Questions and answers

Radio Record, Volume III, Issue 51, 4 July 1930, Page 22

Questions and answers

YC ENT (Wanganui) has built a silkfronted frame to his baby-grand. The mgnet of the speaker touches the frame. Will this damage the tone? A.: Your sill front is really a silk back. The silk touching the magnet will not interfere with the sound. 2. I get a shrill whistle in my set, but it disappears when one of the valves is puhed down in the socket. A.: This, probably, is the result of one of the connections being rather loose. If it is possible for you to tighten’ the socket without getting into difficulties, then tighten the springs. Does the same happen when another valve is placed in the socket? Perhaps it is microphonie detector. Place a lead cap over the glass. 7 D C.D. (Auckland), draws our at- * tention to the fact that the 280 valve rectifier is over-loaded in the Loftin White circuit. A.: It seems to adapt itself to its extra load quite well. Probably the life of the valve will be slightly shortened, but nothing to worry about. It seems to do its job all right. 2. Is there any chance of feeding the field coil of a dynamic speaker without the necessity of a special rectifier? Ps : Yes. The 100-volt D.C. type may be used by connecting it across’. the -++ and — terminals of the rectified current. A 38000 ohm. resistance will be necessary in series, but provision must be made for the extra load by using heavier wire on the secondary, say 382. XM EDLEY (Lyttelton): Your method of connecting the A.C. transformer appears correct, only B — and are connected to centre tap to earth. You will still need bias batteries. If you wish to eliminate them see the method of voltage drop explained in ‘All about the all electric."

VV iLL you give me the number of turns of 1in. tubing to cover 8-125 metres? A.: You will need a special circuit to cover the ultra short waves. See last week’s issue of the "Radio Record." Metres. See, Tickler. 15/30 @eee 6 eoee 5 25/50 Seee 13 eees 7 45/90 esee 25 eeee 10 90/140 .... 50 .... 26 2. Is there any advantage gained by using a .0U015 grid condenser? A.: The smaller the condenser the better the tuning, for the stations are more widely separated.’ K. O.W. (Papatoetoe) recently purchased a "Polar Twin" valve set, and he asks for a wiring plan. : We cannot supply one, and do not where cne can be obtained. 2. The valves are Mullard P.M.3 and P.M.4. Are these correct?-Yes. 8. Is a four-volt accumulator and a dry 45-volt battery all right? A.: You should get better results with 90 volts B. 4, With aerial and earth disconnected the set only hums and begins to crackle from 50 to 100. A.: Does anything of the kind happen when the aerial and the earth are con‘nected to the set? If it does it appears that the leaves of the variable condensers are touching. 5. Why cannot I log Dunedin? I can get all the other main "stations. It may be your locality or the fact that your set is unsensitive at upper wave-leigths. It may improve with the increased voltage. D. W. (Te Archa) asks the optimum combination of valves for his set. He sets out several admirable combinations for his set, probably the best being No. 5, with five 221’s and a 171. He

should not use a 112 in the second Jast stage, as he suggests, as the current required by this valve would probably have drastic results on his transformer. BARNER (Napier).-I have a sevenvalve a.c. set, using two 171’s in the last stage. Could these be changed to 245's? . A.: Sorry, Learner, you cannot. Your set is designed to give 180 volts, and to work 245’s at this pressure means that you are not giving them a chance to reach maximum efficiency. Furthermore, the bias and filament voltage would be wrong. 2. When the volume control is turned up there is a certain jar in the speaker. Why? A.: Probably you are overlooking either the speaker or the valves. Two 171’s will give only good room strength without overloading. Naturally the volume is limited to the capabilities of the set. 8. Whenever there is a foot-fall near the set there is a crackling noise in the speaker; should this be? A.: Certainly not. Examine the connections to the aerial, earth and speaker to see if anything is loose. If this fails to reveal the cause of the trouble, call in the dealer who sold you the set. YEW READER (Hastings) asks how he might fit a pick-up to his set? A.: Certain makes of pick-ups include an attachment that fits to the socket’ of the detector valve. If you cannot get hold of one of these, the tips from the pick-up go to the grid of the detector valve and the other to earth or frame of the set, It is understood that the grid of the valve does not mean the grid leak and condenser which precedes it. 2. I run an extension of 50 feet; am I losing volume. A.: Nothing to speak of, but as it is outside you should be using an output filter. Otherwise a short circuit might result. R J. (Feilding) asks which would give * the better results from a three-valve sereen grid set with accumulators and a charger or a two-valve all-electric set. A.: The three-valve set would give the better results, though the electric set would be the more simple. However, the s.g. set would be little trouble. What are you doing for the "B" battery? 2. Gan the electric light wires be used for an aerial with an electric set? A.: Yes, with the adaptor, but they are not as efficient as the proper aerial. "AACA." (Wellington) lives in an apartment house. Two of the tenants have radio sets and licenses, Can he operate a set without a separate license? A.: No, the regulations require every separate family, whether living in the same house or not and using different wireless sets, must have _ separate licenses, "7 J.S." (Wellington) has the parts of * an old battery set and wishes to make an A.C, short-wave receiver. He asks us for a circuit. A.: A.C. shortwave receivers for amateur construction are not simple, and for this reason we have not, up to this time, published a circuit. If you refer to the shortwave adapter in the Radio Listeners’ Guide, ’» pages 59 and 30, in

"All About the All-Hlectric," you will find a‘ detector unit and an amplifier. Connect the plate terminal’ of the adapter to "P" of the audio transformer and the cathode to earth. Disresard the r.£ and detector stages in the "Wiectric." "W U.D." (Lyttelton), who is intendeing to. build the Loftin-White amplifier, asks the following questions: . 1. Can Philips 1071 valve be used as rectifier ?-Yes. Will it operate the amplifier at maximum voltage and also a Magnavox speaker having a field resistance of 7500 ohms. drawing 40 milliamps. You may use it in the cireuit you suggest, but we would advise the speaker and resistance to be shunted with a 1 mfd. by-pass condenser, 38. Is. this the best position to place the speaker in the power ‘circuit? A.: Yes. We presume you will not forget to add the current drawn by the speaker to the amount drawn by the amplifier, which may be as high as 50 mills., and determine the gauge of wire for your transformer and choke accordingly. You will need heavier wire than * cuit of a 8-valve receiver, preg: ably a detector and two stages of audit? A.: We have not yet published one. Possibly we will. do so this season, ‘The R.F. detector, and first: audio. stages of the B.D. would make an excellent receiver, and it could be used with.a shortwave adaptor, It would be better than detector and two A.F. "Te BEN" (Hastbourne) writes: Since receiving your answers to my queries, I purchased "All About the AllElectric" and obtained a 112 for rectifier and a 171 for power purposes. I " S.F." (Merivale). asks for 7

propose to construct the transformer of 1180 turns S.W.G. )rimary and 36 secondary. Is this all right? A.: It should be quite all right, alough your 112 will be hard put to supPly enough rectified eurrent; still, it should do the trick. ; . 2. You were rather heavy-handed in advising me to make a 100 henry choke. It seems rather high after reading the Guide, ete. ' A.: A high-inductance choke, although ¢osting more to build, has more smoothing properties than a lower one, though probably the 80. henry choke that you propose to make will give you ample smoothing, 3, I am using a 2000 resistor for the 171 valve, and I cannot get the 2250. A.: Get a 400 ohm potentiometer, then connect the moving arm and ove of the sides ai series with it. You wili then a have atijutsable bias over a certain range, 5 at voltage should be on the plate of the 171? A.: From 150 to 180. ‘Be on the safe side and put another 50 turns on the secondary. The voltage can be easily broken down if too high. . I am trying to get 17 watts output with 180 volts on the plate, Am I on the right track? A.; You appear to be. "PUZZLED" (Motueka) asks the folie lowing questions concerning a H.R. 1, At certain places on the tuning dial a sharp click has developed which does not always occur. A.: It sounds ag if the vanes of the condenser touch at certain points. Hxamine these carefully for dust or lack of alignment, _2. The volume does not seem to be right. It comes in full with the rheostate half way on. A.: Don’t let that worry you, many valves operate at maximum efficiency with elje@stat half way on. Does the question of volume have ‘anything to do with the loudspeaker? A.: Not the regulation of volume, if that is what you mean, but, of course.

Some speakers will carry volume very much better than others. 4. I ean get the Australians on the *phones all right. There is no volume on the main stations. A.: A dozen and one things might eause this. Look through our trouble tracking section in this year’s "Guide." It is given on page 128. 5. When tuning to Wellington or Dunedin, neither squeal nor whistle are to be heard. All the other stations howl. Is the set not neutralised properly? A.: Your neighbours will probably be quite happy because the set doesn’t squeal on Wellington or Dunedin, but you are probably on the black list for all others, Don’t let your set squeal, but immediately it bursts into oscillation cut back the reaction, and stop it. The fact that it doesn’t oscillate on Wellington or Dunedin is nothing much to worry about. It means that these two are on higher wavelengthsgthan the others and as the set is mor@insensitive to higher wave-lengths than lower, there is not enough reaction in the circuit to make the set oscillate. You might try increasing the detector voltage. . 6. There is a round thing called a Phasastrol. What is its business in the set? oO A.: It is a method of neutralisation, and quite a good one, too. * A MATEUR" (Te Kuiti) has a .00035 and .00025 variable condenser and wishes to use these, in "Round the World Two." Will there be any additions or alterations to the coils? A.: If you place .001 fixed condenser ‘geries with the .00025, you can use it for tuning jnstead of alterin; the coils. The .000385 can be used for reaction. 2. I have a 240/1 and a 24/1 ratio dial. Which would he the better for the tuning. . . A.: Use the larger for and the smaller for reaction,