Questions and Answers

Radio Record, Volume III, Issue 52, 11 July 1930, Page 26

Questions and Answers

"LNQUIRER" (Wellington) has a tandem cnndenser (.00085) with trim‘mer, and desires to know the number of turns space wound on each coil, necessary for the construction of a 4-valve singledial control B.D. A.: The aerial and the secandary coils require 6S .turns on: a 24-inch former: using 24 D.S.C. wire unspaeéd, If you space .it, wind on 60 turns, but spacing is not necessary when silk or cotton covered wire is used. Tap the aerial at the "twentieth turn, and wind 15 turns on the primary, . Lo 2. Which wire do you advise, D.S.C. or enamelled? oo A.: D.S.C. would be very much easier to work, and give equally as good results. ‘ " Cc." (Wanganui) wants the details of a cone speaker unit. A.: We do not recommend amateurs to try to construct the units. They cost very little, and are not worth the trouble they involve in manufacturing. Purchase

a unit and then make up 4 speaker, such as was described by "Megolim" a short time ago. . "R Vv." (Anckland) has read our arti- » cle on the Loftin-White system, but "cannot get hold of the theory well enough to answer for myself the following questions"; 1. How does one arrive at the values of current and voltage for different points of the set for different valves? A.: It is most involved, and we do not recommend constructors departing from the specifications. Almost all manufacturers make valves with the same characteristics as those recommended. Use these. 2. Could this system be worked with D.C. or_batteries? . A,: Xcs, but it will take a frightful amount of experimental work to evolve a really good get of resistances. If you feel fit to tackle this, we wish you the best of luck. 8. Could you manage to give another article explaining these points or publish a diagram of the resistances for other valves? A.: We will probably follow up with further articles on. the Loftin White. We ecannot promise that we. will experiment with D.C. The principals in America have giyen no details in connection with D.C, valves. They consider D.C. a thing of the past, ° "B D" (RANGIWAHIA) = asks_ the sd following points concerning the two RP. B.D, 1. Which coils.,would give the best results, 2din. diameter or 2in.? » A.: You wouldn’t be able to detect the difference between them, . . 2. How many turns of D.S.C, wire be required in either case for the .00035 condenser? ; A.: Qin. diameter, 98 turns; 2din. diameter, 68 turns. 8. How many primary turns will be required for A409 ?--15. "SUBSCRIBER" (Wairoa) asks if a volt battery can be charged from a G-volt charger. ; A.: Yes; use a 3 ohm variable ance in series and adjust until the right charging rate is secured, 2. Which is the best all-wave set, detector and two audio, or a screen grid four, all wave? A.: For short-wave and all-wave work the screen grid valve has heen successful. Probably the 8.G.4 would be the better. 3. Can a screen grid set, and a 3-valve set be worked satisfactorily from a "B eliminator? A.; Reeent experiments have shown that an eliminator may be used on shortwave. There may be a slight ripple, but nothing much to worry about. \ 4. What is tho best aerial length for an all-wave set? A.: About 60ft. 66 B.C." (Wellington) stated that *when his set is cold a piercing whistle keeps up for 30 seconds or more. A.: It sounds like a microphonic valve, When the set is working commence by giving a sharp tap on the deteetor valve, This will produce a ringing sound. Wait until this dies down, and tap the next valve in the amplifier, and so on. The one which produces the ring will then be found. If the same ring is produced when the valye has been

changed, have a .1 to 15 megohm resigtance placed across the secondary of that transformer, thereby limiting the input to the last valve, Wxcegsive input will produce similar noise. The speaker will cause vibration if the amplifier is too close. A lead cap placed over the valvewill stop the vibration when anti-micro-phonie sockets are used. Use . rigidlyfixed sockets in the amplifier. "IHGINNER" (Gisborne) has a sereengrid four battery set, but cannot gct anything on short-wave. A.: Increase the detector voltage until. the set oscillates. The terminal is usually marked B+ or HT1. The three-posi- (= 3 i|

tion switch is a tap om the aerial coil, so that you can better adjust the length of the aerial to the frequency to be receiyed, The valve you are-using in the last stage requires about 16 volts bias. Rvidently your C batteries are right. You should have given us more information regarding the markings for the battery cable. We do not have diagrams for all commercial sets at our finger-tips. "KTOVICRH" (Whangarei) _ has an &- yalve all-electric set, which uses valves different in make from those specified, in the last stage two valves of different make are used. One 227 is stipulated, but all except the last use 226’s. A.: So long as the types are the same, there is little need to have the same make as specified, though in the last stage, it is better to have two valves of the same make. If good results are expected, the last two valyes must be per: fectly balanced, and this appears to be lacking in your case. The fact that your set stipulates the use of a 227, and you actually use a 226, seems a little con-

fusing, 226 cannot. be used ag detector. Are you sure you haven’t make a misake " a J.M." (Kelburn) asks if two elittin*ators giving the same output can be coupled in series to give double the out--put. A.: In volts, yes. You. may need to -reverse the primary connections of one of them. The primaries, of course, are connected in parallel, that is, hoth adaptors are plugged in A.C. sockets. 2, Where can I obtain a thermo-galya-nometer, and what would be the .approximate cost? . so A.: Carrick-Wedderspoon, Wellington, can supply them. We do not know the ~ 8.- What current. does the "C" valve 8006 in a Philips eliminator 3003, give? A.: .6 milliamps. Its funetion Is to ‘provide voltage, not current, 4, Could I get say 200 yolts 18 milliamps tom the above eliminator? :: Yes. "" A BR." (Feilding) asks for particulars ¢ for valve-bias coils. for "Round-the-World" three to tune over the broad-: cast band. eA Secondary Tickler. Band. 54 1... 24 ..-. 800 180 .... 35 -. 800/500 Use 26 gauge enamelled wire without spacing. . GA." (Stratford) wishes to know if a Tungar charger can be used as a battery eliminator with an electrolytic condenser and choke? A.: Yes, you are not allowing anything for voltage drop in the choke. This, of course, will be very little, but it would be better to nse a 4-volt valve with a resistance. We see you have noted a remark made in these columns that the positive of the charger became the negative of the eliminator, This was an unfortunate slip, for the positive of the battery charger becomes positive of the eliminator. ‘CO BE.L." (Kilbirnie) asks if 3-inch * formers can be used for the coils of "Round-the-World" Three-Yes. 2. The terminal strip shown oi, the diagram is blurred. What should ‘they De A.: In this order: A-, B-+_ detec_tor, B+-- amplifier, O-. A+, B: max. Note that A-, B-, and C+ are connected together. 3. Kindly give particulars of an "ABO" eliminator for this set? A,: A suitable eliminator was deseribed in the 1929 "Radio Listeners’ Guide,’ ’and an "A" eliminator may be made from the specifications of the battery career and choke given in year’s "Guide." " 8." (Blenheim) asks if the,far end * of the aerial were to make good contact with the earth would the R.A. valves burn ont if the machine was runhing all out on distant stations, A.: No. the valves are not in danger while the aerial is shorting to the earth. *"M. * (Napier) asks if a Pilot push- * pull impedance is suitable for a Loftin-White instead of an output transformer? . ,

Questions and Answers --e BADBERS of the "Radio Record" who are in difficulties abotit reception or set construc: tion are invited to write to our "Questions and Answers" department for help. . We particularly. wish to assist those who know little abe it radio, as very oftén there is some very slight trouble which spoils completely one’s enjoyment of the programmes, Correspondents are asked to observe the following courtesies: 1, ‘Write. legibly. 2. Make your questions _ briet and to the point; do not make apologies for writing, and, where possible, tabulate. 8. Do not ask for a reply by. post unless a st.mped anu addressed- envelope is enclosed. iver in these circumstances, we reserve the right to answer any question through our columns, 4, Do not ask us to design circuits ‘or send detailed lay-out diagrams; but we can offer advice regarding circuits, & Address all technical ¢correspondence: "The Technical Editor, P.O. Box 10382, Wellington."

,A.: Providing it will pass the curt, that is, it must be one of the type, and is used in conunction with a 1 mfd. by-pass eonenser, This is placed in series with the speaker. : 2. Please supply the list of material for the detector and -R.f. stages of a Lof-tin-White amplifier. A.: A detector stage is not needed. To tune the local station, a coil and a condenser of ordinary dimensions ‘will do quite well. We are experimenting with RF. stages, and when we haye arrived at a suitable amplifier, we shall deseribe it. . . : "A G." (Lyttelton), concerning the per4 A. forman¢es of his set during the frequency test, asks: , ) During the first frequency test, the ‘speak¢r and amplifier responded between 0 ant. 15,000 vibrations, Is this above 3 e average dynamic? ; A.: The range is hypothetical. If your amplifier would respond over this range it would be truly remarkable. What you heard on the lower frequencies were armonics, and the upper ones--well, we suspect, that 2YA could’ not transmit with any strength notes above 12,000 eycles. 2, Although T live close to 8¥A I find the strength varies. 7 A.: It is purely an atmospheric condiion. "NOvVich" (New Plymouth) is using’ =" a four-valve receiver and finds that it is rather noisy when phones ‘are used. What connections would he need to make to use one stage of audio only? _ A.: Conhect_your phones across the primary (P-+ B--) of the second transformer. ‘Try the secondary and see if results are better. Probably very little will be gained, for the last stage can usually put on the speaker anything: that ean be heard on the first stage on the phos. "J .R." (Oamaru) wishes to know if * it is advisable to convert a certain factory-built battery receiverinto an allelectric, and what would be the cost? A.: Generally speaking, it is not a paying proposition. Note: We thank you for the lock of hair which was enclosed, but as we cannot see any eonnection between your query and it; we are returning it. We trust that it will find its way to the rightful owner. 6 L.P." (Pahiatua) wishes to know * if an A.C. set is harmed if used without an earth. Touching the aerial terminal with the earth does not cut out the signals. A.: It is not unusual for an A.C. set to work better without an earth, but the signals should be cancelled when the earth connection is put on to the aerial terminal if the aerial is on at the same time. Quite often, when interference is bad a set will work better with an earth jn the aerial terminal and the aerial lisconnected. If hoth go on the same termingl and signals still come through then the earth is defective. Ms B.D." (Lyall Bay) wishes to con4ie vert his 1-valve A.C. set to a 2valve. ; A.: You will have to make some radieal alterations for you must use an in: directly heated valve in the last stage. Full instructions were given in "All About the All-Hlectric." ‘D V.W." (Christchurch) asks why ¢ his set develops a very bad A.C. hum when connected to the power amolifier. i |, A.: Reverse the leads to the input and f the set is grounded connect a high jltage tested 1 mfd. condenser in series ground lead. Reverse’ the res on the A.C. point. ‘A H." (Auckland) wishes to know the ¢X* cause and cure of a high-pitched éntinuous whistle when any change is nade in the value of a grid leak over 1 nhegohm. \jA.: The secondary is open. Look to i if the grid condenser is not broken,

Try the *phones test and if there are no elicks other than the first, the condenser is not shorted. It may be kroken inside if this test makes it appear O.K. Reverse the secondary wires in the first audio transformer to stop the first audio valve rectifying. "rI\ McK." (Wanganui) is rebuilding a 8-valve short-wave set a short-wave adaptor, as described in the 1930 "Guide," and asks: 1. Would the .001 and .0025 variable condensers be all right? | _ _A.: Yes, the wave-band would be only slightly different. . 2. Would a CeCo detector valve suitable plate voltage do as detector ?- Yes, quite well. . _ 8. The theoretical and base-board wit. ing differ. In one the grid side of the grid leak is connected to the tuning conenser, and on the other, the coil side. Which is correct? . A.: The coil should be connected che ‘fixed plates, and with the grid of the valve. 4, Looking down on the base of the valve-holder, the top of the secondary goes to F minus. In the base-board the top of the secondary goes to grid. Which is correct? A.: Try the top as shown in the baseboard wiring. The confusion is due to a last-minute alteration in one of the diagrams. We omitted to alter the other. "EK AB." (Hastings) asks for particu*lars for taking "B’’ current from a 280 D.C. main. . A.: Some time ago we a D.C. eliminator. That promise still holds good, and it will be appearing within a few weeks. . .2, Could I make the condenser? A.: It would not be practicable. " R." (New Plymouth) asks if his *combination of yalves is economical as regards "B" consumption.-Yes. 2, The 1980 "Radio Listeners’ Guide" gives the plate current of 630 as .7, the chart accompaning the valve as 30 m.a. total emission. Is this correct? A.: Yes, this is the current when the valve is properly biased. 30 milliamps is the maximum current without bias. Note,-We do not advise you to interfere with the rheostat arrangement. It is quite all right. . "DENTODH" (Dunedin) asks the maximum permissible voltage for a 280 type rectifier, with both plates paralled and used with a _ half-wave rectifier. A.: 280 is to be used only in full-wave circuits for which it is specially designed. We advise you to use 281 for a half-wave. 2. What is the maximum undistorted output of the following valves :- Valve. utput. B448 ww we ee «0450 milliwatts. 0448 eeeoooece 2 watts. B448 wo. eceeees «38.2 watts. 3. Neglecting loss in the rectifier, is the D.C. output voltage equal to the mean or R.M.S. value of the A.C. voltage? A.: Yes, if it is equal to the R.M.S. voltage of the secondary of the power transformer mjnus the drop in the rectifier which varies directly with the inerease in current drawn from it. "W 8." (Inglewood) heard particular*ly strong signals from one of the South Island stations, and asks why they are stronger than those from 2YA. .: This would be a characteristic of the atmosphere on that particular night, The writer has heard Japanese and even Amerienn stations coming’ in as strongly as the YA stations, though, of course an occurrence such as this is very rare. "W R." (Denniston) has built a 38- * valve portable set and is troubled by microphonie noises, What can he do to rectify this? A.: The trouble. no doubt, is a microphonic detector, and the solution is either to replace the valve or place a lead can over it. The fact that it is a portable set would’ tend to make it more microphonie,

2. Can short-wave coils be made for this set, and if so, what are the specification? A.: It is primarily designed as a broadeast set, and to adapt it to shortwave you would have to do more than duplicate the coils. You would have to rebuild the set, using smaller condensers and preferably valve base coils. Specifications for the coils have been previously given in the "Record." $ A J.H." (Napier) wishes to make a : * 4 or 5-yalve all-wave set, incorporating screen-grid détection. A.: We have not published specifications of such a set, for these modern sets ate yery expensive and difficult to build, and then results are uncertain. You may be able to get a kit set that would suit your purpose. The Super-Wasp, with three stages of screen-grid, is specially esigned for DX reception. ‘C F.D." (Wallaceville) has a 6-valve * faetory-built set with which he is using a trickle charger and an eliminator. Reception has suddenly hecome very unnatural. A.: Maybe your "A" battery is done. If you have had it since 1925 then this is the cause. If your battery is only new or tests right, then we advise you to take your whole set in to the agent who sold it to you, especially as you are a beginner we do not advise you to interfere with it. have a short-wave adaptor and can only hear a buzzing noise. What ean I do? . A.: Again we do not advise you to interfere. Take the adaptor with the set to the dealer.

"G I,.R." (Auckland) asks for measure- * ments for an exponential horn to be used with a Baldwin unit. ; A.: hey were given in the *Radio Records." Look up the index in this issue. The original description has been posted to you. "PHASATROL" (Wakefield) wishes ad-« vice on the following points ;-- 1. Would the Browning-Drake coils be suited for the enclosed circuit? A.: Yes, with a .00035 condenser, 73 turns. of 22 enamelled wire on a 2zin. former. ° 2. What is the value of the resistante in the phasatrol? Would it be suitable to use a8 a potentiometer in the circuit? A.: It varies-usually 400 ohms. 8. Are the differential condensers ayailable in New Zéaland? A.: You would see from last week’s igsue that they are. 4. What is the total milliamp. drain of a modern 6-yolt screen-grid valve? A,: Slightly less than 2 milliamps. 5. In my present 4-valve B.D, set I ami using Philips 6385 with R.F. with 40 turns of 28 enamelled wire on the primary. Nentralising is necessary, Why? A.: Probably because of the small elec trode capacity of the valve, and the fact that the impedances are properly matched. Do you think the attachtd circuit is superior to the 4-valve B.D.? A.: Yes, it should be slightly better.