Questions and answers

Radio Record, Volume III, Issue 39, 11 April 1930, Page 34

Questions and answers

A Continuous Howl. WHEN working Auckland I get a continuous howl up to about 8.15 p.m. states "Mo. Whare" (Feilding). . Would this be HKJ heterodying?-Probably. 2. I am re-erecting my aerial and can get one pole 2ft. higher than the other. Which end should be the highest or should they both be level? A.: Having one pole only 2ft. higher than the other would make very little difference, if any, to reception. It is the average height that counts. It is immaterial which end is higher. Inadequate Grid Bias. "TYISTORTION" (Frankton Junction) has connected a Pentode in the last stage of his Cossor Melody Maker and wonders why he is getting distortion. He has 125 volts on the plate. , A.: The pentode is inadequately biased. Connect another 9-volt C battery in series with the one in the set, and you will find that reception is greatly improved. Try varying the tappings to obtain the best results. . Valve Combination. W HAT is the best valve combination for "Round-the-World" Two. I am using dry cells for the A battery, and have to keep filament consumption as low_as possible, states "Dynamic" (Lower Hutt). A,: There are several on a par. The Mullard PM4DX and PM4, and the Osram 410HF and 410LF are as good as any. Amplifier Hum. "PSSREE" (Hawke’s Bay) is using a power amplifier from which he is obtaining excessive hum, although he has employed a D.C. valve in first stage. A.: Apparently the hum is originating in the B supply, and consequently efforts

directed towards removing it from the filament are of no avail. The power pack may not be able to supply the power. It is possible that a condenser has broken down in the B supply. Use a bigger power valve in the last stage, as 609 is incapable of giving efficient service as a power valve.’ It is a general purpose valve, Reception. Erratic. UNTIL recently reception has _ been good, but now it will stop, and the local statign can be heard faintly in the distance. . A.: It sounds very much like an open circuit, and this can be tracked down only by very careful examination and testing by phones and cell method. This has received a considerable amount of attention in previous issues, and in the "Radio Listeners’ Guide." Have your ower packs examined, because a conenser may have broken down therein. 2. I find that better results are obtained by interchanging the detector A415 and the R.F. valve A409, though the former is recommended for the detector socket. A.: That quite often happens. It depends entirely upon your circuit characteristics. General purpose R.F. or audio valves can usually be interchanged, but the power valve should never be tried in any other. socket. The Browning-Drake. A CORRESPONDENT from Christchureh asks the following points concerning this receiver :- i: Have any improvements been made since the circuit was originally published in 1927? A.: Yes, see the 1929 Radio Listeners’ Guide.

2: What types of Mullard 4-volt valves are recommended for this receiver, and what "B" battery voltage is required by each? A.: PM8° Radio requiring from 45 to 90 volts; 4DX Detector taking about 22 volts; first audio either 4DX or PM3 can take 100 volts; the power valve, 254 can take anything up to 150 volts. Bach valye, however, with the exception of the detector, will work on less voltage. 8: What value resistance do these valves require to break 43 volts down to 4 volts?-1 ohm. 4: Would screen-grid valves be any advantage? A.: They improve the distance getting ability of the receiver a little. See Radio Listeners’ Guide. 5: What valve would you advise for the screen-grid?-PM24 in Mullard. It would not make any difference to the 1 ohm résistance. 6: Where can I purchase the H.F. transformers? A.: Fear and Company, Wellington. Price about 25/- a pair. 7: Where can I obtain a suitable audio choke 20 henries inductance? A.: You ean quite easily make one yourself, from details that have already been published in the ‘Radio Record," but Fear and Company will easily make up one for you. A good commercial choke can be obtained for 9/- or 10/-. Do not use the secondary of a burnt-out transformer, as is advocated by some. It has not the inductance and cannot pass the required current. 8. Could I reasonably expect the set to bring in some of the lower-powered stations, when conditions are. favourable? A.: Yes, the set performs very well on stations such as these. 9. Any suggestions will be appreciated. A.: Very full details have been given in the "Radio Listeners’ Guide," but if you have any further difficulties, we shall be only too pleased to assist you. 10. Does the wiring of the primary of the H.F. transformer run in the same direction as that of. the secondary and tickler coils.-Yes. 11. What wavelength can be tuned in on the average aerial? A.: 200 to 250 metres. "A" Bliminator Chokes. WHERE ean I find the details for an "A" choke to make up __ the eliminator described in the "All About the All-Electric’’? asks "R.W.R." (Auckland), A.: This question will be very fully dealt with in the "Radio Listeners’ Guide and Call Book," which will be published early in May. Binocular Coils. I AM building a receiver using binocullar coils and would like to know the following particulars, asks "D.McK." (Wellington). 1. Which is the better type of coil for the aerial, a binocular inductively coupled or a single type coil tapped for selectivity? I am using a screen-grid. A.: The binocular coil is not particularly suitable for the screen-grid valve. Use the tapped solenoid. 2. For reaction should I wind on 40 {urns in two layers in the reverse direction? ‘ A.: Yes, 40 turns will be just about sufficient for the condenser you are using,

but you may have to take off or add a few turns, Amateur Transmitting. WHERE can I obtain a book with the addresses of the Australian amateur transmitters?-"Grid Leak" (Blenheim). : Te Aro Book Depot, Wellington. The book is "Amateur Radio." 2.: When will you publish a complete list of New Zealand experimental and amateur transmitters? .: In the "Radio Listeners’ Guide and Call Book" due out next month. "Round-the-World". Two, (OULD I use a 84/1 transformer for ing) this circuit? asks "A.B." (Feild- . A.: Yes, but it would diminish the signal strength slightly. 2: I have a variable condenser with four fixed and four moving vanes. Will this be all right for tuning?

A.: Your details are not full enough to give a decided Aoswer, but it should be all grieht. Try it. Can I use Bi05? a Yes, in the last stage. 4; Can I expect 55SW?-Yes. Constructional Problem. OAN I obtain a copy of the "Radio Record" containing the first instal. ment of the two stage R.F, Drake? asks "R.L." (Greymouth). A.: We have a few of these on handy. and you_could obtain a copy from the office. However, it is fully described with the latest improvements in the 1929 Radio ne Guide. : uld 20 gauge tinned copper wire do of 18 for "Megohm’s" three-

Questions and Answers dt

HADERS of the "Radio Record" who are in difficulties about reception or set construction are invited to write to our "Questions and Answers" department for help. We particularly wish to assist those who know little about radio, as very often there is some very slight trouble which spoils completely one’s enjoyment of the programmes. Correspondents are asked to observe the following courtesies: 1, Write legibly. 2. Make your questions brief and to the point; do not_make apologies for writing, and, where possible, tabulate. 8. Do not ask for a reply by post unless a stamped and addressed envelope is enclosed. Even in these circumstances, We reserve the right to answer any question through our columns. 4, Do not ask us to desig circuits or send detailed lay-ou diagrams; but we can offer advice regarding circuits, 5. Address all technical corres- ’ pondence: "The Technical Editor, P.O. Box 1082, Wellington."

valve short-wave set? If so, how many turns would be required? A.: It could be used quite well, and . the number of turns should not.be altered, if the wire is well spaced; it must occupy the same length as the 18. Short-Wave Coils, "FE ©." (Auckland) asks for the de ° tails of short-wave coils for a factory-made 3-valve receiver. A.: Those described in the Listeners’ Guide should be all right, with slight adaptation, but the capacity of the condenser must be reduced by means of a .0008 condenser in series. .0008 is not easily obtained, so use a .0005 and a .0003 or .00025 in parallel. We should advise you, however, to obtain the commercial eoil, for these are designed for your circuit. 2: Would I improve matters if I used copper shielding? ‘ A.: Here again we advise you not to tamper with a commercial product., The only way we advise amateurs to interfere with commercial receivers is on the audio ‘side, where they can frequently adapt the wiring to provide different voltages for the anode and grid bias, Rheostats. HAT is the difference between a 80 ohm filament rheostat and a 6 ohm one? asks "G.S." (Wellington). A.: A 30 ohm rheostat will break down the voltage of the current it passes much more than will the 6 ohm one. Usually a 6 ohm rheostat is made of heavier material and will pass greater current. It is used when there are several valves consuming a large amount of filament current, and when it is necessary to break down the voltage supplied to them by a volt or two. The greater the current that flows the less in ohms is the resistance that must be used in the circuit. This explains why a single valve or two. valves using low filament conv

sumption require a 80 ohm rheostat while five or six valves require a 6 ohm one. 2: Does the rheostat act as a switch for both "A" and "B" batteries? A.: Almost without. exception, yes. Set Oscillates Too Freely. I HAVE a 8-valve all-wave set and cannot stop it oscillating, states "H.A." (Waimauku). A.: You have probably too many turns on the tickler coil; try reducing them. Reverse the turns to the tickler if it is separate or to the condenser if the tickler coil is continuous with the secondary. See that the filament negative is earthed, and test your grid condenser. Put the detector voltage on a separate: tapping, say 224. 2: When I put my finger on the grid leak the set squeals. : This indicates that the audio side of the set is intact. A.C. "Round-the-World" Three. [s it possible to use an ordinary bellringing transformer to light the filaments in ‘"‘Round-the-World" Three? asks "L.W." (Auckland). A.: A full explanation of why A. C. cannot be used in D.C. valves appears in the 1980 "Radio Listeners’ Guide," and in "All Abont the All-Electric." If you wished, you could use A.C. valves in "Round-the-World"Two, though a little hum may be introduced when dealing with the high. frequencies. In this case, a bell ringing transformer if it would pass the requisite amount of current, would be all right. . Where ean I obtain details for building a B eliminator? A.: One was deecribed in last year’s "Listener's Guide" a super power nack was described in the "Radio Record" at the end of last year, and the subject will receive attention in the 1980 "Radio Listener’s Guide," If you have not had previous experience, follow the descrip-

es tion in the 1929 "Guide." We do not agree with you when you say it would be | better to buy one. Those who can afford the time to make one can usually turn out a better article than the commercial one at the price. Valve Base Coils, wourp coils wound on valve base coils be suitable for "Round-the-World" Two? asks "B.DJ." (Nelson). If so, what size must I use? A.: The question was very fully dealt with in "QST" for February, 1930, and if you ean get a copy of this magazine you can work this out for yourself. If you cannot locate a copy we shall lend you ours, 2. Would I use the same size choke for both long and short wave? ; A.: The choke is more on the short-waves, so use the short-wave specifications, 38. How many turns would I put on a test tube for this choke? A.: About 150, Wavelengths of Short-wave Coils. "R D." (Te Kuiti) asks the following * questions concerning this subject: 1: What are the wavelengths covered by the Aerola short-wave coils? They are ?? igches in diameter and bound with 8 swg. wire, I think. The secondaries are 8, 7 and 18 turns respectively. They are used with a -001 mfd. condenser. A.: The details are fairly vague. The omission before the word inches prevents us from giving an accurate answer, for we do not know the coils in ‘particular, and consequently can only approximate. If the diameter is 3 inches, the gauge of the wire 18 sgw. spaced, then the bands are most likely 15 to 30 mteres, 30 to 45, and 45 to 60. 2. I wish to construct a short-wave adapter using a stage of untuned S.G, R.F. amplification. Is the enclosed circuit all right?

----- -- -_ A-: Yes, but you would do better to make up the R.F, and detector stages of the se 1een-grid three-valve set described in last year’s "Radio Listeners’ Guide." Aerial Wire. T is the best material to use for } spaces on.a double wire cage aerial? asks "R.E.T." (Whangarei), : A.: Strong bamboo orf cane makes excellent spaces, but the ends must be well bound to prevent splitting, Any hard timber well seasoned and painted will be all right. \ , 2. Should the leads-in be kept at a distance until they reach the aerial terminal? ( A.: Keep them at a distance for so long as it is convenient. Audio Transformer Question. I AM building a short-wave set, states "Shorty" (Stratford), and the specifications state that an audio transformer is to be used. I have a low frequency transformer marked P, G, + B, — G. Can this be used instead of the audio transformer? A.: Yes, they are identical. If the specifications show the transformer marked IP IS, OP OS, then IP=+B,, OP-P, -C=IS, OS=G. If it is designated in the usual American fashion, the markings will be the same on both transformers. 2. Can a valve base be wound for a coil former for "Round-the-Word" Two? How will this affect the wiring? A.: Slight adaptions should be in. the nerial circuit; the tappings. would preSent a little trouble, though it could be overcome. ;