Questions and Answers

Radio Record, Volume II, Issue 34, 8 March 1929, Page 10

Questions and Answers

Power Amplifier Circuit. 'A REQUEST has come in from "A, C. . Amplifier" (Palmerston ‘North), for details of a power amplifier suitable for ta gramophone. To build a real power amplifier would require a great deal of constructive skill, particularly as a transformer which supplies a voltage up to 14,500 is to be used. If, by careless construction, any of the metal part came into contact with any wire carrying this voltage, the operator would probably receive a rather nasty shock, and it is unjikely in any case that the Power Board would allow the construction of such a transformer. However, if the correspondent has an electrician friend who could do the construction work for him, we eould send him a diagram. Probably the best amplifier for his purpose would be the push-pull such as described by Pentode .some months back, and a oa eliminator built to supply the current. In our "Radio Listeners’ Guide’ now jn preparation, is a very comprehensive chapter on transformers, and the correspondent would have no difficulty in constructing one from the specifications. In addition, there are at the present time some very fine power packs on the market, which will enable an eliminator to be puilt with the minimum of effort. In a ‘very short time we shall be reviewing one of these through our columns. Set Without an Earth. CouLD you tell me the reason why my four-valve set (battery operated) fs able to receive far better and with more volume with the earth wire totally discon-

nected from the set? I am able to pick ‘up stations as far away as Japan without an earth, on good loudspeaker strength, and the same station is hardly audible when the earth wire is connected to its proper terminal. For an earth I have three connections all coupled to one wire-one pipe driven in four feet, a sheet of copper buried five feet, and a well pipe, and still they are of no advan-tage-K.D.M. (Ashburton). ANSWER: Undoubtedly, there is something that is giving the effect of an earth or rather giving capacity effect. The aerial and the earth form a huge condenser through the set, and likewise a condenser effect can be produced through any portion of apparatus suitably connected and the ground. If, for example, the accumulator was standing in such a position as to be in direct influence of the ground, a condenser effect would be secured, and the set would operate as though an earth connection were being made. It appears that the real earth is for some reason or other inefficient. Probably, there is some defective connection, so although liberal precautions have been takn to obtain a good earth, it may be inferior to that established through the condenser effect. If this set were slightly deneutralised, providing the earth connection were O.K., it would be found to work with an earth as well as it works now without an earth.

Condenser Reading Inaccurately. E.K., (Levin) writes, stating that * the condenser reading of any one station varies considerably on his set. Sometimes the difference is as little at 2 or 8 degrees, sometimes as much as 20, and then will read "normal" again, so that the correspondent has come to the conclusion: that his condenser dial cannot be slipping. ANSWER: In spite of the conclusion reached by the correspondent, one is inclined to think that either the dial is slipping, or the plates are slipping, with relation to the shaft. If the moving tickler type of reaction is used, the reading will be altered slightly in accordance as the amount of reaction varies. To prevent this, the condenser control reaction described in a recent issue, should be of use, Condensers for an Eliminator. A M.K. (Wellington), writes, stating * that he has purchased a block of 8 2MF condensers for his battery eliminator, which has an input of 110 volts AC, and an input of 140 volts. He has taken the bank apart, and discovers that they are rated to stand only 160 volts D.C. He asks if there is no way in which these may be coupled to give a higher voltage test. ANSWER: It is imperative that the condensers used in an eliminator be of the highest quality, and be able to withstand a far greater voltage than that actually to be worked. Usually the rating of a condenser is about half of the tested voltage, and the rated voltage should be about double of the actual voltage to be worked, In this case, the condenser should be capable of working under 400 volts working conditions: This means that they would actually take about 800 volts. Cheap condensers are usually those with a low voltage test, and before long they will break down, necessitating inconvenience, causing trouble, and putting the owner to a great deal of expense. Set De-neutralised. O* my set (Browning-Drake 5), I can get the whistle or carrier of a station with good volume, yet the music or speech can only just be heard. Can this be rectified? Also, if I touch any part of the cabinet with a wét finger, there is a distinet click in the ’phones . What causes this? Should this be so?-F.J.M. (Pahiatua). ANSWER: It is quite evident that the set has become deneutralised. Probably new valves have been added. and these are the most potent cause of a set becoming deneutralised. he distinct click in the phones. when the finger is applied to the cabinet or the aerial is a sure indication of this. Methods of neutralisation have been frequently described in past issues of the "Radio Record." If the correspondent has any difficulty in reneutralising his set, explicit directions could be given him. Push-Pull Amplifier. W W.M. (Lower Hutt), inquires as to * whether transformers other than Ferranti can be used in the push-pull amplifier described. by "Pentode" some some ago. ANSWER: Yes providing they are of the push-pull type. There are several rood makes on the market at the present time, but some are rather difficult to obtain. The construct«.r should inquire from his dealer what types of push-pull transformers he happens to stock. The correspond---+ asks whether certain transformers that he names would do to replace OP8C specified. by ‘"Pentode." ANSWER: If they are of the push-pull type, yes. The AFS could be replaced

by an ordinary transformer with a reasonably small ratio, 2. Would this amplifier be satisfactory to use with the Screen-Grid Browning Drake as described in the "Record," February 8, page 27. ANSWER: It would be more than suitable; it would be desirable to incorporate the push-pull principle with the Browning-Drake. It would increase tone and quality, and in all should make a very fine and efficient receiver. Induction From Telephone. I J.M. (Thorndon) writes: My indoor * aerial runs along a passage parallel but two feet away from the telephone wires. When I am using the short-wave coil on certain parts of the dial, I can plainly hear the telephone conversation, although there is no connection in any way between wireless and telephone. Does this in any way affect the telephone? . ANSWER: It appears that inductance~ is taking nlace between the telephone lites and the aerial of the short-wave set. Although there is no connection between the two, 2 magnetic field exists, and. when the short-wave set is tuned into resonance with the telephone, the conversation can be transferred from one to the other. No harm is being done to the telephone. Various Points. F. {Bakaia) has a few questions to * ask: 1. Should the length of the Beverage aerial be 600 yards or 600 feet? ANSWHR: The length, to a few yards, does not matter, It should be somewhere between .400 and 600 yards. ; 2. We have a license to operate a set in the house. Would it be necessary to procure another to work a set in a hut a few hundred yards away? ANSWER: No, providing that. the owner is the same in both cases, and that both sets are to be used by him or by his family. 8 Could you give me particulars to build a two-valve screen-grid radio frequency amplifier? ANSWHER: Within a month or two we shall be describing a two-stage screen grid R.F. receiver, using standard coils, but before this is described it will be carefully made up and tested. We have the diagrams, and if the correspondent would like to work on this before it is opportune for us to do so, he may obtain a diaam by writing the "Technical Bditor." 4, Where could I get the necessary patticulars for a transmitter’s license? ANSWER: Apply to the District Telegraph Engineer, Christehurch. Concerning the "B" Eliminator. "LIMINATOR® (Dunedin) wants to 4 know :- . . . 1. Is the series condenser in the diagram of the combined condenser and tickler control reaction type in our issue of February 8, 1929, intended to be a .00026mfd. Also is the fixed: condenser in series with the earth lead from the tickler a .001? ANSWER: A large capacity condenser is necessary, so that the .00025 would be needed. If the set will not readily oscillate with this, put a few more turns on the tickler until it is found to work smoothly. The .001 mfd. is correct. It should in no circumstances be less. 2, Some time back I constructed "Megohm’s" eliminator, using a BH Raytheon tube. The general layout I altered somewhat to make a more compact unit-using all metal construction. My efforts were crowned with success, the instrument having functioned admirably for some months now. I may mention that my receiver is Megohm’s 5-valver with audio incorporated, feeding through an output transformer to a moving-coil speaker of my own construction. This also is successful. I.now desire to increase the voltage delivered to the power valve, say

by about 20 volts, and with this in view I propose rewinding the secondary on the transformer, thus increasing the turns ratio of primary to secondary The laminated core is as specified, i.e., ordinary tinned iron sheet, and under present conditions shows no sign of heating nor does the rectifier. Should I meet with any electrical obstacle in‘ increasing the size of the secondary this way? ANSWER: By rewinding the secondary, the voltage delivered could. be stepped up to a greater voltage than that specified in the original artiele. It will be necessary to rewind as far as the centre tap, ‘then make a neatly soldered connection. Insulate this ‘thoroughly, and wind on 260 turns. Make the centre tap and wind on the same number of turns as are now on the first half of the secondary, that is, the original number plus Double Grid Valves. CAN you please tell me where in New Zealand I can obtain double grid valves, make and price? Thanking you anticipation.-C.C.M. ANSWER: The only suitable make of double grid valves that are obtainable in New Zealand are the Philips 441. Any dealer in Philips products will stock these or be able to obtain them, Coils for a Three-valver, (ouULD you tell me how many turns to put on a 3-inch former for a 2valve set, and how many coils I would need, Where to join them.-DB.R.D. (Christchurch). ANSWER: The aerial coil consists of 56 turns of 22 standard wire guage, enamelled, tapped at the 18th. Instead of one coil, two may be used on the transformer system, that is, 56 turns, comprising the secondary, with 18 turns on the primary. The secondary is connected to the grid of the detector. The "Pentode" Crystal Valve. HAVE not received any other station except 2YA on the "Pentode Crystal and Valve." No batteries were stated in the construction. I am using a B block and 3 dry cells (a 4-volt valve); would grid bias be necessary? If so, could you please inform me as to how I should connect up.-K.A.D. (Kelburn). ANSWER: The ‘correspondent does not state the voltage on his "B" block. This may be quite inadequate for the valve used, and unless these details are given, we can get no further. As for grid bits, very much has been said of this in the Crystal Corner recently. Megohm’s A Battery Charger. [8 writing asking some particulars regarding the’ 367 rectifying valve "Appro" (Otago) ventures some information regarding the A battery charger described by "Megohm" about twelve months ago. He states that he has had to alter the leads from the transformer, finding that by reversing the connections of the first half of the secondary, the output as shown on the ammeter was double. From a sketch accompanying the letter, there is no doubt that the second half of the secondary has been wound on the wrong way, that is, in the opposite dfrection to the primary, so that unless the eonnections were reversed, no current was being delivered from this half of the transformer. When this was put right, and the connections reversed, or as it is, corrected, the ammeter showed its full reading and the battery charger worked NFORMATION regarding the 367 valve has been rather difficult to obtain, but the following are a few of the suggestions offered by the makers: The filament of this valve should be burned on 1.75 volts, and if the correspondent has only nine turns on the filament winding, as he states, it would appear that insufficient voltage is being delivered to the filament. Rectifying valves become very hot, but it is impossible to judge the correct temperature by merely placing a hand near it as does the correspondent. An A.C. meter "is really essential,

To deliver 1.75 volts at least 20 turns of filament winding should be put on, but it would be rather risky to work the filament from this without having tested it. Another question asked by our correspondent is whether certain Continental valves are applicable to his American set. . _ The most simple way of this is to compare the characteristics of the two valves in question, and matching them as nearly as possible, particularly as regards the impedence. ‘The valves mentioned would in this ease be unsuitable. He also asks regarding the accumulator, "Is it quite correct to replace the old electrolyte in an accumulator with fresh acid of 1240 to 1250 specific gravity. Should the battery be fully charged or discharged when the change is made, and does the specific gravity need to be higher for a fully-charged accumulator?" ANSWER: If the electrolyte is an accumulator is to be changed, charge it to the full capacity, and then empty off half the old electrolyte. It would be advisable to rinse out the accumulator with distilled or fresh rain-water. Now, add the fresh solution at the specific gravity mentioned. The accumulator is now fully charge and with full-strength solution. As the battery discharges the specific gravity of the solution becomes less, so that unless the battery is fully charged before the electrolyte is changed, it would be impossible to approximate the proportion of charge that the specific gravity of the electrolyte to be added. Electrolytic Rectifiers. ONCERNING the electrolytic rectifiers, he says: "I notice some correspondents in the "Radio Record" are having bother with electrolytic rectifiers for "A" chargers. No matter how carefully these are made, they are not reliable for any length of time, I find ammonium phosphate the best solution to use; but even with it the charging rate varies as the temperature of the solution rises or falls, and then when the electrodes have been in use for some time, alternating current may start flowing through the battery. May I suggest that anyone wanting to build a charger will be well pleased with their efforts if they construct a reliable, steady valve charger similar to the one I have made.’’ (One is being described for our 1929-30 "Tuisteners’ Guide.’’) Distortion From "All Electric." NOTICH in March ist. "Record" "Questions and Answers" page, that "New Chum" of Foxton. complains of distorted reception and fading. I would like to state that during a stay in Dannevirke, I experienced the same trouble, but was using battery sets. I tested out all YA stations on four sets. and also four speakers, also three aerials and several earths. All sets used different valves and batteries, and I am practically certain that the cause of the trouble was neither in the set nor the transmitter. 2YA was the worst offender, but the other YA’s also were troubled with this distorted fade. -"CRYSTAL" (Wellington. ) (We are rather afraid that in New Chum’s case, neither the locality nor the station is at fault, as "Crystal’s" letter would seem to indicate--Tech-nical Wd.).