QUESTIONS AND ANSWERS

Radio Record, Volume VIII, Issue 19, 16 November 1934, Page 43

QUESTIONS AND ANSWERS

"pyo" (Belmont): I have had a "det" and one audio" se€é which is not very efficient; would it be possible to convert the "Duodyne One" to an r.f., detector receiver, so as. to use the*udio end of my present set as audio amplifier? A. Your suggestion is not practicable. It would be far better to add an extra audio stage to the "Duodyne One" as it stands at present. A high efficiency pentode such as the PM22A would be the most suitable valve to use. It should be resistance coupled to the second section of the 19. A M.K. (Wellington): I.am planning * to build the "Hlfin Four" as described in the 1984 "Radio Guide," but I would like to substitute .a 2A6 for the 55. On reading up the characteristics of the 2A6 I find that diode biasing of the triode unit is not suitable because of cutoff. Now I cannot determine whether the 55 in the circuit is:diode biased. If it is, of course'I cannot use the 2A6, A. In'diode biasing, the cathode of the duo-diode valve is connected directly to earth. The bias voltage for the audio section of the valve is actually provided by the signal, being. greatest when the signal is strongest. Diode biasing can be used with the 2A6, in fact one of the most popular kit-sets marketed ,in Australia during the past year uses a diode biased 2A6. However, in the "Tlfin Four" the 55 is self-biased by. the 2,500 ohm resistor in the cathode lead. If you are planning to use the 2A6 no changes are necessary, though the value of the self-biased resistor could be increased to 5,000 ohms. ; "TP RIG" (Auckland): I am planning to build the "Viking Shortwaver" deseribed in the November, 1933. "Radio Times." I will be obtaining the power from the ’47 output socket of my broadcasting set. Could I use this valve in the "Viking" in place.of the 56? A. Yes, quite easily. 2. What: alterations would be necessary, particularly in regard. to providing bias for the ’47? : . te A. The only alteration would be to provide. the correct bias for the "47. 'T’o do .this connect a 200hm~ wire-wound centre-tapped resistor across the filament terminals of the *47 socket. Between the centre-tap and earth connect a 450ohm wire-wound resistor, by-passed by a dry electrolytic condenser of 10 to 25mfd. capacity. Be sure to connect the red: end of the dry electrolytic to the centretap of the 200hm resistor, and not.to earth. 3. What type of output transformer would I need? | A. The multi-ratio output transformer specified. would be quite suitable. IX2204. (North Auckland): I-have a "Conqueror Superhet" (described in the 1984 "Radio Guide") and it has a peculiarity I would like you to expiain. .As the "A" battery becomes flat.the stations at the low frequency end of the dial (2YA. and 2CO) disappear, followed by stations ot higher frequency. until even 3YA cannot be brought in. 'The stations above the "cut-out". points come in splendidly. © _A, The explanation is quite simple. As the "A" battery voltage becomes low, the filament voltage of the 1A6 is reduced, and the oscillator ceases to functign-at the low frequency end of the dial first of all.

Note.-The Australian and New Zea-: land stations are yery-jammed over certain portions of the lower wave-band-often to such an extent that bad heterodyning interference is created. There is no satisfactory way of obtaining effective separation of stations such as these, The only solution to the trouble! would be a re-allocation of wave-lengths. G R.R. (Masterton): I have a seven*valve a.c. superhet. I am contemplating moving to another street, where I '

understand,‘the power lines cause.a fair amount of interference to reception. How could I eliminate this noise? A. Only by erecting a special type of noise-reducing aerial such ag that described in the November "Radio Times." R F.C, (Oamaru): I have built the *"Piny Tim" all-waver described in the 1934 "‘Radio Guide," and have decided to add two audio frequency stages to it. Is the enclosed circuit diagram correct? (Continued on page 45).

Questions and Answers .

(Continued from page 43.) A. Using two transformer coupled audio stages, you are liable to encounter threshold -howl and general audio instability, If you experience this trouble, -try _connecting a resistor of 50,000 to 250,000 ohms across the secondary of the first audio transformer, Use the highest yalue possible which provides an effective cure. Also, it would be better to use a small power valve (with correct bias) in. place of the 201A you have. at present in the last stage, In addition, you would find results would be much better if you could increase your "B" battery voltage to at least 100 volts. R L.H. (-). We strongly advise you against attempting to use your home made coils in the "Conqueror Superhet," . © for even if the aerial and r.f, coils lined up more or less accurately, there would be little hope of your winding an_ oscillator coil which would track correctly. In any case, the detector-coil in your present: re-

ceiver could definitely not be used. The coil kit we advise you to purebase com‘prises aerial, r.f., and oscillator coils, with two 465 k.c. mica dielectric i.f, transformers,