Questions and answers

Radio Record, Volume IV, Issue 26, 9 January 1931, Page 14

Questions and answers

R B.C. (Napier): I wish to convert a *set into "R. the W. Three." Would an Emco and Ferranti transformer be suitable ?-Yes, 2. What Philips valves would be suitable? Wither four or six volts. A.: Four volt: A415 detector, A409 first audio, B409 second audio. Six volt: A415, A609, and B605. This combination will be an excellent one. 3. What is the material required for an Edison type cell?

A.: Write to A. R. Harris Coy., Christchurch, for full particulars of this cell. AY SO (North Auckland): Could the secondary coil for the H.R. be space wound? . A.: Yes; 80 turns of 24 gauge copper wire spaced by half its own width would be suitable for your purposes. It would be little, if any, better than the unspac ed d.s.e. 2. Using 30 d.s.c. for aerial and regeneraformer primary, how many turns are required to suit 2}in. former? An s.g, valve is used in the r.f.. stage. A.: Try 30 turns tap on the aerial and for the primary 40 turns. If oscillation is troublesome reduce the number of primary turns on the regeneraformer. How many turns and what: size tickler should be used? A.: 1din. diameter and about 40 turns of wire. 4,.: Would anything be gained by using a primary in the regeneraformer of the sereen grid valve? A.: A primary gives much the better results. Look for our description of the two valve screen grid B.D. It is on its way. 5, I intend using swinging coil . re action with the broadcast band of m set and plate voltage control for shor waves. Would this be satisfactory? A.: Yes, but why not use plate voltage control for reaction on long and short waves? 6. What four volt valves would be used with a Daniell cell "A" battery to suit’ a three-valve screen grid set? A.: There are a large number ot valves of different makes that you could use and we do not make it a practice of recommending valves where the makes desired are not stated. Use a special detector and a high gain power valve. See that the filament consumption is under half an amp. 7. Where should fixed condensers be placed in the enclosed circuit? A.: From the screen grid of your valve to earth: capacity 1 mfd. .001 mfds. across the primary of the _ first transformer and if necessary 1 mfd. from B plus r.f, to earth. 8. What value should the grid condenser and leak be? A.: .00025 mfds. and 5 megohms. 9. Is the enclosed cireuit O.K. or should the primary of the r.f. transformer be omitted? A.: The cireuit ‘seems quite a good one, but more will be said about screen grid circuits in forthcoming "R.R.’s" and in the "Guide," WIREMU (Wellington) : Wait for our articles in the circuits for the Crystaphile by "Cathode." 2. Apart from expense do you think the Differential Two would give better results ?-Yes, NEDDO (Whangarei) : I made up the B.D. two r.f., but when I put it on a good amplifier results are disappointing as regards distance. Hyerything is’ quite in order and assembled as in the Listeners’ Guide." A.: Without a definite clue we cannot help you. Have you tried substituting the valves and checking through for weak or defective condensers? Bring the aerial-into the first stage, that is, to the top of. the grid coil of the second

coil. If results are just the same th.u there is something wrong with the first stage. Will. your set oscillate, and have you tried varying the voltages, and what voltage are you using? . A B. (Wellington): I have built a * secreen-grid set, and it has just recently developed crackling. I tried another set with the batteries and found that everything was in order. A.: We presume you tried the same speaker on the other set. If not, that is a very likely cause of your trouble. ook for a broken-down condenser or a loose connection, as this appears to be the trouble. 2. How many turns are needed on a 3in. former’ io suit a .00085 condenser with 24 gauge -d.c.c. wire ?-G5. Oscar (Hikurangi): How can I obtain . volume without distortion. When I advance the rheostat I get an audio howl. I am using two similar highgrade transformers. A.: Try reversing the primary connections to one of the transformers. If this Coes ro ood put the connections back

as they were and incorporate an anti-motor-boating device such as is shown in the accompanying sketch. TEIN (P.N.): What would be the h 5 best combination of yalves for my A.: Five R.C.A. 227’s, two R.C.A,, Ceco, Cunningham, Majestic or other highclass output valyes, and one 280 of these makes, MESspor (Paeroa) : What is the space between the coils in the Differential One ?-1-Sin. 2, What is meant by a differential condenser? Av: One with thrée distinct sets of plates, two fixed and one moving. 38. Would a .00013 condenser do for reaction? A.: If it is differential, yes. 4. Would a pentode valve work in the dudio stage of a differential set? A.: A high gain power valve such as a new B409 would be better. BANUT (Christchurch): Does the average home-built Differential One bring in many short-wave stations on the phones? A.: Yes. you can get stations on a world-wide range on phones, 2. If I built this set using a two-volt valye and about 35 volts plate supply

wvtud its output p.ug directly. into an a.c. or is an output transformer necessary? A.: -Unless you know if there.@s.an input transformer alrcady in the gs ur best plan would be to incorporaté Bn 34 to 1 output transformer-in the adapter. You need 45 volts plate supply. : 8. I get a good deal .of static and interference on my a.c. set. Would the Differential One using an adapter be likely to give much quieter reception?No. BEN ADHEM (Oamaru): Would a Werrand inductor speaker be suitable for the Loftin Four?-Yes, 2. What ratio transformer would be required for it?-1-1. 3. Would a 500 volt working condenser be satisfactory with 650 volts? A.: It is a bit risky. Be on the sale side and yse two 245’s. — ‘ 4. How many secondary turns on a transformer if I use the 245’s? A.: That depends on the core. Transformers were fully described in the 1929 Guide" 425 volts secondary are needed.

Number of Vales ORRESPONDENTS must attach this coupon to all queries sent to the Technical Editor (Box 1082. Wellington). Questions arriving without it are likely to go astray or be delayed. Name of set Name Sete weroereeaeeeeeeeseseserse Address Nom de plume ..........cccceeeee To be kept in subsequent inquiries. Date . Please Note :- (1) Be specific and brief, tabulating, if possible. (2) Write legibly, and on one side (3) We do not design circuits; but accept suggestions for feature articles. Solving trouble, as different from advice, is difficult by correspondence and while letters are given every consideration, answers are not necessarily correct-they are only our opinion based on the matter supplied, which may be quite inadequate. Intricate and involved specifications cannot be supplied without a specialist’s fee,

1; 5. We have some big voltage surges \here. Is a voltage regulation amperite ‘ available? A.: We cannot say for certain if they are, 6. Is there any way of overcoming the undesirable state that is caused when the power is turned on the 250 is lefti for a few seconds without. bias? | A,: You: could incorporate a switch; in the plate circuit of the:250, and turn it on about 30 seconds after the filament. 7 Some months ago I bought a valve which gave symptoms of a loose connection. I replaced it and now after some months’ rest I find it quite satisfactory. Could you explain? A.: Sorry; we cannot. Are you sure it was not in the socket or elsewhere in the set? . ™N. (Gisborne): Can I get better p results with a 65 feet aerial than a"ugpieet one?-Yes. Ross (Woodville): What valves do you recommend for my Americanbuilt receiver? , : Four 221’s and a high gain power valve such as B605 in the last stage, but as you may have difficulty in getting the 221’s you could use 201A’s or A609’s in the four stages. From what you gay, it seems that your valves are at ault. APMOSt (Auckland): Do I have the best combination of valves for my set ?-Yes. 2. Where can I obtain a diagram of an a.c. eliminator, and what will be the total cost? A.: A suitable eliminator will be deseribed in the 1931 "Guide." We cannot yet state what its cost will be. GEARCHER (Kaikohe): I wish to make a battery set suitable for a , radio gramophone. Which are the best ‘circuits? Loftin.Four, H.R. Four, 8.G. ‘B.D. Four, Differential Four? ‘ A.: The Loftin-Four is ruled out on account of its being a.c. The choice probably lies between the B.D. and the Differential Four. Both of these use sereen grid, The Differential would probably be the better for shortwave work. A fair amount will be published on this topic shortly and in the 19381 "Guide." R B, (Nelson).-What current should * the 90-volt Leclanche battery of torch cells deliver?-About 10 m-amps. ‘They read only a volt per cell. Why? A.: It seems that they are more or less polarised, but they are working satisfactorily there is nothing to wory about. If you can, leave them for a while without using them. They may recover, and then be quite all right. 8. Should the area of the zine plates be equal to the carbon rod area? A.: It is not at all material, the zine {eould possibly be a little thinner. The ‘"Wyicker it is the longer it will last. 4, Should commercial or pure zine be used/? A): Commercial zine, which is cheaper, is réally quite satisfactory, though pure zine would last longer. 5. Could a half amp amperite be used with two 4-amp valve in series ?-Yes. 6. Would it be satisfactory with j-amp valve? A.: The filament voltage on this would be about 5.5 volts instead of 5, which would not be altogether satisfactory. RECORDITH (N.P.). I found that static and interference was reduced by at least 75 per cent. without any corresponding loss in volume by using an © underground antenna. This was made sup of two legs of 14-gauge rubber covered wire 60 feet long, slanting away from the receiver, as in the diagram. 2. Is the variable condenser included for any purpose other than to increase selectivity? oe! A.: You have not.stated what variable eondenser. If it is one in the aerial, the selectivity is the main reason for its be-

ing there. It is rarely necessary. 8. Is it necessary to fit a lightning ar ‘ester to this aerial to comply with. the underwriter’s regulations ?-No. 4, Would léad covered wire give any advantage over rubber covered? A.: In this case it would be best. C H.W. (Invercargill)-I intend to * gonstruct a receiver, the circuit of which I enclose, and would like your advice. A.: Why not wait for the screen. grid version of the differential two? It will be very much better, and full constructional details will be given. The circuit you have given would be more or less satisfactory, and it could be improved upon.