Questions and answers

Radio Record, Volume V, Issue 9, 11 September 1931, Page 13

Questions and answers

RANK (Picton) : I wish to purchase a set of new valves for my 5-valve battery set, Can you suggest a better combination than 201A, A.: Yes, Use 221 throughout with the exception of the last stage, and here use one of the B605 class. If you do not have a 6-volt eliminator and have ito rely on dry battery’ you can use the 280 class of valve, with 231 in the last atage. (COUNTDRPOISE (Feilding) : What is the most suitable valve for the Sparrow Hawk One? A.: One of the special detector class, that is, the A415, P.M. 4DX class, 2. In using the Sparrow Hawk One as an all-wave set, what is the most suitable length for the combined aerial and earth? 8. Could a switch on the panel be used in place of the metal shorting bar for switching the small condenser in series with the aerial for shortwave work? es, eu B. BOY (Dunedin): We regret we * do not know anything about the set in question. It has a good name in the States, but has not been brought to New Zealand commercially. If you brought such 9 receiver over on your own account you may have some difficulty with the servicing, although probably the circuit is very little different from any other super-heterodyne. If you purchase one of the better-known makes of super het. the service agencies connected with the distributing firms would always be ready to help you. ‘The set certainly carries a good guarantee, but what use would that be to anyone in this part of the world? ADIO (Wellington): Which is the best set for receiving distant sta-

tions, super-heterodyne or the ordinary receiving set? A.: The super-heterodyne rec¢iver is the more sensitive, although, being more sensitive, it picks up more noise than does the ordinary set.

2. Is 7/20 enamelled wire better as an aerial than 7/22? A.: Theoretically, yes, although there would be very little difference in efficiency; rather too small to be noticeable. If you have an aerial with 7/22, do not pull it down to construct one of 7/20. 8. Which would be the best for a leadin tube, a porcelain or strong glass tube? . A.: There is no appreciable difference. G. (Auckland): Can you account for * the fact that when the third pin connection in my set with the mains is disconnected, as well as my ordinary earth, I can receive KFI, Los Angeles? A.: Your set is stili earthing througn the mains. Actually, the third pin on the wall does not connect with the set proper, only with the shielding. The transformer ig connected only with the two live pins, of which one is earth, so you are still getting the earth return. (TROUBLED (Auckland): When turning the condenser dial 1YA comes in at 35; a little higher, the station will come in again. The set does this on any strong station. A.: Some of the earlier type of a.c. sup-er-heterodynes have this fault It is not serious, although very annoying. It has been overcome in the new sets by the employment of a slightly different ciret3* which use multi-mu valves, 2. Hvery time a deep note is sounded there is a terrible rattle in the speaker. A.: This is due to a resonant peak in your speaker, If it was evident from the onset it is unlikely that it can be cleared up eas ly. Your best plan would pe to consult the agent who sold you the set. As you are not using a well-knowa make it is impossible for us to tell you whether this has occurred to other receivera of the same

8. Does it harm an a.c. set to run it without an earth. A.: No, it is quite in order to do this, as many electric sets obtain their earth return through the mains. I U.D. (Auckland): Can I add more ¢ valves to my 8-valve set? A.: We do not advise you to try, for your set has been designed by a very capable engineer and you would not gain by adding to it. 2, Would I improve my set by using another type of valve A.: No; your set has been désigned for the valves that are in it. Do not alter them. OUNG HAGLE (Gore): I wish to build a shortwave adapter for my T-valve super-heterodyne receiver. Could you supply the details? A.: A super-heterodyne a.c. adapter was described in the 1981 "Guide." It should be attached to the first intermediate frequency stage. P P. (Nelson): What is the size of ¢ aluminium and lead plates for a 1000 m.f.d. electrolitie condenser? A.: We regret we cannot supply this information. If we did we should be

falling foul of the Fire Underwriters’ Association, as we are not permitted to-give instructions for making apparatus of this’ description. nae AINGA O TIA (New Plymouth) : My set squeals very easily and goes into oscillation rather too readily. I am using an eliminator and have tried altering the detector tapping, but withont good results. * Speech and music are mushy. A.: Try the effect of a 2 mfd, .condenser between the B+ and earth. Further, you should not use PM2 throughout, as it is a medium power valve and should be used only in the last stage. — Such valves are sometimes good detec- | tors. If the use of different valves and . the 2 mfd. by-pass condenser is not. effective you will have to locate the tickler coil and take a few turns off it. You certainly have a good list of stations, and it appears that your set is well up to standard. Your aerial system is quite good. We have seen very many inferior. NIGHTHAWE (Lemuka): Although T | had wonderful results with "Spatrow | Hawk One’ I am disappointed with: the pa rnrreneieenenesensecesesecmenaneeseareaiest sess

Ro 0-08 0 gm ecm cemomes (CoRBSSPORDENTS must attach this coupon to all queries seat to the Technical Bditor (Rox 1032, Wellington. Limit three questions, unless letter is accompanied by 1/ee, Name Of Bt ....ssecescccerccevoee Number of valves cvscovcscescovcvces Name oc ccccccvesacnccocoscceccoes Address SOSLOOTOHOOSSSOHOOHSOHOOOS Ae PESHPHSSOHHOSHOOHOHSOSOSHSSOCHSLOOSEO Pe eROTE DESERT OELSAEOHOOSEOSOSHOSONGOSED Nom de plume cecccsccoscececscce To be kept in subsequent inquiries. Date peogeestees te cesoeacecee: 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 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.

HE attention. of correspondents is drawn to the fact that a coupon taken from the ‘Radio Record" must accompany every inquiry. The non-appear-ance of the coupon in any one weel’s issue cannot be taken as a reason for its not being sent. Those who wish not to destroy their paper can, however, transcribe the essential points, Furthermore, we must remind correspondents that queries (limited to three) cannot be answered by post unless accompanied by 1/fee; also no more than three queries can be answered in the ‘Reeord" without 1/- fee. As the volume of queries is tending to increase, we ask correspondents to be as brief as possible. At the present time we have a long waiting list, and sre anxious to get this up to date, but there is only a limited space for this department.

results when I added a further valve. Short-wave coils will not now oscilA.: Are you quite certain that in rewinding your set you did not get the tickler connections reversed? If this is not the cause then we should imagine that the imepedance of your transformer, which is most suitable for a detector valve of high impedance, is not suitable for the valve you are using. Consequently you eannot get it to oscillate on short-wave. ‘I'ry the effect of a .0001 eondenser across the primary. 2. Are the valves likely to be damaged when the detector alone or with a stage of radio frequency is used without grid bias and 90 volts on the plate?-No. K O.P. (Carterton): I am considering * changing my three-stage r.c.c. amplifier to a two-stage r.c. and one-stage push-pull, What are the approximate of B+. and C- voltages required on the centre taps of the output and input pushpull transformers where two 603’s are to be used? A.: Use the highest B voltage you have available, If you ean get 180 volts, so much to the good. If you are relying on batteries use at least 135. Try the double biasing idea; that is, by connecting to the centre tap of the secondaries 60 instead of 30 volts grid bias, ie., you use 135 volts plate. . Instructions with my speaker state that where a single output valve is used a 1 to 1 transformer is to be employed, but where push-pull is used the centre tap of a speaker can be used, and the transformer done away with. Why should the current be passed through the speaker in the case of the push-pull valves and nat in the case of the single valve? A.: The only explanation we can offer is that where push-pull is intended the manufacturer had in mind the double hiasing system, whereby a very small eurrent only is passed through the speaker winding. 8. Would the fitting of an A630 valve to replace an A609 in R.F. of Browning Drake make any appreciable improvement? A.: Fitting an A630 as R.F. is rather uncertain, You must wind a far greater number of turns upon the primary coil, but even then our experience has been that 609 is the safer valve for the first stage-615 if you like. Pp*9mMc (Christchurch) : Will the dia- gram of the S.G. amplifier shown be practicable? A.: Yes, but we think you will not get the results you are expecting. In the first place you are really constructing a booster, and this is not a satisfactory way of adding a screen-grid valve, If you wish to get power, why not pull your set down and use the parts to construct the "Outspan Five," recently described in the "Radio Record’? If you did not want to go to this trouble leave fhe detector and two audio stages of your set, but alter the existing aerial and grid coil of the first R.F. valve. Make the aerial coil entirely separate, and this then can be used for the primary of the intended first stage. The connections then would be as shown in the "Outspan Five.’ Look up the circuit of this set and you will see what is meant. Another good hookup for a screen-grid valve was shown in last week’s Questions and Answers. (CURIEUX (Napier): Thank you for sending along that circuit; we shall certainly try it in the laboratory when we get a chance. Some types of crystals will go better with a low poténtial applied to

them, but it is unusn’’ for a semi--vari-able type to act in : manner, THowever, we shall try oul -. + circuit and publish the result. Thank you. PD*20HB (Taradale) ; I have connected the microphones to my set by_placing it in series with the pick-up. It did not work well like this, so I put hattery in series with the microphone, Now it works fairly satisfactorily; Am I harm-

ing my set through having the battery there? A.: As far as we can see, no, but you should use the correct microphone eircuit, which we publish herewith. You will note that a special micrephone transformer has to be employed. One of these would not be very expensive, and to use it would be the safer method. You can connect the secondary of the microphone transformer to your set. JDEEKSER (Hamilton): I have a 5valve commercial neutrodyne which I wish to convert to the ‘"Radiogram Five." Would it make much difference if I used .0U004 condensers and 3-inch formers ?-No, 2. What would-be the number of turns and the size of the wire using 3-inch formers with .0008 condensers? A.: 52 turns of 24 enamelled or double silk wire. 8. What type of valves do you suggest? If you have to use batteries we shoul -certainly use the new 2-vyolt type, 230, 231, and ‘232. Otherwise use 4 or 6-volt screen-grid valves, special detector, general-purpose first audio, and two 605 tyne of valve in the last stage. 4, What would be the B tappingsB1. B2, B8, ete.? . A.: B1 voltage, usnally 224 volts, B3 the second highest tapping if eliminator is supplied and highest if battery is supplied; B4 also the highest if

eliminator or battery is supplied; B2 is half of B8. 5. You could certainly use the shortwaye choke mentioned instead of the one suggested, 6. Could I adapt a g-amophone pick-up fo a conpnercial super heterodyne? A,: It could bé done, Lut it is not the task for an amateur. You would: take the risk of (throwing the condef%érs out of uligniment. ; B C. (Waitara) -I am considering * building the "Sprrrow Hawk Ditferential Adapter." Is it suitable for u:y set ?-Yes, 2. Is there any particular size or power of the H.F. choke’? A.: No. You could either have them commercially made or build them, Specifications appeared in the 1980 "Radio Guide" and also in the "Radio Recor?" three or four months ago. 3. Which is the detector value in my eircuit? A.: We do not know the layout of your, circuit, but the best plan would be ie get someone who knows something about radio to tell you. You can usually tell by tapping the glass. .The detector will cause a ring in the speaker. Réw RECRUIT (Lower Hutt)-I am *" using a crystal set for a wave trap. Must I take the crystal out of the circuit? A.: Not necessarily; you are not doing any harm by leaving it in. 2. I ean pick up the local station on two wave lengths. Is this in order A.: Yes-the lower reading represents the harmonic, pom (Otahuhu): I recently bought a new B and C Bliminator and find that on various occasions the volume in relation. to the aerial is not. consistent. A.: We simply cannot explain the phenomenal unless your eliminator is coupling back through the mains and using the high tension wires: of the aerial. 2. I am putting 135 volts on the last stage, B406, but the valve warms. Am t injuring it? A.: No, providing you have sufficient grid bias-i.e., 12 volts. 3. Is it necessary to install a separate earth for the eliminator? A.: Theoretically you should have a separate earth, but generally the ordinary earth is quite satisfactory.

} QUPER-HET, (Christchurch) -I have just been trying-out a super-hetero-dyne receiver, and find I can get the two local stations at several points on the dial. Why is this? A.: This is due to cross modulation and shock oscillation (we are calling it that for the want of a better name), features which showed themselves in the earlier super-heterodyne receivers. Your set should not have either of these faults. If he are buying a new receiver specify valves, which will, to a large extent, do away with the complaint. ORTHERN PATROL (Dunedin).--Will a 50-watt transformer be suitable for a transmitter using two 245 valves in push-pull with 350 volts on the plate? A.: If you intend to use only the two 245 from this transformer it will be quite satisfactory. 2. I have two condensers with 28 plates, ,and I want to reduce to .0005 and .00035 sapacity. How many plates would I need take off? , A.: The present value of the condenser is .0005 ; to reduce to .00035 take off your moving four fixed plates. 8. Would choke G in table D, "Radio Guide," be suitable with a 50-watt transformer? . A.: It would be quite satisfactory, but if you are going in for transmitting, why not build the heavier one, as before long you will no doubt increase your power? J ©. (Dunedin): What metal rods are ¢ used in Balkite charger? A.: Lead and Tantelum. 2. I have a four-valve set. Can 1 add another valve as shown in the accompanying sketch? A.: You would gain nothing if you did. You are merely connecting the two output valves in parallel. This method would give you a slightly greater undistorted output, but would not amplify your signals to any greater extent. AMISH (Christecburch): T constructed the S.W. battery set described in the 1930 "Radio Guide," and have had good results, with the exception of a tendency to over-oscillate. I tried a differential condenser instead of the resistance control, but failed to get oscil-

lation of any description. Should f have altered the set in any way? A.: You should have tried the plate lead to the other set of fixed vanes in the differential condenser, bringing the connection from the tickler to the lower set of vanes. Apart from this, however, it is probable that there’ are ine sufficient ‘turns on the coils now that you have installed a differential reaction, which system needs a far greater nume ber of turns than does the ordinary condenser controlled type. W A.O. (Christchurch): You could ° obtain . armoured flex probably from L. B. Seott and Co. or A. E. Strange, of your town. Failing either of these two people Ballingers, Fears or C.A.S. in Wellington, and Johns in Auckland. NOs! (Christchurch); Would I derive any adyantage from putting 2 "A" batteries in parallel? A.: If they were dry cells you would find that they would last longer than two sets of batteries used one after the other. 2. Would it be worth while my building a full-wave crystal set? A.: It is purely an experimental set. You might get good results and you might not. . 8. Are there any 1929 "Guides" to be had? A.: Yes. There are still a few. N=EOPHONE (Wellington): I constructed the Loftin-White amplifier, but it is insensitive. The hum control has no effect and I had to short out the 775 ohm, resistance to get any results. A.: There is something wrong with some of your resistances, probably in the bias chain or the 775 ohm. one. Have them checked over and also check over for any short circuits. Are you quite certain that one of your condensers is not wired in the wrong position? If this were so you would notice the symptoms about which you complain.