Questions and Answers

Radio Record, Volume IV, Issue 28, 23 January 1931, Page 13

Questions and Answers

OVICH" (Masterton): When the volume control is turned ‘almost off is the maximum current from the mains still being used ?-Yes. C. (Timaru): Would it be possible to incorporate a differential condenser in. my commercial three-valve set? -A.: Yes, if you re-design the coils. 2: Would it improve its efficiency? A.: Yes, but it would not be worth while. EGA" (Dunedin): I have a power transformer with a secondary tapping giving four volts at 3.6 amps. Would this tapping be satisfactory for the filament of a single 9485 Phillips valve with a filament voltage of 4 volts, current 9 amp? Will the high amperage burn out the filament? A.: No, it will be quite safe. 2: Have you published a circuit using A.C. screen grid valves in the r.f. stages and incorporating a pentode in

the final audio? I like the tone the pentode valve gives, but so far have not been able to get any information re this circuit. A.: No, we have not published such a circuit. The pentode, which works excellently under proper conditions, is a-rather tricky valve to handle,’

TT ©. (Te Kinga): In what issue was * the article on the Daniell cell charger published? A.: November 7, 1980. "VW Ox" (Kokiri) : Would it be possible to run an all-electric sixvalve combination set from a dynamo? A.: Communicate with Johns, Ltd., Auckland, who specialise in such installations. . IDDLH-MH-REE_ (Napier): When will the audio stages of the ."Differential One" be published ? A.: This week. 2: How would A409, B406 and A425 valves suit the circuit? A.: The first two mentioned would be quite O.K., but the r,f. stage used will be screen grid, and the last-men-tioned valve is a high gain r.f. triode.

. 8: Would the set work :a speaker using a 66K Blue Spot unit? A.: Yes. "IN CEPTOR" (Tariki) : I notice in a recent issue an American listener has logged 740 stations, for which he gives his earth system eredit. Could you explain this more fully? A.: See accompanying diagram. 2: Do you know of a set called the "Inceptor Three," and if so could you give me coil particulars? A.: The set you mention was published by an Hnglish radio magazine and is probably not suitable for N.Z. conditions. "Bune" (Waikato): My set works better on shortwave with the earth disconnected and connected to the metal cabinet. . A.: This is quite usual. 2: What distance can I expect on a three-valve short-wave set? A.: All over the world. 38: Why should my set crackle with aerial and earth disconnected? I have tested my batteries and speaker. A.: It is probable that either your transformers or your grid leak are defective. 4: What size earth wire would I need to reduce resistance losses using 100 feet of 7/22 single wire aerial? A.: 14 gauge single copper wire or 7/22. The earth lead should be as short as possible, 5: Why should I get better volume in the loudspeaker than in my phones? tive. The latier are probably defecve.

6: Should my coils, which are wound with enamelled copper wire, be space wound? A.: Not necessarily, though a slight gain in efficiency would be noticed, 7: Could I use a variable resistance ranging from 0 to 100,000 ohms and capable of carrying two m.amps to secure any "B" voltage from zero up to 45 ?-Yes, ° "QCREEN GRID" (Dunedin) : I have built. the screen grid shortwdve set described in the "Listener’s Guide" and would like to know the following: -1-Why cannot tin be used for shielding? A.: Aluminium and copper are much more satisfactory as they offer a higher resistance to radio frequency currents, and appreciably absorb magnetie lines of force. Tin is not as efficient and its use may cause undesired oscillation.

(CORRESPONDENTS must attach this coupon to all queries sent to the Technical Hditor (Box 1032, Wellington). Guestions arriving without it are likely to go astray or be delayed. Name of set ....csccececescecvescs Number of Vales Seoceesecsecesere Name See cesoceesecessecosessesene Address Coreesecseeosrseseseevesse Nom de plume .eeccocceccvcccess? To be kept in subsequent inquiries. Date becevercassceseczce eocces 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 correspondénce and while letters are given every . consideration, answers are not necessarily correct-they are only pur, opinion based on the matter supplied, which may be uite inadequate. Intricate and involved specifications cannot be supplied without a specialist’s fee.

‘2; I. have only four turns on the tickler coil for the 80-metre band, and IL still cannot control oscillation. The other coils behave remarkably well. A.: Hf you are using tin for shield. ing this may be the cause. ‘Try. aluminium or copper shields, and shield the coils and the screen grid vyalve separately. Also, try reducing B volt. age. "FIAMRAD" (Auckland): What is the size of the variable condenser used in the wave-trap described in a recent issue?-.0005 mfd. 37 see (Wellington): My set is diffi"ilt to control, and goes into osCillation abruptly, : A.: You appear to have tried all the usual remedies. Try taking the grid leak to A-- instead: of having it in parallel with the grid condenser. "DYNAMO" (Awamarino): Enclosed is a plan of my all-wave set. I am troubled with a whistle when using the audio stages. © ' A.: Your lay-out is a poor one and the whole set should be re-designed. 2: On short-wave I can -only receive morse, . A.: An article on short-wave tuning was published recently. This-is a much more delicate business than on the broadcast band. Also, it appears, as you are receiving only morse, that your set is oscillating. .

38: What’ stations should I be capable of receiving at speaker strength? I can receive only 2YA. A.: With an efficient aerial and earth system, the New Zealand "A" class stations, and some of the more powerful. Australians. 4: How can I improve my set? A.: Re-design and re-build it. 5: Would it be an improvement to have more than 90 volts on the plate?. -Yes; try 185. D B. (Dunedin) : Would the chemical * rectifier described in the issue dated January 29 for a transmitter be suitable for a receiver ?-Yes. 2: Is the solution made up of ordinary borax?-Yes. 8: Is ‘it necessary to use distilled water in mixing it? A.: Yes, or you could use rain water. 4: Are the condensers used on motor ignition systems suitable for use in radio sets? A.: Yes. They have a capacity of about 2 mfds. "PNQUISITIVE" (Motueka): Should I receive 2YA on the phones using the "Differential One"? A.:. Yes, quite easily. 2: What would be the approximate cost of the "One" and the "Two"? A.: The "One" about £8/10/- ~The "Two" about £5, 8: Where could I obtain some old five-pronged valyes for the coils?

A.: Any city dealer would probably supply you. ° : J. H.G. (Hawke’s Bay): Iam not tvo clear on how to connect up the rheostat in the "Differential One." A.: You mount a 80-ohm rheostat on the panel, as shown in the photo graph. Leave out the amperite and connect the wires that should have gone:to the amperite one on either side of the rheostat, that is to say, one side of the rheostat connects with the switch and the other side with A-+ or the valve holder. 2: In your diagram, A-++ is connected with the switch and B+ and B- together. Is this right? A.: Yes, it is quite usual for small sets to be designed like this. 8: What value should the rheostat be?-30 ohms.. 4: Could I. get Wellington on this set 12 miles north of Dannevirke? A.: On the phones, yes. IRN" (Wellington). I have two .0003 variable condensers with 17

plates in each. How many plates will I have to take out of each to make a .00025 and a .00014? A.: Reduce the first to thirteen plates and the second to eight. In the latter take out each alternate plate. 2: Could you give me the’size of coils to match these condensers? A.: A table will be published next week, 8: What is the valve combination used in the 5-valve B.D.? A.: Two 112A type valves, r.f., 615 type special detector, a 112A type "first audio, and a 171 or 605 type, second audio, , "WIRELESS" (Invercargill) : There are 25 turns upon my primary and I have taken tappings out of every five. The set will not work from the taps, so I use only ten turns. "Why? A.: It appears that there is a broken connection somewhere. 2: Is a choke necessary in the. single. valve set described in the 1930 "Guide"? A.: It is preferable to use. one, 8: For my earth, which is about 80ft. away, I use an ordinary cold water tap. Is this suitable?

A.: It is suitable, but rather too long. , 4; How.long should my "A" and "B" batteries last on a one-valve set, using it two hours a day? A.: "A" batteries, four or five months; "B" batteries, ten to twelve months. 8: What Australian stations should I get at this time of the year? A.: At least 2FC' and 2BL, and probably 3LO and 4QG, as well as some of the smaller stations. "TTUNGA" (Wanganui): Could I use a tungar bulb to use a "B" charger ?7-Yes. 2: What filament voltage would it use?-24 volts. 8: How many electric light buibs would I need to connect to charge 120\ volts of accumulator? A.: You must use a transformer or you will be falling foul of the regulations. 4: Could .I use a wet rectifier and light bulbs as resistances? 7 A.: Only if a transformer is used rst. "EMP" ( Chch.) : I have completed a Loftin-White amplifier. © With a coil and condenser: across the input it gives excellent reproduction of the local stations. Before I used the amplifier continually I would like your opinion on the following points :-. 1: The insulation between the rectifier filament. winding and the secondary winding has broken down, so, having two spare 24-volt windings on _ the transformer, I joined these to give me a five-volt centre tap winding to use for the rectifier filament. Will the winding, being on the outside, affect the efficiency and will the faulty winding be.O.K. if left dead? A.: The rectifier, being outside, will be quite O.K. The faulty winding, providing it is not short-circuiting, will not take harm. 2: Can I put two more filament windings on top of the present rectifier filament without introducing complica‘tions ?- Yes. 8: The main chain of resistances, especially the 5,500 ohms wire wound’ one, warms up slightly. A.: This is quite usual. 4: The 245 and 280 valves get hot. Is this due to heavy filament current or high plate voltage? A.: To both. It is quite im order. "Purr" (Kurow): How can the capacity of : variable. condenser be determined? A.: From a table given in last] year’s "Radio Guide." 2: I have two condensers in a short-wave adapter and would like to know if they would be suitable for use in the "Differential One." A.: Yes, if they are of the correct capacity. 3: When using my _ short-wave adapter it will oscillate only with defective valves. A.: Try varying the plate voltage and the number of turns on’ the tickler. 4: What is the address of ZL4BS? A.: We do not know. Perhaps some reader will assist. TD. (Auckland): My nine-valye | commercial set has been out of order for some months. The agent has fixed it several times, but a few Gays later it goes wrong again. The

valves have heen tested. The agent thinks the trouble is caused when the lightning arrester gets damp. ~-A.: Take out the lightning arrester and see if reception improves. Again, a defective transformer may be causing the: trouble; also, ask--your local Power Board to test the line voltage for fluctuations. Otherwise, communicate with Frank Wiseman, Queen Street, Auckland. . : W G. (Petone): Power interference '"" "4s very bad on my ac. Would it be as bad on a battery set? ’ A.: Probably not, unless the battery set were as strong as the a.c. set. Power noise is caused not because the set is a.c, (unless it is being brought in through the mains), but because it nas so many stages of amplification, 2: Would I get the trouble on a short-wave set? A.: Not to the same degree as with a broadcast set. I HAVE added another valve to my three-valve set, but the "B" batteries appear to run down too quickly. A.: Are you sure the bias is correct?. You should have about 44 volts in the last valve. Just. check your connections to see that there is not a broken-down. connection 6r any. other fault. 2: The set plops in and out of oscillation. ~ : ‘A.: This points to some trouble in the set other than in the last audio stage. Try u different value grid leak, say 5 megs. and a .00035 grid con‘denser. 8: Could I employ 4 pentode in the ‘last stage? A.: A pentode is not particularly satisfactory for a circuit such as yours. A high gain power valve of the B605 type would be better. 4, Could I use another stage of h.f, screen grid? A.: It would be better if you wanted to do this to construct the five-valve screen grid set we are describing shortly.. You could use the parts in your kit set in this receiver . 5: Could I use Mack’s short-wave adapter with this set?-Yes. H 8. (Te Awamutu): Write to * Fear & O©o., Willis Street, Wellington, who may be able to help you. "F{AMRob" (Northcote): Wlien. a short-wave adapter is plugged into my four-valve set I get a hum in the loudspeaker, and it is impossible to ‘tune in any stations. 3 { _ A.: Examine the grid circuits of the amplifier and see if the "C’" battery is p not dead. Try another value of radio frequency choke, otherwise everything seems quite in order, except that you

may use a 1-mfd. condenser between A- and B-+ detector. 2: Iam using a twin aerial of the Ltype. Is it'O.K.?. : A.: Yes, it is quite a good aerial, though-you are not gaining a great deal by having it.twin wited, as the single wire amounts to.100 feet, 83: Would a two-stage amplifier and a separate unit be better than altering the present broadcast set?-Yes. 4: QOould any additional screening be used on the broadcast set to make it more selective? . I-cannot cut out Brisbane and get Japan, A.:. Your only chance of making the set more selective would be to use a smaller aerial or a .00025 condenser in the aerial. Another method would be to couple the aerial more loosely. J W.D. (Timaru): Can I use a ° -speaker field to break down 480 volts to 483? A.: No, the speakers require a bigger voltage drop than this. Use a 1,500-ohm resistance. _2; Could I wind a choke upon the jron, a sample of which I enclose? A.: No, it is: much too small. "(QQMAR" (Wellington): I have a 1.3 amps charger. How often ‘will I have to charge my "A" battery running a two-valve set using 201A type valves on an average of 80 hours per week? ws. A.: Every three*weeks, but.as you have a charger, every week would be safer. : 2: Could I overcharge my battery? A.: No}; the accumulator will bubble when it is fully charged. 8: Would the charger keep my "A" battery charged using a moving coil 6-volt speaker and the two-valve set on an average of 18 hours per week, the "A" battery to supply the speaker with field. current? A.: Yés, The charging time will depend upon the. speaker used, but on the average for every hour the set is in use the battery should be charged for 14 hours, If should be charged after use each day, if possible. K 8.J. (New Plymouth): I have a * commercially-made six-valve set, oroginally designed as a battery model. Prior to purchasing it, however, it was converted to an all-electric by the use of an eliminator. When purchased the set was fitted with American valves. After a time I replaced these with another make and. then discovered that I could not control volume. I have since been using the following combination: four 201A’s, one A615, and one C603. Is the above combination harmful to my set? A.: No. Have you correct bias on the C603?

2: I recently replaced a failing 201A. with an A609. and volume improved appreciably. Would this alteration be harmful?-No, » : "K ARHHANA" (Wellington): I am having difficulty with the coils of my four-valve short-wave set, which works very well on wavelengths from 100 down to 22, but does not operate with the coils intended to work below that wavelength. .‘The present coils are in accordance with the given specifications, A.: For the 15-28 metre band reduce the turns on the aerial coil from 5} to 4, and increase’ those on the reaction from 54 to 6: Otherwise cut out the aerial coil.and use a midget condenser (.000015) in the aerial circuit. This, however, is not usually needed when the aerial coil is used. 2. The 50-100: metre coils and the upper parts of the 31 to 54 metre coils work on 223 volts or less, but the lower wavelength coils require between 223 and 45 volts.

A.: Use as high a voltage as is consistent with good control of oscillation, 8. I use a 20,000 ohm variable resistance in the B lead to the detector. A.: This should be 200,000 ohms, 4, I have made up the coils supposed to give 9-15 metres in accordance with specifications. These will not oscillate at all except over part of the detector dial when using 90 volts plate. A.: Reduce the turns on the aerial coil from 24 to 14, those on the tuned anode from 2}4-to 2, and increase those on reaction from 23 to 4. Try placing the aerial wire close to the set (after disconnecting it from the aerial terminal),, about 4ft. away, as coupling is evidently too tight. Advance reaction full on and leave for five minutes or so, when the set should oscillate. Note: You did not state the size of your grid condensers. 6. I have made up the coils for 320 to 550 metrés band described in the issue dated September 26, using ex‘tensions four inches high, which come up to the lid of the receiver. The diameter of the extension is 14in, I cannot obtain the specified wire, but have used 30 d.s.c. for the upper coils (aerial and tuned anode) and 40 d.s.c. in the lower. I have made both

rf. and detector coils alike, using 181 turns of 30 d.s.c.‘for the aerial and tuned anode coils, to allow. for the difference .of wire, and 50 turns of d.s.c. for. the grid and reaction coils. A.: This should read grid and tuned anode, and aerial and reaction. . 7, There is hardly any room .on the formers to wind them all flat, so have wound, the 181 turns in three layers of 60 turns each, and the 50 turns in the usual way. A.: Wind the tuned anode and the grid coils flat. The reaction and aerial jumble or bank wound. Keep. the aerial away from the grid. Tunetl anode and grid coils wound with 30 (.s.c. will measure out at 66 turns pe inch, and aerial and reaction wound with 40 d.s.¢c. 142 turns per inch. 8. These coils will os¢illate on all wavelengths except the highest ‘and should therefore receive some stations, but I could find none. ~ There is not. even a sound from 2YA, so there must be something radically wrong. . A.: Rewind the coils, as indicated above. 9. I notice’ that you will shortly be publishing data for .0001 condensers for this set, but I expect ,these will be different from those already calculated for the .00015 condensers. A.: No, there will not be much difference, a