Questions and answers

Radio Record, Volume IV, Issue 39, 10 April 1931, Page 13

Questions and answers

Ai B.L. (N. Auckland): How can I ar- * vive at the correct values of coupling condensers and grid leaks in an r.¢.c. audio. amplifier? * A.: The usual coupling condenser: is anywhere between .001 and .5 mfds, The plate resistance .1 megs, and the grid 5 megs, ; , oe Is it practicable to have r.c.c. couplin an r.f. amplifier receiving it by a"band pass tuner ?-Yes. . 3. Have you the frequency response for a 66R Blue Spot unit? . ; A.: Unfortunately, no. The at 2000 cycles is roughly 16,000 ohms. 4, Have you the frequency response of an Hmmeco Puratone transformer?0. , ARSITH (Te Aroha): Is the enelosed circuit of an rf. oscillator Suitable for testing and matching coils? A.: This would do for aligning condensers or neutralising sets. Phones would not be required; you merely shortcircuit the points where they are shown. When this apparatus is connected to the tnains it will oscillate. Tune this squeal in on a broadcast set at about 1000 kilocycles, by placing the link formed by winding a few turns over the coil and continuing it to form another coil, over We rf. stage. In turn starting with the coil nearest the detector, move each condenser until the loudest signal comes in. Continue this until all the stages have been done, then lock the condensers in that position. For new coils take out one stage r.f. coils and substitute the new coil. Tune in on the oscillator and add or subtract more turns until the reading on the condenser dial is the same as for the others, 7, L.D.X. (Palmerston North): How * many turns are required on a valve base to cover the short-wave band, tuning condenser .00005? A.: This was given only a week or so ago in Q. and A. If you are using differential reaction allow two extra turns on the regeneration coil.

2. When the diameter of the former is given and the gauge of wire specified how is the number of turns on the former calculated? A.: It ean be arrived at by charts published in the "Radio Guide" or by Nagaoka formula. 3. What is an R valve? , A A bright-emitter valve not now used. ° 4, Is there any advantage to be gained by placing a crystal in the grid leak? A.: No. It would act as a high resistance and cause damping. 5.'Can you publish a description of the condenser speaker?

A.: It was published in the "R.R." of July.18 and August 1, 1930 6. What are the causes of overlapping oscillation? _A.: Excessive H.T., too loose a coupling, too great a number of turns on the tickler. 7. The power of Radio Saigon is secured by the current, three phases four threads, 120 volts, ete. What are the four threads? A.: The term "thread" is certainly uncommon. It is probably a free translation of the French word "fil," meaning "wire." Hence a four-wire systemthree phase, one neutral. 8. The circuit sketch with this question is worth trying by reversing the grids, using the control grid for the shield grid and vice versa. Voltage will be critical on the control grid. The circuit No. 9 igs not allowed in New Zealand. 9. In winding an aperiodic transformer on a lin, diameter former one turn of No. 36 Hureka wire represents 1 metre. How many turns would be equivalent to a metre on a valve base former?-A.: 1 3-8, 10. How many plates has a Pilot .000082 mfd. condenser?-Ask your dealer. 11. Will one stage of push-pull give more volume than two stages of straightout audio? ; A.: Not as much, but it handle a greater amount of current without distortion. 12, Is it possible to test the emission ef * valve without specially-built apparaus? A.: Yes; by connecting a m. ammeter in series with the plate lead, and leaving the grid free. te 18. Of what is the following A.: Of a Reinartz transmitter. ; 14, The circuit referred to is a 15. Will you-supply the theoretical] circuit of the Rotorua portable? -4A.: It is the ordinary Browning Drake.

16. Is a three-valve super. het, better than an ordinary three-valve set? A,: The sup. het. needs more valves than three. 17, Can an rf. transformer be constructed for an’ untuned stage by building the primary to have the same resistance as the impedance of the valve?-Yes, 18. What is an if. stage? A.: That portion of a sup. het, receiver which comes between the first and. the pocongery detector. See "Radio Guide, NORTHERN PATROL (Dunedin).-I have two variable condensers each with 28 plates. Are they suitable for the

transmitt i in: ae ec cently in- the 2. Will half-inch copper tubing be suitable for that & ou A.: It will be a bit unwieldy if you are using lower power, but will be ideal for high power. B R.H. (Auckland).-Will eoils of lin. * _ gauge wire spaced be satisfactory for short-wave coils? I see now that most coils are wound with thin wire unspaced, A.: Your coils will be quite satisfactory. The coils nowadays are designed for ease of construction, and are not always fully efficient, The valves are so remarkably efficient that a little can be wasted in the coils. 2. My "B" battery comprises 50 volts, with a tapping for the detector at 30, Should the voltage on the plate of the audio valve be 50 or 202-50. GLAD. (Palmerston North).-I wish to rebuild my set. Oan you tell me a really good 5-valve all-wave circuit? A.: An all-wave circuit of 5 valves would be very unwieldy. For a broadeast set try the Outspan five. Wor an allwave set the Differential Four published last week. . 8 J H.S. (Wellington): How would two * pentode valves go with push-pull? A.: Pentodes valves could be used in push-pull, but there is really no need to do so. They may be difficult to handle, and they would need to be carefully matched to the output transformer. SIGMA (Wellington) : Can an ordinary sliding brass coil be used instead of a honeycomb coil for my crystal set, and would a carborundum erystal. detector be better than the one specified?

A.: The sliding bar coil would be the better. The best type of erystal would be , the semi-permanent type. p#e PUNCH (Christchurch) I ‘intend using 2PM4’s for the "Night Hawk." Will they be satisfactory if used with wet batteries?-Yes, quite, Sat tss 2. Using these valves on dry batteries, . approximately how long should the bat- . teries Iast? A.: The "B" batteries about 7 months and the "A" batteries about 3. 3. Would the transformers I mention be suitable for this set, and was it the same as the one used in the laboratory model?-Yes. RANGrIotTU (Mangawhatu) : How can I improve my earth system consisting of four pipes 3ft. apart three © of which are driven down to water level? A.: Place the radiator near the centre’ of the group and drive the fourth one down to the water level. The pipes would, according to theory, be better if they are. 6ft. apart. , 2. Are the leads to the terminals taken separately or joined together? They can be taken separately or" joined by the terminal. 3. My poles are 42 and 45 feet high. Is it to the detriment of the system to have the nearby pole higher than the more distant one? A.: No, the effective height of your aerial is the average. EK D. (Hastings) : Are the details of my * set correct?-Yes. 2. Gould I increase the selectivity? A.: Four-valve sets of the type you specify are unselective. To improve the selectivity use a wavetrap, or add another stage of x.f, The grid bias on the (eee see, eee a ee es i. tid!

«eee 4 (CORRESPONDENTS 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. Limit three questions. . "4 Name of set woe ereoscecesesooseseee Number of valves Creeoseesseceaeese Name Address THCPSSOSSSOHOSCHOSBSGROSCSSSRS ©0048 CF 06982063 EESS27bE08 PESO SH SHOE HH HSOHOT OTH ESTERASES SHADE SOSTCOOH HRS OEHEOEHOT EE EEEEDSESDSEEEEDENom de plume Corecess veseesagser To be kept in subsequent inquiries. Date SP eer er oer er sesaesererees Please Note :- (1) Ee specific and brief, tabuating, if possible. (2) Write legibly, and on one side of the paper. (8) We do not design cireuits, but accept suggestions for feature articles, Selving 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 spectalist’s fee..

WING. to the increasingly large number of queries that our Technical Department is called upon to answer, we have found it necessary to make some slight alterations in our system. Commencing with our next issue, each correspondent .will be restricted to three questions. Those who wish to have more questions answered, or who desire a reply through the post, must enclose stamps or postal note to the value of 1/-. We regret that this has been necessary, but to a large extent it has been brought upon by some correspondents "overstepping the mark." Letters containing from 12 to 20- queries are not uncommon, and many of these could « have been answered had the correspondent looked up either: back numbers of the "Radio Record" or the "Radio Guide." Here are some interesting facts concerning the numbers of queries we have handled during the last six months: There have been approximately 1,500 letters, representing 5,000 to 10,000 questions. This entails a great deal of work, but where we are helping a correspondent genuinely in difficulty we are pleased to be able to doit. ‘ .

last valve ‘should be greater than that on'the sécond last. M° WHARDE (Feilding): What value "ee c¢ondensers should be used for the rejector shown in the "Guide’’? I want it to work from 300 metres down 'A.: Use one .0005 fixed and one semivariable. 2: How can I eontrol volume? AS: "By using a 500,000 ohms variable resistance: across the aerial and earth. E. L. {Mount Hden): My factory-built set is not as sensitive as it should be. What is wrong with it? A.: It appears to be either a defective s.g. valve, or that your condensers are out of alignment. This is quite a common trouble even with good sets. Have a dealer look et your set with -a view to trimming up the condensers, DUFFER (Auckland) : Can I use the variable condensers from my Cossor Melody Maker for a "Night Hawk Two’? ‘eS,. they would be quite satisfactory, ‘but you must make the broadcast coils to suit. See the tables recently published in. Q. and A. 2, Would the rheostat be suitable, and what is’its value? A.i:It is quite suitable, being about 30 ohms. as.far as we know. The choke, too, would be quite satisfactory. It would be better, however, to use a new panel, . The transformer would be satisfactory, but you cannot use A425. However, try it. 8. Where can I obtain the coils? A.: Several dealers make them up. 4, What is the best sized differential eondenser?-A.: .0002. 5. Will the Night Hawk Two be better than the two-valve -set I am using A.: As far as we know, yes. B G. (Blenheim): Would 400. yards be * too long for the Beverage aerial ?- Noy Is it necessary to insulate the aerial at each post?-Yes. 3. May it pass under trees, etc., without losing efficiency Au? Theoretically it will lose efficiency but when we tried out this aerial it was

a rather rough arrangement which passed through and under the trees, It, however, brought the Americans: in much better than an ordinary aerial. It should, however, come directly to the set. Bend will spoil the directional effect. By this we do not mean that it must come in an absolutely straight line, but there should e no big deviations from the. course. 4. Would it be a good idea to join copper wire to the steel wire over the last few yards of aerial, in order to make a better connection with the aerial termin-al?-Yes. ‘5. A power valve in the last stage makes no apparent difference to reception. Can you account for this? A.: It: depends on the type of power valve you use and the amount of plate voltage and bias you have available. At the same time, no ‘matter how good a power valve is, if you are using a cheap speaker, it. is useless. © This: you have not told us, and so, of course, we cannot help you. ‘ L G. (Taupiry) asks a large number. of * questions for designing details of a set which appeared in an overseas magazine. We cannot undertake, without a specialist’s fee, to supply information such as this. It is quite beyond the scope of Q. and A, . CH (Kakohi) : What is the best com- , bination of Philips’s valves for my . A.: Radio A209, detector A215, audio A209, power B205. [GNORANT (Glenside): I have con. structed the ‘A’ battery charger described in the 1929 "Guide." Can I use a 280 rectifying valve? A.: No, this type of rectifier is intended solely for low current high volt-. age rectification. It is not suitable for an "A" battery charger. . 2. I would like to build a really powerful set. Which is the best you have ‘already published and are you. conteniplating still better ones? A.: The best set we have published is the "Outspan Five.’ We will probably

not be publishing many more sensitive than this because expense and. difliculty of constructing increases rapidly, and most people would rather buy a set when it comes to paying out big sums for parts. Kou! (Auckland) :. What is a good shortwave. set on the market? A.: We are not in a position to state, having ‘tested only Mack’s ‘‘Addaphone" and .the a.c. "Ultimate." The reports of these have already appeared. ‘ 2. Can you publish explanations to the Q signs used-in Amateur Radio? : A.: The more common of these were published last week. We shall be repeating them from time to time. — POWERTONE | (Dunedin): I intend making valve base coils for the broadcast bands. Would you advise the number of turns? : The number of turns were recently published in a panel in the Q. and A. column. _ 2. Is a .00035 condenser satisfactory? A.: Yes, there is no need to remove plates from a .0005 as this also could be used with a suitable number of turns.

When using these on shortwave you must switch a series condenser of small value in ‘the circuit, as shown in the diagram. ACK (P.N.): How can I tell if the by-pass condensers in my set are burnt out?’ ; A.: Your batteries will run down very > quickly and the set will oscillate or howl. Could you suggest a good four or five-valve’ all-wave set for use with batteries? ‘A. There are several commercial makes _available. A circuit of a good all-wave set appeared. in last week’s "R.R." when we described the Differential Four. GRID (South Dunedin): We do not know your set, but it is unlikely than an s.g. valve can be added without considerable alteration. . Would -the "Outspan" give better results, and what could one expect to get with it? What would be the approximate cost of building this set? | : The "Outspan" would give better results. You could get. all the Austral_ians at good speaket strength, the Japanese, and if you are in a good locality, the Americans. The approximate cost would be 410 without speakers. or battery,

_ Next Week’s Issue — will be a ' SPECIAL SHORT-WAVE NUMBER Interesting Articles, Call Signs, S.W. Set, S.W. Topics. Watch for it.