Questions and answers

Radio Record, Volume IV, Issue 19, 21 November 1930, Page 12

Questions and answers

D X. (Dunedin) : Your condenser would * be about a .0008 and you can get particulars of the coils you require in the "Radio Listeners’ Guide." 2. What size coil would tune from 15 to 45 metres on valve base coils with the above condensers? A.: As it stands it is unsuitable, Use a series condenser of .003 and follow the directions recently given in the "Radio Record." 8. Is the following a good valve combination: PM14, PM8, PM4DX, PM4? A.: Try PM4DX and PM8 interchanged, as PM4DX is usually a good detector. S W. (Wadestown): The a.e. — short- * wave adapter from the "Guide" oscillates noisily and I have tried very many expedients, A.: Change the 227 valve as detector and try the effect of- a .00025 condenser in the aerial. Hxamine the reaction condenser for possible shorts. Your baseboard is quite O.K.

2. Is 160 volts too high? A.: Yes, you will have to use a resistance to reduce the voltage to about 50. . 3, A report says that an a.c, adapter is at the best of times only a 50-50 job. Ts this so? : A.: Yes, short-wave adapters of this type are not as good as a d.c. adapter used as an a.e. set. You should, however, be able to get quite a number of stations on this adapter. Note: we do not undertake to reply to questions of this nature by post. B. (Wellington) : My condensers ap- * pear to be a little too big for shortwave. What capacity do you advise? A.: .0001° or .00015, 2. When I turn the moving plates of the reaction condenser_ halfway in or right in the set howls. A,: You will get much help if you read the inquiries from time to time on short-wave. Reduce the reaction, for it appears there are too many turns on the coil or the detector voltage is too high, hence the second oscillation, S O. (Whangarei): I built an a,c. am- ° plifier, but it hums badly. . A.: It appears that your laminations of the power supply are not tight or there is insufficient insulation between the windings. Use a separate winding for the last stage. Use armour filament winding and earth the shielding. In the absence of the actual circuit it is difficult to see what else can be wrong. You should have used a coupon. ECHANICIAN (Wellington): What number of turns and gauge of wire should be, used on the detector coil to match the impedences of a 224 and a 227? A.: The number of secondary turns can be obtained from the "Guide." Primary for the 227 should be made as is described for the Loftin-White and will require about the same number of turns as_ specified. 2. Does the choke coil in the enclosed circuit alter the impedance the valves are working into? .: You should haye stated the inductance of the choke. There. is probably very little alteration. In your circuit direct current would still flow to the windings of the speaker. 3. As valves are very rarely balanced, would 30 ohm potentiometers on either side of the choke coil be of any advantage? A.: It is very rarely that any difference in the valves exists except in theory, unless the valve is actually at fault or defective. Note-You should have been able to obtain most of the particulars for which you ask from the book you have taken the description. [NQUIRER (Wellington) : Which of the two-stage audio stage combinations, (a) or (b), gives the greatest amplification actual? A.: (a) Impedance, 50,000 ohms; G.36 MA/YV. .72 24-1 audio transformer to match valve. (b) Valve 6000 ohms, G. 123} MA/V.2. The steeper the slope the better the valve, so the 2MA/V will be a better slope than the .72 MA/V, and therefore (b) is the better combination.

2. Is there any appreciable gain by using a sereen grid detector? Yes, it allows a greater input to the audio amplifier. A H.F. (Wellington): I have built the ¢ silk diaphragm speaker, and the tone is tinny and the base harsh and weak. The speaker is built according to the snecifications except that the diaphragm could not be pulled back as far as specified. A.: If as you say everything is according to specification, and there is no tension on the reed (this, by the way, is important), we cannot help you. The speaker has been tried many times, and the laboratory model works excellently. 2, Would a 18in. cone give better tone? A.: If you have had trouble with the silk diaphragm and canont rectify, the 13in. cone would. be your safest inyestment. J UMBO (Otago Central): My set of tuning coils will not cover the whole range. Could you redesign them for me? A.: All the specifications you require were given in the "Record" a little while ack. Note.-Correspondents would help us a great deal if they would only watch the columns before asking us to design coils. From time to time we give specifieations which would suit dozens of subsequent inquiriers. A little discrepancy in either the condenser, the size of the former, or the wire, would only alter the tuning bands a very little. 2. Are the connections for my output filter correct? A.: The return from the speaker should go through the condenser to B-++ instead of as you haye shown it. M L. (Hastings). How does a _ tone *control work? It is a combination of condensers and resistances 2. One of my push-pull valves takes one amp. more current than the other. Can I use a resistance in the filament to rectify this? A.: The difference is too little to have any serious effect. Your suggestion will be quite all right. 3. Would I get better low-tone response ‘if I used shunt plate feed? A.: Probably not, but your idea about connecting the primary and secondary should be quite O.IKXK. You would have to experiment to see which ends give the best results. AMATEUR (Dunedin). Can I add regeneration to my present factorybuilt set? A.: Yes, but use a .00025 condenser instead of the proposed midget. A.: Would a super heterodyne shortwave adapter be better than the screen grid set in the 1930 "Guide?" A.: Theoretically speaking the super heterodyne should be much the better, but probably you would find the short-wave adapter easier. ‘ 3. What power valve do you advise in the last stage of the screen grid set?112A type. 4. Do both condensers in this set have to be double spaced? A.: It is preferable to have them both spaced, 5. What is the most efficient system of regeneration control for short-wave sets? A.: Resistance, but there is a considerable amount of yariation in the resis-

tances themselves, and one cannot always be sure of getting the best results, 6. Has the a,c, cone dynamic speaker been described yet? : It is some time since "Pentode" has done any work for us, and so the speaker has been held over. We hope to deseribe. it ourselves as soon as. our scheme of constructional articles, now settled, has run out. : J W.F. (Dunedin). Can you give 1 a circuit for a screen grid yalve to be added to a five-valve B.D.? : Use an ordinary transformer coupled circuit, using the aerial coil at present in use, and an interstage t traus-

former consisting of 77 secéndary turns and 25 primary, slot-wound. This coil should be shielded as described in last week’s "Record." 2. How many turns should be wound m the primary to suit the impedance of 6M927-A.: 11-14, T oF tx (Dunedin). How many fixed 7 and how many moving vanes of the type indicated "are in the .0002 condenser? A.: Hither six of each, or six moving and five fixed. (QUERY (Le Awamutu). My Round o the World Two, which has been funcfor the last two months, is now giving trouble. It will oscillate when first turned on, then stop. If the reaction is advanced, it will oscillate again and then stop., A.: This sounds uncommonly like rundown batteries. Try renewing them. BEGINNER (Mangaweka): How can I improve the volume of American stations? I have a good aerial. A.: Bhe reception of American stations vgry much depends upon your loecality, 4fhe writer has an excellent set and an excellent aerial-earth installation, but can never get a whisper out of them, while the same set only a couple of miles away brings them in regularly at excellent strength. 2. Would a change-over to the valves: recommended by the makers be any improvement when renewing valves? A.: Yes; it should be a great improvement, especially, with a.c. valves. 8. Does the use of enamelled wire give any advantage over the use of tinned wire? A.: Enamelled wiré is: better-see the articles on aerials in this week’s issue. RIPLH GANG (Mount Eden): My a.c. set violently, oscillates. How can I prevent it? a A.: Shunt the bias resister on your detector valve with a, imfd. condenser, use 2 1 mfd. condenser instead of a .1 to by-pass the screen-grid voltage. Try the Cirenit without the pick-up and if this 3§ any improvement take the pick-up into the other side -of the detector. The 171’s require a separate 5-volt winding, the centre tap of which is taken through a 1000 ohms resistance to earth. We cannot imagine the set working at all in the way you have it connected, as you are giving the 171’s only 2.5 volts. 2. On stopping the oscillating, how can I trim up the condensers? A.: Connect a midget balancing condenser across the first stage condenser. 8. Would a centre tap °2000 ohms ls. for the push-pull be better than the choke? A.: No; you should use condensers in series with the voice coil of your speaker, to prevent the flow of d.c. current. Are you quite certain your speaker does not require a 25-1 step-down transformer?

TT RANSMISSION (Hawke’s Bay): Why should the loudspeaker howl in the hand at least 2in. from the reaction condenser? ; _A.: Hither the detector is microphonic or the set is being worked on the verge -of oscillation. Try covering the valve with ecottonwool or substituting it, 2. The speaker and the set was made in 1926. Would a first-class loudspeaker help me? _A,: Your set and speaker are certainly out of date, Probably a new speaker would help matters. 8. I pick up a great deal of noise’ from a transmission line although I have my aerial at right angles. A.: Try the use of a counterpoise earth, that is, another aerial slung a few feet from the ground and immediately under the ordinary aerial, This is the lead-in and should be treated with as much care as an ordinary lead-in. It is connected to the ground plate. NOTH: Your query should have been directed to the address given on the coupon. KAURINUL (North Auckland): A soealled expert attempted to put my -set right and after some interference the set was worse than before. He eventually found it was the speaker, but we cannot use the four valves. Both the third and fourth valves are power valves. A.: This is where the mistake comes in. For the second last valve you should use a A409 (in Philips) and not B406, as -directed. The amount of current from the power valve saturates the transformer and causes the roaring you refer to. There may be something else wrong with your set which we cannot tell unless it is ‘inspected, but try this other valve and if it gives you further trouble pack it up carefully and send it to a reliable city radio house,

A W. (Auckland): My eliminator was 4e left on while the filament current was cut out and the valves now seem paralysed. Do you think they have been damaged? . A.: It is not altogether unlikely, When there is no drain upon an eliminator the voltage rises, frequently to double, because the resistances have now no effect upon the voltage. While it would be difficult to say authoritatively whether your valves are paralysed or not, there is every reason to suspect they are not in first-class order. 2. Would it be advisable to replace all eliminator condensers? A.: Not without testing them. Connect a pair of phones with 150 volts from your eliminator and connect one side of the condenser to negative. The other side of the condenser is then touched on the free phone tag. ‘There should be a good strong click. Take the phone tag away and then place a piece of wire neross the terminals of the condenser.

There should be a distinct flash. On replacing the tag from the earphones there should be another click, and if you replace the tag without first shorting the condenser there will be no further Clicks. You can test it without phones by merely charging the condenser (placing the + on one side and the — on the other), then removing, say, the +, leaving the condenser for a few minutes and shorting it to get another flash. Absence of this flash would indicate that the condenser is leaking. 8. As a "B" and "C" eliminator my eliminator emits a fearful howl. A.: Very likely the condenser across the "©" supply has broken down. 4, Would the a.c. neutrodyne in the 1929 "Guide" be suitable for construction? A,: It is hopelessly out of date. Build the Loftin Four if you want a real good receiver, 5. If I sent you my circuit, would you mark the necessary alterations to convert it to a.c.?-Certainly, Puck (Te Kuiti)-Could a 90-voli wet "B" battery be charged from a 6-volt generator?

A.: Yes; you must split your "B" battery up into four groups, the individual cells in these four groups must be connected positive to positive, and negative to negative. The positive of one group is then connected to the negative of another group, and there will be a positive and a negative free. These are connected to the six-volts generator. 2. I am obtaining a 50-volt generator. Would it do to charge the "B" battery and 6-volt dry battery? A.: That depends upon your rate of charge. We would need a little more information before we could tell you what resistances to use, but it could be done. 8. Can I better the-valve combination in my 6-valve set? ; A.: Use a power valve of the B406 type in. the last stage. Dp™™ (Dunedin).-What are the best valves for my English set for allround use? A.: The s.g. and power valve of your present combination are quite good, but your circuit requires one hf. as detector. 2. I cannot get the same results with oue 99+volt battery as with two 465’s coupled. . A.: We cannot understand thaf, unless there is something wrong with your 99volt battery. Very little drain is taken by your set if it is properly biased. . 8. Is a 50ft. aerial, excluding the lendin sufficient for this set? _ A.:_It depends upon the height. For a 3-valve set you require at least 100 feet of aerial. OWER TONE (Dunedin).-I connected the filaments of my two audio valves with 6 volts a.c. Reception is just as good as with the battery, except for a slight hum. A.: We have heard of that happening although it is very rare. The set is usually most unmanageable when it uses d.c. yalyes in anything but the last stage. 2. Would a 227 be worth trying in the detector? A.: Yes, you would not need to balance ‘ J UNKBOX (Christchurch): Could .00032 midget condenser be converted into .00025? A.: Yes, by removing some of the plates. 2. Is a .00025 grid condenser and a 3 megohm grid leak efficient for short-wave work? A.: The condenser is all right, but use an 8 megohm grid-leak. 3. My valve will not oscillate below 45 volts. A.: This is quite in order. Different valves operate at different voltages. .4, I ean get only one coil to work and even then signals are mushy. A.: We cannot advise you on _ this point unless we know something of the design of your set and the coils.

Kole (Timaru): The specifications you ask for have already been published, but we shall reproduce them in a special table next week. We cannot be continually redesigning coils because correspondents wish to use different gauges of wire. 2. Would it be an advantage to have a midget condenser in series with a 00014 condenser to spread out certain bands ?-Yes. ' 8. What would the value of the condenser be?-.00005. 4, Is a 60ft. aerial too long for shortwave work ?-No. Mer (Westport): What is the best combination of valve for my batteryoperated American set? A.: Four 221’s and one 112. The. iast should be suitably biased and may be of any make. ‘ H. H. (Akaroa): I have had my set of * valves blow out twice. What is the ceause of this? A.: You do not state the circumstances under which they blew out. A fuse between the "A" and "B" battery would prevent this happening again. A fuse

eould quite conveniently be the narrowest strip you could cut from a piece of tinfoil. LJSTENER-IN (Alfredton): I find a new transformer used in my — set whistles when I put my hand on it. Why is this, and why, when the set was overhauled, was a different make of transformer used? A.: A transformer will frequently squeal because the casing should be earthed. Try doing this, and you will have no further trouble on that score. The new transformer was used probably, because it was better than the other one. Al? (Kakahi). A whistle has developed in my loudspeaker, and a slight howl on 2YA. A.: Try replacing the grid leak, and if that is ineffective the following in order: Earth the core of the tranSformer, change the detector valve, examine your wiring very carefully for bad joints, and substitute the grid condenser. This whistling may be due to the complicated wiring caused through the first audio jack. (CONTROL (Gisborne). The information regarding the coils tuned with .000125 condensers on 2in. formers, will be published in a special table next week. D C.D. (Auckiand). I am troubled by * ground noise in the L. White. A.: Probably due to a defect in the resistances. Follow the articles that will be appearing in the R.R., as very many things about the L.W. are going to be said. We suspect very much the .5 megohm leak. Wither try another or have that one measured. Concentrate on (Continued on page 29.)

Questions and Answers

(Continued from page 13.) your resistances, and if necessary have them measured. A P. (Otahu). If you are in doubt * about the wiring of any circuit, fol-_ low the theoretical diagram. This applies to the case in question. 2. How far apart should be. the windings on the coils?-1-8 inch. 3. What is the corerct way of tuning? A.: The dial is turned: round with the set oscillating, and when the whistle isbrought in reaction is slackened until the whistle disappears. Then reaction and tuning are adjusted until the signals’ are: heard. ‘To receive Morse, the set must be oscillating. To receive broadcast it must be on the verge of oscillation. M L.K. (Wellington). Can the super * heterodyne super short-wave receiver described by "Cathode" be employed before a super heterodyne receiver? : No, it is entirely unsuitable for. such a receiver. BEASSPOUNDER (Ngukuhau). Could you give details for a ew. transmitter, and how to get started in the. amateur game? A.: We are at present arranging for a series of articles in this direction. QUALITY (Auckland). Is it worth while changing a A425 to a screen grid ?-Yes. 2. Will the 8002 eliminator supply sufiicient current for a four-valve set using two 405’s in the Jast stages in pushpull? I am using 406 as second audio. A.: Yes, but use a 409 instead of B406 in the first audio, You are placing an

extra drain on the eliminator,:.which ‘s altogether unwarranted, as the g.p. valve can accommodate the grid swing required in the first audio. : ’ 8. Which would be the correct, output transformer to use with a ynamie : spenker, OP38C or 40? A.: Use 80, and take’ out the trange former already in the speaker. MOlWHARE (Feilding). Would the. Daniel cell charger keep my 120 amp. low accumulator up to.scratch? A.: You appear to be’ using low consumption valves, and if you do this it should keep it well charged. Leave the charger on continuously. RAZ DIO (Shannon).: Can the super V het. receiver described by "Cathode" be used as a dic. receiver? Th, A.: Yes; the only changes necessary ei 1. The filament ¢ircuit of the two valves: is completed in the ordinary way, andbrought out to two terminals for connection to the batteries. 2. The grid return of the first detector is made to A plus. 8. The grid return of the oscillator is taken to A minus. Otherwise the re ceiver can be set up exactly as it is.