Questions and Answers

Radio Record, Volume VI, Issue 24, 23 December 1932, Page 22

Questions and Answers

W J. W. (Auckland): What one-valve ¢ short-waver do you recommend a beginner to build? A.: The "Sparrow Hawk," described in the 1931 "Radio Guide." 2. What would be the approximate cost? A.: About £3. A M. (Wanganui): I have a Pilot * Super Wasp, a.c. four-valve. The highest tapping on my power-pack is 220 volts. Do you recommend a pentode or would pushpull do better? A.: You will probably find with the power-pack you are using that your maximum B voltage on load is not over 190 and it would drop even further with a pentode. We do not recommend it. Your best plan would probably be to use type 171A valves in pushpull, double biased, and using class B amplification. 1 4.44 (Auckland): I have constructed a two-valve short-waver, and I find that while it will oscillate perfectly with the aerial off, as soon as I connect it, the set will not oscillate at all. .: Your aerial coupling is too tight. If you ave using a separate aerial coil, take several turns off, but if you are taking the aerial straight into the grid coil, either tap it down the coil or insert a .00005 midget condenser in the aerial. You will find on adjusting this that the set will oscillate, iD R.F. (Invercargill): I wish to sub- * stitute a PM4 for an A409 in my output stage. What value bias resistance shall I use? A.: About 700 ohms. 2. I wish to gang the r.f. and detector condensers, and have a small midget for a trimmer. Which condenser shall I place it across? A.: The detector condenser. You will find that wheu using a triode as detector, | the application of reaction will throw the two condensers well out of step, and con- | stant adjustment of the trimmer will be necessary. This means that you have just the same number of controls as you had before, so why gang them? If you really want to gang them the best solution is to use a screen-grid detector as used in the 1982 "Outspan Four." N.B.-The coupling you are using is perhaps the best for maximum signal strength, but you will find that a separate primary would give you much greater selectivity. 1924 (Auckland): I am enclosing a diagram of my two-valve shortwaver, and am anxious to enlarge it to a four yalve, with as little extra expense as possible. A.: No diagram reached us. However, your suggestion for three stages of audio is definitely ruled out. Your best combination for a four-valve short-waver is sereen-grid r.f., detector (if possible s.g.), and two stages of audio. There will be a new short-wave set in the January "Radio Times," though we have not yet decided on the final circuit. K S. (Hamilton): I have a six-valve * commercial set, Could I alter it to use the new valves? . : No, we certainly do not it. The alterations would be too ex-tensive-in fact, the set would have to be rebuilt, "WRITER" (Oamaru): In my commercial battery short-waver I am using a 222 and three 201A’s, Are these the most suitable valves? A.: You could use a type B605 with advantage in the last stage. "NOBBY". (Auckland): What value potentiometer can I use in the plate

circuit of the last valve as a yolume conrol? Ae 50,000 ohms, Your circuit is correct, . 2. If the secondary of a power transformer gives 250 volts, 60 mils, each side ef a centre tap, what will be the total output from a 280 rectifier? A.: Using condenser input to filter, the drop will be negligible. ‘"PDELBIN ©" (Auckland) : Can you tell me of # better circuit on an aac. S.w. converter than that in the 1931 "Radio Guide"? A.: We have not brought one out since, as home-made converters are rarely satisfactory. , 2. Is the s.g. superhet converter beter A.: Theoretically, and to a certain extent, practically, yes, but we think a simple adaptor is your best plan. ‘"PUZZLHD" (Waikato): I built the . radio superhet, adaptor, as described in the 1931 "Guide," but cannot get it to work with my commercial set. . A,: Your details, are very sparse, but try reversing the connections to the oscillator reaction coil. G H.S.: My aerial runs at right angles * to an 11,000 volts power line, and the interference from these lines causes annoying background noises in my set, I cannot shift my aerial. : A.: You could try a counterpoise, has often proved effective in eliminating such disturbances. This is a wire running parallel to your present aerial, about eight feet from the ground, and carefully insulated. It takes the place of the earth . wire on your set. Using a metal sheathed’ cable for lead-in with the sheath earthg} might help. f Would a Kennedy antenna cut jout some noise? " A.: Probably, but you may lose signal strength as well. W I. (Lower Hutt): Could you give * me the specifications of short-wave coils for the "DX Four," tuned with .00015 condensers and wound. on 1din. formers? _

secondaries are wound with 4U gauge enamel, and ticklers with 26 gauge enamel, All coils are close wound. "(NOILS" (Auckland) : Since 1YA altered its frequency my wavetrap will not cut it out. (Continued on page 23.)

Information Coupon (To be used with all requests for information.) Name of set cosccnccorsscaccntess Model Name Address eeeee PYYTTITIT ITT TTT Tt Titty POSSESS OHSHASHSHSHOSECAOSEOOS SSSCEASOHSSSESSSOHSSSSEAESY PASTS OSHSSHSESOSHSSSSOOCHESSHSSESOSHSESOSHSES POSSE Se SEREHSSASSSESOSSSOSEESESEOEE Nom de plume .iccesvcccedececees To be kept in subsequent inquiries Please Note:- i (1) Be specific and brief, tabu- | lating, if possible. ; (2 Write legibly. and op one side" the paper (3) We do not design circuits, (4) Limit three questions. unless 1/- is enclosed. (5) Postal queries limit three . . questions, elon Stamped envelope and i/- f ~

| Questions and Answers (Continued from page 22.) A.: Rebuild your wave-trap in a simpler > form, using only ove of the binocular | coils. This tuned with a .002 mfd. semi-_ variable condenser, the whole being in series with the aerial, should solye your trouble. 2. When I increase yolume the dynamic speaker jars with every strong note. A.: It sounds as though the moving eoil is rubbing against the ‘pole pieces and needs adjusting. 3. I cannot ‘get stations under 1ZR’s wavelength. A.: Your only plan appears to be to take a few turns off the secondaries. "CURIOUS" (Picton): Are my valves suitable for my four-valye Browning Drake? : Yes, quite suitable. If you wish to the American type use three 2380's and a 2381, ISAPPOINTED" (Hamilton): [I haye a four-yalye commercial set. latest design, but my local station is the only one J can receive at good volume. A.: Your aerial and earth are quite good, so apparently something is wrong with your set. . Possibly the condensers are not correctly Jined up or one of the valyes may be a little "off-colour." PEX" (Napier): | have a five-valve battery set'and employ an eliminator for "B’ supply. Could I take my "AC supply off this also? A.: Your whole eliminator could not supply one-hundredth of the current necessary. For example, your eliminator’s output is probably something like 80 mils, and one 201A takes .25 amps or 250 mils, so you can see how hopeless it would be. "TPINKER" (Humilton): Could you publish details of a mixed oscillator circuit? A.: We wit] do so shortly in the "Times." 2. Are the Wire Tables in the "Radio Guide" correct or well on- the safe side? A.: The carrying capacities are on the safe side.

A.: Band. See. Tick. 16-30 5 5 29-58 12 9 54-100 22 15