Questions and Answers

Radio Record, Volume III, Issue 26, 10 January 1930, Page 25

Questions and Answers

Making Accumulators, CPs you tell. me how I could use old negative accumulator plates to ' ke a "B" battery? asks "A, D -B." (Rae- ’ im A.: See an article by Megohm, in the issue of the "Radio Record" dated July: 29, 1927. Troublé with’ the Browning-Drake. : I AM having trouble with the 2R.F. Browning-Drake, writes L.J.T, (New Plymouth). I am using an electrolytic -"B" eliminator with two tappings, and although I have tried different coils and altered their positions as well ag. the capacity of the neutralising condenser, I eannot prevent the set from oscillating violently.- The boxes for each stage were larger than those specified, otherwise the instructions have been followed. I.have latterly. introduced the parallel feed method, but the set still oscillates with the tickler short-cireuiting. Ishave also tried several valves in -the R.F. stages. A.: Look for an open circuit in the neutralising condenser to either Valve. There may be coupling between the R.F. ‘stages due to too many leads being taken from the same taps of the eliminator. Place audio frequency- chokes in the "B" leads of the transformer, and on the set side of these connect a 1 mfd. condenser to B-. Examine the primary coils, making certain that there is not a short circuit. Reduce the number of plates in the neutralising condensers. to at least three, one moving and two fixed. Valves of the 201A class should be used in the R.F, stages, and require about 15 turns on the primary, In last week’s issue, we published an article dealing with the stabilising of two stages of radio frequency. The correspondent committed . an error when. he made: the shield boxes larger than those specified. In a set such as this there should be no deviation from the specifications given, 2, Which is the best way to neutralise this set? ' A.: Wheh the condenser is of sufficiently small capacity (8 plates) tune in a distant station such as Wellington or Christchurch and increase reaction until the} set oscillates. ‘Adjust the neutraliscondenser controlling the valve nearthe detector the set stops and further advance reaction until oscillation recommences. Again adjust the neutralising condenser, and further advance the tickler. Do this until further movement of the neutralising con- »

denser will not prevent the set oscillating. Leave the condenser in the position which causes the tickler to be advanced to the greatest degree without causing oscillation. Now repeat the process for the other ‘valves. It may be necessary to do this on other stations about the middle of-the dial. 8. Would the eliminator cause this trouble? A.: Yes, it might quite easily be responsible. Coupling takes place between the . radio-frequency. valves,* Insert a choke between the R.F. lead and a bypass condenser between this and Bto keep the radio-frequency currents out of the eliminator. Wireless Literature. CAN you recommend a good book on wireless for'a. beginner wishing fo learn something of the technical side of theory and constructions? asks "H.J.P." (Blenheim). * A.: We know of no better publication for the beginner than our own ‘Radio Listeners’. Guide." There is a rather regrettable deficiency of radio text books. Publications from America such as "Radio News" and its incidental publications, "Radio," and "Radio Broadcast," are excellent periodicals for those who are slightly advanced in wireless technicology, but there has been very little effort on the part of*any American firm to cater for the beginner. Because of the similarity between ours and American conditions and between these and the Eng-_ lish conditions American publications are more valuable to the New Zealand reader than English publications. | ,A Noisy Transformer. I HAVE replaced an audio transformer by one that causes a continual whistle in the speaker. I eannot get beyond the New Zealand stations because of it-‘J.B." (Murchison). A.: Try reversing the connections to. the primary and earth the case and core. "A" Battery Charger. N an old issue of the "Radio Record" an "A" battery charger was described, and in the "Listeners’ Guide" appeared the information for a 1.3 amp. charger. There are a few points I would like cleared up. 1. Is it necessary to put a layer of empire cloth and shellac between each layer of the secondary?

' A.: It is preferable to do this for the sake of greater insulation. ‘ 2. When the primary is wound with 24 D.C.C. wire is it still necessary to place the paper strips between the layers ?-Yes, , 3. Is 18 D.C.C. heavy enough wire for the secondary winding for the 3-6 amp. charger ?-Yes, , 4. Is the secondary winding for the 3-6 amp. charger the same as ‘for the 1.8 charger? A.: With the exception of the heavier wire, yes. . ° 5. At what number of turns does the switch come on to? A.: In the later specification a switch is not employed unless placed in series with the primary. 6. Which is the best method of controlling the output? Resistance lamps or variable resistances? A.: Variable resistances. 7. Would the 3-6 amp. charger be suitable for charging a 4-volt 80-amp. :accumulator, and 154-volt wet. "B’ battery? A.: It would be suitable for. the "A" accumulator, but not for the "B." ‘ 8. Could you tell me the total consumption of my set? I am using Condor valves. A.: The requisite information for the ‘Condor valves is difficult to obtain, so we are afraid we cannot supply you with the information you require. An Eliminator Problem. I HAVE built up the eliminator similar to that in the Listeners’ Guide, but have slightly increased the number of turns on the secondary, and made a few other alterations (here the correspondent enumerates the details of his set). What would be the output at 20 milliamps. drain?-"C.B." (Nelson). A.: About 180 volts. 2: Would it be capable of running two 625A’s in the last stage? A.: Yes, but the voltage may drop to 150 volts. 8: What would be the approximate capacity of a condenser with pape? insulations which measures 4 x 24.x 4in.? A.: Probably about 2 mfds., but it would be unsafe to use this in eliminator work as the voltage test could not be very high. 4: What is the approximate value of a mica condenser 3 x 8 x #in.? A.: Probably 1 or 2 mfds. rated to stand 200 volts working. 5: My eliminator runs for six hours and consumes 1/10th unit. Is this a good performance? A.: Yes. , 4 Amateur Transmitting. Wit you answer the following inquiries concerning various matters? writes "R.D." (Te Kuiti).

1: The power of the usual amateur transmitters, the number of valves and their cost?, ; A.: Communicate with the secretary of the N.Z. A.R.T., c/o Superadio, Ltd, Queen Street, Auckland. Is it advisable to have separate transformers to supply the filament current to alternating current valves with varying voltages? t is ‘necessary to have different windings, but they need not. necessarily be different transformers. 8: Is a condenser in series with the pick-up lead to the detector necessary? .: No. 4: Where can I obtain a copy of the 1928-1929 edition of the Radio Listeners’ Guide? A.: Try Te Aro Book Stores, Wellington. . Improving a Four-Valve Set. I HAVE a screen-grid four-valve sét which I have made up from a popue lar kit. How can I improve my set without great expense?-‘Mons" (Palmere ston). 1. I have heard that better transforms ers than those supplied with the sets can be used. A.: The transformers in this set are reasonably good, and unless they burn \ out, it would not be advisable to change them. Introducing other components into a kit set is not wise unless the owner has a very wide knowledge of radio. We should advise you to dispense with your set and either build another or purchase one of the many excellent heutrodynes. I am using a "B" eliminator, but. on stations below Christchurch reception is harsh and hoarse. Could I remedy this with a different grid. leak? A.: It would be advisable to try another, preferably slightly lower. See that your set has by-pass condensers between the screen grid and earth and between the’ plate and earth. The capacity of the condensers should be about 1 mfd. 8. Could I convert to an ordinary fours. valve set by dispensing with the screens grid‘? Would it require any great alterations, and would-it be an advantage? A.: We would not advise you to tamper with this more or less commercial ree ceiver. 4, I can tune in a large number of carrier waves, but cannot get broadcast from them. Would the inclusion of a pentode valve bring in the music, ete.? A.: Quite probably. The pentode increases volume considerably. Look for an open circuit or a high resistance connec. tion in the aerial circuit, as this may cause you to have difficulty in tuning in the station. 5. This set whistles quite a great deal, Can I fit a neutralising condenser? A.: Neutralising condensers are very rarely required on screen grid sets. In such cases there is rarely more than an extra turn or so at the end of the seve .

ondary Winding and the’ RF, transformer, In your case we should not advise interference. Look for the by-pass condensers previously referred to. Note-If you desire to improve your quality look: to the following points:-The by-pass condensers, the value of the grid leak which should be fairly low 1+2 megohms, the speaker, which frequently causes most of _the trouble, the state of the "C" battery, and the load imposed on the eliminator. If this latter cannot stand up to the drain imposed upon it the voltage will drop and the quality become spoiled. Building an "A" Eliminator, CouLD you supply me with the following data relative to the construction of an "A" eliminator? asks "Eliminator" (Taranaki). 1: What capacity electrolytic condenser to buy? A.: About 2500 mfds, : 2; What capacity metal A.: You need a rectifier that will pass about 4 amps. Communicate with Johns, Ltd, Auckland. 8: What make of choke do you recommend ? ’ ‘A.; You will probably have to make your own, A set of standard stampings (Ballingers and Co., Wellington) filled with 22 DCO wire will probably suffice. 4+ What secondary voltage is necessary to obtain (a) 4 volts rectified ; (b) 6 volts rectified? A.: (a) 6 volts; (b) 10 volts, 5: What size and number of turns of wire is best? _ A.: Use 16 or 18 guage DCC wire and put on about 24 turns for the 4 volt ‘and 35 turns for the 6 volts. A slightly greater voltage drop occurs in the metal _yectifier than the valve and this must be allowed for. Test the final voltage with a voltmeter before connecting up with the set. ; "7: Jf an ABC eliminator is constructed, does the number of primary turns re main constant? . . A.: Yes, but slightly gauge wire is necessary. See a recent article by "Cathode" on the design and construction of power transformers. . List of Stations, . HERD can I obtain a list of New ‘YY Zealand and Australian stations and their times of operation?-"F.F." (Hokixa). A.: The New Zealand stations have been published in recent issues of the "Radio Record.’ The Australian stations may be obtained from .a supplement in the "Australian Wireless Weekly" published.on May 31, 1929. If you eannot secure a copy of this locally, write to the publishers, Elizabeth Street, Sydney. , The Use of 245’s. " V.S.") (Khandallah) asks the. fol- * lowing questions relative to the 245 valve :- 1. With 250 volts on the plate should the grid bias, be 50 or 100 volts? ' A.: 59 volts biases the curve to the mid-point of its straight portion and causes the push-pull amplifiers to act as single valves. The 100 volts causes the bias to.act from the lower curve, and if the 2-valves are regarded as acting together the curve is taken right along the straight parts of both. It is considered on the whole that biasing to the midpoint is slightly superior. On our tests we have found very little difference. 2. What output transformer should be used with a 66P unit? A,: A 1-1 transformer. 3: What,is the impedance of the 66P wnit at 200 cycles? A.: The measurements for this unit have ne*_ been published by the manufacturers, "1 we have not had the opportunity of making these measurements ourselves, 4, What is the maximum undistorted output that the 66P used in the improved linen diaphragm speaker would be able to handle? A.: The exact amount is not known, ‘but it would probably be shout 3 watts.

5. Would harm be done. to a 280 rectifying valve by putting 850 volts on each plate, although the specified maximum is 300. volts. ‘ A. It is not a good practice, but in this case will probably have very little effect. G. In circuit diagrams of "B" eliminators the high tension is sometimes taken from one of the legs of the filament and sometimes from the centre tap of the filament coil, Which is the most satisfactory? Le A.: There is very little difference, and this is in favour of the centre tap. Note.-Full justice to two 245’s in pushpull cannot be done with anything less than a dynamic cone speaker. In fact, the linen diaphragm would hardly be satisfactory. Request for a Diagram. wourn you supply a diagram of a one-valve receiver capable of reaching Wellington, Auckland, and other New Yealand stations on the telephones. I would also like to search for Australian stations -"H.F." (Levin). A.: No. We do not encourage owners of small sets to search for New Zealand and Australian stations, for this constitutes the howling’ valve nuisance. Small sets such as these are not suitable for anything more than local station reception, and when they are employed on the other main stations they cause more annoyance to listeners than do any other type of set. A combination was described in a recent issue of the "Radio Record," ‘This was to be used primarily as a crys« tal and amplifier, but could, with care, be used as a 1-valve receiver, but this is not advised. unless the operator is careful and skilful. D,C. Eliminators. CCORDING to an article published in your paper, it is a difficult matter to construct an "A" eliminator to work from D.C. mains. Several firms, however, have imported some of these, and have been used successfully. It seems to be that there must be a tremendous waste of current, and I should like to use this to light the room even with a table lamp, -W.D." (Reefton). A.: The point of our article was that the operating cost of an "A" eliminatos hardly warranted: it.-In your case, you will require a lamp or lamps dissipating 460 watts. This may quite conveniently be three 150 watt lamps, which may be connected: in series for use in different parts of the room or in different rooms. The valves could be short-circuited when it is desired to use the lights and not the radio set. The lights, of course, would not function as well as lights pro-. perly connected, because the voltage would be lessened (roughly 100 volts each). A 30 ohm rheostat should be eonnected in series with the filaments. Charging a Battery. RATHER unique questions concerning "A" battery charging have been raised by "Fan" (Paerau), 1. Bringing direct current across paddocks 800 yards on poles, the input at a generator 12 amps. at 6 volts, chargitig a 6-volt battery, what would be « the output at the other end? A,: This depends upon the type of wire used. If 7/029, which we recommend, is employed, about 5 amps. ean be delivered at 6 volts... ° 2. I have two 60 amp. batteries and one 74. If I charge these in parallel would they discharge if I left them connected in parallel?-No. My set draws about 4 amps.’ an hour. If three batteries were connected in parallel would the set take an equal amount from each? , A.: Other things being equal, yes. Note: We recommend your using the fairly heavy 7/029 to prevent the wire ‘ breaking by its own weight or by any slight strain that might be imposed upon it. It is necessary to run only one wire, probably the positive (depending on which is earthed at. the generator) and run the other to earth. In charging the

batteries, connect the positive wire to the negative pole. of the battery and the ground to the positive. ° 4, Could you give me a circuit of a short-wave set with two stages of radio frequency ? " A.: Such sets as these are rarely used. We do not happen to have one that we could give you without going into a great deal of detail that is not covered by the scope of "Questions and Answers," A Tip that Worked. R= my query and your reply thereto in the "Radio Record of December 6, 1929, relating ‘to a second aerial for a erystal set: My second aerial has been erected, and the increased. volume of reception fully justifies the addition. I have also added another earth, making five, with beneficial results. The above may be a help to others depending on a erystal set for their wireless entertain-

ment:-

~APG.

(Silverstream).