Questions and Answers

Radio Record, Volume III, Issue 8, 6 September 1929, Page 26

Questions and Answers

In_ Brief. E.Z.C. (Palmerston North).-Broad-cast barl, 98 turns; 100-250 metres, 28 turns; 40-100 meters, 14 turns. Your condenser has a capacity far too large for short wave. ‘Try. .00025. See "Radio Record," August 28, page 30, 31. . Changing Valves. *R B." (Reefton), has recently insert*ed two non-American valves in his American receiver; and he finds that the set goes dead. A,: These. are the wrong valves, for they are designed for tuned anode coupling. Use 199’s, and nothing else, in this receiveh. , . Valves Queries. Js it possible that my valves, which I -have been using for over two years, are not- giving as perfect reproduction as formerly? asks "Valves" (Rodney). A.: After 1000 hours the majority of valves begin to lose@their emission. Some go considerably longer than this, but this is the exteption rather than the rule. 2. What are the symptoms, apart from ‘loss of volume, of valves losing their emission? . A.: The only symptoms are a general weakening of reception. Motor Boating. bad V.S." (Waipukurau) asks the cause * and remedy of motor boating, in a resistance coupled amplifier. A.: Motor boating in resistance coupled amplifiers was fully dealt with in our issue dated August 16, 1929 (page 25). Eliminator Output. "TIXPHRIMENTER" (Auckland) | , raises a point in connection with an answer to "Question-Mark" in our issue of August 9, concerning the output of a Philips eliminator. The B eliminator has an output of 30-85 milliamps. at 150 volts... The A_ eliminator was stated to run four or five valves delivering 2 amps. This latter part of the state..ent is quite obviously a typographical error. The output is not de-

finitely stated’ on these transformers, but would run out somewhere about 5 amps. Dry Batteries for the "A." "QOREENGRID" (Motueka) asks if dry batteries may be used for a three-valve set, the total A. consumption of which is 18 amps.

! A.: Use six dry cells in series parallel, as per the accompanying diagram. Extension Speaker. At what distance is it possible to carry an additional speaker, asks ‘‘Interested" (Tolaga Bay). A, Used with an output filter, a speaker can be used up to 300ft. 2. Would this affect the volume? A.: Not appreciably. 3. Would an extra charge on the batteries be. required ?-No. Battery Potential on Crystal. ' Js At. practicable to add battery potential to a perikon type of crystal detector ?- "Dector" (Wellington). : No. Battery potentials are usually to assist the passage of signals through a carborundum detector. Other types of crystals are too sensitive. ‘R.F. Booster. HAVE built the R.F. booster, writes "A.G.F." (Dunedin), but cannot apply it to my kit set without the danger of blowing the valves.

_ A.: Construct the booster with parallel feed, a diagram of which is appended. This can be quite safely applied to-any receiver not already employing R.F. stages. Where R.F. stages are employed. it will be advisable to increase the audio. This should materially add to the dis-

tance getting ability. of the set, which, however, appears to be not normal. Trouble with R.F. Booster. "(AONSTANTINOPLE" (Matamata) has made the R.F. booster with parallel feed, but cannot get good results. A.: Insert a resistance between 500 and 1000 ohms in the grid lead of the booster. This will neutralise it and enable the filament to be tuned up higher. Primary Cells as Chargers. OULD four or six Le¢lanche cells connected in parallel charge a. 2volt cell if it were connected for 18 hours every second day? We, in the country, are faced with the grave difficulty of getting our batteries to the.charging station. I. have seen quite afew cars disfigured by battery acid. , A:*The Leclanche cell will run for about 20 minutes only without- polarising, so that it will be unsuit..ble for charging an accumulator., If a battery were tightly corked and stood upon a piece of board, or better still, in a small box which were -padded with. some: absorbent material, the difficulty could, to a very large extent, be overcome. ‘Electrolytic Rectifier. I LISTENED to good advice, states C. R. McK. (Tauranga) and purchased an eliminator with an_ electrolytie rectifier. The liquid in these cells has thickened to a jelly; and the voltage has dropped .so low that I cannot receive the Australian stations. Could I recharge with ammonium phosphate, or could I adapt the.wiring so as to use the Ratheon vaive? What should be the strength of ‘this solution? a A: In this case, your good advice was bad advice, A cheap eliminator with electrolytic rectifier is less economical than "B" batteries. Your case is quite typical. All you can do is to empty out the solution and fill the pots with fresh saturated solution of, ammonium phosphate, and if the electrodes are worn replace them. -Keep the aluminium and the lead as pure as possible and float a thin layer of light oil on the surface of the liquid. It would not be possible to adapt the eliminator to use a Ratheon valve, though it might be possible to use the transformer in a half-wave -cireuit

such as that deseribed by Pentode a little while ago. This would not deliver a great amount of current, ; Valve Combinations. "VL." (Canterbury), in his search, for quality, has replaced the A‘M®ican valves in his American receiver With English power valves. He finds now he cannot neutralise the set, and voice is very much distorted. . ; A.: It is quite unnecéssary to ‘use big valves.in any but the last stage.- Their introduction into other stages causes. the "B" battery to run down. rapidly, or "(if an eliminator is used) the voltage to drop. In addition, the windings of the transformers are saturated with current, ‘and ' the distortion occurs. The grid swing to be accommodated by anything but the last stage valve is very small, and any general purposes valve will suffice. There are other considerations, such.’ as matching and amplification that give advantage for special -valves for special positions. For this reason the corres--pondent should havé used three PM5’s where he has used PM6’s if he wishes to use this make of valve. However, in an American set sve would strongly advise’ the use of a 201A valve or its equivalent. Certain makers are now turning out valves with exactly the same characteristics as these, and these only should be used to replace the American valve. ~ Strong Humming. I HAVE completed the small eliminator described in the "Record" some time ago, but find a very strong hum present. How can I reduce this? asks "C.W.M." (Auckland). A.: Insert another audio frequency choke, and by-pass condensor in the lead direct. from the rectifying valve. Strong Audio Howl. ’ ON testing my, set, four valves with pentode in the last stage, a strong

_ andio howl was present, which could ywot_be overcome by changing ,the connecfons. All three trartsformers wert shiélded, and each is of a different make, writes "Home-Built" (Kaitaia). _ A.: The circuit diagram appears 'to. be correct, though the use of a pentode in the last stage of a 4-valve set is not to be recommended. The pentode gives very great amplification of weak signals, but it can be easily overloaded, and this apnears to be the case. ‘The transformers should be arranged with. their axises at right angles to one another, .These two factors appear to be the cause of the trouble. The Browning Drake. TE a four-valve Browning Drake what advantage has a .00025 over a .0005 mfd. eondenser for tuning the regenaformer? asks "W.J.S." | (Dunedin). A.: See "Cathode’s" article in the "Radio Record," dated August 23, under a Pane "The Technician isplattia. I had to put a .05 mfd. condenser. . . 2. Is it necessary to use an output choke with a push-pull amplifier? . At’ Decidedly not. The push-pull transformers are so built that no "B" current passes through the speaker wind.ing.. They are themselves output transformers or output chokes. . 8. How many turns are on secondary coils 24 inches in diameter and tuned with a .0005 condenser? . _ A.:100 turns of 2.2 gauge D.C.C. wire wound without spacing. ~

Push-Puil Amplifier. WiISHING to work a dynamic cone speaker and gramophone "pick-up from my two-valve set, I would like the details concerning. the push-pull amplifier, asks "Watmel’ ’(New Plymouth). 1. Would a two-valve push-pull amplifier give the best results? * Av: A first audio followed by two valves in push-pull would be O.K. 2, Approximate cost of same? A.: Good push-pull. transformers ,can be obtained for 30s. each, while a good audio transformer can ,be obtained for 17s. Three valve sockets each at 3s. 6d., three valves, two at £1 and one at 12s. 6d. , would be necessary. . Be Where ean I obtain diagram of euit . A.: A D.C. push-pull amplifier was described by "Pentode" in our issue of ‘November 2, 1928, and an A.C, pushpull amplifier will be described in our issue of September 20. 5 4, What type of dynamic cone speaker would you recommend? + ‘A:: One which will work directly from the A.C. mains or the 100-200 volt D.C. type, if a power amplifier using rectified A.O, current is used. The field. winding can then be used as one of the chokes. ' Choice of Speaker. " J.D." (Gisborne) ‘asks which is to * be. preferred: an exponential horn -opra linen diaphragm. . ; A.: Much depends on taste, but- the writer prefers the linen diaphragm; but it will have to be made carefully. License Problem. S there a limit tc the number of sets _& person may use with one license providing they are all used at the same house? . . A.: There is no limit, providing this is not 2 public house, and the set used for public or semi-public purposes. 2. If I sell my radio set, can the buyer . nave my license transferred to his name? Guew, oO. | Condenser Problems. | "YARCONI" (Timaru) asks:1, Would I have to alter the capacity of my two yariable condensers if I wished to use a short-wave coil to tune down to 10 metres? " , A.: The condenser tuning the grid coil will a _to be aitered go that the final capacity is no greater than . , and preferably .0001. :

a) v 2, How is the capacity of a tuning condenser altered when the vlates are double spaced, and is theytuning range inareased or decreased? : A.: Capacity is calculated from the formula: 0885 * N X8 ~ @ X 5,000,000 where © signifies capacity in microfarads, number of plates, S the area of one plate, and d the distance between them. Tt.can be seen that the greater the distance between ‘the plates the greater the eapacity, but the variations will be very small, ae ' 8. Are special coils necessary for & sereen prid short-wave set? + .: Yes, their specifications have been. givén on page 101 in one edition, and 110 in the other edition of the 1929-30 "Radio Listeners’ Guide." ~ Filament ‘Windings for 245 Valve. FAYING built the eliminator described in "Qistener’s Guide," "C.R." (Hokitika) wishes now to use the new 245. type valve requiring (filament) + amp, at 2.5 volts. These are being used in push-pull, which means that his wiring must supply half an amp.: He asks if he can take the filament’ current for a 227 from this same winding. A.: The 227 requires 1.75 amperes of current and with the half-amp supplied to the two 245’s would make a total of 2.25 amps.. To supply this a wire of not

less than 16 gauge should be used. This would present mechanical difficulties in the way of winding so that it would be advisable to make two windings, one of 22 sauge and the other of 18 gauge. , Will a B.H. Ratheon rated at 300 volts maximum plate handle the 350 delivered by the eliminator? Not without over-heating. The best plan is to employ two 281 half-wave rectifier as per the accompanying diagram. . A.C, Supply. " A ©." (Christchurch) asks if an Ameri"can receiver used with a step-down transformer could be successfully used on the 230 A.C: mains, A.: Yes, providing the number of cycles per second*is the same, but the chances are that it is not. 2. Why is a metal cabinet better than @ wooden one? A.: Where high voltages have to be handled, as is the case with most modern receivers, especially those employing power transformers, very high magnetic fields are set up, and unless these are shielded from the tuning coils of the set, interference is likely to interrupt recepon. \ 8. Is a magnetic speaker better than the ordinary cone speaker? . A.: They are identical, but some older cones used reed type unit. Airzone Midget Coils. (KoULD you publish. a circuit for 4 three-valve set using the Airzone Midget coils?-H. (Wellington). A.: Airzone Midget coils are ordinary Browning Drake kit. Use this eireuit minus the second audio stage.