Questions and Answers

Radio Record, Volume IV, Issue 11, 26 September 1930, Page 24

Questions and Answers

"AcE" (ikusahid: Both requests arrived within a fortnight and your first one was answered last week, missing only the first week because it arrived late. Correspondents must bear in mind that "Questions and Answers is not kept cpen later than Thursday in the week. Queries arriving after this cannot be answered until the issue bearing date a fortnight hence.

B L.M. (Auckland): I have had my * three-valve set in use for two years and yet both B and C batteries are at full strength. Why? A.: You are indeed fortunate. Your set must be very light on B battery consumption. J.Mcl. (Banks ‘Peninsula): Can anything go wrong electrically with variable condensers? A.: Not unless something mechaniea; happens first. 2. What causes a small high-pitched whistle when the negative terminal of the speaker is touched? A.: This is quite in order. It is due only to an interruption of the circuit. 3. Do you consider — dynamic speaker gives faithful reproduction? -: We cannot make statements about speakers we have not tested. I find that electricity remains in my eliminator after it has been turned off. Why? — A.: It is due to condenser storage. 5. Is it possible for an eliminator to deliver current that is not properly rectied? A.: If the smoothing system is not in order or there is not enough of it a ripple will still remain though the cur rent will be properly rectified. 6. How is grid bias obtained from a B battery eliminator? -: By separating the "A" circuit from the "B" in the set and taking ‘"A-" through a variable resistance of 3000 ohms to "B-"; bring "C-" to "B-" and earth, and shunt the resistance witb a 2 mfd. condenser, 7. Why does a five-valve set with an rc. amplifier give much better signals without the © battery? A.: There .is probably something wrong with the resistances. This should not be. Although the signals may be louder the quality will be very much impaired by the absence of the C battery. Perhaps you are using too much or too little bias, 8. Can distortion arise from a crystal set

A.: The crystal cuts off certain very high and low frequencies, but if a pair of good phones is used there will be no distortion other than this. 9. When tuning the set without an aerial I find that many whistles are received. These can be tuned in the same as stations. A.: What do they sound like when they are tuned in? Probably they are very weak stations which you cannot receive properly without an aerial, hence the whistle. and the failure to receive

signals after the whistle has been tuned to its loudest point. 10. When hail makes a noise on the aerial, why can it. still be heard when the aerial is disconnected? A.: It is due probably to a very strong interference which is caused by heavy rain. This interference is probably so strong that you pick it up on your set without an aerial, It may be due also to moisture causing partial short circuit in your set. 11. When the eliminator or batteries are switched on the current takes a while to find its way to the speaker. A.; This should not be so unless your resistances are very bad. In the eliminator it will take an almost imperceptible time, but with the batteries the signals should be there as soon as the set is switched on. We suspect that some of those high resistances used in the r.e, amplifier are at fault. 12. Could three crystal sets be coupled to an amplifier to give three times the output? A.: Two could be utilised in the fullwave circuit described in the 1929 "Guide," but to our knowledge there is no method of coupling three.

U X. (Te Kuiti): What is the opti~ * mum size for shielded coils and the minimum diameter of the shields for use in a set using 224’s? , A.: Where there is to be high gain you | can reduce the diameter to 14in,, and even lin., making the shield cans the, size of $lb. cocoa tins. . 2. What effect has the shielding on ordinary coils? A.: It increases the self capacity of the, coil and introduces damping. 3. Can you tell me the approximate number of turns for each winding of three coils? A.: Your question is far too vague. We require to know the type of condensers you are using, the gauge of wire you wish to use, and size of former. You will find information on this point in the "Radio Guide," 1930, J W.F. (Dunedin) : I have a set of com- * mercially-made coils in a five-valve neutrodyne, each with 66 turns on the secondary. They will turn to 600 metres. How many will I have to remove to be able to tune below 200 metres? Will this impair reception of the lower frequencies? -: You can remove 10 turns with safety, but you will lose the stations on the lower frequencies, but this sized coil should be able to tune in at 2FC. O wise it will not,impair’ reception. . In which issue was the power "pact described by "Megohm’’? A.: The most recent was described in three issties between December 27 and January 10, These are obtainable, if not from your loca] dealer, direct from the ofce. 8. Where can I obtain the 1980 "Listeners’ Guide’? as I cannot get one in !

| | | | | | | LStENERS must attach this coupon to all queries sent to the Technical Editor (Box 1032, Wellington). Questions arriving without it are likely to go astray or be delayed. Name of set ......... ecesccccccces. Number of Valves .....eeecceseecce Name oem eeceeseeccccccescesescens Address Cec ccecccccccccccacecccece Nom de plume . ..........cccccceceTo be kept in subsequent inquiries. Date cei eie cc cee cc cec cece ewecccecee Please Note :- (1) Be specific and brief, tabulating, if possible. (2) Write legibly, and on one side of the paper, (3) We do not design circuits, but accept suggestions for feature articles, Solving trouble, as 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,

Dunedin "owing to its tremendous sale," as one stationer told me ‘Ax: We really cannot tell you. Perhaps some dealer or bookseller in_ other parts of New. Zealand may be able _ to eu. you. If ayone can locate a few "Guides" they will find a ready sale for them by writing us. : 4, On most stations the set has a sharp whistle and sometimes oscillates though it should not. If I shift the neutralising condensers the squeal stops, but I cannot get the distant.stations. A.: You should adjust your neutralising condensers so that the set is on the verge of oscillation on the highest frequencies. ‘If-you cannot get the results you expect look for the cause in other parts of the set. ‘ EF W.A. (Christchureh): Originally my ¢ eliminator was fitted with a rectifier which worked silently, but has now failed. \ I obtained another of a different make, but it set.up an objectionable hum. I have tried the usual methods of elimjnating this but failed. The only method to overcome the difficulty has been to eonnect the A battery across the terminal. A.: You have not given enough particulars. Do you have an A, B, and C eliminator, and is this trouble present in only the A? What are smoothing arrangements? If you shunted an_ electrolytic condenser of 2000 mfd. across the A terminals the trouble would probably disappear. J ELINCHY (Killinchy): I enclose two audio circuits-one using parallel feed audio and the other using a transformer in a choke capacity circuit. Which is the better and what is the percentage of volume reduction? A.: The parallel feed transformer coup: ling is the better with certain alterations It may be better to take the B-++ teraaipal of your transformer to earth and the "EF" terminal to the C-, though » there is very little difference between this and the method suggested in your sketch.

_ dt would be worth while trying both. \ Number 2, while producing a better tone, has a very marked lack of volume in comparison with the transformer coupled, unless an extra stage is used. 2. Which would reduce a.c. hum the most? A.: There would be little or no difference. 8. How would either circuit compare mn ‘""Megohm’s" double impedance couping? A.: The double impedance coupling would deliver better quality than either unless the highest grade transformers were ‘used in circuit number one, though it would not have the volume possessed by the transformer coupled. 4, Why does music sound from a pickup and come through the speaker at the game time? A.: It is due to parts of the pick-up responding mechanically to the frequeneies Bénerated in the coils. It is rarely -over e, though in good pick-ups it is reduced to a bare minimum. The pickup you mention, although not quite free, is as near so as the writer has heard. 5. Is there much advantage in cranked tone arm? A.: It has certain theoretical advantages, but most high-grade instruments do not use it. 6. Do you consider that fibre gramophone needles reproduce as well as do steel needles? A.: Not quite, as some of the higher frequencies are cut off. However. they are almost as good, especially if electrically recorded records are used. It cerinly pays to use them if the pick-up any tendency to damage the records J. G. (Auckland).-Where could I obtain a blue print for "Round the World Two’? A.: We have not published one, but the description in the diagrams published with the article should give you all the assistance you require,

MeAc (Christchurch): Results on’ the broadcast’ band of the ‘Round the World Two" are good, and on short-wave I have picked up several stations on the speaker, I have now added another stage as described later, but find that the set will not oscillate on short-wave. and even then it is rough, with the result that so far I have only picked up Morse. stations. A.: Try the effect of altering the capacity of the condenser in the aerial, altering the voltages and the grid leak, and the grid condenser if need be. ‘Try’ the use of an output filter. About a month ago in these columns some hints on this point were given. 2. I have tried voltages higher than 465, but the set will not carry it without screeching. thie The output filter should improve is. 8. Do you advise me to shield the set because I cannot, get a wavetrap to work in the usual way? A.: Shielding would be an improvement, but you say in an earlier part of the letter that you can cut out 2YA to receive 3YA at a distance of half a mile. Why talk about shielding the set if you can do this? 4. What are the correct voltages for each lead? A.: B +, detector 224, first audio 45, last valve 90 or more.

Foxx (Hokitika) -A hum, something like the roaring of the sea in the distance, has developed in my set. How can I stop this? A.: As it is an electric commerciallymade set, we would advise you to contact your district agent. 2. If I raise my aerial to 60 feet high and to 450 feet long, would my reception be improved? . A.: The noise background would outweigh any advantages gained. The best ‘aerial would be 60 feet high and about

50 feet long. Perhaps you eould try the Beverage aerial. It is about 500 yards long, 10 feet high, and earthed at the distant end to .00025 condenser. GTATIC (Christchurch).-I_ wish to change my seven-valve a.c. receiver to a short-wave set. What would be the cost? . | A.: It would not be practicable to do it. You should obtain a short-wave adaptor, such as the Addaphone. , F G. (Kelso).-I found the amplifica- | * tion in the "L.W." much louder and clearer than the transformer coupling. _ At present I am using D5 as a power _ valve, and do not know whether to use the | 224 or 245 as last audio? A.: The only practicable combination

for the L.W. is a 224, followed by a 245. Other valves can be used if they are the same or of similar type, though, if you depart from specifications, you must do the experimenting yourself, 2. In constructing a transformer I could fit only 62 lammations through the centre. Is this-satisfactory? A.: Yes, if they are tightly packed. _ 8. I have been using the tuner of the 2 r.f. B:D..in connection with this amplifier, but I need a variable grid leak for the plate and grid return of the amplifier valve instead of a .5 meg. resistance, but I find this more or less unsatisfactory? A.: Take the leads of the L.W. from the audio side of the first audio trans-former-that is, you must use a detector and transformer if you wish to get good results from a tuner of the nature indicated. 4, The heater element of the screen grid valve seems to vary. Does this affect the amplifier? A.: Most certainly, either your valve is defective or there is a defect in that filament circuit. AZEL" (Christchurch): Is a log ol 21 stations poor for an 8-valve a.c. set? A.:It is not too good, but of course your locality may be bad. Your aerial is not a good one. If you wish to use

a "T"’ aerial the lead in must come trom the exact centre, otherwise you will have to use an "I" aerial, taking the lead-in from one end. . 2. If I am surrounded by other aerials does this affect reception? _ A.: Other. aerials will screen yours toa certain extent, especially if yours is a poor one. ; 8. Does the fact that the speaker is by the set make any difference?-No. 4, I cannot receive ‘any American sta: tions. Why? A.: It is due probably to the poor aerial you appear to have: For DX reception the first qualification is a faultless aerial and earth system. 5. What is the average life of valves? My set has been in use for a year with an average use of four hours daily. A.: About 1000 hours. Yours appear to have had well over that, so probably a new set would strengthen up. recep. tion. "H Z." (Napier): Would you supply ¢ details of broadcast coils for PCJ four? A.: Coil No. 1: Secondary 195 turn: No. 82 D.S.C. tunes 320-550 metres. Re action coil 50 turns 36 to 40 ga. D.S.C. jumble wound in a pile. Coil No. 2: Secondary 96 turns of 28 D.S.C. tunes 205-350 metres. Reaction 30 turns 36

to 40 ga. D.S,C. jumble ‘wound. The rf. and det. coils are the same, Bring the aerial to the top of the secondary through: a .00015 condenser.. The coils are figured for celluloid extensions to the valve bases, and for .00015 mfd. maximum eondensers. 2. I have separated the coil windings. That is, the aerial and grid and the T. Anode and Reaction, about 1/6 of an inch instead of 1/16 as they recommend. Would this increased spacing affect the tuning in any way . A.:. Yes, slightly; and the set will not be so sensitive. Why not follow instructions? HAT" (Masterton): I have a six- ' valve set with the following com-bination-three radio DEL 610, detector L610, first audio P610, pushpull two P625. Iam having trouble with a continuous whistle in the pushpull stage. With a resistance in the grid leads it still whistles. When I take out one valve the volume increases slightly, but tone is inferior. If I replace: it and take out the other the set motor-boats. When I cross the wires the opposite happens. I suspect the transformer. A.: There are two or three interesting .points which suggest trouble. In the first case have you realised that your set is drawing about 70 m.amps, and at 250 volts, unless you have a super eliminator you will meet trouble here for very few: commercial eliminators will deliver this. If . you have 250 volts on the plate.as 625 requires, you will need a bias of 30 volts. Do you have that? Failure in either one or both of these directions will cause the symptoms spoken of, although the fact of crossing the wires over seems. to point. to trouble in one side of the split secondary. This could be tested by a cell and phones or voltmeter test. You should not be using DEP in the first audio stage as this reuires nearly 10 m.amps.. L610,° requirg 8.5 m.amps., will handle the output sufficiently well. If DEP is not biased

by 9 volts you are .getting a far greater drain. than 10 m.amps.' and of course eausing more trouble, "7 B." (Otago): Can I use a B elimin- * ator with my four-valve set? A.:In all probability yes, though there may be a slight background no stronger than barely audible. 2. In making the two-stage crystal amplifier described in the "Listeners’ Guide," where does the grid return go? A.: To the Cl terminal. FULECTRON (Wellington): I cannot receive American stations. When should I look for them and when do they close down? A.: Follow the DX notes and you will see when they are received. The usual time is in the afternoons and up till about 7.30 p.m., when they close down, that is, about midnight on the Pacific Coast. 2. Would Day’s Bay be a good -plaee for receiving the Americans? A.: Owing to power leakages and the screening of the hills there are far better places than Day’s Bay. 3. If I erected a larger aerial would the static increase in the same relation as signal strength? A.: Providing your aerial is 40 feet high and 60 feet long any further: increase would increase the noise level disproportionately to signal. S D. (Hikurangi): I have a Bluespot ° 66K. How ean I make this into a four volt dynamic? A.: It cannot be accomplished. 2. In my four-valve set I am using two PM4’s, RCA112, and an American 112 last stage. Is this a good combination? A.: You would have had a difficulty had you tried to get a worse one. You are using four power valves. The combination should be detector A415, PM 4DX, L410, or 221 in the American series. For the first and second audio you should use two PM8’s, 409’s, L410’s or 221’s. You can use your 112 for the last stage. . 8 Wher I increase the battery from the last valve the set squeals. Why? A.: Probably -because you are using 80 many’ audio stages and are reaching saturation point, 4. In this week’s "Radio Record" I notice several correspondents ask questions in connection with a third audio stage. You discountenance same, but I have had success. A.: You are really very lucky, but we would like to hear your set in operation, especially with the valve combination specified. I A.L. (Taumarunui): Where is the * neutralising condenser on my set? A.: For information write ©. and A. Odlin, Wellington. 2. I am troubled with distortion. Are the valves wrong? I am using P625 in the last stage, with 9 volts bias. A.: The grid bias on the power valve is altogether wrong. P625 requires about 15 volts when worked with 140 volts annode, although to obtain anything like maximum efficiency from this valve it requires 200 volts, with 20 or 25 volts bias. You should use P625.A., with 15 vok bias. 8. Are wet batteries better than dry? .: They are more economical, but require a little care, o B.C." (Auckland): I have a five-*-**valve set. What are the best valves to use? A.: Four 221’s American type, and one medium power valve, such as 112. PM256, 605, P610. "(NONTROL" (Invercargill) : Occasionally on switching on our set a moaning whistle comes through, which disappears when the speaker is lifted from the table. A.: This is due to a miergphonic detector. If it is an a.c. set see if there are any other five-pronged valves other than those with a connection to the top

and interchange. them ‘with the detector. If the set is not a.c. change the detector with another one of the same type. M," (Motueka): Can the parts * enumerated in my letter be used in R. the W. Two? A.: Yes, the variable condenser with nine moving plates and eight fixed has‘a capacity of approximately .00085, and can be used in R. the W. for reaction. 2. I have a fair amount of wire, of the sample I enclose. Can it be used?

A.: It is about 17 gauge, and can be used for the coils and the wiring of the se BEN ADHEM (Oamaru): My 1930 Guide short-wave set works well on long waves and brings in the American stations. Short-wave telephony _ stations are very’ weak. Would 30 feet of inside antenna be better than an aerial with a steel mast 30 feet high?-No. 2. I use an Hnglish s.g. valve. Might it improve matters to mount it horizontally and shorten the plate lead? A.: Quite possibly it would be better. 3. The maximum detector plate voltage is about 45 and it will oscillate between 23. The same valve used ina Cossor set will work with 135 volts on the plate. A.: This ig due to the difference in the circuits. It is. diffieult to explain why you, are not getting the short-wave stations, particularly as your set oscillates well. Many correspondents have written us telling us of their success with this very popular receiver. Short-wave tuning requires a good deal of practice and skill. Ifyou could raise your aerial to 40 feet, yet keeping it short, it might perhaps be better. "{NOLUMBIA" (Picton) : We have been using a five-valve battery set for néarly three years and lately the stations have been coming in with very little volume, althotigh the batteries are kept well charged. The rheostat makes very little difference. vt A.: Have you changed the valves since you purchased your set? If not that is the cause of your weakening reception. When you change them use 221 in place of 201A’s and a power valve of the 171A type in the last stage. W H.A. (Dunedin): Can you tell me ® the most suitable valve combina-

tion for my six-valve receiver for distance, (b) for quality.?, ’ (a) A.: Use (a) five 221’s or their equiyalents, and a 112 type power valve; jand (b) 171A type power valve. 2. Is my present combination a good one? A.: Yes. If you want to get distant stations try improving your aerial and earth equipment. : [ICKLER (Invercargill): Is an aerial 110 feet too long for short-wave reception? , A.: Yes, use a series condenser of 2. When I connect the "B" supply a spark ean be seen for the first time. What does this signify? . A.: That the condensers are charging. 8, I have obtained: plug-in coils made specially for the set.. Is it possible that one set will cover the whole short-wave band? : oh A.: Three or four coils will be necessary to tune between 25 metres and 120. You will have a difficulty in getting. lower than 25 metres with a .0005 condenser. ‘ 4, Sometimes I get 4ZP, Invercargill, on top of Wellington, but nowhere else except on its proper setting. Why? A.: It is probably a-reflected frequency.

AERIAL (Christchurch): Is an aerial 85 feet high at one end and 30 feet at the other, 50 feet long, two wires separated by six feet spreaders, with a 25ft. lead-in, a good one? j A.: It would be better if you could get it higher and slightly longer, but if you are cramped for room you have probably erected the best aerial- you can. B.K.Z.Y.". (Auckland): I wish to ¢ build a very sensitive receiver to get the Australian stations in broad ‘daylight. I am thinking of using a circui using four stages of screen grid followed by a regenerative detector. Would the fourth stage of the’ s.g.r.f. ‘tend. to make it unstable? i A.: You are certainly aiming to get & super-sensitive receiver, though. you will probably be troubled with instability, as four stages .or even three take a great deal of managing. . 2. Would it be too sensitive and pick up too much noise? A.: It would probably pick up its fair share of that and no doubt you would be able to hear more static than stations. 8. If it were working satisfactorily could I pick up two New York stations? A.: You are very ambitious, but we are sorry we cannot help you in this respect. We have no idea what the set would be capable of doing. It should be very powerful. 4, Would five tuned circuits give sufiicient selectivity for New Zealand conditions ?-Yes. . 6. Have you any suggestions to A.: Yes, two stages of screen-. \grid should be ample. You will probably have a great deal of trouble with four stages. Try the two first and if they work you can add the others. "QD." (Auckland): I am enclosing a cutting from your paper giving the specification of coils for the Cossor Melody Maker. ‘This is in response to a request from a correspondent who asked you for them recently. , A.: Thanks, "S.D." We could have located them, but our correspondent par. ticularly wanted valve base coils so we referred him to suitable ones. We appreciate your action. "A T.C." (Wellington) ; Can I elimin4° ate Wellington using R. the W. Three with a wave trap? A.: You should be able to. 2. Are ‘any particular transformers required? A.: Any good quality ones can be used

andthe ot the one you specify would be quite 8. Are any particular makes necessary for the other components? A.: Not providing good quality ones are used. 4, What is the diameter of the former for the coils (3in.) and the letters on the terminal strip? A.:A -, B + detector, B +- first audio, Cc -, A +, B + power. A- ¢ +8 and B. — are joined. _ What is the resistance of the grid leak k? A.: For shortwave use about 9 megohms, for broadcast 2. 6. What is the value of the rheostat? \-30 ohms. "7 What type of jack is it? A.: A combined jack and filament switch. 8. What is the position of the .0005 condenser mentioned. A.: 0005 should read .0035. reaction.