Questions and Answers

Radio Record, Volume IV, Issue 31, 13 February 1931, Page 14

Questions and Answers

G P.K,. (Te Aroha).-I have a seven- * valve. a.c, commercial set, and am troubled with annoying gurgle when using the pick-up, which overates from the first audio, Apparently no grid-bias is used in this set, circuit of which is attached. A.: It is probable that one of the bypass condensers has broken down. ‘Try first earthing the pick-up, Grid-bias is certainly: used, and is automatic, 2. Where should the pick-up be connected ? Av: To the grid and cathode of the detector yalye. Special adaptors which fit under ‘the base of this valve are obtainable. ; -"ORBINE* (Oamaru).-lIs -the choke _ ~~ coil described in the January 30 issue for the "Differential" series for both shortwave and broadcast work? ° A.: Yes, it. would be suitable, though a choke. for use on both high and medium frequencies is only. a compromise. For best efficiency incorporate a. shortwave choke and switch. .. 7 Would a transformer ration 4421 be available? Av: If you are building the "‘differential four’ this would be slightly high for best quality, though it could be incorpor.ated in the last audio ‘stage. Two volt valves are quite suitable, and types have been mentioned in the articles on this receiver. 3. ‘Will: the set when completed: be suitable for receiving all New Zealand stations and the principal Australian on the broadeast band on the loudspeaker?. ° A.: Yes; see the reply to another correspondent concerning the results obtained from the "Differential Two." | 4; Will the set be suitable for operatins a moving coil speaker. and for reproducing gramophone records? A.: We would not advise the'use of the. former. -A ‘pick-up connected in’ the detector stage would, however, give quite good volume for the latter. 5, Will you be publishing in the’ near future particulars of a three or four-yalve all-electric set suitable for amateur construction? , A.: In. all probability, yes. =. Was the coil diagram omitted from the differential article in issue of January 30? A.: Unfortunatelv, yes. Space was at a premium and it was too late to amend the text. Jt appeared. however, in the issue dated December 26, 1930. on a circuit. .: In-our opinion the "Differential Four’ is far in advance.of. the circuit enclosed, and is a more. powerful re-, ceiver. © ~ os J §. (Christchurch) asks for an opinion A "QUERY" (Te Awamutu) wishes to construct an s.w. adaptor for a six-valve a.ec, set. and desires to know if one will be described in the 1931 "Guide," ‘and if so, would it be more efficient than that published in the 1930 "Guide." A.: Yes, an s.w. adaptor will-be described in the 1931 "Guide," and. is of better design, . .

IGHT AWK" (Temuka).- Would the Cossor valves 210 R.C.- and 220P, he suitable for the. "Night Hawk?’-Yes. 2. Which ‘is the most suitable ratio for shortwaye work-5-1 "OF. Ast : If y you are intending to add ahdther audio stage later, the’ latter would be better, otherwise use the 5-1, ron G.E.- (Carterton) .-Is the following ‘combination * of : valves" a good ‘one for. a six-valve battery set? A..: Yes, except that a. power vale, 1.4, the one nearest the speaker, could with ad-. vantage be incorporated. . This should ; be of the 171 or 112A type. : R. AR, ‘(Wellingtori) ¢ Owing to lack’ of room I am compelled to use a short aérial.. I can get four or five Aus-. tralian stations . only at fair speaker strength on good nights, and wish to add another: stage of amplification and wavetrap, Is the circuit of the .former,, which I inclose, suitable? | A.: No. Your set is not designed to use a booster, and it would give more trouble than it would be worth. An. excellent wavetrap will be deseribed in ae 1931 "Guide. " . H.B. (Auckland) 7 I have an. eliminator supplying 150 volts at 30. mills., and I wish to change my three+’ valve screen grid. to.2,¢.. Which would be the best combination of Philips valves?’ A.: 1442, £415, B405... ’ ‘ 2, As my eliminator is a "BR" unit only, is there any way of obtaining grid-. bias from it? AL: Yes, though there will bea resulting drop in’ the plate ,potential on the valves... Between your "B" terminal, and "A" connect a 8000 ohms variable resistance. Senarate within the set all "A-" and "B-’ leads. The bias resistor can be adjusted to suit the valve used. 2. Could a pentode valve ‘be used in the last stage if converted to: a.c., and if so what valve would be suitable? _A.:.Yes, type B443, — . "Bue (Lower Hutt)-: I have -a ‘com-’ ercial three valve, and am at pre--sent using an inside aerial which gives me excellent volume from 2YA.. 1YA ‘and’ 3YA also come in faintly, but there is dlways a whistling noise in’ the speaker: when tuning in. A.: Your set is obviously not powerful to pick up the distant stations on’ the aerial you are using. Consequently you are forcing your set,-and by undue use of reaction the set is oscillating. : 2. Will this whistling. be eliminated when I erect an outside aérial? A.: If you erect an efficient one it will to ‘a large extent be eliminated, because your signal pick-up will ba much stronger. . Is the enclosed sample of wire I am for both the aerial and earth suitable ?- Yes, but not :for a: permanent . aerial. Use 7/22.

"CONSTANT READDR" (Gisborne) ; I have a home-made Tungar bulb battery charger which smokes slightly and becomes very hot while operating. A.: The number of turns you are using are approximately correct. It is possible, however, that the insulation separating the winding has broken. down or a short-circut has oeeurred. It wll pro-short-circuit: has occurred, It will probably be necessary to dismantle the transformer and re-wind it. Carefully insulate it with Empire cloth...

"605" (Southland).--Wihat would’ be . the best combination of Philips’ valves for my five-valve neutrodyne. A.: Four 609 type and one B605. 2. Can I utilise three 201A’s, one A615, and one PM6? A.: Yes; but a PM256 would be better in the last stage. 3. What is the correct grid-leak?-2 megs, $ Where can I obtain a diagram of this wr We do not know.

LACK CAT" (Blenheim)-I have a a .001 variable condenser with, 22 plates. How many will I have to remove to make a .0003? A.: The condenser is certainly not 001. It is approximately correct as it is, though the removal of three plates would give you the required capacity, "THER" (Westport) --I am using 221 valves in my set. . What type of power valve do you A.: A171 or a B603, or PM256. Is extra "B" and "O" voltage necessary? A.: Yes, you should use at least. 135 volts on the plate and approximately 15 volts grid bias. 2. My speaker is of the old-fashioned horn type, but a very good reproducer. Would it be necessary to substitute. a eone type if I insert a power valve? A.: No; if you have any fear of the speaker windings burning out insert an output filter. 8. What should be the life of an A" battery ? A.: Using 221’s and a-power valve, about three-months. They would last longer, however, if _you connect them in series-parallel, FE. GC, (Auckland) Having studied the eoil data in this week’s issue, I-fail to find Aan ‘answer to my. question of two weeks: ago. It appears to me that you try to dodge giving a clear answer: to some of your readers. Perhaps you. give ‘your direct subscribers the preference.. My. Set requires one coil for the r.f. stage’and one for the detector stage, but the coils in your table appear to be wound om one former, ‘Would the same number of turns do for'the coil in the detector stage? A.: Yes. Our-table was designed’ and: as our’queries are numerous and our ‘time limited,’ we certainly cannot undertake to design coils ‘for individual requirements.’ As for your remarks’ that direct subscribers receive preference, do you ‘imagine that upon receiving a query we hunt through files just to see if the-in-quirer is a yearly subscriber? If you: do,;. you are mistaken. "OWL" (Greymouth) : I have both a 84-1. and 5-1 audio transformer. Which would you suggest would be the better for use in the "Night Hawk" Two? Aw: . The 5-1. 2. You suggest using a 5meg, grid-leak, and I notice that the 2meg. is advertised for this particular set. Which is correct? A.: The value is not very critical, and ‘depends to a large-extent upon the yalve you are using as detector. A 5meg. would be more sensitive, but a 2 or 3meg. would give a better tone. 8. Would it be of advantage to have @ pig-tail on my variable condenser? If so. to where is it soldered? A.: Hither to one of the moving plates or to the shaft which rotates them and to the framework to which the terminal connestod with the moving vanes is at4, Will the set operate a loudspeaker? A.: In a poor reception locality in Wellington we have received all the YA stations at good loudspeaker volume, and 2FC and 2B at fair speaker volume, -besides a number of. others.