Questions and Answers

Radio Record, Volume III, Issue 50, 27 June 1930, Page 26

Questions and Answers

"I S." (Hawera) asks the correct * method of neutralising an electric set. : A.: These sets are best left alone by amateurs. They have to be neutralised when the set is switched on, and the very high voltages generated are likely to cause bad shock and even death. 2. How ean I tell the filament pins of a 5-pin base? A.: A diagram of a base indicating the connections was given in our issue of September 20, 1929. 8. Why is it necessary to place a transformer between a gramophone pick-up and the detector valve in this set? A.: The gramophone output is very much weaker than the radio signals of

an average station stepped up by the R.F. amplifier. An input transformer increases the voltage from the pick-up and gives louder signals. 4, I am told not to remove the valve from the set with the current turned on. Is.this harmful? A.: Yes, the power pack is constructed so. that it will supply the exact amount of current at the correct voltage when all the valves are operating. If you take out one of them, a surge is caused and the other valves are overloaded. . "R A.S." (Wellington) is making an ° A. and B. eliminator and wishes to use 25 S.W.G. wire. He wants to know the number of turns. A.: The number of turns is the same as for 26 enamelled wire. D.C.C. will take up a greater space, and the window will probably have to be larger. You should leave this until you have finished the transformer. ‘S G.S." (Foxton) wants to known * the most suitable eliminator for a 5-valve set. He has tried one which caused the set to motor-boat. A.: The set motor-boated because the eliminator was not capable of supplying sufficient current. An eliminator supplying 150 volts and 40 to 50 milliamps. would be required for: the job. This would allow for the use of a power valve in the last stage. "H. C." (Wellington) asks for the e : of a short-wave ada tor using valve-base coils, . A.: Use the circuit described on page 103 of the 1929 "‘Radio Listeners’ Guide" and the coil specifications given on page 90 of the 1930 "Guide." . "H W.Y." (Herekino) states that al*though we have specified that grid and plate wires should not be parallel, we have made them so in the "Round the World" series. , A.: Although we examined these circuits fairly carefuly, we could not see a case in point. ; 2. If it is unavoidable that they run parallel, how far apart should they be? A.: At least two inches, but they Should be kept as short as possible. 8. What does "split primary’? mean, and how does this method of neutralisation apply to the Browning Drake? A.: A diagram on the next page shows the connections. It can be seen that the primary is divided into two equal parts, and the B plus tap taken to the centre. 4, Will 30 gauge SWG be all right for the choke for a short-wave set? A.: Yes, it will be more bulky than the smaller wire specified. Battery consumption is not affected by the gauge of wire. If it-is too fine, the amount of eurrent drawn by the yalve will burn out the wire. 5. To neutralise the Browning-Drake I shut off the rheostat controlling the

R.I. valve, and then cannot get a whisper even of 2YA, My neighbours cannot hear the set, when I turn the neutralising condenser. A.: A regenerative receiver is neutralised not by dimming the filament of the R.F. valve, but by tuning to a station toward the centre of the dial, advancing the reaction until the set bursts into oscillation, and then adjusting the neutralising condenser until the squeal either disappears ors becomes low in intensity. If it disappears, the reaction is further advanced and the procedure repeated. 6. The layout of the Browning Drake says "do not crowd the R.F. side." Exactly what does this mean? A.: It is really a precaution against placing the R.F. transformer too near to the detector transformer. So long as the parts are fairly close together with these two coils six inches or more apart the stage is not crowded. 7. Is it possible to use .00014 condenser and .00025 condenser for Round the the World Two?-Yes. "K W." (Napier) asks us the sizes * for the sound box of a gramophone cabinet. A.: Sorry, F.W., we can tell you what you want to know about electric gramophones, but not about mechanical ones. Try one of the gramophone shops. 2. Would you give me the dimensions of an exponential horn. A.: They were fully given about three weeks ago in the "Radio Record." "pP OP" (Nelson) wants to know if * there are any stations between 500 and 550 metres. A.: Yes. 7ZL, Hobart, on 516. "Ke de R." (Wellington) wants to * know the following concerning the Hammarlund Roberts Four in the ‘Radio. Listeners’ Guide": 1. The capacity of the aerial and secondary condensers-.000385. 2. Detector and R.F. rheostats-20 or 30 ohms. "R 8.0." (Wellington) has a six-valve * set with three stages of radio frequency, but he cannot cut out 2YA. He has tried several wave traps, and encloses the circuit of one which reduces the interference from 15 degrees to 8 degrees. A.: With three stages of R.F. you should be able to eliminate Wellington in favour of 4Zl without difficulty. It seems as though the design of your receiver is inherently wrong. If you tried one of the formodensers used in the Radecord wave-trap in series with the aerial as well as the wave-trap depicted, you might further limit the interference band. "L H.H." (Auckland) asks, should the * aerial be higher at any one end?

A.: It should be horizontal, if possible. It makes very little difference which end.is the higher, though it is generally recognised that the leg -in ‘end should be the lower. "Wy Ev." (Blenheim) asks jon teri ing an aluminium base. . 1, Which is the best way to clean this metal? A.: Clean with Bon Ami. 2. Can aluminium be painted, and what is the procedure? A.: It can be painted with any metal paint. . "L. G." (Geraldine) asks if he can get better reception by having his lead-in directly down to the set or from an angle? A.:. The lead-in should come down directly if possible, or equally as good is a lead-in the set end of which travels back toward to distant end. "S W." (Queenstown) has a set of * plug-in coils and cannot reach the minimum wave-length of one of them. A.: Take a few turns off the coil. W.L." (Dunedin) has made the * Rotorua Portable, and had great success with it. He wishes to. know if the Tongariro Portable is better. A.: It is a more powerful receiver, but requires considerable care in the making. It is difficult to say what you would get with it, but with the loop the original model could bring- in the three main New Zealand stations at speaker strength. ARKED" (Wanganui) asks’ the following questions concerning "A" eliminators :-- 1. I am using a 4-valve receiver the. filaments of do not take a great deal of current, but I find that the Australian stations and even some of the New Zealand are weak. A.: Why not test the voltage of the smoothed current, for it seems that either the choke will not pass }sufficient current or the charger wil not deliver enough for the valves, «This charger is rated at 6 volts 4 amp., so that unless there is a defect, it should supply enough current. Keep the rheostat as little in the circuit as possible. The A-+ on the charger is in reality A- when used as a battery eliminator. 2. Would it make any difference by having the leads of the "B" and "C" eliminator and the "A" eliminator from the same plug?-No. 8. How many valves would I be able to use with such an eliminator? A.: It depends on how much "A"; current they are going to take. Youy will Bet Just under 4 amp.

\ AXFLI" (Invercargill) asks what stations other than New Zealand d Australian can be expected with a powerful all-electric set? A.: Japanese, after about 9 p.m., and the Americans in the early evening between 5: and 8. , "NEUTRON" (Christchurch) asks the date of the crystal and valve to work from the A.C. mains.-November 16, 1928, . 2: Would you give me the address of "Keen" (Hastbourne) ? A.: We could not undertake to do this, but if "Keen" likes to write us and express his willingness we shall forward it to you. ; "K JT.’ (Hawke’s Bay) asks if the Loftin White can be made with a "BY eliminator and an "A" accumulator . A.t No, the Loftin White has been designed for the 245 or 250 valve and

The split-primary method of neutralisation referred to on previous page. tihe A.C. screen-grid. If you like to experiment, you might possibly evolve a circuit. L.M." (Hawera) has a well-known * kit set and wishes to add two more valves. A.: They would have to be radio frequency screen-grid valves, and this would involve a wide knowledge of radio construction. We do not advise ‘any amateur to undertake a task such as this. "W. B.B." (Auckland) asks the following questions concerning power-pack construction :- 1: Do more turns have to be put on the secondary than on the primary if high voltage is required? A.: Yes, the voltage is developed by the secondary if directly proportional to the number of turns. 2: I,require only 180 volts, but I wish ¥o provide for 500. Will I be using ny more current than if I wound for 18 only? A.: No. The remainder of the secondary, unless connected with a load or short-circuited, would not draw any eurrent. 8: When the power-pack is made I shall have to get new valves. Can you recommend better ones than A415, A409, and B403? A.: No, these valves will take the maximum 150. You could, of course, use a 245 in the last stage with a 250 volts on the plate, but this would not be warranted with a three-valve set. ‘Tnless you propose to use a power plifier there is no point in providing for 500 volts. . Why cannot the current from the | of the filament transformer be rectified and used on ordinary battery valves? A.: It can, but extra turns must be wound on to allow for the drop in the

rectifier, For the number of turns see ‘the details of the "A" battery charger described in the 1930 Radio Listeners’ Guide. You would, of course, have to provide a large capacity smoothing condenser and a choke. 5. Will an electrolytic rectifier work instead of a valve rectifier? A.: Yes, it is satisfactory, but messy. 6. Will there be need for a separate rectifier for each voltage taken from the transformer? A.: If you intend to take filament current there will be need for two rectifiers, one for the high voltage and one for the low, on the high voltage side only. After rectification the voltage can be broken down by means of resistances. 7 Is a commercial filter necessary or can a circuit of chokes and condensers be used? A.: A circuit of chokes and condensers is a filter. "J C." (Hastings) has made up the * amplifier described in "All About the All-electric, and is meeting with the following difficulties :- 1. A lack of volume, even when used with a pick-up. Signals distorted as though choked. A.: It seems as though some mistake has been made or the valves are unbalanced. It is essential in making an amplifier such as this that the emission of the valves be exactly the same. You should have them tested and matched. There may possibly be a defect in the grid circuit: a short circuit or a wire in the wrong place. The resistors may not be able to carry the current. 2. Music and volume is equal to an ordinary amplifier when using only one valve in push-pull. A.: This indicated that the valves are not balanced, or are wrongly biased. 8. I tried 100,000 ohms resistance in series with the grid leaks and this cleared up the music slightly. A.: This is really only a compromise for instability and not a cure. 4. Is the 409 output transformer suitable for both speakers mentioned? A.: Yes, but there must be a further matching transformer in the speakers themselves. ; 5. Is a 230-volt lamp in series with the field of a D.C. speaker necessary? A.: Most of this type of speaker is designed to work on 100 to 200 volts D.C., so if you are using D.C. mains of 230 volts a variable resistance of 1000 ohms will be needed. . 6. Can the new Loftin White amplifier be used with 230 volts D.C.? A.: No, but the principle can be applied. Experiment would be necessary.