No.1How To MakeFour Doerle

Short Wave Sets

No.5Beginners'Radio

Dictionary

No.9Simple ElectricalExperiments

No.2How To Make The

Most Popular All-Wave1- and 2-Tube Receivers

No.6How To HaveFun WithRadio

No.3AlternatingCurrent

for Beginners

No.7How To Read

RadioDiagrams

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No.4AllAboutAerials

IRADIOIk!8EGINNERS!"'?l~~'\~~"""I'! 1 '.~:2:\~'::ft.~_./

No.8RadioFor

Beginners

Order book9 by number - remit by check, money order, cashor unused U. S. Post'age Stamps. Books are mailed POSTPAID.

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HOW TO READ

DIO])IAGRAMS8Y RDiJe~~ Elch6e"!l

RADIO PUBLICATIONS. 2S~BROADWAY, NEWYORK,NY:

INTRODUCTIONIF you are the average beginner in radio, you prefer picttlte'_.diagraIIl~ tothe schematic diagrams, which use symbols. The symbols are unfamiliar

to ytJu and, therefore, bewildering. But when, after two or three hours'study, you have learned to understand the symbols, you will find schematicdiagrams far simpler to read and easier to trace than the pictorial diagrams,with their confusing mazes of wiring. .

Symbols are really the short-hand of radio. With a few simple lines,they can show any unit used in radio. work. Not only that, but they lendthemselves to better arrangement, and more clearly show the relationshipbetween various parts. Charts of many of the most frequently employedsymbols are given on the following seven pages.

In each of the charts, the name and derivation of each symbol is shown.wherever possible. For example, the curlicue which symbolises a coil canbe traced to the turns of wire which comprise a coil; the. Audio FrequencyTransiormer symbol shows two such windings in relation to a core; etc.

Numerous simple diagrams of simple radio circuits are also shown.To familiarize you with schematic (symbolic) circuits, each has below it::he corresponding picture diagram. As an aid to learning to Iead theschematics, it is suggested that you lay a piece of paper over the picturedi:.LgTamand the textual explanation which accompanies each drawing.

If you are unable to read it, refer to the picture diagram. The symhols::lil(~picrures are in precisely the same position. in so far as possiblc. Try tore.,d the schematic diagram with the aid of the picture diagram.

l\iter having familiarized yourself with the symbol chart, you shouldbe ,"hlc to take the point of your pencil and reason out what happcns inthe circuit. In Figure 2, for example. you should be able to say, "Theantcnna is connected to one side of the phones and the cat-whisker of the'crystal detector. The other side of the phones and the crystal of the detectorare connected to ground."

To get the most benefit from the "Explanation" pages, it is suggestedthat you follow a certain procedure.

First, as has been said, study the symbol charts given in the precedingpages, until you are familiar with the meaning of the most conlll1unly usedsYfllbols.

Second, Take some sheets of p'l-per and cover up all of a symbol pageexcept for the symbols themselves. Try to write the name of the part eachsym bol repre~nts.

Third, take some sheets of paper and cover up all of a symhol pagee:;<:ceptfor the names of the units. Try to draw the symbol for each namegIven.

Fourth. have someone call off the names to you in no regular order.Draw the. symbol for each and write the name of the unit under it. Checkand practise as above.

2 ROlli to Read Radio Dial1"Grtle

AL"TERNA"TOR

~

LOOP ARCAERIAL

~

~~~-e- J]

AMMETER AN"TENNA OR@- 1 AERIAL.

@ T---0 T

ALTERNATOR - The upper sym-bol shown is based upon thebrushes and slip-rings of an alter-nating current generator. Thelower symbol, much used becauseit is more easily drawn, has atits center the symbol for alter-nating current, which indicatesthe type of current which thegenerator produces.

AMMETER-While the upper sym-bol is still used, the lower one isfavored by most draftsmen. Theletter "A" at its center shows thatthe instrument represented is anammeter. Were it a milliammeter,the symbol would show an "MA";a voltmeter, a "V"; and a milli-voltmeter an "MV". Thus eachunit is readily identified.

ANTENNA OR AERIAL-The sin-gle long line which projects atthe bottom of this symbol rep-resents the lead-in wire, while

the other two lines which angleupward from it indicate the feed-.ers from an old-fashioned multi-wire antenna, represented by theflat line across the top. Thoughantennas of -this sort are no long-er widely used for radio recep-tion, the symbol has persisted. Itis used to indicate an antenna oraerial irrespective of design.

LOOP AERIAL-Most loop aerialswere simply a piece of wire space-wound on a large rectangularframe. Therefore the svmbol wasa rough representation of this,showing two turns of the loopand a pair of connections.

"A" BATTERY-The symbol rep-sents a number of ceffs in series.

ARC-The arc, much used in thedays of spark transmission, wasa simple drawing of the two elec-trodes, with an X, indicating thearc, between them.

"B" B~TTERY - The symbol forthe HB" battery is much like thatfor the "1\" battery, except thatmore cells are shown, for the "A"usually consists of from 1 to 3cells in series, while the "B" mayhave from 15 cells (for a 22~-volt batt.ery) to 120 cells (for alBO-volt battery). Each pair oflines means one cell. Not all ofthese cells are drawn, however,the voltage being indicated byfigures.

BUZZER-As a buzzer consists ofa winding around an iron corewhich attracts an arm used tocomplete the circuit, the symbolshows this arrangement clearly.The parallel lines are the core;the arm makes contact at the littlearrow head.

FIXED CONDENSER-No matterwhether it is fiat, tubular or of

~

.1 fi". . ':' .

. 8".·

any other shape, a fixed condenserconsists, essentially, of two me-tallic plates, with a dielectric(non-conductor) between them.Therefore the symbol shows con-nections to two t;Jlates, with aspace between them, to indicatethat there is no connection be-tween the plates. The capacity ofthe condenser is shown by figures.

VARIABLE CONDENSER - Irre.spective of its size or shape, avariable condenser consists of twoor more metal plates with a dielec.tric (generally air) between them.The symbol may be much likethat for the fixed condenser, witha diagonal arrow added to showvariability or, as in the lower sym-bol, one of the lines may becurved and provided with an arrowhead, to show that it is a rotaryplate or group of plates.

.1T

--I-L II

T_-1':• I TRIMMER--i t CONDENSER 1111~T +-!-

]TRIMMER CONDENSER-A trim-mer condenser is variable, butnot as widely nor as easily asa standard variable condenser, forits shaft is slotted for a screw-driver instead of being providedwith a knob. The decrease in var-iability is indicated hy omittingthe head of the arrow.

CONNECTION - When two wiresare brought together and a dropof solder applied, the connectionlooks much as it appears in thesymbol.

NO CONNECTION-Sometimes, indrawing a diagram, it is neces-sary to cross lines over eachother. To make it clear that thereis no connection between thewires these lines represent, one ofthem is curved where it crossesthe other, to indicate that there isspace or insulation between them.

COIL-A coil consists of a numberof turns of wire, and the symbol

][VARIABLEINDUCTANCE~

---.. ~ CRYSTAL

Jl~ +for a coil is therefore nothingmore than a line with a numberof loops in it, to represent theturns. The inductance of the coilis given in figures; the same sym-bol is used to indicate a radio-frequency choke coil.

VARIABLE INDUCTANCE-Somecoils may have any predeterminednumber of their coils connectedinto or out of the circuit at will.This can be done by means ofone or more sliders, as shownabove, or by means of a numberof connections made betweenvarious turns and a number ofswitch taps. A movable metal armallows the circuit to be made tothe desired tap. This is shownin the lower part of the illustra-tion.

COUPLED COILS-Sometimes, asin radio-frequency transformers,it is desired to have two coils soarranged that there is inductive

coupling (a form of magneticlinkage without direct connec-tion) between them. When thisis done, the coils are placed closelytogether, as 'the symbol shows.

COUPLED COILS, VARIABLE-W~en the coupling between twoCOlis, the variability is indicatedsliding or rotating one of thecoils, the variability is indicatedby the arrow run diagonally acrossthe symbol, as shown.

CRYSTAL DETECTOR-This unitgenerally consists of a mineralcrystal used in conjunction witha fine wire "cat's whisker" whichis moved about its surface untila sensitive spot is found. Theheavy line in the symbol indicatesthe crystal; the arrow head, thecat's whisker. Even if the cat'swhisker is fixed, the svmbol re-mains the same. .

SPARK GAP-Once widely used inradio, this is now seldom if ever

employed. The symbol merely rep-resents the two electrodes of thegap.

QUENCHED GAP-Like the abovethis has virtually faded from th~radio picture., It had large platesto cool the gap, and these areindicated in the symbol. It ismuch like the battery symbol, ex-cept that the lines are all of thesame thickness.

GROU~D - In normal receivingpractice, the ground connectionmay be a water pipe, the frameof a building, a radiator, a pipedriven into moist soil, or any-thing which makes good contactwith moist earth-as do all of theunits mentioned. So the symbolis highly simplified. It is merelya little cross-section of a piece ofground, showing a few layers ofsoil. It is always given the tri-angular fOlm to make it readilyrecognizable.

SPARK GAP KEY GROUND

1~

1f 'V. .

QUENCHED LOUDSPEAKER MICROPHONEGAP

~

.J..~ t- w ,-T ' '.

8 How to Read Radio Diagrmn.

CONDENSER 1RESISTOR. PHONOGRAPH

MICROPHONE PICK·UP, ~ '~\T ,PIEZO-£L£CTRIC VARIABLE If ""000 "" 'iCRYSTAl. l. RESISTOR

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~0 I,. ___ ~_l

KEY -A key is an easily operatedswitch, normally in the open po-sition. And that is precisely whatthe symbol shows.

LOUD SPEAKER-If of the mag-netic type, it may be representedsimply by a horn, as shown inthe schematic illustration. In -rep-resenting a dynamic speaker, thesymbol shows the big fieldwinding, the metal core (parallelIi,nes), and the small voice coilwhich is attached to the papercone.

MICROPHONE - The double-but-ton carbon microphone is sym-bolized by two short, thick pointsto indicate the carbon buttons,with a long, thin line, Tepresent-ing the diaphragm, between them.For other microphones, seeTransmitter.

CONDENSER MICROPHONE - Acondenser with a thin and a thickplate (much like a battery) may

be used to indicate a condensermicrophone. Its position in thecircuit helps show what it is.

PHONOGRAPH PICK·UP - Themagnetic pick-up consists of acoil of wire surrounding an arma-ture to which the needle is at-tached, as the upper symbolshows. The crystal pick-up hasthe needle attached to a piezo-electric crystal which is clampedbetween two contact plates. Ineach, the arrow head indicates theneedle.'

PIEZO.ELECTRIC CRYSTAL-Different from the detector crys-tal, this one generates electricitywhen. subjected to strains. As thesymbol indicates, it is clampedbetween two metal plates. Thesame symbol sometimes is used,for a crystal microphone.

RESISTOR-You can readily under-stand that, as every conductor of-fers a certain amount of resis-

tance to the passage of elech'lccurrent, a longer wire (of thesame siz~ and material) offersmore resIstance than a shorte1'wire. Therefore the symbol forres!stan.ce is the jagged line,,!hlch IS longer than a stnightlane w~uld be in reaching fromone poant to another. The samesymbol is used irrespective of thematerial or desi~n of the resistorand its resistance is shown i~fi~res.

RESISTOR (VARIABLE) - Onemeans of showing that a resistoris variahlc is by putting an anowdiagonally across it, as shown.But sometimcs the resistor is ofa particular typc. It may be con-ncct~d at both ends, with a mov-able contact on the resistance ele·ment. Thi~·.is shown at the lowerpOTtion of' the illustration. Attimes this same symbol, with oneof the ends unconnccted, is usedinstead of the other symbol, to

show an ordinary variable resistor-particularly when tbecircuitcan be opened enm-e1y b),l rIJOvingthe arm from the element.

TWISTED PAIR --- Twisted wiresoften used for lead-ins and fo;hea,ter connections, are shownmerely by a pair of intertwinedlines, all in the iIIustr(ation on tbepreceding page.

SWITCH-A convenient means ofopening or closinR' a circuit.

TELEPHONE RECEIVERS - Thissymbol is merely a simplifieddrawing of a pair of head-phones(the two small circles). Thecurved line which joins them rep-resents both the head-band andthe connection between thephoncs.

TRANSMITTER (MICROPHONE)This is the diaR'ram most usuallyused to indicate any type of trans-mitter or microphone except thedouble-button carbon type. Let.

]~\.IE

~]l~ ~ ~

~,7·WI

~e~inl{ generally sho'ws whetherIt l11dlcates a crystal, single-buttoncarl?on, .dynamic, velocity or ca-pacity microphone. Circuit connec-tions may also indicate this.

TRANSFORMER (AUDIO FRE-QUENCY)-An audio frequencytransformer consists of two coilsof wire wound on the same iron~ore, and this is readily followed111.the s:ymbol. There are the twocOlIs, with the parallel lines be-t~een th:-I1J, to represent the core.1he primary is distinguishedfrom th~ secpndary by beingd;:lwn wltl; fewer turns in many~lIag,rams; In others the primary~~nlarked "~ •• and the secondary,S, Occasl0naJly no special:neans. of distinguishing is used,111which case the primary is thewinding connected to the platecir.cuit of one tube, the secondarybemg connected to the grid cir-cuit of another.

AUTOFORMER-This is simply atransformer with a single winding,tapped so that one part of it may ,be used as the primary, the otherpart as the secondary. The symbolshows this clearly.

TRANSFORMER (POWER) - Att:ansformer designed to handlehigh voltage and considerable cur-~ent. Used to step up line voltageI? power packs. May have addi-tional step-down windings forfilaments of tubes.

TRANSFORMER (PUSH-PULL)-In some. circuits, the plate ofone tube IS used to feed the gridsof t~o tubes connected in "push-pull, and a special type of trans-former, with a center-tapped sec-ondary, is used. This push-pull in-put transformer is shown at thetop of the illustration. If theplates of these tubes are to beconnected to a loud speaker, apush-pull output transformer

(Continued gn Page 28) J

'\' ou know the symbol for theneTlal. H:ere it is, in Figure I, con-nected .rIght to one side of a crys-tal which "detects" the signals theaerial picks up.In order to. hear these signals,

you ne~d a pair of phones, so theother Side of the crystaL is connect-ed to this unit., Both sides of the phones must beconnected, for best results, so theother side of the phones is con-nected to ground.That is .'cally all there is to read-

ing diagrams-you simply have tofigure out what has to be done andconnect up the units t6 do it.Every circuit must be complete;

there must NEVER be any "looseends" in radio.One of the most important of all

C~lcu~tS is .the antenna (or aerial)ClrcUlt. ThiS consists of the aerialwire and the ground, and any ulOitsconnected between them. .Later, in other books, you will

learn of circuits which operatewith.out any apparent g-round con-nectl0n, but the ground is alwaysthere. For example in electric setsit may be one sid~ of the electri~lighting lines.Antennas may be anywhere from

?5 to 100 feet long; generaJly speak-Ir.g, the longer the antenna, then;ore numero\ls and stronger theSignals you will pick up, you willhav:- more trouble separating ones\atl0n fron; another, and you willget more II1terference and staticnoises, on the broadcast band.The phones should be 2,000 ohms

or upward; 75 ohm phones, such asMe used on telephones, will Dot do.

Is the diagram in Figure 2 thesame as the preceding one?True, it has the same parts, but

if you inspect it closely, you willset" that they are connected differ-ently. It is NOT the same.In the preceding diagram, the

crystal detector and the headphoneswere in series. In the diagram onthis page, the crystal detector andhead-phones are in parallel.Trace the circuit. The antenna

connects to Olle side of both thecrystal and the phones. The otherside of the crystal and the phonesconnect to the ground.This makes entirely different ac-

tion take place in the circuit.To understand this, you must

know that a radio wave is alternat-ing current, shaped or "modulated"by a lower ("audio") frequency. In01 der to work the phones, this ndiowave must be rectified or changedinto direct current. The detector.whether it is a crystal or a tube,does this.The crystal offers great resistance

to one-half of the radio wave, butpasses the other half freely. Thatis known as "detector action."In the preceding circuit, half of

the wave is retarded, and, the otherhalf passes through the crystal andthe phones. That is what alwayshappens in series circuits of anytype -- the same current passesthrough all the units which are con-nected in series.In the circuit on' this page, the

half of ,the wave that passes throughthe, crystal goes directly to ground;nearly all of the half that is retarded~asses through the phones.

"

What have we in Figure 37 Anew clement -- a fixed eondenser--added to the circuit of Figure 1.No device made by man' is per-

fect. Thcrcfore, sOllie of the radiofrequency CUTrcnt gets by the de-tector .. The phones offer a greatdeal of opposition to the passal1;eof radio frequency currents, and thismakes the circuit WO'l'k less perfect-ly than we would wish.But currents of this sort, thou~h

they do not pass freely through cotlsof many turns of fine wire (such asthe windings of a phone or a pairof phones) do pass very easilythrough condensers.For this reason the condenser has

been added. I t affords a free pathfor the radio frequency currentsand, equally important, it keeps themfrom pas3in~ through the phones.They would not injure the phones

in any way, but they would makereception weaker, for one-half ofthe alternating current radio wavewould tend to. cancel out the otherhalf. It would not cancel it com-pletely, because the half that leakedthrough the detector would be muchweaker than the half that passedeasily through the detector, but itwould make reception poorer. Thecondenser takes care of it.You will see that the condenser

is marked ".001 mfd." That is themeasure of its capacity -- what lay-men call "its size." Sometimes acondenser of greater or lesser ca-pacity will work better, depertdingupon the phones and crystal used.Experiment -- tryin~ one condenserafter another -- WIll show whichworks best.

).001.-M~.

The diagram, Figure 4, is wrong.A mistake has been put into it de-liberately. Do you see what it is?True, it is the circuit of Figure

2, with a condenser added as in Fig.ure 3, but there is something differ-ent.The condenser is across the crys-

tal as well as across the phones.Remember what you read aboutparallel circuits when studying theexplanation of Figure 2.In this circuit, the radio frequency

currents would not only be by-passed around the phone, but aroundthe crystal as well, at least in part.For that reason the crystal could notfunction as perfectly as though thecondenser were not there, thoughone station might be heard.

TUNED CIRCUITSAll of the circuits given thus far

have been untuned circuits. That is,they will pick up any -signals which,reach their antennas, but will beefficient only on a wave which hap-pens to be of approximately thesame length as the antenna. If tworadio waves are being received atthe same time, the circuits givencannot be adjusted to cut one sig-nal out so that the other can beheard in comfort.A tuned circuit is needed to adjust

the receiving circuit to resonancewith the various stations' waves.This is done most simply by addinga coil of wire in series with theaerial; the more turns of wire thatare added, other things being equal,the longer the wave-lengths whichwill be received.

, Do you recognize this circuit inFigu,e 5?Just as Figure 3 was developed

from Figure I, so the circuit. on thispage is developed from FIgure 3.It is the same as Figure 3 exceptthat an extra unit has been ~,dded;This new unit is a single-slide

tuner, connected between the an-tenna and the crystal. It could havebeen connected between the phonesand the ground instead, bu: the re-sults are usually (although not al-ways) better when it is connectedas shown.A single-slide tuner is simply a

coil of wire with a slider arrangedto be movable along it, to makecontact with any of the turns ofthe coil. By moving the slider,more or fewer of the turns can beconnected in the circuit, at will.\Vhen making a tuner that em-

ploys sliders, it is easiest to use a.nin!ouL1ted wire. Enameled wIre ISperfectly satisfactory' and costs lessthan cotton covered or silk covered.\\711en the coil is made, the insula-tion must be scraped off. the wirewhere the slider touches it. Thiscan be done easily with a piece ofsandpaper held o\,er the end of afinger and Tubbed back and forthalong the coil. following the paththe slider will take.The coil can be wound on an

empty oatmeal box. Put on about150 turns of No. 22 wire. Wind theturns close together if you use animulated wil e; if you use a wirethat is not insulated, you will haveto leave a tiny space bet"'een eachturn so that the wire does !lot shortcircuit. This is hard to do neatly.

In Figure 6, we have somethinga little different. -If the coil were omitted, this dia-

gram would be exactly the sameas the one shown in Figure 3. Butthe coil is connected between theaerial and ground. I t is in parallelwith the detector-phone circuit, andthus is actually a part of the cir-cuit. -There is a fact called "resonance".

V,hen, a circuit is in resonanceVl'ith an incoming wave, the naturalfrequency (or wavelength) of thecircuit is the same as that of thewave being received, and the resultis that the signal is received strong-ly. Moving the slider along the coilpermits the circuit to be brought toresonance, just as in Figure 5.Another explanation of this ac-

tion is that when the coil is cor-rectly tuned by means of the slider,it offers a very high resistance tothe passage 01 the one particularwave to which it is tUfled, and butlittle resistance to all other waves.Thus the wave which is wanted isforced to go through the crystaland phones, while the others flowthrough the coil to the ground andtherefore do not interfere.The secoRd explanation is easier

tv understand, and while it meansmuch the same as the first one, isnot quite so accurate technically andtherefore is less commonly used.However, it may give the readera clearer understanding of the ac-tion of the circuit,Resonance is one of the parts of

radio which is often difficult forthe beginner to understand, and itis beyond the scope of this volume.

, Except for one detail, Figure 7IS ,the same as Figure 6. Instead ofa slide tuner, It has a tapped coil.The tapped coil can be easily

mounted, because the connectionsto the various turns can be madeto switch points inserted throughthe ~anel, ~nd used together witha switch arm.By tapping the coil correcth' an\'

desired number of turns ma~' beconnected into the circuit-but note\'ery turn has to be tapped. Ii thiswere not possible, a 100-turn coilwould need 100 taps, and this wouldbe extremely in~oll\·enient. Onl\' 19taps are needed to use from 1 to 100tl1rns of a 100 turn coil.In winding the coil, make one turn

of the wire, then twist 'I little pieceof the wire so it stands up from the~oil, and wind the next turn, t,,;st-mg a loop on it in the same way.Do this on each of the first tenturns. Then wind nine turns inthe regular way, make a Jittle loopon the 20th, nine more regular turnsand the 30th turn with a loop in it,and so forth, jor the rest of thecoil, tapping each tenth tI:rn.Connect the first ten taps (the

ones on every turn) to one groupof switch points, which will be con-tacted by one switch arm. Connectthe other fJine taps (the ones onevery tenth turn) to a second groupof switch points, to be contacted bya second switch arm. Make' yourrough tuning adjustments with thesecond switch, the fine adjustmentswith the first switch. If you num-ber the switch points, )'OU can makea record of where ever)' station istuned in.

Notice the similarity betweenFigure 8 and Figure 6.A two-slide tuner has replaced the

single-slide tuner.One end of the coil is connected

to ground; the antenna connects toone slider and the detector. to theother. In Figure 6, the detector wasconnected directly to the antenna,ard when fhe slider tuned the an-tenna circuit, it also tuned the de-tector circuit. Now these two cir-'cuits may be tuned separately, eachby means of the slider to which itis connected.This will give appreciably better

results than the single-slide tuner,uecause each circuit can be tunedseparately, instead. of a compro-mise reached. The single-slide tun-er simply tunes to a point which isbetween the bcst ,tuning of the de-tector circuit and the best tuningor the antenna circuit..-\. tuner ot this sort is made ex-

actly the same as a single-slide .tu~-er with but one exceptIOn. ThIS ISth~t there are two rods, e<:.ch withits slider. It is connected as shownin the figure on this page.At- first one might think that the

two sliders could be mounted onthe same rod but this is not so.The reason is that for some stationsthe antenna slider will have to benearer the end of the coil and thatfor other stations the detector slid-er will have to be nearer the endof the coil. If they were on thesame rod. this could not be donewithout stopping to change connec-tions-a great nuisance.The two-slide tuner is used in

most of the so-called "long dis-tance" crystal sets.

Here is something new again!Figure 9 shows the first two-cir-cuit set which has been presentedin these pages. All the others havebeen single-circuit diagrams. for ~heantenna circuit was coupled direct-ly to the detector circuit.In this circuit, however, you will

see that there are two coils insteadof one. The two coils together forma radio-frequency transformer ofsorts. The autenna and ground areconnected to the primary, which isinside the secondary. The detector-secondary, which may be a simplesino-Ie-slide tuner but which is morec3si'ly made as a tapped coil, asshown. ,Notice the arrow running through

the two coils. That, as you havelearned, indicates that the couplingbetween the primary and secondarycan be varied at \vill. This is usual-ly done by winding the primary- on'a smaller form, which fits insidethe secorldary and md\1nted on ashaft which can be rotated by meansof a knob.This circuit is a great improve-

m~nt over those previously given.for although it does not make theset more sensitive than, the onegiven in Figure 8, it makes it fa.rmore selectIve. In other words, Ithelps the listener tune-out ullwa·n-ted stations.Not only does it make it possible

for the antenna circuit and the de-tector circuit to be tunc,! perfectly;it also enables the listener to de-crease the amount of energy fedfrom the antenna circuit to the de-tector circuit. Thus, when two sta-

(Coutinued on Page 31)

Here, too. is a new element enter-ing the circuit; it is the variablecondenser. It is connected bet.ween!he aerial and the antenna coil, an'dlS used to tune the anl.enna circuit,as seen in Figure 10.Coils with sliders or with taps are

m~re of a bother to tune, for theshd~r must be pushed back andforth. ~r if a tapped ·coil is used,t';Vo sWltch~s must be manipulated.~or are coJls of these types as effi-Clcnt as might be desired, for thereare loss~s of energy due to currentsset up m the turns which are cutout of the circuit.Pntting a condenser in series with

the circuit is the electrical equiva-le~t of. taking some turns off thecoIl. Usmg a smaller condenser hasthe effect of taking off more turnsthan USing a larger condensep.Therefore the use of a variable con-denser perm:ts the user to securethe effect of taking off a fraction ofa turn, a co;pplete turn, a {tw turns,or a l~r&,e number of turns, simplyby tWlstmg the condenser shaft tomove the plates in and out of mesh.Belter yet, with the slider or with

the tapped coil, every adjustmentmeant changing the coil by one com-plete tUTn: There are no such stepsm the adjustment of the variablecondenser-the variatiOh is smoothand gradual instead of being insteps. The user can secure the ef-fects of rem0\'!ng 1/10 of a turnor 1534 turns with equal ease. A~a result. the circuit can be tunedm.ore accurately than with taps orslJders.Like fixed condensers, variable

condensers are rated as to capacity(Continued on Page 24)

As you see in Figure 11, anothervariable condenser has been addedto the circuit, to tune the secondarycoil. A condenser in parallel with icoil acts in precisely the oppositeway that a condenser in series does.That is, as the capacity is increased,this acts like adding more turns.Now, in this circuit, no taps or

sliders at all are needed to tune theantenna circuit 01:- the detector cir-cuit. They are brought t.o reso-nance with the incoming wave bymeans of the condensers.Circuits of this sort are seldom

used with crystal detectors, for vari-able condensers are not as cheap assliders or switches. They are, how-ever, used in vacuum tube circuits,which will be considered r-ext.Before goin'g on to vacuum tube

circuits, however, review Figures 1to 11, inclusive. Be sure that you• 'understand- how connections aremade between units, and the func-tions of the various units you havestudied tbs far.Every tuned ·radio circuit has ca-

pacity and inductance. A coil is anir.ductance, and even if there is nocondenser used with it, some little"distributed capacity" exists betweenthe turns of the coil.Even in the-circuit of Figures 1

and 2 where no condensers or coilsare u'sed, inductance and capacityexist-the former, in the windingsof the phones; the latter betweenaerial and ground, and in the wir-ing of the set, particularly in thephone cords.Remember that there must be in-

duc,tance and capacity in all the cir-cuits which follow.

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II,IIIIIIII

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EXPLANATIONYou recogniz~ the symbol, Fig-

ure 12, for a simple three-elementvacuum tube, which contains gridfilament and plate. 'Connected to this tube are two

b~tteries--the "A" battery to pro-nde current to heat the filament·th "B" b I'e attery to supp y voltageand current to the plate circuit.The filament is a wire which is

so treated tnat it emits a cloud ofelectrQns when it is heated by thepassage of current from the "A"battery. ,:!"he .electrical charge ofelect~ons !s saId to be negative,It I~ .a faft that unlike charges of

electrICIty attract each other. There-fore the electrons are attracted bythe plate which, as you see, is con-nected to the positive end of the"B" batterv.An explanation of how and why

current flows in the plate circuitwould be long, and technically be-yond the scope of this book. How-eyer. yOll can believe that electricityfrom the "B" battery flows throughthe filament. from the filament tothe pl~te, from. the plate throughany COIls or resIstances which maybe between the plate and the bat-tery, and back to the battery. Thecircuit just described is the externalplate circuit.There is another equally impor-

tant circuit in the tube--the gridcircuit. The electrons from the fila-ment must pass through the gridto reach the plate, and some ofthem are attracted to the grid.These must flow back to the fila-ment through any coils or resistorsin the external grid circuit.The third circuit is the filament

(Continue<! on Page 26)

EXPLANATIONYou are not expccted to under-

stand ~his diagram thoroughly, foryou wIll never build a set of thesort. &hown in Figure 13. It uses aspeCial type of detector which iscalled a "diode" as it has but twoelements instead of the three justdescribed.I~ i~ gi,:"en for a special purpose,

for It IS VIrtually the link betweenthe crystal detector and the three-element vacuum tube describedupo,? the preceding page.Diodes came out before three-ele-

ment tubes and. as the latter werefar !TI0re elncient, wcre soon dis-contmued. But a few years ago,they came back strong in certaincircuits. (such as for use as seconddeteeto·r in superheterodynes withautomatic volume control). How-~ver, th~y .are never used today10 the ~lre~l1t ~hown on this page.In thiS CIrCUIt, the tube functions

as ~ simple rectifier or detector.~otl~e .how similar the secondaryCIrCUIt IS to that given in Figure11. Except for the fact that it hasa' two-element tube instead of acrystal, and that the antenna andground are directly connected itis the same. 'Compare this diode tube, which

has only plate and filament, to thethree-element tube shown in Fig-ure 12. The. grid, added by Dr. Leede Forest, IS what really put radioon the map. That little grid actsas a ,:alve .. When t.1H.' incomingwave gIves It a negative charge, itretards the electron flow from fila-ment to plate; when it is given apositive charge it accelerates theflow of electrons from filament to

(Continued on Page 25)

Fi~nrc 14 pres('nls the simplest ofstalld;ml tllbe circuits. Compare ittt) th(' (Jlle Rivcn in Figure 13.Alsl) refrr back to Figure 12 ant!

the rxplanation there<Jf, you willsee that the tunilll{ unit and an ad-ditional resistance and fixed con-dcns('r have becn added to the tun-inl{ units (coil and variable conden-arr) in the /STidcircuit. You will alsonolke Ihat the headphones havebecn added to the plate circuit. Anda variable rcsistance, has been putinto the filament circuit, as was ex-plain('d.The resistor (usually from 2,000-

000 to 6,000.000 ohms-or, more sim-ply. 2 to 6 meg-ohms) completes thegrid circuit, allowing the electronsto leak slnwly back to the filament.But this resistor, which passes thedirect current of electrons, offersIt emendolls resistance to the pass-age of the high-frequency alterna-ting currents which are the radiowaves. and it is these radio waveswhich must reach the grid in order10 make the tube operate.As you were told in the explana-

tion of Figure 3, the high-frequencycurrents flow e,asily through a con-denser. For this rC'ason, a fixedcondenser (generally .0,0025 mfd.) isconnected in parallel with the re-sistor. The condenser is called agrid condem;er, and the resistor iscalled a grid leak, from their uses.As yOll read in the explanation ofFil\'ure 2, these elements are con-nected in parallel.Direct currents cannot flow

through cOT'densers, sO the gridleak is used to' afford a path for the,electrons; alternating ctlrrents can,

(Continued on Page 29)

Figure 15 shows the same set asthat presented in Figure 14, butwith a great improvement added.This new feature is regeneration,and the set shown ii known as asingle-circuit regenerative set.As you were told in the explana-

tion of Figure 3, some of the radiofrequency wave escapes past thedetector. Up until this drcuit, suchcurrents have been by-passed toground by means of a fixed con-denser across the phones, and theenergy wasted. This circuit no long-er wastes the energy.Notice the coil between the plate

of the tube and the phones. It is aradio-frequency choke coil, and of-fers high resistance to radio fre-quency currents, though it permitsthe detected audio-frequency part ofthe signal and the plate currentfrom the battery to pass withoutappreciable hindrance.The radio-frequency currents, im-

peded by the choke, have to gosomewhere and, as you have been

told, they pass through condensera.Therefore, a va'riable condenser iaplaced bctween the plate and thegrid coil. The radio·frequency cur-rents are thus caused to flowthrough this coil again. This haathe effect of reducing the resistanceof the grid circuit, and making theset far more efficient.The more the plates of the feed-

ba,ck condenser, controlling thiaaction, are in mesh, the more the ef-fect of the R. F. (radio frequency)currents, because the condenser'acapacity is increased.A fixed condenser cannot be used

for this purpose because if the cap-acity is too low, there is not enoughfeed-back, and if its capacity is toohigh, there is t,oo much feed-backand the set howls or whistles.In fact, it becomes a transmitter

when it whistles, and if a key isconnected between the tuning coiland the ground, it can be used tosend signals in Continental orMorse code.

Fig. 16EXPLANATION

One of the greatest drawbacks ofthe set shown in Figure 15 was thefact that ,every time it was careless-ly tuned, it howled. ,nrl that thesehowls (caused by electric:al oscilla-tions in the tube) radiated andcould be heard for h!ocks. Thisruined reception for everyone try-ing to listen-in in the S2.me area asthe set.Another of the set's disadvan-

tages was its inability to tune verysharply. Two powerful or nearbystations, broadcasting on almost thesame frequency (i.e., wave-length)would interfere with each other;their signals could not be separated.To overcome these difficulties, the

circuit was developed as indicatedin Figure !6, on this page. An an-tenna coil and a grid coil replacethe single tuning coil that was usedin the preccding circuit, and thisnew two-coil unit was called a two-circuit tuner. It has, as you see, acoil in the antenna circuit and aseparate coil in the grid circuit.These two coils are wound 0;1 tL~

same form, and are separated 1:;:1' (l

fraction of an inch. The greatertheir separation, the more sharplythe set will tune, and the less it willradiate; but if they are too far apart,the set becomes less sensitive andloses volume.Heretofore. we have always tuned

the coils in the set, but this circuitmakes use of an untuned antennacoil, which serves merely to trans-fe:i' .ene'gy from the antenna to thegrid circuit. It has com parativelyfew taros; if the grid coil is, say,120 turns on a 2-inch form, the an-tenna coil will have only from 5 to15 turns. -A set of this sort is remarkably

efficient, and this circuit is the basisof many sensitive short-wave setsin use today. It can be built at acost for parts of less than $5.00.

(Continued from Page 18)in microfarads. In the case of thev?riabIc condenser, the capacitygiven is the capacity with the platescompletely meshed. It is decreasedas the' plates are brought out ofmesh.

The three-circuit regenerative set,shown in Figure 17, operates in-much the same way as the two-circuit set of Figure 16.Notice, however, that there is this

difference: The choke coil andfeed-back condenser are omi'ttcd.Instead there is an additional coil,known as the tickler, which servesthe same purpose. The plate cur-rent, together with its R. F. com-ponent, must pass through the tick-ler coil to get from the plate to thebattery.As you have already learned, when

alternating current is flowingthrough one coil, it induces (Le.sets up) a like current in anothercoil placed close to it. That is howenergy is transferred from the an,tenna coil to the grid coil. In exactlythe same way, the R.F. in the platecircuit is fed back to the grid cir-cuit by means of the tickler coit.The amount of feed-back is con-

trolled by having the tickler 'coilmounted on a shaft which can berotated bv means of a knob. Thereis the greatest amount of inductive

coupling (trans fer of electrical en-ergy) between the tickler coil audgrid coil when the two coils are inthe same direction; this cpupling isdecreased more and more as thetickler is turned from this position.The tickler is usually wound on asmaller coil than the grid coil, sothat it can fit partly inside it, andhas from 1.1 to 34 as many turnsof wire.NOTE ON TUBE FILAMENTSThe detector grid returns shown

in these diagrams are connected tothe positive side of the tube fila-ments ("A" +). Such connectionsare used with the -01 A, 27, 30, 56,76 and other tubes. The grid re-tmns at -OOA tubes are made tothe negative side of the filaments("A"-).

(Continued from Page 21)plate. When the degree of the pos-itive or negative charge maintainedon it is increased or decreasecl, sim- •ila·r action takes place.You wi!! understand this better

when you have studied the nextdiagrain, Figure 14.

QJ

FiC. 18

EXPLANATIONHere, in Figure 18, is the circuit

just described, but with an impor-tant addition-a stage of audio-fre-quency amplili!=ation. . .Adding this stage does not make

the set any more sensitive Of' se-lective; it makes it louder. Whilethe precedinR sets will work head-

• phones with f,\'ood volume, they arenot loud enough to operate a loudspeaker. The addition of a stageof A.F. (audio-frequency) ~mplifi-cation will permit a speaker to beoperated at low volume; two stage,will give ample ,volume for anyhome, as you 'Viii be told later.The phones are no longer in the

detector plate circuit; instead thereis the primary of an A.F. transfor-mer which is by-passed with a smallfixed condt"nser to afford a path fOf'the R.F. to slip by without passingthrough the condenser windings,just as the phones were.The secondary of this transformer

is the grid coiL of the audio stage.As there is no need to detect inthis stage, the grid ~teak and gridcondenser, used in the detectorstage, are omitted.

The plate circuit of this A.F. stageis completed through a loud speak-er or, if you prefer, a pair oi phones.No by-pass condenser is neededacross the A.F. stage's output.One thing you must~noticc 'care-

fully is the way in which the gridcircuit of this stage is completed.The return, instead of being to thepositive filament lead (i.e., wire)which is the correct grid return forrr.ost del ector tuhes, is made to thenegative filament lead. This isnecessary in all A.F. amplifier tubes.This can he aone by connecting

them to the negative side of thefilament in single-stage amplifiers,but two-stage amplifiers, handlingstronger sig-nals, need a greaternegative gTid bias.

(Continued from Page 20)circuit, consisting simply of the fila-ment, the "A" battery, and any re-sistor that may be in series withthem to regulate the filament tem-p.erature and thus the emission ofthe electrons.

Figure 19 shows a two-stageaudio amplifier connected to thetuner and detector of Figure 17.This circuit will work a loud speak-er with perfectly satisfactory vol-ume, and good tone ..It has several features not previ-

ouslv discussed.First, you will notice, it makes

use of "C" batteries-that is, bat-teries in the grid circuits-of theamplifier tub~s. These are drawnjust as are "A" and "B",batteries;their use in the circuit marks themas "C' batteries.The positive terminal of the "C"

battery is always connected to thefilament circuit, just as is the nega-tive terminal of the "B" battery.You will also notice how the "B"and "C" 1:>atteriesare tapped to pro-vide the correct "B" voltages fordetector, 'first audio and secondaudio plates, and the correct "C"voltages for the two audio stages.Normally, higher "B" and "C"

voltages are used on the secondaudio stage than on the first, forthis second tube must handle a

greatly amplified signal, and re-quires more power. In fact, special.l20wer tubes, such as the 20, 31, 33,112-A and others give best tonewhen used in the output stage.While the volume can be con-

trolled by adjusting the filamentrheostats (variable resistors in thefilament circuit~) or the tickler coil,this is not the best method. A farbetter volume control is the resistorR connected across the secondaryof the first A.F. transformer. Mov-ing the slider-arm along this resis-tor reduces the volume the morenearly it approaches the grid endof the resistor. There is practicallyfull volume when the arm is at the"C" battery end of the resistor.Another feature of this circuit is

the output choke. The plate, cur-rent of a power tube is so great thatit might damage the windings of theloud speaker .• It is therefore causedto flow through the A.F. choke con-nected in the plate circuit of .thelast tube. This choke passes the

(Continued on Page 32)

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Figure 20 represents a stage ofpush-pull amplification, as used inan audio output stage. It can be usedin place of the last stage shown inthe preceding diagram, Figure 19.It makes use of two tubes for a

single audio amplifier stage. Theyare connected into the circuit bymeans of special push-pull trans-formers. The push-pull input trans-former has a center-tapped secon-dary; the push-pull output trans-former, a center-tapped primary.When the transformers are con-

nected as shown, a negative bias iskept on the grids of both tubes inthe push-pull stage, and a positivebias on their plates. But when thesignal impresses a more. negativecharge on one grid, it makes theother less negative, and vice versa.Thus one tube is "pushing" whilethe other is "pulling" the signal.The output of such a stage is far

greater than that of the same twotuhes with their grids connected to-gether and their plates connectedtogether would be.

(Continued from Page 8)shown in, the lower part of the mediately clear, refer to Coil andillustration, is used. Its primary Transformer, Audio Frequency.is center-tapped. Between two The choke differs from the audiopush-pull stages;, an inter-stage frequet:lcy transformer in that itpush-pull transformer, with both has but a single winding.primary. an<L•.aecondary ~enter- VOL TMETER _ S th 1_tapped, IS used. .. ee e exp ana

CHOKE COIL (AUDIO FRE- tlOl1 gIven for Ammeter. .QUENCY) - Either of the two DYNAMO or MOTOR-ThIS sym-symbols shown may be used to ~ol represents two brushes touch-designate this unit, for each mg the commutator of a D.C.shows that the coil is wound upon generator, or a motor.an iron cor!'!. If this is not im- (Continued on Page 30)

EXPLANATIONAnother type of amplifier is seen

in Figure 21. This is a resistancecoupled amplifier. It does not 1,lsetransformers, as did those previouslydiscussed, but employs fixed resist-ors in the plate and grid drquts ofthe tubes, as shown.Although its accepted name is

"resistance coupled" it would bemore accurate to call it condensercoupled, for the true coupling isthrough the fixed condenseTs whichcouple the plate of each tube to thegrid of the next.As you have gathered, when a

signal is flowing through an ampli-fier, its audio component is a pulsa-tion which acts much like' an al-ternating current. The plate re-sistors offer opposition to the pass-age of these pulsations, which areimpressed upon the condensers(known as coupling condensers, butmore often as blocking condensers)and thus are conveyed along to thegrids of the following tubes.The tonal quality it is possible to

secure through resistance couplingis far better than that obtainablethrough the use of cheap or even

most moderately priced transform-eTS, but the amplification (increaseof signal strength) per stage is notso great. Therefore, instead of twostages of transformer coupled, threestages of resistance coupled ampli-fication are needed.However, resistors and fixed con-

densers are inexpensive, and thereis generally a saving of expense.The plate resistors are 250,000 ohms,the grid resistors 1 to 2 megohms,and the coupling condensers, .006mfd. to .25 mM.An A.F. choke is generally used

in the output, as few resistors willcarry the current supplied in theplate circuit of the power tube.

(Continued from Page 22)so the grid condenser affords theirpath.The radio waves Teaching the grid

through this condenser vary thecharge on the grid many thousandsof times per second. The grid con-trols the plate current, and thus thesignal is heard in the phones. Asthe plate current is far greater thanthe radio currents which control it,the signal is more loudly heard.

In the few preceding diagrams,we have been considering audio fre-quency amplifiers. As you were toldthey do not make the set moresensitive or selective.The diagram, Figure 22; on thispage, shows another type of ampli-fiers-a radio frequency amplifier-which does add to the set's sensitiv-ity and selectivity.Refer back to Figure 16. The an-

tenna coil and grid coil used in thatcircuit are the same as those 'usedin the RF. stage here. The detectorstage (the second tube and its as-sociated units) makes use of a coilassembly similar to the three-cir-cuit tuner of Figure 17, but with afew more tnrns on its primary. Theprimary, which was the antennacoil in Figure 17, has become the

(Continued from Page 28)VAC;UUM TUBE - The tube mostfrequently used in battery cir-cuits has three elements, the fila-ment, grid and plate, and is repre-sented as shown at the top of theillustration. Qneof the most com-

RF. (radio trequency) stage's platecoil in Figure 22. Having no gridleak and condenser, the tube doesnot act as a detector. Certain bias-sing (ndt used) coufdalso make itdetect. -If a standard three-element tube,

such as the OIA, is used in the RF.stage, there is the problem of givingit the correct grid bias. It will beinefficient if the bias is too positive,and will cause howling noises tocome from the phones or loudspeaker if it is too negative. There-fore, a potentiometer type of vari-able resistance may be used, con-nected as at R in the diagram.There are many other ways of

keeping the RF. stage from oscil-lating (the cause of howling), butthe best is to use a screen grid tube.

mon tubes for electric (houselighting lines) operation hascathode, grid and plate.· Thecathode (corresponding to thefilament) is heated by a separatepart called the heater, which isnot considered as an element.

Figure 23, herewith, is preciselythe same circuit as that of Figure22, except that a screen-grid (four-element) tube has been substitutedfor th,e three-element tube of theearlier diagram.The added element is the screen

grid. The element called the gridin preceding .explanations remainsthe same as heretofore, but its namechanges. It is now called the con-trol grid to distinguish it from thescreen-grid, for its function is toenable the incoming wave to con-trol the electron flow in the tube.The addition of the screen grid

serves two purposes. It makes itimpossible for the tube to oscillateand so eliminates the chanc;e ofhowling originating in the RF.stage; it also can be made to am-plify the signal much more thancan a three-element tube.Its grid bias may be obtained by

connecting the grid retnrn to thenegative ~ide oJ the filament bat-tery, or to a "C" battery, as shownin the illustration. The positive bias

applied to the screen grid may beobtained from a tap on the HB" bat-tery which sU'pplies the plates of thetu bes in the circuit. You will noticethat the voltage applied to thescreen ,grid is lower than that ap-plied to the plate.The screen grid attracts elec-

trons, too, and some current flowsin the screen grid circuit, but as itis an open coil or mesh, and as theplate is at a higher potential, most. of the electrons are attractedthrough the screen grid to the plate.The screen grid prong is in the

position occupied by the grid prongof a three-element tube.

(Continued from Page 17)tions are heard, and their signalscannot be completely separated, thelistener can tune until the wantedstation is louder than the unwantedone, and then move the sliding coilof the loose-coupler (as this unit iscalled) until the unwanted stationis no longer heard.

TOPllIMA!l:1o~ t!>lAr:T

II _'8 •'--f 4$V~~

Figure 24 shows a HB" and HC"eliminator or power pack.The transformer at the left is a

power transformer, and its primaryis connected to the A. C. electric.1ig·ht lines of the house. It has two'secondaries. One of .these steps upthe voltage to supply HB" and HC"voltage to the set. The other steps'the volta!!;e down to heat the fila-ment of the rectifier tube.In this diagram, a full-wave recti-

ner is used. It is really just twodiodes (See Figure 13) in the sametube and using the same filamentfor both plates. It enables the powerpack to make use of both halves ofthe altern:lting current cycle.The rectifier changes the A.C. to

pulsating D.C. - and the set needssmooth D.C. Therefore one or more. at:dio chokes are put in series withthe high voltage lead, as. shown. Inaddition, large capacity fixed con-densers are connected between thehigh voltage and low voltage leads,as shown. These condensers may beof any capacity from about 2 mfd.

up. The larger they are, the betterthe filter, and the more it costs.To get different voltages for the

various sta::o:es of the set, a voltagedivider, R. is used. As it is connectedacross -.tIe lines, it always dra'Nssome current throu!Yh itself, and i~therefore often calred a Hbleeder."This resistor is usually providedwith contacts which can be fast~nedat any desir.ed points, so that the,desired voltages may be tapped off.It works much like the potentiome-ter resistance described in Figure 22.Other fixed condensers, usuall;y

about 1 mfd. are used to by-passeach tap to gro\lnd.

(Continuedl from Page 27)plate current but impedes the A.F.pulsations, which flow through thelarge condenser C and the speaker,and so ba::k to the plate circuit.A switch has also been added in

the "A" battery lead, so that the setcan be turned on and off withouthaving to readiust the rheostats.