Questions and Answers

Radio Record, Volume V, Issue 41, 22 April 1932, Page 10

Questions and Answers

RAstyus (Christchurch). --What is. meant by stations heterodyning? A.: For a full explanation of the term, "heterédyning" see chapter 2 of the 1932 "Radio Guide." When approximate frequencies are generated at the same time a third frequency, known as the beat or heterodyne frequency, is set up. Coming down to mother earth it means that if two stations have wavelengths approximately the same, and can be simultaneously brought in by your set, a third wavelength would be set up, which sounds in your speaker as a whistle. 2. Could I use a .0001 condenser with the "Sparrow Hawk" one without altering the coils ?-Yes, DTD®TECTO (Auckland).-If you wish to construct a pushpull output transferformer your best plan would be to consult the chapter on output transformers in the 1982 "Radio Quide." We could give you the specifications, but there are many factors entering into the design of these transformers which, unless you are aware of them, would give you trouble. It would be quite satisfactory to use two 201A’s in pushpull with a.c. on the filament, but you would not get a very great output. 8.R. (Christchurch) -What yalves * would you recommend for a fourvalve B.D.? _ A.: The 221 type of valve (or the 201A) in all sockets except the last, where you should use a semi-power valve of the BG6O5 class. M §.A. (Pahiatua).-Is it possible to * eradicate the interference caused by brushes on a dynamo? A.: Yes; the procedure was fully deseribed in the "R.R." of August 14, 1931. 2. The volume from.my five-valve set is weak. Could you advise? A: Look through the "Causes of Weak Volume,’’ outlined in the "Radio Guide.’ In the absence of further particulars we can do yery little to help you POUBLE JAY (Trentham).-Will a "B" and "C" eliminator do for the Multi Mu ‘Three described in the "Guide"? A.: No, the output of this eliminator is insufficient for a pentode yalve, which takes 30 mamps at 250 volts. the types recommended 2. Would a trickle charger be of any use for a filament supply?-No. The proper method of obtaining power supply is by constructing a special powerpack to suit your requirements. Particulars of the powerpack can be obtained from the article on the subject. ey B. (Clive) -Of the two sets you * mention we have tested the latter, which is British made, and are quite satisfied with the results of our test, It is an excellent product. We know nothing of the other one. 7 Which are the best valves for any set? A.: Rather a sweeping question. Use y the manufac- — turers -the best known makes only. B. (Christchurch).-Can specially * matched coils instead of the specified intermediate be used for the Cathode Super? A.: Yes; it is not imperative that the specified intermediates be used. So long as the condensers and coils resonate on 2 suitable intermediate frequency, any condenser coil combination can be used, the connections remaining unaltered. 1 o2uc (Christchurch).-I wish to erect ~ two aerials. What is the minimum between the parallel flat tops to prevent mutual induction ?-Six feet. 2. The leads in are not parallel. Can they be hrought any closer than the minimium distance in one? A.: It depends,what you mean by not parallel. The best plan is to keep them as far away as possible, and if they hanrpen to be closer ihan desirable you at ley.

have the satisfaction or knowing you have done your best. 8. Are three insulators a foot apart sufficient ?-Yes. HA™ (Timaru) : We could supply you with a circuit only. In the "Radio Times" electric sets will be. described from time to time, and you should find one to suit your requirements. CPYSTAL (Dunedin): I find I can get. better results on my multivalve a.c, set by using a erystal between the top of the grid coil and the grid of the valve. A.: How are you bringing about detection otherwise? You are not using a grid leak and condenser and for power grid detection the valne of the resistance in the cathode of the 224 should be at least 20,000 ohms. Power detection is the best to use when a pick-up is incorporated, wee PXNAMO (Waitomo Caves) : Wher I remove one of the push-pull valves the set works as well as with two. A.: A set with one of the push-pull valves removed will always work, but the volume the set can handle will be considerably reduced and usually the quality suffers, 2. Since adding p.p. the first audio valve will not take any negative grid bias, whereas before it would take 34 volts. A.: Try raising the voltage on the audio valve and decouple the plate supply by the means of a 600 ohms noninductive resistance by-passed to earth. 3. Would it be advantageous to incorporate p.p. after the second det. of the Super Six? "a A.: Yes; a 33-1 ratio transformer should be used. EF H. (Gisborne): I cannot get the * "Radiogram Five" to work. A.: Cheek up on the grid leak. connections thus: The "G’’ terminal of the regeneraformer should go to the side of the grid condenser which is not connected with the valve. The other side of the grid condenser connects with the valve and with the grid leak. Reverting to your sketch, the connections should be from "G’? to the centre terminal of the combined grid-leak condenser, from the lower end of the grid terminal of the valve; not as shown. Make certain you have these connections right. If they are correct, there is nothing we can suggest other than systematically searching your set for faults. Are you quite certain you have the connections of the sereening grid eorrect ? In an American valve the grid is on top, whereas in the English the plate is on top. It seems to us, however, that your trouble lies in the grid circuit of the detector. TW. (Auckland): Some time a I * built the "Super Six," but the tuwhing has been very broad. . A.: This is apparently due to an incorrect loop. Use the best Litz wire and connect the loop by as short a lead as possible to the appropriate terminals on the set. ‘The oscillator dial should be slightly out of step with the r.f. dial. RALrO (Wellington): We. are sorry AY we cannot discriminate — between commercially-made receivers, | L A. (Wellington) : If a radio engineer * has attended to your set and left it unsatisfactory, there is not a great possibility of our being able to advise you what to do by correspondence. The best plan is to worry him until you get what you want. . + "Continued on ‘pagé’ 22.)

Questions and Answers

(Continued from page 10.) JYAMRoB (Ashburton): Your circuit is not quite correct. Replace R2 with a fixed condenser of the value of 00025. Place R2 between the choke and the plate prong in the socket. The value of R2 is 50,000 ohms, and that of R1 100,000 ohms variable. Once you have arrived: at the correct value of Ri, it will not be necessary to make further adjustments, so this need not be panel mounted, Your circuit is now eorrect, but you may find a difficulty in operating the adaptor with the sg. r.f. valve. It ‘would not be advisable to replace the first tuning cireuit with a choke. [TRANSFORMER (Karori): As many important points enter into the congiruction of an output transformer, you would be wise to consult the 1932 "Radio Guide" before setting about it, You can then couple a pentode to the 66K. For best results you could not use the core of your transformer. W.S. (Auckland) : I propose to use & * super het. with the second harmonic oscillator system. What would be a suitable if. to use?-50 kc. 2. What should be frequency range of a tuned circuit in the grid cireuit of the combined oscillator mixer be? A.: Approximately 250 to 725 kiloeycles. In other words, half the.normal frequency range minus the intermediate frequency. 3. Should the plate coil be coupled to the secondary of the first transformer or to the tuned circuit previously referred to? A.: To the latter. C B (Waitara): Why not consult the °* "Radio Guide," where you would find out particulars you want. Using 26 ds.c, on 2 Zin, former, and a .0005 variable condenser, you will require 72 turns on the secondaries, 30 turns on the aerial, 25 on the primary, and 25 on the re generation coil. A "B" and "cy? eliminator is quite capable of supplying enough power to operate your set. 624 is quite suitable, H. D.C. (Wanganui): We think your * scheme is not commercial. FLLLTOP (Wellington) : Would it be an improvement to the B.D. set to substitute the present close wound coils with space wound ones?-No. S M.B. (Whakatane): I am using 2 e eommercial amplifier with two mov-ing-coil speakers, but the quality is very poor, A.: One of two things may be wrong. There may be faults within the amplifier, particularly in the grid circuit of any one of the valves, or (2) incorrectly matched impedances. To us the latter seems more feasible. If you are using two speakers they should be connected in parallel, preferably through & specially matched output transformer. We recommend, particularly if you wish to do public address work, to eonstruct an output transformer such as that degcribed in the 1932 ‘Radio Guide." You would then have a better chance of matching up the impedances of the valves to the apeaker. 2. A bad state of resonance exists between microphone and amplifier. © A.: This sounds like a poor microphone transformer. We hope you are using a special transformer and not the one provided with the set.