Questions and answers

Radio Record, Volume IV, Issue 27, 16 January 1931, Page 14

Questions and answers

"A UDIO" (iiketahuna): Where can * TI obtain a circuit and lay-out diagram of a super het, shortwave set? A.: "Radio News," October, 1930. and in addition will probably be published in the 1931 "Guide." 2. Could the r.f. detector, and audio stages of a receiver, be built and the super het. shortwave adapter’ after ward added to it, having all the com ponents on one baseboard and panel:

A.: Yes, if room were allowed for it. 8. Would all the tuning condensers of the broadcast action be needed if a C.A. set was used to build a super het. set? A.: Probably, yes. . , 4, Can a differential condenser be used to control reaction in the shortwave set described in the "Guide" ?- Yes.. A H.M. (Timaru): I have two vari * able condensers and wish to know the capacity of each. A.: That eomposed of 14 plates would be of capacity .00025 to .0003. ‘mfd. The other would be of .00015 to .0002 mfd. 2. The condenser I need is either a 00035 or a .0005 mfd. Could I alter the ones I have to the required capac ity? A.: No, but the 14 plate one would be O.K. "TJ AMMARLUND" (Taihape): How many volts "C" battery does an Osram P625A require at 185 volts "B"? -10-12. . 2. Is the 66R Blue Spot loudspeaker unit an efficient one? A.: Yes, we have tested it and found ii excellent. 38. Are my 15/6 transformers 001! ones ?- Yes. 4, Is an output filter necessary when using the speaker 25 feet from the set? A.: It is safer; but not essential. 5. I would like to make my set !! little stronger on the weaker stations. A.: Use a stage of screen-grid radio frequency. An article concerning this will be published shortly "A LUTA" (Dunedin) I have. an American five-valve battery set using five'D.V. 5-valves. What valves do you recommend to replace rhe above? A. :201A’s or 221’s with a B605 in the last stage. This should be suitably biassed. As no provision is made for this in the set you are using, you will have to make the necessary altera tions. L B. (Christchurch). When wiring * a house for loudspeakers, using bell wire, should the wires be twisted or should they run parallel? A.: It is immaterial-parallel for economy. "(\HOKH" (Gisborne), What type of reaction control do you’ recommend with a four-valve broadcast set using one stage screen-grid radio fre quency? A.: The rotating tickler method is quite suitable, but if an efficient variable resistance is obtained, control is slightly better. It is necessary that the resistance be a good one, though. 2. What does the value of the variable resistance depend on? ~

A.: The operating plate volley of the detector, the plate eurrent, and the "B" voltage, 3. Where could I obtain a circuit for a two-valve rf. amplifier employing ordinary valves?

A.: We would advise you to wait for the articles on the 1931 B.D. to be published shortly. Although the r.f. stages are screen-grid you could use the lay-out, and employ. ordinary valves. 4. What is the best method of hand ling the volume? A power-valve in the last audio stage or two matched valves in parallel? A.: The power-valve. .F, (Dunedin): I have a five-valve commercial set, and I notice that the valves after twelve months’ use are becoming dull. Could my valve com bination be’ improved upon? A.: You mention an A605 in the see ond audio stage. Should this not be B605? If this is the valve you mean it should be suitably biassed. Othet wise the valves are quite suitable. "VOLUME" (Gisborne): Lately 1 have noticed a decrease in volume from the YA stations, but I find thui distant stations are easily tuned in. In fact, the other evening I received WGY far louder than Wellington. A.: We are afraid we cannot. help you. You seem to have tested everything likely to be at fault. It seems eurious that you should receive distant stations so well, and the YA’s with reduced volume. D T. (Oamaru): Will a Leclanche * sal-ammoniac battery charge an accumulator? A.: No, use Daniell cells. 2.: Will I get better results with my B.D. set using 135 volts than with 90?

i A.: Yes, but do not add a new 45volt "B" battery to two that have alreadv been in use for some time. Ts last week’s issue a correspondent sent us an inquiry concerning jallelectric d.c. sets. At the time we knew of no agents for these, but since then two correspondents have written in. One has an Attwater-Kent D.C. 7valve combination, which is. quite satisfactory, and the other a "Silvertone" receiver, the agent for which is 8S. B. Moe, Civic Buildings, Auckland. "QCREEN-GRID" (Ngaio) : Is the en--closed circuit . correct for two stages of radio-frequency using the original Loftin-White amplifier?-Yes. 2.: Is it necessary to have separate condensers with couplers or will a triple variable condenser do? A.: The latter will do if the spindle is wrallel to the front panel. "PUZZLED" (Auckland): I have an ‘ all-electric receiver. Distance reception has been splendid and_ the

q » Valves and voltages test out O.K. Why ( / \ 1 is it posisble to take a spark off the aerial and earth terminals‘or any other part of the set with the earth wire which is attached to the usual water pipe? This is when the power plug is disconnected from the mains. A;: Owing to condenser discharge you mav get such a spark, but only once, Your best plan would: be to mention this phenomenon to the agent. C,@. (Feilding) : Why is the plug in eoil of the differential one to be wound with 30 s.w.g. wire and the fixed with 24? A.: On this valve base there have to be more turns. Consequently the wire should be finer, ./2.: Would 4.¢.c. wire be as efficient as d.s.c.?-Yes. : /3.: Which would be the better tuning condenser, 2.00085 or .0005? A.: Possibly the .00035, though the other would be quite O.K. 4.: What diameter is the plug-in coil? A.: The same as that of the valve base. Jy F. (Westport).-I am about a mile . from a power station, and very: close to high-tension lines. Interfer- . enee is very bad, but immediately the power cuts off reception is clear. A.: You should contact the engineer. in charge of the powerhouse. The noise could be stopped by shunting two mfds. condensers across the brushers and to earth. We had more.to say on this topic in our issue of December 13, 1929. 2. I have a counterpoise, and have connected it with the ground terminal, | which does not appear to make any difference. . A.: Have you removed the earth, for the counterpoise alone must be on the ground terminal. We cannot understand how, if correctly adjusted, there is’ no effect whatever. EB J. (Timaru).-How many turns of * 26-gauge wire must I use on valve base coils to cover the short-wave bands with .00125 condenser? A.: 26-gauge wire is not practicable ‘as the coil. loses efficiency through its incorrect shape:factor. You should have used 30-gauge. ; J AKA (Wellington).-Which would be the more efficient set, the Differential Two or "R, the W. Two"? A.: Probably the Differential Two. .gPERPLEXED (Whangarei).-I wish "MW ‘to break into amateur transmitting. ‘so-can I make or have made an a.¢, generator to supply the current? A.: Yes, we should advise you to write to Johns, Ltd., Auckland, who would be able to put you on to the track of a suitable generator. It would not be a practicable proposition to make one yourself. ' AUSSIE (Wanganui).-How can I erect a counterpoise aerial under an umbrella aerial? A.: Run the wires out immediately under the true aerial. A better aerial to combat interference would ‘be an ordinary Lor T one run at right angles . to the source of the noise. You could then run the counterpoise underneath this in the orthodox fashion. 2, How much wire wouid be re- . quired for the counterpoise earth? Af¢ Ags mutch as for the umbrella aeriaL .

W. ©. (Raurue).-Thanks for sending along: the coil particulars, but we regret we cannot give them any publicity. Number 2 is probably the easier to manipulate, but for short-wave" work a condenser control regeneration would. be the better. W. H.S. (Wellington).-Would a half wave metal rectifier be . suitable" for an A eliminator, and could: the output voltage be regulated by having a tapped input or a resistance in the output? A.: The rectifier to which you refer is probably a full wave one, and will be quite suitable for your purpose. A resistance in the output would probably be the best method for breaking down the voltage. 2, Would the A eliminator described in the 1930 "Guide" be more satisfactory? A.: There is little difference, 3. Will 16 volts be too much for an electrolytic condenser maximum voltage 12? A.: Probably yes, although if these condensers break down they are selfhealing. 4, Which would be the better plan in winding a transformer, to put half: the secondary and half the primary on each leg, or put the primary..on one and: secondary on the other? © =~ A.: The former is better, though .the best plan is to wind the transformers, ‘such as described in the "Guide,". that is. all the windings on one leg..~ 5. Which is the best method. of output B voltages-tapped potentiometer or separate resistances ?-The ‘latter. 6. Which is the most satisfactory of obtaining grid -bias battery, voltage drop, or separate eliminator? A.: Theoretically the separate eliminator, but the voltage drop is widely used and perfectly satisfactory. 7. Will the eliminator work with all the windings on one transformer? A.: It depends upon the wattage. Find out the total number of watts consumed by the secondary and then allowing about 90 per cent, efficiency, ealeulate the number of watts to be taken by the primary. Look up the table in the "Guide" and see if the transformer will carry the load. D S. (Market Cross): Your idea for * a motor and a generator is quite a good one. ‘The 300 volts and 120 mamps provided will be quite satisfactory for the amplifier we: mention, with the exception that P625 valves should be used in the last stage. We will send you along a diagram of the amplifier within the next two or three days. EK. W.P. (Invereargill: Are valves drawing more filament current when the rheostats are down or up? A.: The more resistance there is in the circuit the less current the valves are drawing. 2. Why is there. more noise in the speaker when using a B eliminator than when using batteries? A.: Noise should not be noticeable in a properly designed eliminator, though they are rarely quite as silent as. batteries. 8. Is a 409 a better valve for the first audio than 415?-Yes. 4, I put a sereen grid valve in the first stage, but results were not as good.as using a .201A. A.: Did you.design the circuit especially for the ‘screen-grid valve? If you just: put it in, an ordinary socket you ‘would not get the pick-up you should do,

5. What is the average life of a power valve? A.: 1000-1500 hours. . 6. Do "B" patteries make circuit when the filament current is switched off?-No.