Questions and answers

Radio Record, Volume IV, Issue 36, 20 March 1931, Page 15

Questions and answers

"PROB" (Otago): Would a water pipe 12ft. away from my set make a good earth? If not, what would? Aes It depends how soon the water pipe itself goes to earth, If it does so almost immediately it would be quite efficient, otherwise why not try the Pierce earth system? eS. (Westport): When I switch on ¥. my set there is a sound in my loudspeaker which resemblés a clock chiming. This fades out after about one minute. A.: This is probably due to the spring mounting of your detector valve, which vibrates when a slight jar, such as that occasioned ‘when the set is switched on, is communicated to it. There is nothing wrong. J A. (Ohbristchurch): I have a fivee valve American set, but find there is no provision for grid bias. Where do I connect this? ‘The g.b. terminal on, the transformer is connected to "A A.: Break this connection and take it to "O-," connecting "O-4." to "Am." 2. What alterations are necessary to use a power valve taking 135 volts "B" in the last stage? At present I am using 90 volts on both audios. "~~ A.: Do not wire your audio stages as 135 volts is quite suitable for both. See, however, that you apply the correct grid bias to each stage. "EXPERIMENTAL" (Oamaru): I have a variable condensed with 15 plates. What is its capacity and how many turns do I need on a valve base coil to match it, I wish to make the "Night Hawk Two." A.: 00025 mfd. See the coil table published in a recent issue. ) OSEPH Dunedin): I intend to ¢onstruct the 1-valve shortwave receiyer and would like to know if I could ase a power valve? A.: No, this would not be suitable. You require a valve of the A415 type.

2. Would an aerial 40 feet high at the far end, 20 feet high at the lead-in end and 60 feet long be suitable for shortwave reception?-~Yes, but it would be better higher. 3. How should I construct the coil? A.: You give no details whatever of the type of former you desire to use. See the "Radio Guide’ for instructions on coil formation, M A. GRAVEN (Wellington): ° Would an £210 valve deliver as much volume as an H210 when used in the Differential One? A.: No, the latter is more suitable. KAYGEE (Wellington): Are the shortwave adapters I have seen advertised really efficient, and whieh one would be the best for my set? Is any technical knowledge necessary ta fit one of these? A.; The only commercial ‘shortwave adapter we have tested is the "Addiphone," made by Mack’s Radio, Kent Terrace, Wellington, It gives excellent results on shortwave, and technical knowledge ig mot necessary to operate it. 2. Gan it be operated from the ed. mains? A.: Not satisfactorily. It is more usual to have batteries supplying the current to the two valves used by the adapter. Qscak (Marton Junction): I have ‘two 171 valves of different makes in push-pull, When I turn the volume on fully a blue haze spreads over the interior of one of them. Is the valve defective? A.: It seems that the valve is defective, though without saying how the volume ig controlled we cannot offer much explanation. If it is controlled by varying any of the constants of the valve in question it can be interpretated in terms of electron emission and the gaa in the valve. Quite probably it has not been thoroughly exhausted, but so long as it

works there is nothing to worry about. Possibly the life may be shorter than another valve. 2. Why can I hear musie when the speaker has been disconnected A.: That is due to one of the laminations in the transformer being loose. The signal in the winding of the transformer pulsates backwards and forwards as it does in the speaker winding, but whereas in the latter a diaphra or armature is moved, in the former the loose lamination responds. " TPxPO (Blenheim) : Is my valve combination economical for both filament and plate voltages?-Yes. ‘ 2. My set will not oscillate on the 27 to 58 metres coil. A.: This is probably a dead spot. Try the effect of a midget variable condenser in series with the aerial. TPERMINAL (Dunedin): I have built the detector of R, the W. Two, but I cannot get it to oscillate except when using the largest tuning coil, A.: Apart from increasing the grid leak to 10 megs., trying another valve and increasing the battery voltage to 674 we can suggest little, as R, the W. Two has given perfect satisfaction to hundreds of constructors. | We do not in: necessary to moze nS 2 to the tickles, e__

2. How many turns should be on a coil designed to bring in the 80 metres band? A.: Using a .00015 condenser with a -eoil 3in. in diameter, the number of turns should be 20. AY ‘DIO. (Manawatu) ; Approximately what would be the range of a coil of 24 turns of 22 enamelled wire on a UX valve base? A.: Probably 15/25 metres. 2. What would be the approximate range of a coil with 32 turns? A.: About 20 to 35 metres. GIMPLE (Petone) : I have constructed the Daniells Oell "A" battery, but cannot get results. A.: You should connect the set up to the charger and take a reading when it is on load. A set using such little "A" current as yours does, the battery should work quite well. We found it would deliver between a 3 and 4 amp. without difficulty, and your set requires only 4.amp. Are you sure you connected it round the right way-that is, the copper to the positive terminal? And we presume you connected your "B-" to "A" og is usual with "A" batteries? D H.A. (Dunedin): I have a three- * valve commercial receiver, but my tion on distant stations is not as as it was a short time ago. Oan you explain? ; A.: If your batteries are quite in order it would seem. that your aerial and earth require attention. The earth particularly becomes corroded and the lead ‘wire makes a very poor contact. Examine your aerial insulators and be quite sure there is not a short circuit across them. It pays to let the aerial down occasionally and examine it. 2. On looking over the set I find the reaction control has two fixed terminals and ons moving. Should the extra

terminal be connected anywhere? -. A.: No. It ig on the same plate ag the other fixed condenser. -Two are provided merely for convenience, . The’ set breaks into oscillation when the volume control is turned down. Can this be avoided? A.: If by the volume control you mean the filament rheostat it is impossible to contro] the oscillation unless you use the detector voltage. Hach valve has an optimum filament temperature at which it works best. This may be even a volt less than the rated filament voltage, consequently your set will oscillate easier when the filament is at this temperature, 4, Will a pentode valve work more successfully than on ordinary valve that is only 90 volts : Yes, a pentode can be quite efon 90 volts. but for general purposes. we are inclined to favour the high-gain power valve. . Will a heavy aluminium sereen be substantial than the one provided? one? Yes, but it is not worth changing ov. "What speaker would you advise for sensitivity tone? A.: A well-known make of light cone. 7 I have some sal ammoniae. Can I use this for my accumulator? A.: Most certainly not; # you put it anywhere near your would ruin, them.

R D. (Pukekohe) --I have constructed * ‘a Daniel charger, but it is quite dead. I poured the acid solution into the porous pot, and put in a handful of bluestone. = A.: That is where you made the mistake. The bluestone does not go into the sulphuri¢ acid, but with rain water and is used upon the outside of the porous pot with the copper. Only the sulphurie acid solution and the zine are inside the porous pots. A.X. (Waikato).-How is the cireuit *- appearing on page 143 of the 192930 "Radio Listeners’ Guide" neutralised? A.: By the RT, which act as grid suppressors. Do not, however, contemplate building this receiver, as it is quite out of date. 6 ERY 2. I have a battery set is ‘very much the same, There is a fixed condenser between the detector plate and A-. Should this be connected between the plate and A-, which is earthed. A.: Not necessarily. ‘The function’ of the by-pass condenser is to convey audio current to earth, and the high-frequency resistance of the "A" battery can be ignored. Ag far ag these currents are ¢coucerned, it is a dead short to earth, .. 8. Is the circuit I enclose correct, and could it be neutralised? post A.: It is correct as far as it goes, but it 4g fairly old-fashioned. Reflex cirettits are quite out of date, and you would: be better to use a modern three-valve receiver, It could be neutralised by the split primary method illustrated elsewhere or by a grid suppressor of 500 ohms. 4, at is the best type of crystal, for reflex work? . A,: A earborundum would prove best in the long run. 5. Is the advice reganfing rejuvenating valves in the "Listeners’ Guide" correct’

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 ..ccccccscosccceecoese Number of Vales sccorceoesssvese0 Name 0 6000560000 0F00sOSdG FOS ° AGGre88 — seesocccoee oonwate Coe cee ease ee CSOT OSS SWECRS TESTIS TNT POUUTUEUIT TEC TIT Nom de plume eesoecesoeneaensese To be kept in subsequent inquiries. Date PUPUTE LIC Please Noterx- 0! 8 SET (4) Be specifi dnd brief, imbu~ lating, if possible. Z (2) Write legibly, and on oné side of the paper. (3) We do not design eireuits, but aecept .suggestions r feature articles. Solving trouble, as different from advice, is difficult by correspondence and while letters are given ¢évery consideration, answers are not necessarily ecorrect-they are only our opinion based on the matter supplied, which may be quite inadequate. Intricate and involved specifications cannot be supplied without a specialisi’s fee,

A.® Yes, it is certainly correct, but it applies to the American valyes-201A’s, 199’s, 171’s, 210’s and 200A’s ‘The procedure, however, is not widely practised. ‘6, Will it harm valves that have 4 normal emission ?-Yes. 7. Is there any method of finding the ratio of a transformer? A.: It can be done easily enough with instruments, but not with any degree of accuracy by an amateur. Kowal (Christchurch): : I cannot get the wave-trap described by "A.A.H." to work. How can the condenser make any difference when it is ‘only connected with the 10 turns of 18 d.c.e wire? A... Unfortunately we told you in that article to connect them up to the wrong winding. It was a mistake that was cor--rected a week or so ago, and by making those alterations you should be able to get the set to work. The fact that you are using a .0005 condenser. should not affect the results. MYStirieD (N.P.): I am building the: differential series for.a shortwave set only. Are two .00016 tuning condensers satisfactory to work with the short-wave coils described in the ‘Reeord’’? A.: It would be better to use a .0001, though. you could use a .00016 with very little alteration in the tuning. The coils for this sized condenser were described afew. days ago. Add a little to the tickler. windings.

‘Which is the better size for differential reaction, 00018 or .0002?-The iatter. ) 8. Should the two h.f, chokes specified be a special short-wave type? A.: No, only the one the detector. 4, Would an aerial of 90 feet be suitable for this: set?-Yes, 5. Should I leave out the plug and socket arrangement ?-Yes. . Do you know of anything else important I should do? A.: No, you.seem to have lighted upon the special points that enter into the construction of a short-wave set. TTRANSMITTHR (Kerepeeti): Could you give me the circuit of a goo short-wave receiver employing pentode and screen grid valves? : The all-wave Kestrel three will- be shortly, and this will make up into a good shoes ware receiver. 2. Where eould obtain a book on transmitting? A.: "Handy’s Handbook" is a standard work which can be obtained at least from the Te Aro Book, Depot, Wellington. ~ 8. How can I work out the number of spaces required in a condenser to give it any given capacity ? A,: That is not easy to do, but vou will find ‘the full information. in ‘‘Handy’s Handbook." * H.R. (Auckland).-I am using a * three-valve set which oscillates smoothly, but I cannot control the audio oscillation, although the set may be a

long way from oscillation I get a continual squeak in the speaker. A.: Try reversing the primary connections to one of the transformers, If this is not effectual, use the anti-motor boating device illustrated herewith. Beye (Waikato): Could I use audio amplification for my three-valve commercial set? A.:-No, your set is probably a detector and two audio, in which case it is not easy to add another stage. If, however, it has one. high frequency stage, an external audio stage can be added. How many turns of 24 d.s.c. wire on a 8in. former are equal to 80 turns of 26 gauge wire on the same size former ?- ‘8. Is a log of 50 broadeast stations good for my set, which uses an earth consisting of four pipes each driven Sft. into the earth? The system is watered by a leaky car radiator, A.: Your log is quite a presentable one, and your earth system good. You are certainly getting all to be expected. Would using a number of earth wires as illustrated in the diagram of the Pierce earth system put the oscillations out of phase if the wires were not of the same length?-No. J G. Dunedin): What size. of -wire * would I use in making a rheostat of 30 ohms when wound on an ebonite tube 2}in. in diameter and 3-8in. long? A.: You could use 26 gauge Hureka wire, ‘which has a resistance of about ohms a yard, so you will require about 17 yards, or 85 turns. To get it into the 38-8 space you could not use sufficiently heavy wire. .

2. How -.many plates of. the size en-. closed are necessary to obtain the .00035 aerial condensers? ; 12 fixed and 13 moving, 8. Could you name a compound to immerse fixed condensers in after, they are made so as to prevent moisture from interfering with capacity? A.: An ordinary wax is quite -satisfactory, 4, Would the insulating compound, such as transformer wax, or other compounds used in transformers, be satisfactory? . :_ Generally speaking, yes. 5° is the best for condenser | plates, brass or aluminium ? . A.: There is very little difference, though the more expensive condensers use rass. ; AC" (Johnsonville) : My B.D. is very efficient. Would it be worth while changing it to the "Outspan Five’? A.; The "Outspan Five" is certainly the more sensitive set, though if ‘your B.D. is giving you good it is hardly worth 2. How many moving plates would I have to remove to make my S8.L.F. .0005, thirty-three plate condenser, into a .00035 ?-Seven. 8. Would this condenser be just as efficient as a straight-out .00035? AL: Hivery bit as efficient, G ©. (Greymouth): I have built the ¢ "Differential Three," but have very little success. I have to add another 40 turns on the reaction coil before the set will oscillate, and now the s.g. valve makes yery little difference. : Is the r.f. choke fully. efficient, and check very carefully the connections to the first coil? Trace out the internal wiring of the coil, but be quite sure that you have brought out each wire to the eorrect pin. It is not infrequent that the number of turns on the reaction coil has to be increased. Very much depends upon the valve and upon the capaun of the reaction condenser. BEND." (Wellington); How many turns of 30 d.s.c. wire. must I wind on a valve base eqil to tune between 200 and 600 metres to the .00035 condenser? A.: 140 turns. 2. Would the aboye coils placed in 3in. diameter aluminium shields be too close to the metal?-WNo. 4, What Philips A.C. valve is most suitable for-the anode bend detector? A.: H415 or E409. 5. Is. the circuit I enclose satisfactory ? A.: It is not the best, as the audio and radio valyes should be piassed. Furthermore, the intricate tuning arrangément would probably give trouble. Your best plan would be to try the "Radiogram Five." deseribed in the 1931 "Guide." . 6. Would a-.5 mfd. condenser do to bypass £415 cathode to earth? A.: In your diagram you have shown it connected directly to earth. If you mean to place it across the pick-up, then it is. not needed. ‘The pick-up would work quite satisfactorily in the cathode. 7. How should I proceed to balance the ‘cireuits using the three .00085 condensers ganged? : By trial and error. First of all your three condensers together until the signal is at its maximum strength, ther loosen the shaft and balance each cir-. cuit separately. If the condensers require any adjustments separately the coils must: be altered. The condenser that is in farthest, controls the coil which has the least number of turns, and turns must be removed from the other coils until all three condensers are in line. P. M. (Ponsonby) : Would the s.g. valve overload the detector valve on the local? A.: Yes; especially if you are close to the local The best plan is to incorporate the system shown in the "Outspan Five" of: connecting the aerial through a .0001 blocking condenser to the plate "of the sereen grid valye. For local work you can: connect the aerial on here and turn out the s.g, valve ,

— 2. Using a four volt s.g. valve, would’ T need to place a 15. ohms resistance i the negative lead if o six volt accumulat is employed? A.: Yes. , ' 8. If a rheostat is also in that position, what must I do? A.: Do not take any chances. Connect the resistors in series with the rheostat. 4, Using the set mostly for shortwave, is it better to place a midget condenser of one fixed and one moving plate across the detector coil to make easier tuning? A.: No, tuning will not be difficult if the fixed condenser is employed as is described. For shortwave work, however, it will be preferable-to employ a separate rheostat for the detector valve. ~5. Which will be connected to "B-"", "A" or $6 A A.: It does ast matter which ; preferably "A-", 6. I am not too sure about the coils. ‘ A,: They will be described next week. RID" (Dunedin) : Reception has b continuously marred by a roay¥pg on all but the local station. 1 notice that when the cover was taken from the s.g. valve the noise diminished. Has statie been bad generally recently? A.: It sounds as though one of the grid resistances in your set has broken down, and the set is now oscillating. We would advise you to have it seen to by a local radio technician. Your valve: combination is-a mixture... For your set we advise the use of either of the first two makes of valves enumerated in your letter. 2. The aerial is 50ft. long and 30ft. high, using iron poles. The earth ‘passes about 2 foot below where the lead-in enters the house, and is connected ‘to a waterpipe. Would this arrangement have a detrimental effect upon reception?-No. 3. Would the Pierce earth system be of any use here? If so, how far apart should the -pipes- be placed, and of what use is the car radiator? A.: In our opinion the Pierce ear system to be of definite value, must ‘be installed with the pipes 6ft. apart. The leaky car radiator is provided to keep the ground moist. 4.: What is the best lightning insulator for the lead-in? A.: Your ‘question is not quite clear. We speak of lightning arresters, and insulators, which have nothing to do with lightning. A good lightning arrester should be installed outside, and because of this must be moisture proof, 5. How can the wavelength of a station be.determined from the dial setting? a

\ Av: Hither by constructing’ graph, pwhich is a more or less cumbersome busijless, or by approximation. ‘By -the latiter method the settings of several stations are noted, and if a new and unknown station is logged the wavelength -can be worked out from the station’s setting in comparison with nearby stations. Thus, if you tune in a station half-way between 2BL on 850 metres, and 4QG in 3895 metres, you can be certain that wavelength is. about 372 metres. ©. ; 6. What. can I do to improve the tance-getting ability of my set, which is not as good as it should be? . A.: Reetify the fault referred to in question 1, 7. Oan I do.anything to reduce the statie noise? Coat me . A.: Ordinary static you can do to reduce, but we suspect in your case noise is arising because of the trouble previously referred to. BEGINNER (Port Chalmers) -I am using a three-valve battery set 13 miles from Dunedin. My aerial is 20ft. igh, and total length of 100ft. Would a ‘hyeher aerial improve distant. reception ? .: Yes, increase the height to 40ft. if possible. 2. Would a shorter aerial sharpen the tuning of 2YA and yet not affect distant reception? . A.: No. Distant would be wealrened. Why not incorporate a wavetrap ° G L. (Auckland).-What' is a good outAe put filter circuit: incorporating a transformer if necessary? A.: A low-frequency ; choke the speaker terminal with a 2-mfd. in:one of the speaker leads, Otherwise use a special output transformer. . 2. Could the B.D. Four be made more efficient ? 7 . A.: Yes, the "Outspan Five," using two stages of s.g., is much more sensitive.

3. Can the coil of the wavetrap de seribed by "A.A.H." be wound ona lowloss former?-Yes. 4, Could you tell me of a good grid bias yoltage control for my set, and how is it connected A.: The taps on an ordinary grid bias battery give fine enough adjustment. C+ is connected to A-, and. O- to GB on the transformer. . Son 5. Is it an improvement to use a condenser in series with the earth?-No, 6. What. ratio transformer should I use?-A.: 38$-1.

7. I have been advised to use a variable condenser in the aerial.. What capacity should this be, and cait a wavetrap be used with it as well? A.: About .0003mfd. A formodenser is much cheaper, and answers the purpose quite well, It can be used in a wavetrap such as that to be described in the 1931 "Radio Guide,"- which will be published on March 25. eae 8, Should I use bright or dull emitter valves? Of what type should they -be? A.: Bright emitters have gone out some years ago. Use an r.f. det. Lf. and power, type valve. 9, Are soldered joints more efficient if bound with insulation tape?

A.: No, not at all, but if exposed to the atmosphere corrosion is prevented. 10. Should I use single strand or. flex Glazeite for wiring?-NSingle. 11. How is the primary and_ secondary wound on‘a‘spider-web former?" = A.: By winding primary and secondary continuous and them cutting the wire to separate them. ; 12. What type of neutraliser is best used in a B.D. circuit? : A.:. Split primary, but then it is not the B.D.: 18. What resistance should the rheostats for the B.D, Four have? A.: It depends, of course, on the voltage and amperage of your filament supply, and on the voltage and the drain of the valves used. 30 ohms. is quite a usual value. J H. (Wellington)-Has an a.c, -10- * valve set, and eannot bring in America. . A.: Thig is probably due to your local- . 2. Do you recommend any other valves for this set? . A.: A set of R.C.A, valves should you better results, Af M.H. (Dunedin) -I have. a .2-valve * BD, and a gramophone amplifier. Can I put the output from the set through the latter? st

Av: If your two-valyer is an r.f. det.yes. If not, you are jiable. to get motorboating. : 2.-Should the two be connected, as shown in the accompanying diagram?- ; 3. At present I am not using the rf.c. | Will this be necessary when using the amplifier ?7-Yes. . Can I. incorporate differential reaction in this circuit? If so, is the diagram correct?-Yes. 5. ’m not sure whether the lay-out of my set is correct or not. Where can I gecure an efficient lay-out? A.: See the lay-out for the diff. 3, but omit the a.f. stage. W A.P. (Haleombe).-Has a T-valve . * a.c, commercial set using an aerial 76ft. long and 35ft. high, with a galvanised iron pipe driven 5ft. in the ground for an earth. He receives several American very faintly, and desires to know how he can improve volume. ’ A.: Is your earth lead short, and _ the ground surrounding the pipe damp? Your aerial may be shielded and not properly insulated. If you have had your valves over a year get them tested, J B. (Auckland): Why do I get better ° results from a 45-volt battery block after 24 years’ use than from an eliminator? / A.: This is probably due to the lack of by-passing in your set. Try a .006 condenser from "B--" to earth. Hxactly what do you mean by better results? Is the signal strength from the eliminator less than from the. batteries or is it less silent? . . My set is 24 years old, but gives good results. Could I expect better results from a modern three-valve set such as you describe in the "Record"? A.: When people are getting results we are loath to advise them to change to another circuit. In our opinion the "Differential Three’ has been the best set of its size yet described, but if you are a little unfortunate and could not get it.to go properly you would ‘be disappointed because you had changed. This w do not want you to be.

8. Can you tell me where to get adiagram of a booster suitable for my set? A.: We do not advise the use of boosters of any description. ‘They are decidedly a very unsatisfactory and poor compromise. Far the better ‘plan would ‘be to ‘build another stage to your set such as we have described. . Z." (Penrose).-I have a fivye-valve ~* neutrodyne, which sets up a scratchy noise when tunéd ‘to a Station. I have to tune it out slightly to get decent music,: What- causes this? gOS es _ Av: It sounds ‘as though the grid bias is not adequate, or the set may be going into oscillation, Furthermore, your valves may be deteriorating, or your batteries may be low. eae ee 2. How many turns would I need on the coils to match condensers with 15 plates? A,: Use a tenth more than those specified for the .00035 condensers ‘mentioned in ne eet we th ye ag a . at is the capacity of the Lissen differential y eS A.: They probably manufacture several, .0002 is about the average: ~ * 4, Have you any back numbers ofthe "R.R." dated December 20? . A.: A few. " "DEB" (Helensville) : Could you supply me with a circuit. for -a-two-valve short-wave set? ico A.: Your. best plan would be’ to make a battery adapter for your present set. One has been described in the 1930 Guide," and it should cost no more than £5 complete. "QPHAKDR" (Whangarei): ‘ Would you explain the full meaning: of kilocyeles attached to stations?*'= ~ *"=- A,: An explanation has been made ‘véry fully in the "Radio. Guide." Kvery jtation transmits an’ electro-magnetic wave which goes out on a definite number of times in a second. The number of times. the peak of this wave leaves the tramsmitting station is known as the fre‘quency. Thus from 2YA there -are- 720 waves every second. If now, the distance ‘(Concluded on page 31.).-

Questions and Answers (Continued from page 15.)

bebween one wave and the next one is measured, the wavelength will be given. It is usual to make this: measurément in metres, for it is a more scientific measure than anything else, We should find in the case of 2YA that the wavelength was 416m, The product of wavelength and frequency is always a constant, i,e., 800,000, the velocity in metres per second of electro-magnetic waves. "DADIO FAN" (Wadestown): Before converting my 2r.f. B:D. into the "Outspan Five," I would like one or two questions cleared up. What would be the

LE TE AE EE A A TO ET eee eee number, of turns for primary and second-. ary of the two coils using 24. and "34 d.s.c. wire for a condenser .0005 and .00035 ? A.: For .000385,-100 turns see, and .40 turns primary, 60 tickler; for .0005, by turns, 30 turns primary, 50 tickler,) 2. Would 5m.f. condenser be safe -for use instead of the consider two imfd. condenser ?-A.: Y 8. In the theoretical diagram of the "Outspan,"’ the B plus sereen goes. to the top of the valves, and nowhere else, but in the reading matter it appears that top of the valve goes to the second and third coils, which I take it to be the grid. You are just a little confused between the English and American. -type valves. Our diagranis were prepared. for English valves, which has the plate at the top of the screen, and the sereen coming to the plate terminal of the valve socket. If. you want to use American. valves, the top terminal will certainly be ‘the grid, and connected to the top of the coils, while the grid terminal on the valve socket will become the s. grid. UND" (Westport) : What capacity, fixed condenser must I have in series with my .0005 to reduce it to .00035 ?-.0002. 8. What, capacity fixed condensix would I require to put in my push-pull. stage to by-pass any high frequency current, and how are the ‘fixed condensers connected? Your question is not very cleaz. By-pass condensers are used in push-pull stages only in the output filter, and across the biasing resistance in the case of a.c. sets, The value is usually one or two mfds. If you want it to act’ as a‘tone control the value will be .025 mfds., and is used in series with a 50,000 ohms

rheostat across the grids of the push-pull valves. A D, (Wellington): I can put only 44 *volts bias on A409, which I use in the second last* stage of: my two-valve. set. The correct bias for the last. stage valve is 15-volts, and. it makes no dit ference at all if T vary this, A.: In the ease of the first valve, 44 yolts will be ample, because 9 volts only are taken when 150 volts are on the plate. Are you quite certain in the case of the output valve that you have the bias voltage connected round the right way? The Jast transformer you are using appears to be questionable. Try substituting it with one such as AF3 or Al, "PUZZLED" (Wellington) : I converte ed a home-made three-valve set to an all-wave set of English design, using plug-in coils. ‘There is an increase of volume on. the local station, and although I can get the carriers of the more distant stations, they are really weaker than before. A.: Your circuit seems perfectly Sate isfactory. The. coil specifications are likewise in order. Providng your valves and. batteries. are satisfactory, your aerial and earth system beyond question, your set should function perfectly. on distant: stations. 2. What will reduce the hum from an eliminator? A.: Hum in an eliminator is due soinetimes to a defective by-pass condenser or ehoke. _ We think in your case the better plan would-be to contact the firm © who supplied the eliminator. , I use a good cone speaker, but when the impedance switth is up something betwen a whistle and a groan ensues, though when the switch is down the speaker is all that is to be desired of it. A.: Then why put it up? It would seem that either the speaker and the last valve are not matched when the switch is up, or there is something defective in-the loading coil controlled by the switch. +] I am using two PM4 valves in the audio A.: This should not be, and is possibly a good deal to do with your trouble. You are saturating the primary of the first audio transformer by the high anode current of the PM4. This should be used only in the last stage. Replace it by a g.p. valve such as PM3. K. J.A. (Wellington: I have built the 4-valve B.D.; and I wish to convert it to alel operation, * so built the B éliminator described in 1927. I now wish to change over to complete a.c. operation. Where can I obtain an all-wave circuit? A.: In the 19381 "Guide" there is a description of the "Radiogram Five," and from it you should be able to obtain particulars of the r.f. and detector stages. You could use a pentode following. the first transformer instead of the valve shown. ' We shall bring out a complete three-valve a.c. circuit when qe come to do the a.c. "Kestrel," but this is some way in the future. 2. I intend placing heater windings on the existing primary transformer. Would a 80 gauge primary stand the extra drain? If so, approximately how many turns would I need for four volts? .: You are cutting pretty fine by using the 80 gauge, but we think it would just do it. The number of turns would be 380, Can I supply all three or any twdé of ‘above valves from one heater winding? A.: The best plan would be to put 442 and W415 on one winding, and C443 on another, although the three could be taken from one winding without unduly overloading it. Can I obtain gri bias from this : eliminator? . : Yes, but it will reduce the B voltage "by the amount taken by the bias. In your case, however, you appear to have -plenty to spare.