QUESTIONS AND ANSWERS

Radio Record, Volume V, Issue 15, 23 October 1931, Page 14

QUESTIONS AND ANSWERS

FRANK

KEE

The Technical Kditor will, through these columns, be pleased to help readers experiencing trouble with their sets. Queries are limited to three-for more than this a shilling fee is charged, and a similar fee is payable for queries answered by post. Supplying layouts. circuits and solutions of intricate, theoretical problems is beyond the scope of this service. A coupon ‘must accompany all requests for information. Nonappearance of the coupon in any issue cannot be regarded as a reason for its not being used. . Address all queries, The Technical Editor, Box 1032, Wellington.

YUSHPULL = (Auckland).-I intend using an ordinary audio transformer as input to a pushpull stage. Will you show how the grid bias is to be connected ? A.: The two resistances are used across the secondary, as was shown in the "Diagnosis" a short time back. The other connections are then the same; you merely regard the centre tap of the resistance as the centre tap of the transformers and the two ends, of course, are the same as when a transformer is used. XPERIMENTER (Dunedin). — We have no technical data concerningthe speaker you mention. 2. Where can J obtain the paper to make the cone? A.: Any well-known dealer could obtain it for you. 8. I have a four-valve B.D. with a 245 ‘in the last stage; 180 volts are used on the plate. Will this be suitable to operate a dynamic speaker? °

A.: With 180 volts only you would be better to use a valve of the P2G5A class, and work this at maximum. LECTRO ((Kelso).-Is the LoftinWhite, with #443 as output sufficient to work a dynamic speaker? ‘It will overload a cone, but when I connect it to a dynamic, the field current of which makes very little difference, the volume level falls considerably. A.: There ig something wrong with your speaker or field supply. The Lof-tin-White will give you excellent volume from the ordinary dynamic speaker. When you connect the field coil you should notice a decided step-up in volume. Have you tested your accumulator with a meter when it is on load? 2. The power transformer is 60 watts and does not overload. Would this be sufficient to supply the field eurrent of a high voltage dynamic speaker? A.: No; the field coil takes a fairly heavy load,’ and your transformer does not have a great deal of power to spare.

AN-1927 . (Napier) -I have an old battery set with R.O.C. amplification. What valves do you, suggest with a view to economy in running? A.: Use 221’s in all stages except the last, ‘where you can use one of the highgain power-valve type valves. 2. What values of resistance are usually used? A.: The resistances in. the plate circuit are generally 100,000 ohms. ‘Those in the’ grid circuit 250,000 ohms. Could s.g, valves be used, and if so, how? ° A.: 8.G. valves could be used, but the change would be uneconomic. The set would have to be completely redesigned on the radio ‘side. TT EB. (Cambridge)-Are PM3, and PM4 suitable valves for the "Kestrel: Three’ ?-Yes. but M4D is a better detector than PM8. 2. Are details of an a.c. version of the "Kestrel Three" available? -A.: Not at the present time. We hope to ‘have them ready shortly. QGUPER SIX (N.P.).-Will the super six be more powerful than the "Outspan Wive’’? A.? Considerably so, but it must he used from a loop antenna, and this will offset, to some extent, the abnormal sensitivity of the receiver. 2. Will it: be single dial control, and when will it be out A.: It will have two dials, and we hope will be out next week. 8. What will be the cost, including the valves? A.: Somewhere in the vicinity of £14. A $C. (Christchurch) : Why do I blow out so many valves? ' A.: Your voltages are too high, due, probably to an incorrect number of turns on the primary winding. Your best plan is to communicate with the agents who sold you the set and get their service man to put a meter in the circuit. B. B. (Gisborne): What is the solution used in a balkite electrolytic charger? A.: Sulphurie acid specific gravity 1200. 2. Should I use 201A valves in my set? A.: You can use them in all stages excent, the last. where we should advise you to use one of the high gain power valves, 8. Would a wavetrap separate the local stations from 2ZW?-We think so. Dpxeew (Wellington): When I switch on my set a loud plop comes from the speaker, and then the music is heard. At other times when the set is switched on the items come through without the plon, . A.: This, may be due to your set oscillating, or to a defective transformer, or valve, J. B:C, (Petone): LI have ag five-valve. . sét, but when I put the speaker on

\ the cabinet a loud whine is heard in the speaker. A.: This is caused through a microphonie valve. Place a leather cap or a cocoa tin inverted over the detector. 2. If I wish to build the "Outspan Five" could I do so by using R.C.C. amplification? A.: Yes, the "Outspan’" would be quite suitable for resistance capacity coupling to the audio, but you would have to use three valves instead of the two we specify for transformer coupling. NOISE (Christchurch) : Refer back to a short article in the "R.R." a couple of weeks ago, in which it is pointed out that when the dual wavetrap was first described a. wire was left out of the layout diagram. . 2. Is it possible to charge an "A" bat- ) tery from the generator of a motorbicycle? A.: Yes, but watch the charging rate. It should not be more than 4 or 5 amps. 8. What is the method of connection? A.: Disconnect the two wires now going to the accumulator on your motorbicycle and take them instead to the "A" battery. Start the engine running and leave it on until the battery is charged. It will possibly be a rather expensive method of charging an accumulator, but otherwise it should be satisfactory. Adjust the charger to a high rate and set the motor-bicycle running slowly. C T.H. (Denniston): Is the pentode * valves better than the make I mention, and if so would they be used in my American a.c. set? A.: You do not give ts very many particulars about your receiver. We presume it is-one of. the-a.c. type. You cannot use a pentode in it, and must replace

’ (To be used with all requests for information.) Information Coupon t Name of set "shee erocoseeesees Number of valves seeoceeesaecessees Name Address ste ew oeweseseseoeesereoene teeseesceceeosesecseoosser: "PPP OHT CHEESE ESESOSHEOTEESEEEDOLO RED OS TTPO POSH OOH eM SR ERERESEESESEBSO OE EE* Nom de plume .....cecccccescees To be kept in subsequent inquiries Date epeprescece Please Note:(1) Be specific and briet. tabulating, if possible. (2) Write legibly, and on one side of the paper. (3) We do not design circuits, but accept suggestions tor feature articles .

~Wwith the valves as they were originally. The make of valve you mention is a standard first-class one, and by using this you cannot go wrong. R M. (Grey): I have an a.c. set, the * electricity being taken from a light socket, When the power is on at the light and the set off I _ find that I can ‘get a. spark by touching the earth wire to the chassis. Should this be so? A.: It depends on the situation of the set switch. If it is in the primary lead there should not be a_ spark. If it is on the secondary it is quite possible. Have you tried turning the plug connecting your set with the lighting system around? In any ease, the defect is not serious, and there is no need to worry about it. : C R.M. (Christchurch): Patent earths * are rarely a success. We cannot quite understand your particular installation, but we think, however, that by burying the whole outfit and carrying the lead to the set, you will be doing mvre or less rightly. JpUPLEXx (Auckland) : Concerning the Advance receiver in this year’s "Guide," the theoretical diagram shows the fixed condenser connected to rf. Cl as .1. Should this be 1mfd.? A.: It is really immaterial which is used. When the paper condenser was used the necessary alteration to the diagram was overlooked. 2. The list of parts shows two .5 mfds. condensers, but I can trace one only in the circuit. A.: Only one .5 condenser is required -probably it was intended to use another where a 1 mfd. has been placed. These small discrepancies occur through drawing diagrams before the finish of experiments in order to save time, but actually the working of the set is not affected. 8. My variable condensers have 23 plates and 11 plates respectively. What are the present values, and what value of fixed condensers is required in series to adapt these to a shortwave set? A.: You have not told us what make. of condenser you are using, nor the size of the plates. The 23 plate condensers will probably be a .00035 and the eleven plates one .00015. In this case the ye series condensers are .00014 and C B. (Auckland): Oscillation in the ° "Advance" s.w. set will take place only on: certain places of the two tuning condensers. One has to strike a ecombination between the two condensers before the set will oscillate properly. The set has been made exactly to specifications. 7

A.: Making the set "exactly to speci;cations" still allows the constructor a considerable margin of difference between his own set and the original, but such differences do not necessarily prevent good results from being obtained. Your trouble with oscillation is unusual in that there are so many patches around the dials. The patches are known as "holes" and are caused chiefly by poor or unsuitable r.f. chokes, so we suggest your making up or purchasing anothor choke different from the one you are using. This applies particularly to the choke associate with the tickler, which has to be about 80 millihenries. © Since

the publication of the "Guide" it is possible to obtain 50,000 wire wound Colvern potentiometers, and you are recommended to use one of these for the detector screen. Fear and Co., of Wellington, stock them in case you have difficulty in obtaining them lozally. You can also try connecting the 2 mfd. condenser across the screen potentiometer from the arm to the earth instead of as shown. ‘The reaction plop can be reduced by using a .0001 fixed reaction condenser in place of the .0005, if this has not already been done. This, of course, reduces reaction on all coils. Make sure that all resistances are good, especially 1000 ohms to the tickler and the .5 grid. By a little perseverance you will be rewarded with a very fine receiver. Many constructors have built it, and been highly satisfied. @IMPLUX (Waikanae) : I want to use a erystal set in Waikanae, 37 miles from Wellington. Which cireuit of those I enclose would be the most suitable? A.: We think you will not get a crystal set to work in Waikanae; 25 miles is the very outside range of 2YA. Probably your number 8 cireuit would be the best. 2. I intend to use a .00035 variable condenser. How many turns will be needed on the coil? -: On a 2in. coil using 26 d.s.e wire; ahout 80-turns. 3. Will cardboard former do for the coils? A.: Yes, but Exelon would be better. S D. (Auckland): You probably have a ~* loose connection. If you cannot locate it yourself, get in touch with a service man. :

AMBITIOUS (Tolaga Bay): I am contemplating constructing the Trinadyne battery set described in the "R:R.," using 280 and 281 valves. Would this receiver give good loudspeaker reception of 2YA at 7 p.m. all the year round?

A.: We are doubtful. The particular circuit may not be suitable for your requirements. Why not stretch a point and make up a set such as the "Kestrel Three"? Under these circumstances you would get good results. You could use 230 and 231 types of valves in this circuit. . . 2. I intend using the resistor method of obtaining bias. What should the value of the resistances be? A.: Don’t; with batteries it is uneconomical. You are dissipating valuable energy through resistances. Use a "C" battery; it is cheaper. 8. How long should a set of standard upright "B" batteries and an air cell last with an average use of, say, three hours nightly? A.: The "B" batteries should last approximately 7 months, and the air cell, we believe, about two years. 8. Could you explain super-regenera-tion? A.: Super-regeneratives have the advantage of the abnormal sensitivity, which most users of regenerative detctors will have noticed to exist, at the moment when the detector valve goes into, or emerges from, oscillation. It is, of course, impossible to maintain the required condition by moving the reaction control backward and forward by hand, so the result is achieved automatically by varying the grid potential. ‘ If the variations were made at an audible frequency, the commencement and cessation of oscillation would be audible as a note of a frequency determined by the rapidity of the variation. By choosing suitable circuit constants, however, the variation may be made at a frequency higher than 10 k.c., which is about the upper limit of audibility. As a matter of fact, super-regenerative receivers used on the broadcast reception, apart from the fact that their use on an ordinary aerial is prohibited, suffer somewhat from the use of the super-audible oscillation, but in the reception of extremely shortwave lengths. this receiver is most successful. 4, What is the equivalent in the nearest s.w.g. wire of 60, 25 and 9 turns of 30 dec. b. and s. wire on a Iin. former? A.: The equivalent of 30 B. and S. is 88 s.w.g. The number of turns can remain the same and also you can use number 32 s.w.g. wire instead of 33 without any ill-effect. 5. Could you inform me how the r.f. current is introduced or coupled to the transmission line of a wired wireless telephone? A.: Modulated r.f. currents are usually introduced into wired wireless systems by tuned rf. transformer. Although the writer is not familiar with such systems, it is understood that it is the usual thing to tune both primary and secondary of the transformer, although in some instances only the secondary is tuned. The rest of the line is, of course, simply in series with the secondary.

HIRD GRID (Greymouth) -What effect has (a) the nlate eurrent of (1) the oscillator, (2) the modulator, (b) the mu of the (1) oscillator. (2) moduintor, (ec) the impedence of (1) the oscillator to the modulator, have on (A) the

output,.(B) the percentage of modulation in a Hartley transmitter with Heising modulation? . A.: You make our head swim with those A’s, B’s, 1’s and 2’s. Still, let us see if we can get them right without gem ting them mixed up. _ A,: (A1) The greater the plate current the oscillator, provided adequate excitation is present, meang increased output. Unless this is balanced by corresponding increase in the capacity of the modulator it is also likely to result in a reduction of the modulation percentage, (A2) The plate current of the modulator has in itself little effect on operation. A greater variation in plate current, however (i.¢e., from maximum to minimum of an audio cycle) will increase either the percentage of modulation or the'size of the oscillator valve which can be. effec: tively modulated. Obviously a greater variation in current will-in certain cases only be possible with a greater average current, so that the average current may be of importance indirectly. (B1) The mu or amplification factor of the oscillator is, within limits, not of yery great importance. Generally speaks ing, @ low mu valve requires a greater grid excitation than one having a higher amplification constant. On the other hand, however, it is probable that, for @ iven plate voltage, a higher cutput can be obtained from a low mu _ valve if sufficient excitation is provided. Hxtremel low mu valves usually require too mu excitation to make good oscillators: (8?) See C2 below. . C1) The impedance of the in relation to its amplification constant is some indication of the overall efficiency of the valve. A valve of high impedance, the mu of which is not correspondingly high, ig likely to require too much excitation (and grid current) in comparison with its output to be a satisfactory proposition. (C2) The impedance of the modulator is of importance as a guide to the plate current variations which can be exp ed, since these variations determine percentage of modulation ag ¢ above. (Continued on page 29.)

D.X. NOTEPAPER. a

Makes sending for verifications easy. Per two dozen lots, 1/6 posted. Per 6 dozen lots, 4/- posted. Special paper for members of the D.X. Club. State your station number when ordering. Non members ean still get the ordinary notepaper paper from office only. -. Write to Box 1032, Wellington.

(Questions and Answers

(Continued from page 15.) low impedance low-mu valve can be exweeted to have high plate current and accommodate a large grid swing. Under these circumstances the variations in a plate current will be considerably greater than with a corresponding valve of high impedance (excepting, of course, the pentode). 2. Can the extent to which a valve can be safely overloaded under constant working conditions be determined by its maximum anode dissipation, and can a valve be overloaded to a greater extent in a transmitter than in a receiver? A.: A valve should in no case be seriously overloaded, in whatever cireumstances it is used. The conditions ruling in a transmitter are, however, entirely different from the case of a receiver, In the receiver practically. the whole of the power supplied is dissipated at the plate, so that the input is determined by the permissible plate dissipation. With a properly adjusted transmitter, however, a substantial percentage of the input is dissipated in the aerial as radiation or in a succeeding grid circuit as excitation for an amplifier, Thus, roughly speaking,

4atir the dissipation is limited to the difference between input and output. It is entirely possible to run the input on a valve rated, say, 20 watts anode dissipation as high as 50 watts without the slightest danger of overloading. 38. How is anode dissipation and _percentage of modulation calculated (at the transmitting end) ? A.: Anode dissipation can be approximately measured by feeding the output of the oscillator into a dummy aerial, the losses in which can.be measured. The difference between such losses and the input will then be an approximate guide to the dissipation. Percentage of modulation is not readily measured with the equipment ayailable to the average amateur, The most practical instrument for’ this purpose is an adapted vacuum tube peak voltmeter, but a description of the construction and operation of such an instrument is beyond the scope of these pages. A study of Q.S.T. files for 1929 will provide all necessary information on the point, The purpose of a Heising modulator is simply, by. drawing more or less current corresponding to audio variations, to decrease or increase the voltage applied to the plate of the oscillator, and consequently, its output. Hence the possibility of over-

loading the oscillator does not enter into the question. 4. In a Hartley circuit with Heising modulation would it be any advantage to have a higher voltage on one valve than on the other? A.: Do secure a high percentage of modulation (higher than 60 per cent.), it is necessary to apply a higher voltage to the modulator than the oscillator, this being usually accomplished by a resistance inserted in series with the oscillator. There is no novelty in this idea. 5. What is the principle of the Telefunken modulation ? A.: Telefunken modulation, by operating on the input to the oscillator has the advantage of requiring a less ambitious modulating system. ‘This type of modulation appears to be rather hard on the valves employed, and_ is considered less satisfactory than the Heising or constant current system. 6. How is double tuning of the if. stages of a super-heterodyne earried out, and does it cut out getting one station in two places (not necessarily on the same set of coils), ie, at (1) the sum and (2) the difference of the frequency of the station and that of the local oscillator? A.: Simply by turning both primary and secondary of the intermediate transformers, coupling being by mutual induction. The "dowble tuning" effect of a superhet. employing a low frequency is not affected. Both primary and secondary are tuned to the same frequency. 7. What does the load impedance of (a) the oscillator, (b) the modulator depend on in a Hartley with Heising modulation, and about what do these figures generally amount to? A.: The load impedance of the oscillator is determined by the tuned eireuit and either (a) the aerial rediation resistance or (b) the input impedance of a succeeding amplifier-in parallel; the latter factors being also affected by the degree of coupling to the plate coil of the oscillator. Owing to the wide variations in the conditions likely to be encountered, it is not possible to estimate the probable load impedance. The load impedance of the modulator comprises the feed choke and the plate resistance of the oscillator in parallel. Generally speaking, it will be fairly low, a few hundred ohms. being a probable quantity. 8. How would (a) a pentode, (b) a sereen grid valve go as (1) oscillator. (2) modulator? _ A.: There is no particular advantage in using either pentode-or a screen-grid yalve as an oscillator; either makes a good amplifier in a master-oscillator out-

fit, and the pentoae makes quire a satis factory modulator for a low power outfit. 9, There are several instruments for measuring r.f. current. Are there any to measure r.f. voltage? A.: A yacuum tube voltmeter is the instrument you require. This is simply a plate-bend detector (or, for less accuracy, but greater sensitivity, a grid detector), the input being between grid and filament; measurements are taken with 4 low reading milliameter in the plate clrcuit. The instrument requires to be cali. brated from a source of known a.c. volltage. [These questions, although interesting. are quite beyond the scope of "Questions and Answers." To make matters worse, "Third Grid" sends down nine questions and asks us to spread them over three weeks. He certainly interprets the words "free service" in a very liberal manner. We are not particularly anxious. that other radio enthusiasts should interpret the terms in the same meaning. We wish to help the average chap to get a_littk better reception, not to delve into the intrieacies of radio for the benefit of those who have a sufficient knowledge to hunt these things up for themselves, and cer. tainly. we cannot entertain splitting queries up into several weeks. If you have more than three questions to ask don’t seek to dodge the issue, pay you shilling-but these are "Specialist" ques: tions.-~Tech. Bid.]

EDM (Wanganui): My aerial consists of two poles about 40 feet high with 90 feet between, including the leadin. Would -I obtain greater _ signal strength if I erected one pole 80 feet high? | . A.: While it is impossible to: give any definite ruling on the matter, we are inclined to believe that you would. Theoretically the best aerial you can have is a vertical one. GMITHY (Wellington): Look for the description of the super six next week. It is an adaptation of the circuit to which you refer. W B. (Pio Pio) : Could you suggest an ¢ improvement in my valves? I use an American set with 615 as detector, A609 in all the other stages, except the last, where I use B605, The set has not the punch that it had formerly, though all the batteries are Ai, and I find I get much better reception with a B605 in the first r.f. stage. A,: Providing your valves are not too old, they should work in your circuit quite well. For most circuits of your type the American class 221 valve is best, at least in the two radio stages. The use of other valves generally means that the set has to be re-neutralised. QPsox (Pio Pio): Your circuit is quite orthodox, and should be satisfactory. Seeing that it employs a_preselector circuit, you should, to a large extent, overcome the selectivity trouble, but of course this will mean a_ certain loss in volume. Is it worth it? Why not build a three-valve set of the "Kestrel Three" type, which would give you better selectivity than the two-valve type, and certainly a greater pick-up. It would not be necessary, with the circuit you contemplate making, to use a midget condenser in the aerial.

ALL WAVE (Shannon): I intend building the Electric Radiogram Five. Will the 100-watt power pack described, in the "Radio Guide" be suitable?-: Quite. 2. Will the r.f. choke described in the "Radio Record," January 30, be suitable. -yYes. ; 3. What should be the d.c. resistance for the r.f.c. in the electric Radiogram? A.: Not in excess of 300 ohms. V R.H. (Auckland) : I am building the e "Kestrel Three." Is a screen-grid valve advisable? A.: We ean thoroughly recommend a screen-grid valve. If you want to use a screen-grid valve of the same make as you are now using, use a 642, but you must use a 50 ohms, resistance in the filament circuit, for it is a four-volts valve. 2. I have only 110 volts "B." Will this be sufficient? A.: It will be quite satisfactory. You should get good results from the "Kestrel Three" with this voltage. R D. (Banks Peninsula): Is a four- * valve Browning Drake superior to the IIlammarlund Roberts of the same number of valyes?-No. 2. Would it be possible to add a second stage of radio frequency to either of these sets. and how would this be accomplished? A.: The five-valve Browning Drake was fully deseribed in thé "Radio Record" and 1980 "Guide." In last week’s issue we explained how to add an s.g. valve to the T.R. Four. . HORT SUP. (Palclutha): I am ¢onstructing the battery version of a super het. shortwave adapter. In my set "A+" and "B-’ are joined together, and for volume control a potentiometer is used across the "A’ battery, the sliding arm being earthed. Could you give me the number of turns for valve base coils using 201A valve and .00015 condensers? A.: A diagram which should prove successful as a battery s.w. adapter is given herewith. The potentiometer used in your broadeast set as a volume control should be set to the position of maximum sensitivity, but just short of the oscillation point if the receiver is capable of being

thrown into: oscillation. Suitable coils ae winding on to valve bases are as folow i-

R A.H. (Auckland): I recently com- * pleted, according to the specifications, the shortwave a.c. super het converter, and have been unable to get the set to function. Could you give me:-1. The approximate range in metres of each pair of coils? A.: The commonest difficulties with super het. shortwave adapters are the failure of the oscillator to function and mistuning of the preselector provided by the broadcast set. Failure to oscillate may result from too low a plate voltage, insufficient reaction turns on reaction winding connected in a revérse manner, The remedies are obyious. Tuning of the preselector is fairly broad, and it is usually possible to hear something, even when mistuned, making it possible to adjust it for better results. In case of difficulty the aerial can be hooked on just ahead of the pre-selector and the weak broadcast stations tuned in, the pre-selector being adjusted on this. The ranges of the coils are as follow:-The 80 metre band from 65 to 100 metres ; the 40 metre coils from 30 to 65 metres; the 20 metre coil from 15 to 30 meters. 2, What indication is given when the ser is oscillating? A.: Little indication is given when the oscillator is functioning except increased background noise. The milliammeter in the plate lead will, however, dip downwards when the valves commence to oscillate. TUICE (Auckland): How many turns and what gauge of wire should be put on a 2in. former to use with a midget condenser of 5 plates spaced a 1-1G6in., as used for an 80-metre wavemeter? A.: 35 turns of 24 D.S.C. wire. 2. Can you give me the circuit of a monitor for the 8Q-metre band using a crystal detector? : A satisfactory monitor for code work cannot be built to employ a crystal detector. The crystal will respond only to whatever modulation happens to be present in the transmission. If the monitor is required merely to keep a check on phone transmitter, a shielded crystal set (with a smaller coil, of course) with a small pick-wp coil in series with the tuning coil, and outside,the can, may be used. UE: (Te Kuiti): What inductance will furnish optimum load yalue for a 224 valve used ag :- 1. A space charge a.f. amplifier, or following a screen grid detector? 2. Used in the ordinary way? A.: While the a.c. resistance of a screen grid valye may vary considerably according to the use to which it is put, it is, in any case, extremely high. It is therefore not so much a matter vf obtaining optimum load yalue as of maintaining a sufficiently high inductance with the a.f. choke in the plate cireuit in carrying direct circuit to give satisfactory frequency response. It is doubtful whether there is any commercial choke which, when carrying the plate current of an a.g. valve will maintain an inductance higher than 200 henries. In any

case a choke of higher inductance would be likely to have a self-capacity so high as to by-pass audio frequencies. 2. It is difficult to understand what is meant by "the ordinary way." ‘The ordinary way to use a 224 is as an uf. amplifier, in which case it is usually followed by a tuned circuit, the inductance of which is determined by the capacity of, the tuning condenser employed and the waveband to be covered by the combination. R G.R. (Frankton Junction): Could "Ye you forward me a diagram of a modulated oscillator for aligning ganged condensers? Would my circuit be .satisfactory ? A.: The circuit submitted would be satisfactory if a constant source of af. current-e.g., a frequency test gramophone record and a pickup-were available. A pentode might be found desirable in a modulator seeing that there is no prior amplification from pickup or microphones. It is possible that if only one receiver is to be ganged equally satisfactory results could be obtained at less cost by temporarily wiring midget condensers in parallel with the tuning condensers and tuning them until the system is in resonance. Which ever of the midget condensers appears to require least capacity to bring the circuit into resonance, is an indicator as to which coil should have its turns reduced. The process must be continued until all the midget condensers are in line when the set is in resonance. For precision work a modulated oscillator in which the modulation is constant in frequency and extent is necessary. A suitable cireuit is shown above and if further particulars of this are required, they can be furnished. Not every audio transformer suits this use, and two or three should be tried. grow (Timaru): I am desirous of constructing a four-valve portable radio set. Could you supply me witb a suitable eircuit ? A. : Circuits were described about 12 months ago in the "R.R." Our new model will be described in the course of a couple of months or so. What appears to us to be a fairly good four-valve set was deseribed in

Aerial Oscillator Band coil metres Jl Il2 8 on L5 80 .§ 24. 6 20 12 40 4 12 8 10 8 20 2 6 2 5 5