Questions and Answers

Radio Record, Volume VI, Issue 14, 14 October 1932, Page 10

Questions and Answers

K A. (Woburn): I am building an out- ¢ put transformer to match a 66R speaker toa 247 valve. How many turns would I put on the primary of the output transformer with a-core of 1 x 1? A.: You Will require 2500 turns, but if you can get’ more on, so much the better. 2. If I wish at some time to use a different valve to the 247, would this necessitate altering the number of turns on the primary? A.: No. he idea with the primary is to get the impedance as high as possible, and make all adjustments to the secondary. ’ 3. I believe there will be marketed shortly an indirectly heated version of the 247. What is the optimum load impedance of this valve?-A.: 7000 ohms. p=. (Waiuku): The harmonics appear to be missing from the music with my speaker. ‘The cabinet vibrates continuously, no matter what volume is handled, Would a lining of non-reson-ant material improve matters? A.: The enbinet work of your set is probably not heavy enough. Vibration should not take place, as this will spoil the tone. A lining of non-resonant material would probably improve matters a the speaker were mounted against this.

2. I find that the coupling resistances in the grid of the pentode and earth is 5 megohms. What difference would one of, say, a $ to 1 megohm make? A.: Five megohms is too high for this position. It should be 3-1 megohm. In what countries are international reply paid coupons not acceptable? A.: We do not known. Inquire at the Post Office. F H.G. (Waitotura Valley): My set * was some time ago converted into a four valye set in order to gave re placing a transformer. IJIowever, I now wish to revert to the original. llow shall I go about it? A.: If you do not know anything about radio construction you would more likely get into difficulties. The connections from the transformer are apparently now taken to the detector stage, cutting out the first audio. These should go P to the plate of the audio valve, B+ to the maximum B4- voltage available. The plate of the detector valve will now connect with P of the transformer nearest it, and B-+- will go off to a suitable detector tapping, ie., about 25-40 volts. 2. There is a 34-1 transformer in che set at present. Could I use a 3-1 with it? A.: Yes, this is quite suitable. | 8. If I get a new set of valves, four 6 -609’s, one 605, will the set want reneutralising? A.: In all probability yes. As we do not have a circuit of your set we cannot tell you how to go about the job. If you use 221’s instead of the ones already in use and a 605 in the last stage you will probably not have to re-neutralise the set. PNTODE (Papakura): About 135 turns of 80 gauge enamelled wire. 2. I am using the superhet. shortwave convertor. Could I take the 90 volts for it from the 90 volts tapping on the voltage divider of my broadcast set?-Yes. GPARKS (Dunedin) : From whom can I obtain the Admiralty hand-book of wireless telegraphy? A.: The Te Aro Book Depot, Wellington. — 3. Would an engineering. course help me to obtain the wireless operator’s ticket? A.: You would be better to obtain a special course catering for the wireless operator, There are several available.

h U 1 (Mamaku) : I am using 221’s and 3605 in my receiver. Would a special detector like A415 improve reception? A.: It ig. very doubtful if the improvement would be noticeable. 2. How would the 1982 "Outspan" compare with my five valve for performance and economy ? A:: It would be superior. 3. How would this set compare. with the "Super Six" for distance on the broadcast band? A.: It would, if used on an aerial, against the "Super Six’ on a loop, be much superior, otherwise the "Super Six" would win. — (CQURIEUX (Napier): I can hear quite a number of shortwave amateurs on my crystal set. Would I. receive them better by tuning to their correct wavelength? A.: They are not harmonics; a harmonic can only come in on a_ shorter wavelength. They are either spurious frequencies or you happen to be in the shock area of those transmitters. . 2. What are the coil specifications for the 80 metres bands using a .0005 mfds. tuning condenser? ,

A.: On a 2in. former you will require 15 turns. 8. Is a "Pix" advantageous to dxing on a crystal set?-No. D F. (Glenside) : Our set has one fault °-Wellington comes in half way over the dial. Would you advise us to sell the set?

A.: A wavetrap would probably he auite effective and would keep out;2YA. We would not advise you to sell the set, as if its value lies only in a local station receiver you would not get more than a few pounds for it. The set could be made more selective, and if the wavetrap is not efficient we would advise you to get in touch with the agent, whose engineers would probably be able to do something for you. W G.H. (Waikanae): The volume con- © trol of my set will not work well. A.: It is rather difficult for us to pick the fault with your volume control.,as we cannot see how it is connected in the circuit. It sounds as though it is a control on the voltage of one of the valves, and in that ease the trouble should disappear if the resistance were replaced. 2. I have charged the battery all day and all night. and then turned the set on at Sp.m., and bv 9 or 9.30 it will gradually vo down until nothing can be heard. A.: Your battery has either not had a full charge-it takes more than 24 hours to charge a battery fully-or it has come to the end of its useful Jife. and conseauently will not hold the charge, Do you have a hydrometer? Jf vou have, test it on the completion of the charge, and if it runs down within a few hours, the battery is useless. A BRIAL (Dunedin): I have a 2-valve all-electric set incornorating a dry rectifier. The tone is not the hest. there being p vinnie in the music and speech. A.: Probably you need more smoothing in the "R" circuit. Test your filter conflensers and filter choke. W.T. (Wellington): I am_ conteme plating building an a.c, loca] sta‘tion receiver, that will be reasonably selective, and would like a circuit employing an r.f. and detector valve, with pentode output. A.: The ideal cireuit appeared in the last.month’s issue of the "Radio Times." It is called "The Link Three," and comprises a band-pass filter, multi-mu, r.f,, screen-grid power detector and pentode. No; the set you enclose would not be satisfactory. You would need a wavetrap.

UZZLED (lL. Hutt): I am. using "Round the World Three" with a power unit and battery charger, as described in the "Radio Record." The set has given good results’ lately, when a loud rushing noise started and tone and volume deteriorated. The switch controlling the primary side of the transformer, or a switch controlling a light is switched on and off, or vice versa, when the trouble comes on, it immediately ceases, and operates normally again, A.: The trouble is so unusual that it would be almost impossible to tell you what is wrong with your set without examining both the power unit: and the set itself. It seems that there is. something wrong with the power unit, probably a broken-down condenser or a defective resistance. Maybe the rectifier valve has petered out. \ , X450C.: How could I add a resistance coupled 227 to my present: circuit to be transformer coupled to a 247? A.+ Such a hook-up would not be satisfactory, as resistance coupling: should be employed in the 47 valve. To do this you would use two orthodox resistance coupled stages. ‘ 2, Will the 227 alter the tone of the pentode ?-No. _ 8, Will a 34-1 transformer be suitable for the purpose mentioned above?. A.: We do not advise, you to use a transformer. : . . 4, Have I sufficient voltage available? A.: Probably. . 5. What difference would a 235 make in place of a 224 rf. valve? A.: Unless your set is specially designed for the 225 it will not make any difference.

6. Could you give me the approximare specifications for rf. and detector coils for the following bands, to be wound on Qin. formers and tuned by a’.00016 condenser? .

Primary, secondary, tickler, in that order, Reaction condenser .00015, Secondaries wire 24 enamelled, primary and tickler 26 enamelled. We are posting you the circuit you require. EN (Auckland): I desire to make a erystal detector using a carborundum .permanent detector. and I understand this needs a stabiliser. What are the connections? A.: They are shown in the accompanying diagram. 2, The value of the potentiometer and the yoltage of the battery? A.: The potentiometer is usually 400 ohms and the battery 14 volts. A condenser is not needed.

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