Pub. Co 676 RESISTORS/ CONDENSERS Radio Publications 702 Supreme Publications 644, 677 Technifax 690, 695 SUBSTITUTION BOX RESIN PREPARATION FOR RADIO PARTS See Diagram Above LOOSE DIAL CORDS Amperite Co. Inside Front Cover HERE'S a sure cure for those slipping Dumont Electric Co., Inc 676 AN inexpensive substitution box can be made in the service shop, from standard dial cords. Obtain 3 ozs. of ether and 2 Hammarlund Mfg. Co 662 ozs. of powdered resin "rosin ") from International Resistance Co 655 parts. Servicemen that have never used an (or National Union Radio Corp 659 instrument of this kind have no idea what the drug store. Just enough resin is die - Solar Mfg. Corp 677 a time saver it is. solved in the ether to form a thick liquid. Sprague Products Co 681 This is applied to the dial cord with a small Here is a circuit of one that I have been brush. The ether will evaporate leaving the RADIO SETS using for some time, and which has proved resin on the cord. Your supply must be very satisfactory. Small cards can be typed kept sealed or, Harrison Radio Co 648 and due to the evaporation of Midwest Radio Corp 684 placed above each pointer. the ether, it will return to a solid form. Radio Electric Service Co 667 GEORGE HUrp iAaTEa, EARL GARNER, Detroit, Mich. Knight8town, Ind, RADIO SUPPLY HOUSES Allied Radio Corp 687 Burstein-Applebee Co 647 BOOK REVIEWS Radolek Co 703 .. n:emmmnnmmmmnnni:uununnmmnmmnnuwuuumnnmuunnmmnnnuununn wuuuunnunnr.ananinonwunmm Sears, Roebuck & Company 645 YOUR CAREER IN RADIO, by Norman V. INTRODUCTION TO ELECTRICAL MA- SCHOOLS Carlisle and Conrad C. Rice (1941). Published by CHINES, by A. W. Hirst (1940). Published by E. P. Dutton & Co., Inc. Cloth covers, else 8% z Capitol Radio Eng. Inst. 646 Blackie and Son, Ltd., London, England. Cloth 3) ins., 189 pages. Price $2.00. covers, size 6 z 9 ins., profusely illustrated, 122 Coyne Radio School - 646 Here Lincoln Engineering School 646 is another of those books which tell about pages. Price, $2.25. National Radio Institute 641 the possibilities for employment in the various To judge by the title, this book contains little National Schools 646 fields of radio. Treatment is mainly about broad- of interest for the average radio man, and even Port Arthur College 644 casting, including television. the publishers of this book refer to it as an Radio Training Assoc. 65S The book breaks away from the conventional engineering textbook. RCA Institutes 646 method of presentation by being built in story The fact of the matter is, however. that "In- Sprayberry Academy of Radio form around the conducted tour of 3 representa. troduction to Electrical Machines" should be on Inside Back Corer five boys. from a high school, through a large the shelf of every serious radio man; if in addi- broadcast station. The boys are given an oppor- tion the reader is familiar with algebra, then SOUND & RECORDING EQUIPMENT tunity to fire questions at and receive direct we add, "positively." A fair impression of the answers from the various technicians. studio per scope of the book may be obtained by review of Amplifier Co. of America 662, 677, 684 .onnel, etc., employed in a sight or sound its chapter titles: Atlas Sound 680 putting Electromagnetism and Electro- Corporation program on the air. It is sugar- coated knowledge, magnetic Induction; The Magnetic Circuit and Jensen Radio Mfg. Co 663 about radie broadcasting. embryo Lake Radio Sales Co 66, for the broad- Magnetic Calculations ; The Electromagnetic Ma- Lifetime Corp.. The 662 cast worker. chine; Insulation and Insulating Materials; John Meck Industries 670 Losses- Heating and Ventilation of Machines; Meissner Mfg. Company 665 RUTHERFORD, by A. S. Eve. Published by Wave Form and Harmonic Analysis. Regal Amplifier Mfg. Corp 657 Macmillan Company. Cambridge, England. Cloth We would like to see the author do a com- Universal Microphone Co., Ltd. 671 covers, size 10 z 634 ins., 436 pages. Price $5.00. panion book on Transformers. University Laboratories 670 This authorized biography constitutes the life INTERNATIONAL ELECTROTECHNICAL and letters of Lord Rutherford. The book is based VOCABULARY, (Reproduced 1940). Published on the collection of books. papers, etc., in the TEST EQUIPMENT by Unwin Brothers, Ltd., London, England. Cavendish Laboratory, University of Cambridge. Available in the U.S.A. from American Recommended Standards Radio City Products Co 658 for the reading table of ad- Association, New York, N. Y. Cloth covers. size vanced book RCA Manufacturing Co.. Inc.....Back Cover technicians. this delineates the work 11% z 8 ins., 211 pages. Price 82.50. 663 of a scientist who Readritè Meter Works made many outstanding con- This photo-offset book contains no illustrations Supreme Instruments Corp. 671 tributions to his chosen field of endeavor. 643 but it does include definitions of all the major Triplett Electrical Isatrument Co. Among the discoveries credited to work by the terms the technician will Weston Elec. Instrument Corp 677 author are the following: be likely to encounter Magnetic Detector in reading articles in English and in French; (work which Marconi completed) ; co-worker with the terms but not Sir their definitions are further TUBES J. J. Thomson at Cambridge in determining translated in Esperanto, German. Italian and laws governing the flow of electricity in a gas Spanish. Hygrade Sylvania Corp 675 (ion theory); discovery naming and of uranium Students who may have occasion to double. alpha and beta rays, the first -known radioactive check on original technical literature in foreign MISCELLANEOUS gas (thorium emination). languages will welcome this authoritative work. Atlas Press Co 684 An index in 6 languages greatly facilitate cross - CHAMBERS TECHNICAL DICTIONARY, references. The is Classified Section 694 Edited book divided into groups as by C. F. Tweney and L. E. C. Hughes. follows: Fundamental Definitions; Machines and Flight Magazine 695 Published by The Macmillan W. W. Grainger, Inc. 681 Co., New York. Transformers; Switchboards and Apparatus for Cloth covers. size 8% z 5% ins., 957 pages, ot- Connections Lancaster, Allwine & Rommel 684 easional illustrations. and Regulations; Apparatus for Moody, Willard 694 Price $5.00. Scientific and Industrial Measurements; Produc- A staff of 80 Onan, D. W., & Sons 646 experts under the editorship of tion, Transmission, and Distribution of Energy Messrs. Tweney and Hughes has compiled in Premium Distributing Service 690 Electric Traction ; Electromechanical Applica- Radio Servicemen of America, Inc 686 this extensive work all the major terms used in tions; Electric Heating Applications; Lighting; Walter L. Schott Co 671 pure and applied science, medicine. the chief Electrochemistry; Telegraphy and Telephony; Wellworth Trading Co 680 manufacturing industries, engineering, construc- Radio Communications; Radiology ; Electro- Woodstock Typewriter Co 680 tion and the mechanical trades. In this rapidly - biology. expanding technological world, this work stands as a beacon. in clearly presenting accepted definitions of existing terms, written by specialists NEXT MONTH . for specialists. A radio Serviceman from Shanghai tells (While every precaution is taken to insure The topics of interest to radio technicians about servicing radio sets in China. accuracy, we cannot guarantee against the which are covered in this book include the follow- A coil manufacturer gives the "inside" possibility of an occasional change or omis- ing: Acoustics, Dielectrics. Electrical Communi- story on coils in Frequency Moducation cation, Electrical Engineering. Measuring Instru- receivers. sion in the preparation of this index.) . A custom set builder tells how to make ments, Oscilloscopy, Photoelectric Cells, Physics, a "personal" radio transmitter. Radio and Television. 704 Printed In the V.S.A. RADIO -CRAFT for MAY, 1941 asi OVER 100 EXPERIMENTS plus Many Other Special Features ALL DESIGNED TO HELP YOU MAKE FAST PROGRESS!

SPRAYBERRY Training Marts right at the beginning of Radio . unfolds each subject in a simplified, logical, understandable style. You easily learn Television Frequency Modulation, Signal Tracing, Mobile Radio (Auto -Tank), Aviation Radio, Electronics, Facsimile Radio, Radio Set Repair and Installation Work. You learn quickly in -our spare hours . . . AT HOME OR AT ('AMP.

Training Prepares You for a Business of Your Own , . . Or Good Radio Jobs at Excellent Pay! The great, fascinating and progressive field of Radio and Television offers many opportunities to the man who wants to get ahead. There is good money to be made. My Training is designed to give you quick access to these opportunities. SPRAY - BERRY Methods are thorough and practical. Your Training will not interfere with your present work. You Get Professional Test Equipment plus Experimental Outfits! Includes 146 RADII) !'ARTS for building a complete Receiver, RADIO TOOLS and a modern TESTER -ANALYZER. You also receive S BIG SHIPMENTS OF RADIO EQUIPMENT for conducting actual experiments with your own hands, I show yea how to create Radio defects . how to correct them. This practical "beforehand" experience will come in handy again and again when you are called on to do actual Radio repair work later on.

READ WHAT THESE FELLOWS SAY ABOUT THE NEW PRACTICAL SPRAYBERRY All TRAINING Equipment "GROSSED BETWEEN $ISO AND $200" "Since finishing your Ira in big I've darted in the Radin Becomes repair business. 1 have about all tile work 1 can take r of. I have grossed between $150 and $200 in the last bee

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City State 'Tear uff this coupmt, mi: il iu utrtlupt or paste un penny postcard. T PUSH BUTTONS THE Qßld WAY!

RCA Station Allocator

Covers all push -button setting and alignment work. $ 50 Operates from either self -contained batteries or a -c. Stock No. 171 -Price

The RC ' Station Allocator is an indispensable instru- Only one adjustment is ment for required for each frequency. service dealer who really plans to "cash in" What's more, thanks to the Allocator's magnetite core on his g eatesi business- building opportunity in years. inductances and polystyrene condensers, these With it, you can handle adjust- more push button receiver ments stay put. Weight is only 51/2 lbs. re- jobs with batteries. tuning iv less time, with less effort and at greater Size is 5" high, 81/2' long and 31/2" deep. profits. Moreover, it will serve as a useful shop instrument long after the present reallocation is forgotten. See the Allocator at your RCA Test Equipment Dis- tributor's today. Prepare now for faster work and bigger The Allocator is speedy, accurate, portable. Eight push profits! buttons can quickly be set up to desired frequencies. Operation is with or without modulation. It then serves SPECIFICATIONS as an oscillator for rapid identification of desired PUSH -BUTTON RANGES: stations Buttons 1 and 2 during the re- tuning of receivers. Two (approximately) 405-825 kc. buttons can be Buttons 3, 4, and 5 600-1185 kc. tuned to i -f frequencies if desired, thus providing remark- Buttons 6, 7, and 8 820-1700 kc. able flexibility for all alignment as well as pushbutton TUBE COMPLEMENT: setting work. The Allocator operates from either a -c 1 RCA -1R5 asr- f oscillator, or self-contained i RCA-6H6 for a-c rectification, batteries. 1 neon tube for a -f oscillator and pilot lamp.

Another RCA Time - Sating Money -Maker RCA Junior VOLTOHMYST Costing only a little more than an ordinary volt -ohm- the addition of an isolated rectifier -type a -c voltmeter! meter, the RCA Junior VoltOhmyst gives you electronic Special protective features guard against meter burn-out push -pull operation with all the time and money- saving for d -c volts and ohms measurements. It is an instrument features of the famous Rider VoltOhmyst circuits plus no wide -awake technician can afford to miss! $34 9 5 Price, Complete Stock No. 165

RCA MANUFACTURING CO., Inc., Camden, N J. A Ssrv.av e. th Radie Lo.p. .: n. In Canada: RCA VICTOR CONAraUy. lid., .r - 'Weal