QUESTIONS AND ANSWERS

Radio Record, Volume IX, Issue 2, 19 July 1935, Page 53

QUESTIONS AND ANSWERS

"QICREEN . GRID" (Palmerston North): I would like to build the rf, and detector stages of the a.c. "Sky Hawk" and use them as a shortwave adaptor for my broadcast set. Could I use a pair of 24A’s instead of the 58 and 57, as shown in my circuit? A.: Yes, these valves will work quite well, The plate resistor of the detector valve should be reduced to 100,000 ohme. You: will not get as much gain using resistance coupling as if you used the /eviginal. choke coupling, and also the \yhaximum voltage as applied by the pofentiometer to the screen of the detector valve would be too high. Try including a 50,000 ohm resistor between the positive end of the potentiometer and _ its present junction with the 50,000 ohm voltage dropping resistor shown in your circuit. You may find it necessary to put two turns on each reaction winding instead of one to obtain smooth oscillation over all bands. Be sure to use an r.f. choke designed for shortwave operation. "A THOS" (Hikurangi): You should have sent along the circuits of the sets you have built, not the point-to-point wiring diagrams. In adding another valve to your first set you have connected the grid of the second directly to the plate of the first. The latter has no plate voltage applied to it, which is just as well, as otherwise it would soon be ruined. You would get far better results -by building up a set such as the "Beginner’s All-wave One," described in the \ July "Radio Times." If necessary you ‘gould add a transformet-coupled audio stage to it later. We can make very little of your second query, but if the lead-in of the outside aerial is passing close to the portable receiver, this would explain the effect you mention. B €. (Hamilton): When I advance the * volume contro] on my _ five-valve superhet, the set becomes very noisy. Also, a hissing sound is heard, and it is very difficult to listen to weak stations. Why is this? . A.: Simply because when you advance your volume control the set becomes

more sensitive and brings in more static and interference. Valve hiss also increases. : 2, I cannot advance the volume contro! full on strong stations without the set. appearing to choke, A.: Any moderately powerful set not incorporating A.V.C. will overload’ on powerful locals if the volume control’ is © advanced too far. The fault ean be cured in your set by incorporating automatie volume control, and also by using a potentiometer for the diode load resistor to give manual control of volume. However, you will find your set will operate perfectly satisfactorily if you take care not to overload it on powerful stations. AM" (Westport): The cireuit you enclose would operate a speaker, cither a cone type or a permanent mag- . net dynamic. However, we certainly do not advise you to use a pentode in the first audio stage. An ordinary type 80 triode will give you all the gain you want. ‘The best valves to usa would be a pair of 30’s in the detector and audio stages and a small pentode such as a PM22A in the output. "PEREECTONE" (Levin): The poor selectivity of the set you are using is probably due to the fact that you are using coils with high impedance primaries. Wither use a small semi-variable condenser in series with the aerial or put on another aerial primary winding of, say, 20 turns, close-wound below the secondary; 45 turns would be sufficient for the other two r.f. primaries; these should ‘be wound over the bottom ends of the secondaries with a layer of Empire cloth or insulating tape separating the windings. Use a fine gauge of wire such as 32 d.s.c, or enamelled for all windings. NOTH: We are pleased to know you are getting such fine results from the "Air-Ace," UTODYND" (Dunedin): Sorry, but we cannot undertake to design superhet coils for individual requirements. Besides, we do not advise you to wind your own, as you would find it almost impossible to obtain accurate tracking without the use of proper equipment. XBR" ( ): If you have the LF. transformers accurately trimmed to 165 k.e., with a reliable oscillator, and ther have the whole set re-aligned, the whistles you are experiencing will disappear, The converter you mention could not be modified to tune from 90 metres downward with a single coil, Another set of coils would be required, together with a wave-change switch. If cleaned up, the wire you mention should still be quite serviceable for use as an aerial, "TGNORANT" (Onehunga): An aerial about 80 to 35.feet high, 70 feet overall, and with a flat top running at rightangles to the power lines and as, far’ from them as possible, would be the most suitable "L" type aerial for your set. The lead-in could be run round the walls of the room on the small type insulators’ made for this purpose, or elsé, | as you suggest, it could be run under the ©

house and up through the floor boards. Be careful to keep the lead-in — spaced well away from any pipes "under the house. : AX" (South Otago) : I am building the "Trans-Tasman Three," and would like to know if the set can be used on the short waves, If so, what are the coil details? A.: You would find tuning extremely difficult; in fact, you would need a good vernier dial to be able to pick up ‘anything at all, because the capacity of the condenser gang is too large for shortwave listening .on a set such as this. However, if you care to try it, the following pair of coils should give eoverage of the main short-wave band, from about 20 to 50 metres, Aerial coil: Primary 8 turns (wound 1-8in.. below secondary), secondary 5 turns. Detector coil: Primary 3 turns (interwound with secondary), secondary 5 turns, reaction 8 turns (wound 1-8in, below secondary). Use 22 or 24 gauge wite-any covering would do-for the grid windings, and finer wire, if you have it on hand, for the primary and reaction windings. If not, then the heavier gauge will do tor them all,

Information Coupon (To be used with a]l requests for information.) Name of set Model Name Addregs Nom de plume To be kept in subsequent inquirieg. Please Note X (1) Be_ specific and brief, tabu- lating jf 'possible. (2) Write legibly and on oue side 'of 'the paper; (3) We do not design circuits; (4) Limit three questions unlesg 1/ is encloged; (5) Posta] queries limit 3 quee tions, @nclose stamped and nddressed 'en velope and 1/: fee: VIbiiziibiibiibiip IIbIIbIIi HI