Questions and answers

Radio Record, Volume III, Issue 36, 21 March 1930, Page 28

Questions and answers

Single versus Parallel Wire Aerials. « TLC." (Wellington) asks yegard- * ing aerials: Do you consider an ferial of two parallel wires 25ft. long more efficient than a single wire aerial of the same Jength? .: They are if separated by about 6ft., otherwise the difference does not make worth while putting up the extra wire. Valve Combination. I HAVE an American set, and wish to know the best of valves, asks "F.H.G." A.: Use 201A Radiotron or Cunningham with each of the first five stages, and in the last use a power valve of ‘the 171A type. It is not necessary to use an American yalve in this socket. Any of the medium power valves will do. Distortion, I HAVE a six-valve set, but reception has been distorted. ia am using two DEP610 valves in the last two stages, tate "T,,.T.H." (Bay of Islands) : You are probably using a_power that ig too small, Try the 171 type (P6254 Marconi) with about 40 volts bias, if you are using the full 180 volts, 27 if you are using 135 volts, and 15 if you are using 90 volts. You may have a defective valve, which can be tested by an expert. The speaker may not be able to carry the load, or the "C" battery may be run down. Short-wave Problem. A.M." (Auckland) asks concerning * a short-wave receiver :- 1. Why is my circuit legal when the short antenna is connected directly into the grid circuit? 1

A.: In effect, it is loose coupled, for it has a variable condenser in the aerial. 2. When working certain stations reception is spoiled by a loud shriek which can be eliminated only by loosening the reaction, but in a moment it is "back again, Can it be that these stations are hetrodyned by nearby morse stations? A.: That is probably the trouble. These stations may possibly be amateurs working in the neighbourhood. We advise you to contact with the secretary of the Amateur Radio Transmitters’ Association, Auckland, and enlist their cooperation. If you could identify the morse stations it would help you. 8. I frequently hear stations KGO and WGY on short waves. ‘What are the designations of the stations broadcasting these on short wave? " A.: KGO is broadcast on short wave by W6XN and W6AF, WGY by 2XAF and 2XAD. "Round-the-World" Two. wat is the correct number of turns on the coils? asks "RR." (Christehurch), A.: There are five coils to tune between 10 and 70 metres; 4, 5, 6, 8, and 12 rns. "L. H." (Mercer) asks: Could he use an re and "©" eliminator with the "Round-the-World’?’ Three? A.:: If the "A" current were sufiiciently smooth, yes. 2. In making tappings for the short wave and broadcast coils, would a twist tinned siuftice ?-Yes. 8. What type of neutralising condenser would you advise? Would a Lissen be all right?

A.: Any of the well-known makes. The one you mention is O.K. Legal Points. Mest I have a separate primary on a 1-valve anode-bend set, or can I connect it directly to the grid. A.: You can connect it directly to the grid coil, providing you have a neutralising condenser in the aerial. Be careful in operating the set to prevent annoyance. 2: Is. auto-coupling considered direct coupling as far as the regulations are concerned ? A.: Auto-coupling is usually approved only when there is a series condenser in the aerial. The P. and T. Department in approving a set usually take into account the liability of the set to oscillate, hence they look with disfavour on such as employ a large number of reaction turns, and have a high voltage on the plate. | 8: By using two general purpose valves in parallel in the last stage, could I bias back and so get greater swing? A.: It is not advisable. The system of paralleling output valves has long since passed out of date. Far better to use one single power valve. 4: Would the impedance of the valves parallel be doubled or halved ?-Halv- ed, . 5: Can you tell me the make of a transformer to match a medium high impedance detector? A.: One with a very low ratio and made by a reputable firm should suit your requirements. Cost of Upkeep. AM perhaps one of your very rare readers, states "M.S." (Otago), for I have no radio. We are hoping to get a, small set, say, three valves, but have been told that the upkeep is about £10 ayear. Is this so? A.: No. It is about half this, if care ig taken in the choice of batteries. Use 90 volts "B" and employ the large size batteries.. Use four "A" batteries in-

stead of two, and connect them by the series parallel method shown in the accompanying diagram. Omit the two last cells. Push-Pull Transformers. ‘ OULD you give me the details. of pah-pull suitable for the 245 valve? asks "A.B." A.: We do not advise you to make these, as they are not expensive, but involve a great deal of intricate work if home-constructed. 2. What should be used in the amplifier preceding, these? A.: Two 227’s R.O.C. coupled. Use 4 megohm eid leak in the grid circuit, a .01 coupling condenser, and a choke coil of about 100 heuries in the plate circuit,

A Crystal Set Amplifier. | you may be interested to hear from # successful constructor ofthe A.C, amplifier in the cre) er There -is no trace of hum valves for rectifier and amplifier. The volume from a crystal sét is ample. Would you clear up the _ following points :-- 1. There was not much room in the window of a transformer for three filament windings, so ean I take eurrent for two, .) amp. valves off one 5-6-volt winding? Yes, providing the gauge is not less than 22 s.w.g. two valves in cascade will not operate successfully unless a special A.C. valve is used in the first 8 oe : There is bad backlash with a circuit using separate valves to detect and amplify in a short-wave circuit. A.: This ig probably inherent in the, circuit which attempts to utilise HF amplification. This is not ufusual. Capacity of Variable Condensers, | I HAVE a variable condenser wit! R fixed plates and 12 moving. Cold I' use this in making up the All-waye! "Round-the-World" Two as illustrated in the "Record." -_ A.: It will be a .0005, which is too large for short-wave work. It will be necessary for you to obtain a .0082 conenser . 2. How is the capacity of a variable condenser calculated? : It can be done only with elaborate Short-wave Kit. Set Problem. HAVE a combination broadcast and short-wave kit set which has been working well on the broadcast band, but I cannot get the set to oscillate on the ;

.L." (Mere Mere). A.: It is very difficult to say without seeing your set. Your best plan would probably be to contact the dealer who sold you the set. We suggest that the "B" voltage on the detector is too low. This is the positive tapping nearest the detector, and it will probably pay you to shift this up a tapping or two to get greater "B" voltage. If you do not have at least a rudimentary technical knowledge of-radio we would not advise you to attempt any further alterations. Have you had your detector valve tested? . 2.: Why can I get better results with the lid off? ' A.: The cabinet acts as a shield, and by lifting the lid you are getting a certain amount of pick-up from the coils. "Round-the-World Two." considering building the "Round-the-World Two," writes "L.N.W." (Auckland), and wish to know the following :-- 1. Is it practicable to use two .00025 condensers? , A.: The .00025 may be tried in the tuning circuit by placing a .001 fixed condenser in series with it. . 2. I have an audio transformer op which the terminals aré marked 1, 2, 3, wavelength. What is the matter?

4. Which are the primary and which are seconadry? A.: Probably 1 and 2 are primary, 3 and 4 secondary. Make the following vetification: Connect up a pair of ’phones or a voltmeter and a ‘cell as shown in the accompanying diagram. If this is correct there should be a click or a reading between 1 and 2, nothing between 2 and 3, and a click of lesser intensity or a smaller reading between 3. and 4. If this is not correct, you can find out for yourself which is which from these particulars. 5. Is the two-valve suitable for an amplifier with a gramophone pick-up? A, sit will make quite a good starting plac t an electric gramophone, You will gé&t fair results if used with a light speaker. It will probably be equal to a table or small console model mechanical gramophone. Interference. Wwe have an electric set and the passing trams cause a great amount of static. How ean this be reduced? writes "GS." (Wanganui Hast). A.: Use a shorter aerial and run it at: right angles to the tram lines. Use a counterpoise earth if an earth of any description is necessary. Try the cage aerial described in a recent issue of the "Radio Record." Communicate with the tramway authorities, who may be able to do something for you. Power Transformer. 1 AM going to construct a transformer to step 230 volts down to 110. What gauge of wire should I use and how many turns are needed?-"H.H.L." (Wellington). A.: We can give you the number of turns for a 1din. core-820 primary and

420 secondary, but not the guage of wire until we know the amount of current to be supplied. Get strips 34 inches long and find out the size of the window when the windings are finished. Probably 4 28 gauge primary and a 30 gauge secondary would suit your requirements. Screen-Grid Detectér Adaptor. I HAYVHE constructed the screen-grid de- ’ tector circuit described in the "Record" and am using it as an adaptor. Results are poor.. What can I do? asks "M.W." (Kimbolton). . A.: We advise you to use your screengrid valve in the screen-grid adaptor described in the Radio Listeners’ Guide. The screen-grid detector, too, was merely an experimental set for those who have had plenty of experiencé. The screengrid adaptor, however, has been fully described for the home constructor. Valve Replacement. I HAVE an American factory built set which has been very poor on outside stations lately. I intend replacing the valve, but, having a 6-volt accumulator, think it advisable to use 6-volt valves. I cannot quite see why 5-volt valves should be used with a 6-volt accumulator, asks "G.H.B." (Christchurch), A.: Undoubtedly, you require a new set of valves. Use the 5 volt valves, for the

set incorporates resistances in the filament leads to break the 6 volts supplied by the accumulator down to 5. If you use .6 volt valves. the resistance will still be in the cireuit, and will break down the voltage. That is, they will be running under maximum efficiency. It is more important to use 5-volt valves in the radio and’ detector stages than in the audio. For the power valve any 6-volt medium valve may be used, if it is suitably biased. Insufficient Data, HAVE built a 1-valve set, but it will not oscillate, states somebody (name unknown). A.: The data you have given us is insufficient. Send along the theoretical diagram or the details given with the set, and then perhaps we can help you. The fact that 2YA comes in at 150 means that your secondary coil is just about right. You dial is graduated to 180, whereas the usual dial goes to 90. Bring the tickler coil nearer the secondary. Ya Choke Coil. HAT is the gauge and, number of turns for the choke coil used in the 1928 "Radio Record" short-wave adaptor? "A,D.W." (Whangarei). A.: 100 turns of 80 gauge wire wound on a 5/8 former. The winding will oceupy about 3% inches. Is Morse Necessary? J§ a Morse certificate necessary for operating a small "B" class transmitter? If so, why? "Radio Maniac" (Christchurch), A.: Yes, it is stipulated that everyone who applies for.a license must have a working knowle’’e of Morse, as most amateur transmission is conducted in this

code. Hiveryone operating must be proficient in it, ; Sereen Grid Problem. AN you enlighten me upon. the following relative to a screen-grid set? writes "R.D." (Te Kuiti). 1. To send me the diagram and correct resistances needed to add a resistance capacity coupled stage, and criticise the accompanying diagram. 'A.: The accompanying diagram gives all the particulars. ‘Three stages of R.C.C. coupling should be followed

by a push-pull stage in order to prevent audio feed-back and howl. Your diagram is quite all right. 2. An explanation of the methods of preventing the set from howling. A.:; You should have defined your trouble more accurately, giving us an indication of what caused the set to howl, and under what conditions this took place. However we have devoted an article in this week’s issue: to your trouble. Phasatrol Neutralization. Woop you supply me with the following information relative to phasatrol method of neutralization, asks "Phasatrol" (Nelson). 1: What is the best way to insert a phasatrol in the Browning Drake? A.: As you suggest, the method employed in the "Beginner Tree." 2: Does the phasatrol make the set less efficient? . A.: Slightly, it introduces a certain amount of damping. ; 8: I cannot tune out 2YA with the radio valve shut out. A.: The phasatrol will not neutralize completely. Have you tried the method of neutralizing described with the A.C. B.D. in September last year? 4: I havé inserted a grid leak and condenser in the lead of the RF valve.

This seems to improve the tone considerably. A.: It ought not; there is something wrong with the set. Try bias on the radio’valve. . 5: I wish to incorporate an audio output filter. What do I do? A.: Buy or make an audio choke of 50 heneries and connect this between the output terminals. Insert a two mfd. condenser in series with one of the speaker leads and the speaker terminals.

not use an old audio transformer. They are not designed for power valves and the result is saturation of the cgne.' Pentode’s Dynamic Cone. I purpose using this speaker as a choke in’ Megohm’s Power Pack for Super Fowar valves, states "Eliminator," Dunein, . 1: Would it be possible for. me to usé this suitably rewound as the choke in the power pack? A.: Yes. Fill the space at your disposal with 84 SWG wire and put a resistance in series before the speaker

field if it does not break the voltage down sufficiently. ‘ 2: Is there any objection to having the speaker detachéd at some distance from the set?--No. . In Brief. G.H.B. (Pieton).-The amp-hour capacity of your battery is probably 80. Thanks for your consideration, but we do not accept payment for replies so are returning your P.N. Motor-Boating. WHEN I bring the set into reaction it ' Starts motor-boating, I have various leaks across: the transformers, but as soon as the s.g. valve is inserted the motor-boating starts -A.A. (Wanganui). A.: Have you tried a + megohm leak in series with the first transformer, if necessary in combination with another across the primary? An "anti-mobo" device ig a combination of series and parallel resistances arranged in this manfer. Audio Oscillation. "FINROUBLD" (Eketahuna) states that since replacing his second quality transformers with better ones he has been troubled with strong audio oscillation. The set hag much more power. A.: Although we do not particularly like the transformers you are using, we think the trouble can be lessened, if not cured, by reversing the connections to the primary of one of them. ‘ Input and Output. wovrp you define the terms "input" ton) and "output"? asks V.H. (LyttelA.: "Input" refers to those signals fed into the apparatus in question and "output" those delivered by it. The input to a speaker is the signals fed from the last valve whereas the output is all the sounds that one hears as a result. One is the "going in," the other the "going out." A False Start. I AM informed the layout of the Round the World Three is wrong owing to the valve filament connections being crossed. complains W.H.P. (Rangiora). A.: Sorry, your information is not cor rect, but if you don’t like them crossed bunch them and take a pair to each valve inturn. Try both systems and see which is the easier to follow out and. which gives the better results.