Questions and Answers

Radio Record, Volume IV, Issue 30, 6 February 1931, Page 13

Questions and Answers

PYikSLD (Wellington) : I can pick up Wellington only fgintly on the Ditferential Two, very m@th fainter than I d with the one-valver. f= .: If you had it going well on the one*valver you may be quite certain that it is a wrong connection, a defective transformer, or valve. Have your transformer tested and check your wiring from the diagram of the "Night Hawk" published last week, 2. The differential condenser does not affect this set. A.: Did it not affect the one-valver? If the trouble was there in the smaller set it should be eradicated before a twovalver is contemplated. In our experimental model we §xperienced the same difficulty and traced it down to a wrong eonnection in the eoil. Check over your eoil very. carefully, and, if it is still at fault, take it along to a dealer who builds these receivers and ask him to try it out in one. The-set as we describe works splendidly, and if you have followed the directions carefully and have efficient components it is impossible to encounter difficulty. 8. I am using HL610 as detector and PM6 as audio. Is this combination satisfactory ?-~Yes. : CvuRIO (Dunedin): I have had my set for 18 months, but lately the local es has been coming in all round the 2. o . A.: It seems as though your valves. want renewing. 2. Do you think a wavetrap can be operated with the set? . A.: It should-not be necessary two stages of r.f.

oe 8. Could you give me a diagram for an efficient wavetrap? I have not had success with any yet. A.: You have been very unlucky. Wavetraps published in the "R.R." have worked splendidly, and the most recent one contributed by a country listener. is particularly efficient. LIO (Devonport): My set. has a considerable a.c. hum. Kindly supply some probable causes and cures. It is an 8-valve a.c. A.: Experience has shown us that it is pure folly for us to.attempt to direct anyone into the intricacies of an a,c. set, consequently we advise you to take your set to a dealer and ask him to put it in first-class order. Pp#TER (Auckland): I can receive N.Z. and Australian stations on the long-wave, but can only get a few N.Z. short-wave amateurs, The set is a threevalve all wave. A.: If you used a special detector you might do better. Why not try the 3valve differential circuit that we shall be bringing out shortly? NvcHUM (Nelson): Is my set in eorder when the speaker leads spar, when coming into contact with the meta part of the cabinet? The two last valves lit up brightly when this happened. A.: It is quite in order for the speaker to spark when brought into contact with the frame. We cannot understand your valves lighting brightly without burning out. It seems as though B- is connected to A-- and A- connected to the frame. If A- is to the frame, see that A- and B- are connected together.

2. Could you name a good combination of Radiotron valves for my set? A.: The best combination would be five 221A’s and the 171A. The 221’s are rather hard to obtain, and you may have to use 201A’s, E B.C. (Auckland): What is the best, * length of aerial for a short-wave receiver? A.: Thirty to forty feet. As high and short as possible. 2. What is the advantage of the aerial Oop A.: The aerial Cop is a splendidly designed lightning arrester. It can be installed outside, is absolutely impervious to the weather, and if affected by lightning the fuse can be replaced. 8. If I encase the condensers to keep out moisture will the stability of tuning be affected?-No, the idea is quite good. 4, How can eliminator hum be overcome? . A.: In the, 1931 "Guide" we shall have quite an amount to say about this problem. It is possible generally to overcome hum on short-wave by using an extra choke coupled by a large capacity condenser in the detector lead. An efficient transformer immediately following the detector is essential. P W.B. (Wanganui): What would be * the best Mullard four-volt valve combination for my B.D. Three? A.: PM38, 4DX, and PM4. 2. Why is it that I can receive some of the Australian stations better than 1YA? ‘ as A.: This is due probably to your loca8. Would it harm a new 45-volt block to put it in series with one whose voltage is down to 39?--No. H A.P. (Taihape): My Daniells charg- ° er is not functioning properly. The battery has gone down since it was connected, the zines have eaten away, and erystals of bluestone are forming on both sides of the porous pot. ; A.: The reason for the zine to eat away and the crystals ‘to be formed on the porous pot. is that the solution is too strong. Add a little more water to the sulphuric acide solution and scrape the crystals off the porous pot. Keep the charger eontinuously on the battery and make sure that the lead coming from the copper is connected with the positive terminal of the battery. A B.A. (Auckland) : We are affected by * passing motor-buses whose ignition system causes severe interference. A.: Complain to the manager of* the bus company and ask if it is not possible for the ignition system to be bypassed with condensers. This is not expensive and would be indeed effective. Failing this communicate with the District Radio Inspector. It might be as well to check up with other listeners along the bus route and see if they experience -the same trouble. Two complaints will always be more effective than one. If necessary we could supply a. diagram showing how condensers should be connected across the ignition system. 2. Does a tiled roof have the same eapacity effect as an iron one?

A.: Not so long as it is dry, but immediately it becomes wet it provides a fairly good path to earth. S. Is my present installation a good on we Not particularly. The aerial should be higher and the lead-in brought directly to the set, instead of having to climb round the inside walls. The fact that it is only Tit. from the ground at one end is really quite inefficient. A aerial would be on the chimney and down the side of the house to the set. K. ‘V.D. (Rotorua): What are the cor- * rect valves to use in my American six-valve set? A.: Five Radiotrons .221’s or 201A’s. and one power valve of the 171A type which is not necessarily Radiotron, See: from the carton that accompanies the valye that it is biased correctly. 2. Where could I procure an instruetion book for the set? ' A.: Write FP. and A, Odlin, Welling on. 8. I have a five-valve commercial set. How can I add a stage of screen-grid r. 74 A.: With a commercial set it is worth while to try to incorporate one of grhese valves, © 221’s be better than 201A's?A.: The 221’s would be better, buf. they a1 are probably difficult to obtain, What power valve should I use? A.: Any make of power valve and one of the 171A type. 4, Can I make the amperite for @ detector valve and a rheostat for audio stages? A.: As you are using a six-volt valve: as detector: you will not need an amr perite. If, however; you change over. to a five-volt valve you will have to use an amperite in the detector stage and

ORRESPONDENTS 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 ccocccccecccseececeess Number of ValeS ..cosccseeseese00 Name 0290000080000 OSSD IE SCHKPSAESIO Address = acecoce ere] 200508 S0COODTIO OOS SHOSFIOTCIOSISCS DOOM o 000600000 0090090000868 000098088008 99S Nom de PIUMC ceccovccccsecssever To be kept in subsequent inquiries. Date ASSP eSeseSHoceser TO eeeoe Please Note:(1) Be specific and brief, tabulating, if possible. (2) Write legibly, and on one side of the paper. (8) We do not design eircuits, 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 volved specifications cannot be supplied _ without a specialist’s fee.

could use your rheostat for the audio Stages. It would be preferable, however, to use the rheostat for the r.f. stages. "y HISTLE" (Wellington), Why does my set burst into oscillation when tuned to 8YA? Am. I oscillationg or is it someone else? ° A.: It sounds very much as though you have a persistent howler on 3YA. Take off the aerial. ‘If the howl is not there you can rest assured that it ig ~ an outside set. If it is, reduce the detector voltage. There is no reason why it should oscillate on 83YA ond on no other station. I am using a five megs, grid-leak. A.: Try reducing it to two megs., as this value should give you better overall results, ; TNTERESTED (Bastbourne): I can get excellent results from New Zealand and Australian stations, but am unable to get American, Do you think it is the locality or my aerial, which is below the roof of the house? a A.: Probably both. Your locality is notoriously bad, and your aerial is very poor if it is below the level of your house, » ‘It will possibly be shielded by the roof,

H L.M. (Whangarei): Which is the * most satisfactory for all-round work, the detector, screen grid r.f., and one stage of audio, or detector and two stages of audio? Both circuits are the differential three, A.: For quality use the latter with a good power valve. For distance getting, the former. I have-at present a straight-out four-valve circuit of 1927 vintage. -Would the Differential Three with a screen grid rf. be better? A.: That is rather hard to say, probably yes, or at least as good, but if you have a four-valve set on hand why not make the four-valve differential receiver? 3. Which is the optimum condenser to use'in connection with the .0001 mfd. fixed condenser to reduce the combination for short-wave work? A.: .00035 would be better, as a lower resultant capacity can be obtained. F G. (Wanganui): I want to use a * . telephone wire which runs from one hil] to another to hang my aerial on. It is 90 feet straight above me. I have permission to do so, but would the connection interfere with either the telephone or the wireless set? A.: It is a little bit risky, for you might interfere with the telephone, and also you might pick up some of the conversation on your wireless. However, you could try it. Use as many insulators as you can between the telephone and your wireless aerial, ALW. (Invercargill): Lately I no-° * ticed that the set worked better without’ the rif. ‘valve. I cannot get the "C". battery to.work properly, although . I have tried different values of bias and reversing the connection. A.: It seems that your valves should be tested. Although the valve lights up it is no criterion that it works correctly.

Test. the ‘secondary of your. last transformer and check over the grid return of that valve. "('ONDENSER" (Hamilton): What does it mean by'saying a’ "three coil tuner"? ; ‘' : A.: A tuning device using three coils. The first connected to the aerial and the earth, the second to the grid'of the valve and A+, the’ third to the plate of the valve.and the choke or transformer or *phones. : . 2, In my sketch. there seems.to be no B- connection. ; A.: A- and B- are connected. Note: Your exact requirements. were not very clearly stated in your letter. If you intend to build this set let us counsel you to attempt something more moilern, say one of the ‘Differential ‘series. : These old sets are quite out of date. Kor (Timaru): What number of turns must I put on a Qin. coil to be used with a .0005 condenser for a Differential "Two ?-64, ‘ , 2. In the second instalment of the Dit. One are the specifications for a’ .0005 or .00035 condenser ?-.000355. . 8. I intend. building a’ four-valye version of the Dif. series. How far would a tuning condenser be from the’ left-hand side to balance the condenser on the r.f. stage? , A.: The reaction condenser should balance the r.f. condenser. Both these will be 4in. from the sides. . If you desire to balence one tuning condenser against the other you must design a layout to suit yourself, It’s easy.

D X. (Dunedin) : I have come into pos- * session of a Super-sonie short-wave adapter. Is it any good? ' A.: Super-sonie adapters are usually quite satisfactory, ‘especially where the carrier wave is fairly strong. . Where, however, it is weak it is not easily picked up. oi. 2. What alterations, if any, would have to be carried out in the wiring to /use English screen-grid valves? A.: You could’ use English valves quite well. by: watching mainly the filament voltage and remembering that the plate is at the top of the yalve, ‘NOTE: A full description of a superhet. adapter that has been tried by us will appear in the 1931 "Guide." C C:T. (Wellington): Is not an aerial: * erected in the same direction as 2YA’s transmitting aerial the best for all-round reception? ‘ : A.: For the average "L" or "I" aerial direction has no effect. The only practical aerial that has directional effect is the Beverage. 2. What are the chief symptoms of the 280 type rectifying valve losing its emission? .

A.: A general loss of power due to the yoltage petering out. 3. A loud hum accompanied by a gurgly reception is experienced when my a.c. chassis js not earthed, A.: This is quite in order, because when the earth is removed the set is on the verge of oscillation. The set is then noisy, but it is more sensitive than normally. 4, Is it not practicable to take out "B" voltage leads from my set to work the sup.-heterodyne short-wave unit? Could specification be. supplied for a power pack using the 201A as rectifier? : You will need a separate: power nack and could use the 201A as rectifier. The 226 would. however, be a better valve. These points are being fully dealt with in the 1931 "Guide," which is well under way. {5 RID (Tamatua).-Would a car generator be quite satisfactory for charging a_six-volt radio battery ?-Yes, 2. Does the amateur reading at either slow or fast charge indicate the voltage is six? A.: The voltage is really slightly more but as the car battery is practically the same as a‘radio battery, you need not worry about the'yoltage bsing wrong. Do not, however, attempt to charge a radio battery ata rate greater than 2 amps,

8. Why are American and English cir- | cuits unsuitable for New Zealand condi- | tions, and why is it that there are so many Aniprlean sets on the New Zealand maret? ' A.: American circuits are not really unsuitable for New Zealand . conditions except where the aerial, is directly coupled with the grid. As Wnglish circnits usually incorporate this feature, and as,’ too, they usually have a loading coil for the longer wavglengths, they are unsuitable. American s.c, sets are usually designed to operate from 110 volts, and as we are 7 ay y. a step down transformer must é used. [TRICKLE CHARGER (Khandallah).Could you explain the band pass system and the B.P.F. unit in the "Record’’? A.: A B.P.F. gnit was described when the Akrana Foui*was described about six months ago. . _2. In the "Guide" you give the 873. ¥’ tifying valve as 2:4 yolts. Is+this not ineorrect? A.: Yes. It was such on the specifications that originally reached us, and consequently it was impossible for us to detect the error. We are, nevertheless, sorry it occurred. It should be 4. D 8.D. (Wanganui) : I have. encounter- * ed trouble with the "L.W. Three." When trying the amplifier with a pickup there is a loud popping sound in the phones which can be made fast or’ slower by adjusting the hum bucking potentiometer. The 100,000 and 5475 ohm resistances become fairly hot. A.: It appears that your trouble is in the bias chain. Try different gridleaks. The 100,000 ohms resistance should not heat. Try rémoving it. Some of the new circuits connect the half meg. grid-leak directly to the B plus terminal of thé output transformer and try altering the value of the bias gridleak. However, this should. not be necessary as the set should, if made from our description. work perfectly. We experienced a little trouble, but experi- / menting and diligent search Cleared these obstacles away. Have the sereen-grid valve tested and if possible try another one in its socket, as they vary a great deal. We advise you to get the amplifier working: perfectly before using a radio stage. No trouble ‘should be experienced once the amplifier is in good order, "TON" (Christchurch) : I completed the L.W. Three and on the whole it gives good results. I have not been able to balance out all the hum. A.: See our replies to the above cor respondent. Your trouble probably lies in a defective grid-leak resistance. 2. The selectivity and sensitivity seem poor. I have to use a wave trap to get

) 2A, and when 8YA is tuned in to its the set i4 unstable. _ A.: It-may. be necessary to use a wave trap on Wellington if you are not -very elose to 3YA. One stage of r.f. is not particularly selective, nor, for that matter, sensitive. _ The fact that. your set’ is unstable when tuned to 3YA suggests that there are still too many turns on the radio frequency transformers. Reduce the turns to about 70, 8. Would you advise two stages to remedy these defects? P A.: Two stages would certainly give you greater sensitivity and selectivity, ut it might be as well to get your amplifier working perfectly before attempting the extra stage.. . Note-The other points mentioned in your letter are quite in order. The correspondent points out that he had a break-down between the .280 filament sejnding and the secondary. There is a yevery high voltage between these two ‘windings and it is wise to put two layers of Wmpire cloth between them. | "AMBITIOUS" (Gisborne): What pause wire and how many turns could I use on (a) the field coils (b) the voice coil of a dynamic speaker? A.: The voice coil will depend upon the impedance, of your last valve or valves. Had you told us what they are we might have helped you. ‘The specifications for the field coil: are :-37, turns of 38 gauge enamelled wire. .3lbs will-be need-

i ed and they will occupy 92 layers. The current pass will be 32 amps, 2. Why does my set distort on everything except gramophone record broadcast? ; A.: We can think of no adequate reason why this should be. You may be getting microphone blast, but that is most unlikely, 8. Where can I obtain details for making a small generator, of say 500 volts? A.: Write to Johns Ltd, Auckland, who seem to have a great deal to do with this type of apparatus, 4. My motor sometimes causes distrubne to listeners. How can I remedy is A.: That subject was fairly fully dealt with in an article which appeared. some months ago. We reproduce a diagram which should help you, oo R T,. (Shannon): How many turns will . I need: upon the primary of a r.f. transformer which has an inside diameter "o£. 13? ; A.: Wind about 26 turns on a ‘Tin. ormer and take out the centre tap, Place this inside the low ‘potential’ end of the grid coil.. The tickler on the same sized former should be about 20 or 25 ‘turns placed under the grid end; 30 gauge d.s.c. would be quite in order and for the r.f. . valve a 221 would be preferable to a 201A. : "TUBE" (Invercargill) ; Which aerial would give me the best results for direction, one due east and west, or one north and south? . A.: For short aerials direction effect

ean. be ignored. If there is any gain it would be on the one going east and west. ’ 2. II use a "DL" aerial is it absolutely necessary that the lead-in wire should be taken ‘from the middle of the aerial?-. Yes, _ 8. Are glass insulators the best for the ae "PUNGAR" (Wanganui): What are the number of turns on the valve base coils for the POJ Four? A.: Last week we gave a: fairly comprehensive table. You should be able to get what you require from this.

2. In the gravity type Daniell’s cell how are the copper sulphate and distilled water kept from mixing?-They are not. . Using a transformer without any centre tap what would be the connections to a tungar bulb to charge my "B" pbattery? A.: See the accompanying circuit. 4, Does the gravity type Daniell cell work as well as the porous pot, and would it give 4 an amp. A.: The porous pot is the better and would deliver 4 amp. 5. How many amps. would my set take using the enumerated valves? A.: You have not stated the "B" voltage nor the bias, but we should estimate it at about 12-15 mamps., using a maximum of 185 volts. _. IL use a .00025 instead of a .0001 grid condenser in the P.C.J. Four without making a great deal of difference? A.: Yes, you would need to make the alterations in the coils as were given last week. 2. Would the P.C.J. Four work satisfactorily on shortwave and broadcast? — ex . "ATOI Whare" (Feilding): My valves ; are becoming weak and I want to replace them, Would PM5 be better than my existing detector? A.: If you want to use Mullard, PM6D would be the best. 2. Would 221’s be better than the valve I use now? A.: In all probability, yes. 8. There is a remote chance of my getting near a power line. Would an eliminator supplying 180 volts and 30 mills. supply the above-mentioned valves and a 603 with enough current?-Yes. 4. What does impedance, amplification factor, slope, and undistorted output mean in everyday language? ; A.: Impedance is the resistance of the valve. Generally speaking the lower it is the easier the valve will work in your circuit. ‘The aim of valve-makers is to reduce impedance. Amplifaction factor implies the number of times the valve will multiply the signal. The higher. this is the better. Generally, however, the higher the amplification factor the higher the impedance, so that for valves in different parts of the set one has to be sacrificed. In the r.f, portion it is impedance, in the a.f. part it is amplification factor. Slope is the real measure of the efficiency of the valve. The steeper the slope measured in amps per volt the bet-

. ter the valve. Undistorted output is the amount of current. that can be supplied without distorting, ,