QUESTIONS AND ANSWERS

Radio Record, Volume V, Issue 16, 30 October 1931, Page 15

QUESTIONS AND ANSWERS

FRANK

KEE

The Technical Editor will, through these columns, be pleased to help readers experiencing trouble with their sets. Queries are limited to three-for more than this a shilling fee is charged, and a similar fee is payable for queries answered by post. Supplying layouts. circuits and solutions of intricate theoretical problems is beyond the scope of this service. A coupon must accompany all requests for information. Nonappearance of the coupon in any issue cannot be regarded as a reason fer its not being used. Address all queries, The Technical Editor, Box 1032, Wellington.

pis. (Otago): Can I eouple a s.w. oscillator direct to the screen-grid " of the first detector in a super het. ? It s best this way. ~: It is quite satisfactory to make this connection, although it is somewhat unusual, Generally a super het. will work best when coupled with the grid and grid return of the intermediate stages. 2. Which is the best coupling? I have tried out a dozen different» methods. A.: Generally the inductive method. 8.. Why have some oscillators condenssers and leaks in the grid circuit? ' A.: Almost any kind-of oscillator cireuit may be used for the super heterodyne. Some employ a tuned plate having a tuning condenser across the plate coil instead ‘of across the grid coil. Most have tuning condensers across both coils. In all oscillator circuits it is necessary to use either the tuning condenser or «Separate fixed condensers. as blocking condensers so that the direct high voltage _ for the plate circuit eannot reach the grid circuit which is connected to the filament of the value. Some super heterodyne receivers do away with the separate oscillator entirely making the first detector do the frequency changing. 4, Is not 80 kc. too low a frequency for a shortwave super-het.? A.: 30 k.e, is considered to be the lowest frequency at which super-hets. will ‘work satisfactorily. 5. Could you publish an article on oscillator design? A.: We find it advisable to give superhets. a fairly wide berth as when they are made by amateur.constructors the sets almost invariably fall foul of the Post and Telegraph Department. .We have already been brought to book on this account. 6. Are there many modulated c.w. stations on the air?-Yes.

7. Would a super-het. pick up ec.w.?Yes. H00k-uP (Matamata) : In which positions in a super het. are the variablemu valyes most suitable-the r.f. or the if. stages?-The r.f. stages. 2. The set I purpose building calls for a variable resistance to control the s.g. voltage on the detector. If I use a multimu yalve would I be able to eliminate this contro. A.: It ‘would be better to use a 224 as detector, and control the voltage as shown in your diagram. 8. Would a 500,000 ohms. tone control be suitable for controlling the variable bias of the multi-mu valves?-Yes.

4, Is 14/36 twin flex suitable for wiring a.e. filament ?-Yes, 5. Which end of an "LL" type aerial should point to the station desired? A.: The directional properties of the ordinary aerial are not strong. Generally it is considered that the Jead-in should be at the set end and the free end pointing toward the station. 6. Can you give me the specifications for a power transformer developing 383) volts on each side of a centre tap_at 130 mamps with heater voltages of 2.5 at 16 amps and 2.5 volts at 3.5 amps, and 5 volts at 2 amps? oo --A.: 100 watts transformer will just serve the purpose. You will need a primary of 1140 turns of 24 gauge enamelled wire, a secondary with 1930 turns on either side of the centre tap, using gauge wire. You will require five separate 2.5 windings consisting of number 16 gauge d.e.c. wire, and, for the 5-volt winding. you will require 30 turns of number 18 gauge. You will find more particulars about this transformer in the 1931 "Radio uide. D-D* (Dunedin) : Is there any method of adding a third audio stage to my present set without causing instability ? A.: Three transformer. stages are impracticable. If you must use three stages we would recommend r.c.c. method of coupling, but then the three valves will do no more than the work of two valves used in transformer coupling. You could add another valve by turning the last stage into push-pull, It is impracticable to try

to work the three audio stages by employ ing more by-pass condensers, ete. 2. Does increasing the air gap in an a.f. choke increase efficiency? A.: There is an optimum gap for every choke, and if you do not happen to have it right, increasing or decréasing it, as the case may be, may enable you to stumble across the optimum dimension. We were interested to hear about your

aerial and your letter has been passed on to the DX editor. PowsR PACK (Oamaru): Where could I obtain a circuit and specifications of a good powerful a.c. set using pentode valves? A.: You do not state if you want t.r.f. or superhet. In the near future we shall

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a -- be bringing out a very powerful a.c. super het, and you should find this of interest. 2. I am building the power pack described in the "Guide." Will this run an &c. set? A.! Tt ‘will run the average set with gix or seven valves. S B. (Taramaki): Questions such as yours have been answered many times in the "Radio Record." A fairly complete table was published in our issue dated April 2. .D*"4 (Pahiatua) : Is it permissible for a licensed electrical wireman to service an all-electric set without first passing the radiotricians’ examination which has been recently inaugurated ?-Yes. OWL (Greymouth): Could I utilise a .00025 variable condenser in a erystal set? If so, what will be the coil specifications? A.: On a 3in. former you will require about 62 turns of 26 d.s.c. wire, and bring your aerial in at about the 20th turn. "A BRESTER (Newtown) : Both of your sketches are correct, though theoretically "B" would be the ‘better. There is really no difference, however, in the results obtained, and in the measure of safety involved. W J.W. (Auckland): Your set is very * unselective, and we think there is probably something wrong with it. We not know anything about the set itself, and we would advise ycu to get in touch with the agent who sold it to you and ask him if that is usual with that type of set. To improve the selectivity it would be necessary to use extra tuned cireuit, comprising a coil and condenser, something like a wavetrap, and you would have to operate the dial of this in gonjunction | with the dial of your own Bet."

Foxy (Hokitika); In replacing the valves in your a.c, set use the same type as you take out. Be sure, however, that you use a well-known make. 2, I have a small erystal set and cannot pick up the two Greymouth "B" stations, which are about 25 miles away. A.: 25 miles is the limit of a crystal for a station such as 2YA. You are beyond the service area of two small sta‘tions such as those to which you refer.: 3. Occasionally I see in the "R.R." where someone has logged a fairly large number of stations on a crystal set. Is this true? Do they have to use valves? A.: We hope it is true. Reception of many stations on a erystal set is really dependent upon many things, not the least of which is the location of the set itself and the particular crystal used. It is impossible to forecast anything but the reception of the local station when cononl

structing crystal sets, as they are designed for that only. Sometimes a crystal and valve can be employed to receive distant stations. PORTABLE (Gisborne): What resistance would I need to drop the voltage from 8 to 2 if .38 amps were passing? -3 ohms. Thank you for telling us of your results from following our advice. We were very interested indeed to know that you got Retter results through shifting your aerial, C W.W. (Auckland): I have built the * "Outspan Five" and am more than pleased with its performance. I wish to use a power detector, but as soon as I increase the plate voltage it starts to motor-boat. , A.: If you cannot overcome the motorboating by any other means, use an anti-motor-boating device such as has been illustrated in our columys from time to time. This question, of motor-boating has been dealt with very fully in "Questions and Answers in Radio," a booklet which will be available next week. 2. I wish to insert a jack to use a pickup and a s.w. adapter, and would like to know where to put them for best: results, A.: Connect the pick-up between the grid and the grid-return of your detector valve. A shortwave adapter is bettez =

connected between the plate and "B+." of the detector valve. watts (Nelson): In calenlating the vaitage of a transformer secondary, with 300 volts on either side of a centre tap, is the wattage 30 or 60 when 100 mils. are drawn? A.: You always consider the voltage that is actually being delivered. After having passed the rectifier, plus any drop in the rectifier itself. Although actually there is a potential of 600 volts, yet you are supplying only 300, or if you like to consider that you are supplying the full 600, then only half the load is being supplied. In your case you are using 80 watts only. 2. Can you tell me the approximate drain of the average dynamic speaker with a resistance of 2500 ohms? Is it too large a drain for the average powerpack? A.: It is usually in the vicinity of 50 m,. amps, and is too large for the average power pack unless specially designed to accommodate it. It can, however, be used as a choke. There will be a fairly big drop to it. 8. Is 125 amps the highest value of the UX 280 rectifier, and would it stand a steady drain? A.: It is surprising just what that valve will pass. We think it will be quite safe to use it as you suggest. D-eX22Nw (Tasman): Some time ago you told me it would be all right to use 18 gauge wire for the housewiring, but that it would be safer to use an output filter. I do not quite understand the meaning of "safer." . A.: If you'do not tise an output filter you are passing. high voltage current through your house, and it is possible that either the wires. may short-circuit due to corrosion and cause a spark, or someone may touch them and get a shock. Were they to short, of course, you would not damage anything, but you would have some difficulty, probably, in locating your circuit. Battery current should always be taken by the shortest route possible, and it is not wise to take it all round your house before it gets to the plate of the _ valve. We do not happen to know who is the agent for your battery. Apply to Johns, Ltd., Auckland; they may know something about it. (CAPACITY (Invercargill) : Your question is beyond the scope of Q. and A., and there is not sufficient interest in the subject to warrant preparation of a special article. pX*1800 (Invercargill): Can I earth the moving plates of the reaction condenser in the Differential Four circuit? A.: Yes, you may use the hook-up ' ghown for the Radiogram Wive without » any loss in efficiency. ,

-- 2. In this week’s issue you describe how to add a stage of s.g.zr.f. to an H.R. receiv- ~\ er. Is this also applicable to a Browning Drake ?-Yes, : 8. I have an S625 valve which has three pins at one end and two at the other. At the two-pin, end is the s.g. and plate. One of these pins is marked "A" while the other is not marked at all. I presume the pin marked "A" is the anode, Is this correct ?-YXes. ‘ U NO (Nelson): There is a crackling noise in the audio stages of my set. I think one of the transformers has gone, but which one? ° A.: To test your transformers take a piece of wire and short-circuit first the primary, then the secondary of the first transformer. If the noise disappears when you bridge the primary you know the transformer is causing the trouble. If the trouble persists bridge the primary of the second transformer, and in this manner you will find out which transformer is at fault. You ean replace by using a high quality 34-1 transformer. Do not worry about the third cord on your speaker. It is for use with push-pull valves¢ (COUNTRY (Frankton Junction): Can my five-valve receiver be modernised in any way? A.: It could, but modernising an old commercially-made set is not a practicable proposition. 2. For shortwave work what would be the most efficient single valve set or adapter to be worked in conjunction with my present set? A.: One of the type of adapters which plug into the detector socket. The Add-a-phone commercial receiver is the only one we have tested. Ux (Te Kuiti): What are the characteristics of a Mazda A,C.H.L. and A.C.P.I. valves? A.: A.C.H.L., impedance 18,500 ohms, amplification factor 35, filament current / 1 amp, mutual conductance 3, anode current at 120 volts 4.5, grid-bias at 100 (Concluded on page 29.) ‘ .

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-_- Questions and Answers

(Continued from page 16.) volts 3, filament voltage 4. A.C.P.I., im‘pedance 2000, amplification factor 5, filament current 1 amp., mutual conductance 2.5, anode current at 120 volts 25 mils., grid-bias at 150 volts 35. 2, Do you know anything about the Cosmos power pack?-No. 8. What would you consider to be a satisfactory former wound on a 1fin. former to match UY¥224? A.: 40 to 50 turns of 34 gauge B, and D&= (Mataura).-In referring to back correspondence for replies, please repeat your question and our answer, as we cannot keep them all in mind, and hunting through back numbers of the "RR." takes time. Your trouble still appears to be due to neutralisation, a loose cygiinection or a defective valve, particuIMly the detector. . My aerial is 32 feet at one end a 40ft. lead-in; 80 feet passes over a tree 20 feet high. Is the aerial affected? A.: An aerial is much better if kept away from trees, etc. In this case it would be better to get your aerial higher FLECTRON (Banks Péninsula) : Could the stage of s.g. described in the "RR." for the H.R, be applied in the same way for the B.D. Four?-Yes, 2. What "CO" batteries should be used in the r.f. and last audio stages of the B.D., using B605 as power valve? _ A.: With 135 volts on the plate B605 requires about 15 volts. A valve in the re stages rarely requires more than 14 Volts. COTTY (Auckland).-See our notice below concerning the Super Six.