Questions and answers

Radio Record, Volume IV, Issue 33, 27 February 1931, Page 13

Questions and answers

(CULDE-SAC (Auckland): I have the Loftin Four about’ half-built, but IT was put off completing it by vcry competent authority, it being said that it was almost impossible to keep the h.f. out of the audio. A.: While admitting that the Loftin Four is not foolproof and should be undertaken only by those who have had eonstructional experience, still we maintain that it will work-for the simple reason we had it working. The h.f. can be kept out of the audio by the methods shown. Some of the correspondents have been successful in its construction; others have encountered difficulty. uur Own experiments have shown that difficulty can very easily crop up, and is sometimes very hard to locate, C P. (B. of Plenty): I am always in2 terested to read anything in your paper which may help to get good reception from all N.Z. stations and so forth. I would very much like you to send me a clear diagram of a good static eliminator or trap. A.: We very. much wish we could oblige you. If anyone could work out a suitable circuit it would be the last thing he need do, for the invention of that "would earry with it a very comfortable income. Engineers have been trying jever since radio was, to find such a cirbut so far nothing practicable has been devised that will cut down static without cutting down signal strength. H W. (Waipiro Bay): A nearby light9 ing plant badly interferes with reception. How can it be overcome? A.: In the "Radio Record" dated September 13, 1929, we went very fully into the problem. You should approach the hotelkeeper and suggest the methods outJined in that article. Try first the two condensers across the brushes, then the chokes could be introduced if that was not successful. GHORTY (Taranaki): I recently completed the four-valve screen grid short-wave set described in the 1930

"Guide" but can not get a sign of a signal. A.: You did not give us anything to work upon. Will the set oscillate? Have you checked all your connections by the phones and cell method? Have you checked the components, such as transformers and condensers? 2. What is the voltage on the screen and on the audio valves? , A,: The voltage on the screen is hal that on the plate of the same valve. The audio stage can carry the full "B" voltage available, but should be suitably biased if it does this. TRIER (Christchurch) : I was to construct a crystal set with a honeycomb coil capable of bringing in. our two local stations, the diameter to be 2%in., and I am using 13 pegs. How man turns of wire should I complete before taking a loop? A.: If you'do not wish to ‘use a condenser, take about 5 loops out altogether, that means one in every 30. This, however, is a very rough way of tuning. It is far better to use condensers shunted across the whole coil. For a .0005 condenser 80 turns will be required. PROTON (Gisborne-: What will be the extra cost of converting my four-valve screen grid set into the Outspan Five? A.: About £8, but it will.depend to a great extent upon the coils used. 2. Will I need to make a new set of coils? A.: You could use your old ones, but it would be preferable to make a new set. You could use your existing coils as first and third in the Outspan, making a new one for the second, but you would have to be very careful about the way you connected it up. ae Can an ordinary condenser be gang~-’ e@ A.: If it has a long. enough spindle, yes. It need not be hollow. 4, I removed a moving vane from a .0001 moving condenser and as a result there is about 40 per cent. less noise. Why is this? A.: It was probably touching the fixed vanes. e« The value of the conden- | ser will not be appreciably altered. TPERRACH (Kelburn-: I am making the R. the W. Three. Could a .00025 grid condenser with a .0008 variable condenser be used instead of those specified 7?-Yes. 2. Does the wire from the aerial terminal go to the fixed or moving plates of the neutralising condenser? A.: Hither, preferably the moving plate, 8. Should I put anti-microphonie valve sockets in every position? .A.: No, only in the detector stage. L J.L. (Auckland-: What diameter and ¢ number of turns are required for the two stage rf. B.D, with .0005 condensers and .00037, with a .00035 regeneration? A.: Using 24 gauge d.s.c. wire 57 turns for the .00035 condensers on a 24 in. former, and 82 turns for the .00037 condenser. Reaction will require about 25 turns and if you are using 200 "A" as detector the grid return must go to * the negative,

Jj AKA (Hataitai): What is the best combination of valves to use with an American neutrodyne set? A.: Four 201A’s 2201’s or their equivalents and a B605 type power valve in the last stage. ‘ This must be suitably biassed, Your set can be easily altered to take grid bias. , 2. Would 24 or 26 gauge wire be suitable for housewiring to use several speakers ?-Yes. ~) UMBO (Otago-: Are. the particulars given in the short-wave tables in’ the "Record"? for aerial and secondary both? _ A.: Yes, the same coils can be used in two positions. The primary terminal can be done away with by coupling

directly on to the grid terminal. In the case of the aerial transformer a condenser must be used. 8. Can I use a d.s.c. wire instead of enamelled ?-yYes, 4, Can I use potentiometer control for grid bias with a detector? A.: Yes, see the diagram. "NX K. (Wellington): What would be

""4e the cost to charge my "A" battery with a 1.8 amp charger? A.: As near as we can reckon it out you would get 100 hours for 1 unit, which, if over the minimum in the Wellington district costs you a penny. 2. Is there any difference between charging from the hot-point or from the electric light as regards cost? A.: Unless you have two meters, no. 8. What difference does it make if the field of the moving coil speaker is just run on the bobbin or should the turns be put on in separate layers? A.: It is quite in order to run them on without layers. H. P. (Blenheim) ; I wish to convert my ¢ short-wave set. to that in the 1930 "Guide." Will the valves be satisfactory ?-Yes. 2. Will I need to alter the value of the resistances, fixed condensers, etc.? A.: That we cannot tell you unless we know the value of the existing ones. "DAN" (Auckland); Can I use my commercially-made six-valve a.¢. set for short-wave? . A.: Yes, by using a sup. het. shortwave adaptor. , 2. What alterations would haye to be made to the set itself?-None. Would it be better to have a separate get for short-wave?-No. J P. (Waihi): On my super. het. I * picked up 2YA on a frequency that

was not a harmonic, Can you explain this? A.: It is not uncommon with shortwave receivers to pick up a strong station on several frequencies. This is what you heard. waar are the best Philips valves for my 5-valve American set? A.: Four -609’s and B605. ' 2. Are my present batteries correct90 volts "B" and 9 volis "CO"? A.: It would be better if you could inerease the "B" to 135 or more. you use Bé605 er must increase the "CO" battery to 12. ; 8. What is the life of my "A" battery -the set is in use for three to .four hours a day? , A.: If you use dry cells they will only jast a few weeks with the combination you now have in your set. With the combination we have suggested you might get six weeks’ service. Your best plan would be, if you cannot use an accumulator, to have 12 batteries arranged in series parallel. "TNCEPTOR" (Tariki): Would you recommend two 7 to 1 transformers in the one set? A.: Certainly not. When you use two transformers the ratios should not be higher than 3% to 1. 2. Is there any limit to the weight. of copper used for an earth?-No, Ared counts, not bulk. F A. (Kaipara): Would you send me ¢ cireuits of the most powerful four and five-valve portable radio e A.: We recently designed a "Home’-' and Country Three," and you could add © another stage to this. We eannot design circuits to order. We believe the Te Aro Book Depot has a few.

(CORRESPONDENTS 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 ..c5cccococccggooceeees | Number of Vales ccooseocecseceace Name 00006009080 8ADAISOSSSIOOTSOSIG Address 090000800000 e008 oee0ecso 600000000000 S00000000990908 soov verde 00 0sapeecaensenseacscesenee Nom de piume , 22.20000000000000000 To be kept in subsequent inquiries. Date Coneneooe servos see SOPIOnee 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.

ENTACOIL" (Dunedin): A fourvalve B.D. built from the- 1929 "Guide" does not have any kick. A.: This we cannot explain, as the information is too meagre. It has given excellent results generally. 2. What Mullard valves shall. I use in’ each stage? In this order; PM5X, 6D, 5X, 6. ; 8. "Guide" says UX199’s. What is the Mullard equivalent? A.: PM5X is an up-to-date version of the UX199. . 4, What is meant by ey in a valve? A.: We don’t quite know what is meant without the context. It may have been a misprint for grid bias or may mean grid to plate capacity, possibly general purpose. 5. I have two .0005 capacity variable condensers. If used with 56 turns are they as good as .00035 with 86 turns?es, 6. I have only three tappings on the eliminator, while the set calls for four. Can I use the maximum on both first and second audio?-Yes. 7. Are the primary and the tickler windings both of 30 gauge?-Yes. 8. I have 18° turns on the tickler and 16 on the primary.. Will this suit the valve recommended? A.: Yes, but you might have to reduce the number on the primary to 14. 9. How would I know whether to add or take off windings by the performance of this set? A.:-If it oscillates too readily there are too many turns on the tickler. Conversely, if. it does not oscillate there are too few. The performance of the set in relation to the primary can be gauged only by experiment. 10: With the valves that I tried, r.f, rheostat and detector rheostat, wee about half on and the audio has to be full on. If this is normal, why have an audio rheostat? A.: It is quite normal for some valves. The ‘audio rheostat in your case is not necessary.

11. What resistance should the three rheostats be? ' A. 80 ohms. S. (Khandallah):.I have recently * constructed a nine-valve a.c. broadcast super het. and have encountered trouble. The intermediate frequency amplifiers absolutely refuse to amplify. The set works beautifully with the if. transformers eut out. The first. and second 1.1. stages if cannected to the oscillator and detector by the secondaries each show a definite peak. How would it be connected to a Loftin-White? -: We do not know at present of any successful method of coupling the

super het. to the Loftin-White amplifier as it is extremely difficult to obtain a constant impedance throughout the audio frequency range for the 224 in the second detector plate circuit. The selectivity of the if. amplifier is in itself sufficient to somewhat suppress the higher audio frequencies in the range of 2000 to 4000 cycles; therefore we suggest that you use a method of coupling the second detector plate circuit as shown in the. diagram. This method will be very popular this year. The second det. plate circuit is isolated by a choke ,and two .00015 mfd. condensers. The second det, is impedance coupled by what would appear to be a new method

to the new 245 push-pull valves. Owing to the difficulty in obtaining a constant impedance throughout the audio frequency range of the 224 second det. plate circuit the impedance is in effect made eonstant by a 40,000 ohms _ resistance -aecross one section of the coupling choke. Owing to the suppression of the higher audia frequency due to the selectivity of the if. amplifier these frequencies must be reinforced in the a.f. amplifier. This is accomplished by the small resonating circuit comprising a small condenser shunting a coil, as seen, in series with the 40,000 ohms resistance, equalising resistances across the 245 grid circuit.

This resonating circuit is proportioned so that it will boost the higher audio frequencies which are somewhat suppressed by the if. amplifier to exactly the extent to which they were suppressed. NOTE.-A question such as this is really outside the scope of Questions and Answers. J B. (Murchison) : From your descrip- £ tion either of two things appears to be wrong: (1) You haye connected the battery up in the reverse way. (2) The battery terminal markings are wrong. This is a mistake that sometimes happens. C T. (Wellington) : I have a five-valve * neutrodyne set and am troubled with hum. I wish to try out a shortwave adapter, but the hum is too strong on headphones. A.: The usual procedure to make such an eliminator humless is to use a 25 to 50-henry choke in the detector lead and use a two mfd. by-pass condenser from the set side of this to B-. Full particulars about smoothing an eliminator for short- ware will be found in the 1931 "Guide." P. W.D. (Auckland): Could you make’ more clear the coil specifications for the Outspan Five? A.: Yes. Aerial coil, primary 25 turns 22 d.s.c... secondary 75 turns 26 d.s.es First rf... primary 30 turns 32 d.s.e.; secondary 80 turns 26 gauge. Regeneraformer, primary 30.turns 32 gauge d.s.c. ; secondary 75 turns 26 gauge d.s.c. Tickler, 30 ‘turns of 82 d.s.c. These are for a .00085 or a .0008 condenser. 2. Would aluminium cans 4in, high be large enough for the coil screens? — ves. 3. In the body of the article 26 gauge is given for the secondary coil, in list of components it is given as 24, Would either do? The list. of components should ‘have read 26, but 24 gauge will do quite well. Could the grid-leak, be taken across’ to: instead of across to the‘ grid condenser as shown?-Yes. 5. Would this set be suitable for con-. necting to a pick-up, ‘and, if so, where do you recommend at should be connected? A.: It will be quite suitable for a pickup, but if best results are required a push-pull amplifier’ should be included. Before very long we shall bring out a set with a push-pull amplifier,’ and you. could incorporate this in the Outspan | Five. The pick-up should be connected |

between the grid of the detector valve and A-, >: . % What means of volume ’control do you advise? A.: For local station work use the second aerial terminal. Volume on all other stations but the local can quite easily be controlled by dimming the filaments of the r.f, valve. When using the auxiliary of the aerial terminal turn out the filament of the r.f. by the rheostat provided. 8. The photographs are badly reproduced in my copy. Would it be possible to get a better one? A.: Our printers were not at all kind to us last week in the way that they brought .up the blocks of the Outspan Five. We shall look out the best one we can and send it along to you. 9. I take it that all the earths shown are connected to the shield. .: Yes, A- can be connected to the shield or the wiring carried out as is shown in the wiring photograph. The earth’s points are shown more to keep the diagram simple than to indicate actual connections to the shield. Ww L.F. (Invercargill): Could I. fit a * tone control to my set? A.: Yes, you need a2 500,000 ohm resist+ ance and a .002 mfd condenser. These are connected in series between the grid and filament return of the last valve. The tone can be controlled by varying the resistance. 2. Do I have to change both coils in & screen-grid set for short-wave ?-Yes, 3. Is resistance capacity eoupling as efficient as transformer coupling?

eens ) A.: Resistance capacity has not the { 2mplification that transformer — has,: ; though the former may have a_ better / frequency response, . ‘ W 8S. .(Dunedin): I have constructed * a short-wave adapter, using enclosed circuit. I cannot receive any stations although I can tune in the carrier wave. As soon ag I cut out the whistle, the station disappears. A.: Use a ‘vernier contro] and, if necessary redesign your coils so as to use a smaller. capacity condenser for. both "tuning and regeneration. Try another valve, preferably one of the special detectors, "QYD." (Mercer): Is not the :"Differential Two": as described in thei "Radio Record" the same set as the "Night Hawk"? If so, why is a 2mfd. condenser and a 50,000 ohms resistance specified for the "Two" and not for. the "Night Hawk’? © A.: The resistance and condenser were: used in the "Differential Two" to make unnecessary a further tap. The "Night Hawk" is not quite the same as_ the "Differential Two," but is intended to be a complete version of that set The Differential series was described so. that constructors may: start with a small set and work through to a big one,. The "Night Hawk," on the other hand, was designed for those who wanted no more and no less than a two-valve receiver. — 2. Why is a 2mfd. gridleak specified in one and a five in the other? A.: A gridleak is more a matter of _ eonstructor’s opinion. A 5 meg. leak gives a greater sensitivity and the.2 meg. better quality. . 8. Why is 30 d.s.c. wire for use with a .00085 condenser in the "Dif. Two" and No. 28 d.s.c. wire in the "Night Hawk"? A.: Hither gauge of wire can be used. . . , 4 The lay-out of the sets vary. Why? A.: For the reason we have already ‘made clear. 5. If. the "Dif. Four" is being built for all-wave work, which r.f. choke is most suitable? A.: It depends upon what band the constructor wants to compromise. Ti he wants short-wave work primarily, a short-wave choke should be: used; .and the same applies to broadcast. "TAY" (Kaikoura): Is my valve combination satisfactory? -A.: Yes, it is quite in order. 2. Volume is lessened when I put the aerial on the stud marked "long aerial." Why? A.: Usually a condenser is connected in series with this stud so as to reduce the effective capacity of an over-long aerial. You do not need a smaller aerial. 2. When the sulphuric solution in a Daniell. charger weakens, should water only be used to replenish it, or another solution made? : A.: It is far better to tip it out and miake a fresh solution. : 8. The blue stone lasts generally about a’ week without any addition. Is this in order ?-Yes. 4, Could a short-wave adapter be added bey my set, and how much would. it eos A.: You could build one for about £3. 5. Would a water-pipe earth. be less efficient as a ground than a sunken Ford radiator? A.: This depends very much upon the way each installation is fixed up, but other things being equal, the Ford radiator should be the best. ‘ H BH. (Greymouth) : I cannot carry the * full current of my B.D. without p howling. ° A.: It sounds as though a by-pass condenser has broken down. If these don’t happen to be in your set they should be. One between B-+- r.f. and A-, and a 2 wr condenser between B- detector an -~~. .

tind 2. The tone is muffled. The "B" batteries are about five months old and the tone is not .affected by the "OC" battery. A.: It sounds as though your batteries are fairly well down. The "O" battery might be properly flat, but it should make ‘some difference. At the same time a con: denser between B-+- and earth may help. 8. Sometimes I cannot tone down the volume control. At other times I can hardly get the stations. A;: This again sounds like a. defective by-pass condenser or a defective valve. 4, Would it be possible to redesign the set, using the one in the 1929 Guide?

A.: Yes; it would probably be better for you to do this. Check up your transformers and see if they are quite all right. 5. Would it be possible to use a shortwave adapter with the-set? _ Av: Yes, but get it going properly be.fore you start doing anything like this. The same comment applies to your remarks about the screen-grid valve. "QHARCHER" (Kaikahoe): I am enclosing a lay-out of the B.D. Four which I have built, but the audio valve will not light. A.: The filament wiring is altogether wrong. To start with, you have the radio frequency valve in series before the others. Follow a layout such as that given in the 1929 "Guide." 2. I have two condensers, one with 27 plates and one with 25. How can I find the capacities? . A.: They are both abott The capacity can be found by a formula. 2. The "Dif, Four," I have been told, will not be a selective set. Is this so?

'A.: No four-valve set, unless specially designed, is selective. The "Dif. Four" will be more selective than the "B,D. Four." _. 8. Is it necessary to have the baseboard for the "Dif. Four" 10in. wide? : No. The width ean be made to depend upon the size of components used. With the coils reduced to an 13 in. diameter all sizes Gan. be very much reduced. © Possibly you could fit them well. on’rn 8in, base. .4 Can you advise me the best book to increase my knowledge from the elementary stage?

A.: If you have acquired the knowledge presented in the "Listeners’ Guide" you will find the next step: up in Moorecrore, "Hlements of Radio Communicaion. W. (Pakaraka): Am I doing my set *any harm by switching from phones to loudspeaker without shutting off the current?-No, but you might get a shock. EARNING" (Wellington): For distant stations ‘what would be the best earth-a zine water pipe about 9ft. in the earth with the earth wire fastened by an earth clamp or a piece of flat copper about 6.x 12 with the earth wire soldered to it and buried 6ft. in the ground? & A.: Possibly the latter. 2. What would be the best earth wire -a piece of 7-stranded insulated electric light wire or a single-stranded copper wire?-The former. UZZLED" (Christchurch): What would make my "B" batteries run down in less than a week? ---

A.: It sounds very much like a brokendown condenser or some wires shorting within the set, 2. Would the valves be likely to cause: the trouble? 'A.: Not unless they were internal], shorting, which is unlikely if they will. operate, . 8. Is the combination a good one? A.: Yes, it is a good combination, and the "A" consumption is very low.