Questions and Answers

Radio Record, Volume V, Issue 42, 29 April 1932, Page 10

Questions and Answers

GlPSY MOTH (Mount Eden): Would I notice & great increase in yolume if I were to increase the plate voltage from 90 to 135? A.: We think, yes. 2. With 224 volt. det. and a 5 meg. grid leak there is a gentle step into oscillation on the two shortwave coils and on the others (the broadcast coils) under the same conditions, the oscillation is rather rough. A.: Your best plan would be to incorporate potentiometer grid return. You could then alter the setting to include reaction on all coils. The valve of the grid condenser for broadcast work should be about .0003. 8. I have lead two wires of 22 d.ce. for a distance of about 40 feet from my. speaker. Is this imposing a strain on the set? A.: No, but it would be advisable to use. an output transformer, to restrict the d.c. current to the set and not allow it to wander through the wires. A giagram shows how to connect such a er. [Toney (New Plymouth): What size condenser do you wish to tune the Secondary with? We cannot give you coil details without this. N H.P. (Nelson): Assuming that the same ¢ signal voltage were fed into both systems, and that the valves were working at their fullest capacity what would give the greatest undistorted volume-two pentodes in pushpull or two 250’s in pushpull? A.: The undistorted output of two 247’s in pushpull would be approximately 7.5 watts. Dhat of two 250's approximately 14 watts. C M. (Lower Hutt): I cannot get a pick- * up to function by connecting it to the grid cireuit of my detector valve. How is rectification accomplished in my eightvalve set? A.: If the customary condenser and gridleak hre missing, power detection is employed, and the best way to insert a pickup is to break the grid return between the bottom of the coil and grid bias and put your pickup there. The condenser and resistance are used for neutralisation reasons. The specifications for a filament transformer to supply six 227’s, one 415, and one 608 are core l}in x lifin. Primary 1140 turns of 24 gauge enamelled wire. Secondary, three separate windings of No. 16 gauge d.c.c. Hach winding should have 15 turns. These can be used in parallel to supply the six 227’s. The 415 is a dc. valve and quite unsuitable for a.c. operations. Tor ‘the 603 you will require one winding of 18 gauge dec. wire with 35 turns. For further particulars see the "Radio Guide." W H.W. (Dunedin) : I have made a shorte wave adapter, and am using same from a "B" and "C" eliminator. There is a bad hum. A.: This is due to the sensitive shortwave detector picking up hum through the plate supply from the eliminator. Your best plan is to incorporate another choke and condense in the "B"’ lead. A commercial one of the B3 type would do, and use a 2mfds. condenser between the set end of the choke and earth. B= (Rangiahua): I have difficulty in separating the stations on my fiveyalve set. A.: Your best plan would be to construct a band-pass filter, an extra tuning unit such as was described in the 1982 "Radio al The estimated cost would be about J B.G. (Lower Hutt): What is.the ratio ¢ of the two audio transformers in the shorbwave receiver described in the 1930 "Radio Guide" ?-34-1, 2. Would Osram four-volt valves be suitable in the first and second stages ?-Yes, R C, (Nelson): It would not be practic- ¢ able to add a radio stage to a shortwave receiver, such as "Round the World Two." If you want to use a three-valve set, with a radio stage, construct the "Kestrel Three," the sereen-grid r.f. amplifier. his was described in the ‘Radio Record" about five months ago. You could use your own coils in this set,

rpexour (Sumner): Is it possible to con-. . struct the coils used in the i.t. stages of the "Cathode Super"? A.: Yes, providing they are of the bandpass type, any intermediate coils resonating at 175 kilocycles will be satisfactory. We have made arrangements for "Cathode" to describe, some time in the future, suitable coils for this set. Bank wound coils would not be much inferior to the honeycomb ones described. However, your best plan would ibe to look out for the article. It will appear. probably in the "Radio Times." UPDPR SIX (Reefton): I have constructed the "Super Six," but there is far too much noise. All the components have been completely tested. The stations are tuned in with a shrill whistle which does not leave on some of the stations even when properly tuned in. A.: Such a condition as whistling on some stations is often caused by the fact that the receiver is sufficiently. sensitive to respond to two signals on proximately the same frequency transmitted by two widely-separated’ Stations.

There is no real remedy as far as the receiver is concerned. Other causes may b an open ground at the shield and contact, poor contact in the mixer and ic?" and demodulator circuits, incorrect location of the control grid lead in the first detector and if, stages, and parasatie oscillation generated by the oscillator valve. If the noise you complain of is a constant crackling, try removing the loop. If it continues, then there is either a loose connection or a defective component in your set. But you must bear in mind that the "Super Six" is really a sensitive set, and if there is any noise due to power leakages, ete. there is a very good chance of picking up. D U. (Paeroa): Iam on 2 hill surrounded eby trees. How can I best erect my aerial? A.: Without a sketch of your locality it would seem to us that the best plan would be to run the aerial from one of the higher trees to a mast on your house. The lead-in could be taken from a point several feet away from the house end, However, end the aerial at this point, and. merely support it to the mast by ordinary wire well insulated from. the aerial wire. TT RURO (Auckland) : Is it possible to add short-wave to my ossor Melody Maker? A.: Yes, special short-wave cozls are provided for this receiver. 2, Could I improve my earth, the wire of which is 15 feet long, and is tapped to a water-pipe which runs under the house for a further 26 feet before it enters the ground, which is very rocky? A.: Is it not possible to take the wire direct from your set to a galvanised pipe or series of galvanised pipes driven into the ground and kept moist with salt water? IH (Manunui): What would be the correct output transformer for two dynamie speakers, wired in parallel, to be matched to two 250 valves in pushpull? A.: It would be a better plan to wire two speakers in series. However, for parallel operation the ratio should be approxi-~ mately 27-1. If the two speakers ave wired in series the ratio sohuld be 20-1. 2. A Blue Spot speaker is wired in parallel with a dynamic. This also alters the impedance. Would it be better if it were taken off the stage earlier? A.: Yes; but surely if you are using two 250’s in pushpull it is unnecessary at a (Continued on page 22.)

« Questions and Answers (Continued from page 10.) pre stage to use two 245’s in pushpull. At the most you should use the J71A class of valve in this pre-power stage. Generally speaking, the medium small power valves are quite satisfactory for this position, Your best plan would be to use an output transformer with a large number of variable taps,’so that you may use the tap which best suits the combination. W. FIEND (Auckland): Please explain * modulation, and unmodulated aerial power. "A.: For a full description of modulation, wee the 1932 "Radio Guide." Modulation ja that process by which the microphone reurrent, which is sound waves turned into electric waves, is imposed on the high frequency current, generated by the transmitting station. This latter is the carrier wave, while the former are the carried waves. -Unmodulated aerial power is the rating of the transmitter irrespective of modulation; that is, the output is the rating of the carrier wave of the station-that generated by the oscillators and associated apparatus. 2. Would a square or circular aerial be worth experimenting with? A.: No; the irectional effect of the average aerial is so small that it .is noi worth worrying about. PVA. (Wellington): What length of brass tuibing is required for the Hartley transmitter? A.: Approximately 5} feet. RONA. (Napier): I recently coupled a / peutode directly to my inductor dynamie speaker, and volume and tone were pleasing, but I read in the ‘Times’ that it is not good for the speaker. I put an output transformer in the output lead, but had to de-couple the auxiliary grid and plate of the pentode to stop oscillation, The yolame then dropped by half. What should rs . A.: Apparently the impedance of the pentode and that of your inducipr dynamic speaker--you do not say which one it ismore or less match. The best plan would be to incorporate an audio choke and 4 4mfd. blocking condenser in the output choke circuit -in contrast to the output transformer circuit. The extra lead cannot be utilised exeept for valves in pushpull. See the "Radio Times" (May) for a fuller explanation. HARGER (West Coast): I wish to charge my wet "Ts" batteries in blocks of 45 yolts from a 60 amperes dynamo. How could I break down the volts and amperes to suit my charging rate? A.: You will require a rheostat of 1000 ohms that will pass 100 milliamps. Adjust this until you get,the volts and amperes correct. Arrange your batteries in parallel blocks. 2. I eregted a Beverage aerial, but find that it goes better without the earthed ‘resistance in the end. A.: This seems as though there is a leakage to carth in some part of the system, amd the aerial is acting then only as an ordinary one. As far as your cable is coneerned, "B-" is usually black, otberwise all right. . .