Questions and answers

Radio Record, Volume IV, Issue 32, 20 February 1931, Page 13

Questions and answers

(Written during the absence of the* Technical Hditor.) CRYSTAL (Wellington): Yes, you can reach Christchurch with a erystal set by boarding the ferry steamer. ACHELOR (Petone): No, marcelle + or pexmanent waves cannot be received on this circuit. S#O BI-WAVDR (Auckland); To see short-wavers in action go to any of | our beaches on a fine day and you will see p AcomEnr (Lower Hutt): We have questioned several dealers, and they inform us that they do not buy burntout Valves, NE-VALVER (Otago): You cannot get the set out of oscillation; your neighbours confirm this; haye you tried dropping bricks on it? (OPASSIOA (Dunedin) : What, haven’t you heard the Broadcast Band performing. Well, listen-in on any silent night GAz SO (N. Auckland): How is an > ordinary valve rf. added to the ) » "Night, Hawk" receiver? A.: When we bring out the "Kestrel" Differential Three we shall explain how this extra valve is added. We will then give you the coil specifications you require. 2, Which value differential condenser would you recommend for smoothest reaction in an all-wave set? : We have found .0002 to be of very value. ANGUS (Wanganui) : What do you think of the enclosed four-valye short-wave receiver? A: The diagram seems quite satisfactery, but in the 1931 Guide you find all the information you require on this subject, for we are bringing out a

four-valve a.c. receiver with special power pack, 7, 1.D.X. (P.N.): Could you supply * me with a shunt-feed method of coupling the audio to the detector of a Schnell circuit? a A.: See the accompanying diagram, 2. Is there any other method of eliminating audio howl except reversing the leads, etc.? A.: Yes, it can be eliminated by doing away with all stray capacity and coup-

ling between the leads and with the generous use of chokes and condensers. 8. Would it be possible to include a list of the Australian "Hams" in the 1931 "Guide? A.: We are afraid not; such a_list would occupy far too much space. They can be obtained from the "Radio Amateur Cali Book Magazine." . . 4 Would any of the receivers described in the "Guide" use other valves than sereen grid for r.f. amplifier? . A.: Probably not. Such circuits ‘received a good thrashing in previous "Guides," RAP OFAN (Cambridge): I am haying difficulty with the L.W. The plate current is about 50 amps, but the plate becomes red hot. Is this in order? A.: There is something radically wrong as by running the plate red hot you will soon melt it off. See that none of the resistors are earthing where they should not, and try another grid leak (same value), resistance in the bias chain. I cannot reduce the hum, although I have used 8 extra mfds. in smoothing. A.: This again points to a shorted resistance or a defective resistance in the bias chain. It should not be necessary to have to purchase the extra by-passing condensers, as all the smoothing is brought about in the circuit as we published it. 3. I am using a choke condenser output, The speaker return is connected to the centre tap of the filament. A.: That connection seems to be wrong. The centre tap of the filament should eonnect with the main chain of resistances and bias resistance only. ‘The speaker return should go to earth. From the plate of the valve the current should pass through the blocking condenser (used in the choke condenser output), and then through the speaker. The high voltage current should come up through the choke and blecked by the condenser

from getting to the speaker, should go to the plate of the valve. 4, Any interference with the grid circuit of the 224 results in a violent crashing in the speaker, A.: This is in order, H B. (Woodyille).-Some time ago * TI built a screen grid all-waye set, but now want -to.use differential reaction, Can I do so? . . A.: Yes, in a very short we be publishing the "Iestrel three." This will be the circuit your are requiring. HSMONT (Taranaki).--We regret we eannot do anything for you. Yours is clearly @ case where a service agent should be consulted. NOVICE (Wellington).-Could I increase the length of my aerial by 25 feet_and make it into the "TD" type? : You would me gaining nothing. . 2. I am troubled with fringe and audio howl, and the set is going in and ont of oscillation. _ A.; This is due probably to the method of reaction you are using. The Schoell type type of circuit,of which the moving plates of the condenser are connected to earth, is -better, Your batteries are far too low (25 volts instead of 60), and this alone would introduce considerable amvynt of noise and howl. COUNTERPOISH (Christchureh) — Which is the better type of wire for a counteypoise earth single or stranded? -Stranded. 2. I had a wavetrap constructed by 2 competent man from your last publication, but it is no good. A.: That is different from our experience and the experience of many of our readers. A wavetrap usually needs careful tuning, and must be made very carefully. We aré bringing out an excellent one in the "Guide," and perhaps you might get this one to work better. TTUBBY (Auckland): Would a four or five sets give me the chief New Zealand and Australian stations? A.: If it were of good design, yes. 2. Could you name such a set, preferably super-heterodyne? A.: Super-heterodynes have more valves than this. We have tested only one super-het., and that is the "Majestic." It is an excellent type of machine. 8. Could you give me some hints about picking out a reliable machine? A.: The first thing that anyone buying ga set must consider is the price. ‘ou will have to sort out sevéral in the vicinity of the sum you are willing to pay. The first test is for tone,.Satisfy yourself that you really like the music and speech that comes from the set. Above all, do not be guided by an overpreponderance of bass. A listener very soon tires of this. Hear it on the very notes, such as violin and piccolo, and of the very deep drum notes. These latter should be very clearly defined and not a jumbled mass. Hear what the set can do in the way of bringing in the main New Zealand and Australian stations. On a good evening, say, at about 10 o’clock, they should all come in very well, although much depends upon atmospheric conditions. For this reason you will require to hear the set under differ-

ent conditions. Do not strive to get the yery distant stations, such as the American, as these certainly. depend upon your locality. A set that can be relied upon to bring in the main Australian stations is all that is required, For selectivity your set should be able to eliminate the local station and bring in one close by, In Auckland you. should be able to get 2BL with only a slight background of the local, In Wellington 4YA and FULERPH (Reefton): I am having trouble with my reflex set, I cannot properly neutralise it and I have put the trouble down to the disappearance of grid bias owing to the valve being a six-volt one. " A.: This is probably incorrect. We think a more likely cause of your trouble to the erystal detector being at fault. In any case, reflex sets are very much out of date, so why not use your parts to make the "Kestre] Three’ or any other type of modern set?_ 2 I-require 9 volts "OC" battery on the last valye even with 75 volts on the plate. . ‘ _ A.: This is probably in order as you have a power valve of the 171A. which requires a good deal of biasing, 8. I_ can get only 75 volts from my d,c. eliminator. A.: There may be a leak through one of the condensers or a defective resistance. It is only a case of examining it carefully or having it tested by an expert. HISTLE (Masterton): My set when turned on develops a whistle, which gradually disappears. Sometimes I notice a slight fading in reception. Would shortening the aerial or placing a fixed ORRESPONDENTS must attach this coupon to all queries sent to the Technical Hditor (Box 1032, Wellington). Questions arriving without it are likely to go astray or be delayed. Name of set acoeeeseseeeesesenceer Number of Vales .ece e00sgesesees? Name SOPSHHSSHSSHASSSSSSSOSSSOSOSEED Address 2002000000 8099 ses eseeeoNe?¢ 999000559409 8S638 8006 SHS SOOSSSESTOSOE ROCHOSP OE ROEESSE HS SESHESOHEDOSESENO2E Nom de plume ..cccos eeereoseesses, To be kept in subsequent inguiries. Date BOSETOAETERE SE REDE EHESEOD Please Note:(1) Be specific and brief, tabulating, if possible. (2) Write legibly, and on one side rt) e paper. (83) We do not design circuits, : ut accept suggestions for feature articles, Solving trouble, ag different from advice, is difficult by correspondence and while letters are given every consideration, answers are not necessarily correct-they are only our opinion based on the matter supplied, which may be quite inadequate, Intricate and involved: specifications cannot, be supplied | without a specialist’s fee.

eondenser of .0001 between the aerial and the aerial terminal have any effect? A.: It is an expert’s job to make any adjustment to an ac. set. We should imagine that there is-trouble in the’bias resistors of the r.f, valves. Placing a condenser in series with the aerial will not have the desired effect.’ 2. Would line voltage or current surges cause trouble? A.: It is most unlikely... 3. Have you any other idea as to what the trouble may be? : A.: Other than the bias resistors ‘and the valves themselves there is little we can suggest.. The set should be overhauled with. a modern fault-finding. instrnument, P. N. (Hokianga) : Is my diagram. showing how to add an extra stage ‘of sa ‘erid rf. in order? : The valves would be better transcoupled as we described this week for the Outspan Five. 3. Should the coils and valves be screened? A.: The cans should be screened with an inverted can, and the valves only if oscillation cannot be controlled by other means. Is it necessary. to have the coils a minimum definite distance from the screen?. A.: About 2in. would be ample, 4, Should the secondary coils have the same number of turns ?-Yes, 5. © plus connected. to A plus, is this eorrect? A.: It is done with some sets, but the usual practice is to connect A minus to C minus. ' 6 My log comprises 60 stations in America, Japan, New Zealand, and Australia. ‘Is this satisfactory? A.: Yes; it is excellent. 7%. With the addition of a second sereen grid valve, would the set be more selective ?-Yes, ; ; S.¢ (Masterton): I have an American e neutrodyne set in which I have four valves that are not American. My secon radio and detector 2 minus A615, and I

znye a four and-a half "B" battery with A.: In these older American sets payticularly it is not advisable to use anything but American valves, at least in the radio and detector sockets. It would have been better had you used 201A’s or 221A’s in these stages where you have 615’s. The 615-is an excellent valve as a’ detector, but .it is not possible, especially in a set of your type, always to get good results when two are used. Your "O" battery should be increased to nine volts. OorLs {Hketahuna) ; Can a short-wave a.c. receiver be constructed. using ‘valve base coils? . Where can I obtain a circuit? A.: It could certainly be made, but would need special care. We-do not have a suitable circuit, though: a fourvalve one will appear in the "Guide." iD W.D. (Invereargill): Is, an output * filter of any advantage over an ordinary direct-coupled speaker ?-No. 2. What value of choke would I need for this? A.: About 30 or 40 henrys. 2. How many turns of wire would I take to make this choke? . A.: 5000 turns of 36 s.w.g. on a core Sin. square. No gap. — 8. Why does my set give an annoying growl when coming into oscillation? A.: This sounds like threshold howl. Shunt a 200 ohm potentiometer across the "A" battery leads to the detector and take the centre arm to the grid-leak, as shown in the diagram in the next column, 4, Should I be able to receive American stations on my set with an aerial 80 feet and 30 feet high? A.:-This depends upon your location. 5. Could I use one stage of trans-former-coupled audio with a stage of ree. following? A.: It would be better with.one stage of r.e.c. with a transformer following. POWER (Christchurch) : I have -made two chokes as described in the "Radio Record’ twelve months. ago.

Could these«be used instead of the chokes specified in the deseription of the L.W.? -Yes 2. I wish to run two 245’s in parallel and a 280 rectifier. Would this be satisfactory? — A.: Yes, We publish on page 14 the amended diagram of this cireuit for your information, TRICKLE CHARGER" (Khandallah); What is the mést satisfactory power valve or valves for the Loftin Four? A.: A single 245 will give quite a reasonable output, but if you are not

impartial to high tones you would find that one of the pentodes an excellent one. 2. Would 1250 turns be corrett for a single 245 valve with a transformer core dimensions of 14 by 1 3-8 as deseribed in the "Radio Record’ on December 57 . A.t No. For full-wave rectification 85u0 turns centre-tapped are required. F B. (Hokianga): My set has develop- * . ed a loud erackling noise. A.: It sounds, like a defective transformer, a- broken speaker cord, or a broken-down ‘condenser. If your set is a commercial one send it in to an agent to have it &ttended to. 2. I get.a shock -when-I touch: the speaker : This is quite in order as there is want high voltage floating about them. 3. A friend of mine has a set similar to my. own, but the switches work in opposite ways. ; This is probably :in order, the indication being that in assembling either yours or his, the switch has been turned upside down. QOXXZ (Timaru): Could I use a frame aerial and connect one wire to ‘the aerial and one to the earth? A.: No; a frame aerial must be connected in ad special way to the grid and filament return of your set whith takes the place of the first coil. Litt WILLIE (Timaru): I have _ & seven-valve American set which has decreased in volume: and is distorting on all stations, What is the matter? How is your bias battery? Is it up to full: ‘strength? Have your valves tested. 2. My speaker has developed a rattle on the high notes. .: This sounds something like grid bias being at fault. What is the height of poles from whieh the best reception is obtainable? A.: 40 to 50 feet is ample. A J:H. (P.N.): What is the best valve combination for my five-valve American set? A.: Four 221’s and one B605, CONTACT (Nelson): We recommend the American valves that you name for replacement in your set. Would a combination of valves of different makes give me better results than if they were all of the one make? A.: Vidlves should be all of the one make, with generally the exception of the last valye, which is not so important,

3. Wiho are the General Dlectric Co. , and what make and tube is the improved valve they are bringing out? A.: The General Blectric’ Co. are one of the biggest electrical manufacturers in the U.S., and for that matter in the world. We have no further details of the valves than have been published, 4, Would a resonater be a useful ad-. junct to my set?. A.: In all probability yes. 5. Is.a similar piece of apparatus already embodied in it ?-No, PROSPECT (Mount Eden) : The valves of my set are enclosed in a can of tin. Do you consider that a shield of aluminium would be better? A: For the valves, no. 2. "Have any people in. N.Z,. installed the Pearee earth, and what have been their _Tesults? : We have heard from quite a numben of readers who have had great sucee | with this system. . JY POOKY" (Dunedin): Is there any means of attaching headphones to a seven-valve all-electric? : Yes. Use.a special output transformer. An audio transformer would. be quite all right, the output being connected to the primary, and the headphones ito the secondary. Otherwise a’ variable resistance shunted across the headphones would give you the desired result, " LECTRIC" (Dunedin) : Why does my nine-valve a.c. set give purer reception on the harmonic than on the actual frequency of a station. Reproduction .on the latter sounds distorted.

ie I find also that the more distant the station the better’ the tone, , A.: It appears that your set is too power- , and the exceptional signal strength is overloading the detector, causing dis--tortion. For those stations on which reception is better on the harmonic, try reducing the length of your aerial, taking it off altogether; or putting the .0002 fixed condenser in series with it. N° GO (New Plymouth): Musie and speech are blurred, and a_ sifting noise appears in my speaker when it is ‘ called upon to handle any volume. I am using a 112 as rectifier. A.: The 112 has probably expended its useful life. Renew it and probably your reception will improve. Do you know anything about my speaker? A.: It can be adjusted by the factory representatives (A.W.W.), Wellington, i should not be touched by anyone ri e, . . ‘A ERIAL" (Port Chalmers): My set . is a seven-yalve battery-operated one, Should it be able to receive stations beyond 2YA? A.: Yes, with an efficient aerial and earth system you should be able to get all the YA -stations, all the Australian "A" class, and quite a number of the more powerful "B": class on the loud. speaker, This is provided, of course, that your locality is normal. 2. would two aerials be better than one? A.: No. Brect an efficient aerial, sucb as that described: in the 1931 "Guide," which will be published shortly. 8. What would be the best method of increasing volume? A.: If you cannot receive stations beyond 2YA, and your aerial and earth system is at all efficient, your set is. no’ working correctly. Take it to an expert for overhaul, 4, What should the earth lead be con--nected to for best results? ®e .

A.: It should be as short as possible. not necessarily insulated, and connected to a water pipe or to a copper or galvanised iron rod driven into damp ground. Connections should be soldered, "BEGIN NER" (Auckland) : If two-volt valves were used in the "Night Hawk" would the "B" battery voltage have to be reduced? ‘ .2.No. *, 2. How many turns are required for the coil, and at which points are these connected on the UY valve holder? A.: The making of the coil was fully described in the issues dated December 6 and January 380. 8. Should this set be able to bring in Wellington on the loudspeaker? A.: Yes, see reply to above quéry.

"7 ITTLE WILLIN" (Oamaru); I am using a four-volt "A" battery for six-volt valves. Would it improve reproduction if I use a six-volt battery? A.: Not very much, but sensitivity would be greatly increased. . In fact, working on a four-volt battery, distance results would be almost impossible. I change to a six-volt battery and find no improvement would it harm the valves to change back to four volt? A.: No, certainly not. But you won’t change back,