Questions and answers

Radio Record, Volume III, Issue 35, 14 March 1930, Page 28

Questions and answers

Double Grid Valves. "WEGINNER". (Napier) asks if a double grid valve can be used in the audio stage of a two-valve BrowningDiake, A.: It is not suitable for anything other than a single-valve receiver. This valve, although operating on very low plate voltage, and giving remarkable amplification, cannot_handle loud signals. Where can I get a circuit of the set? oe A.: It has been published in the "Radio Listeners’ Guide’ as the 4-valve BD. Omit the last two stages. 3. Would this set work an exponential] harn with any satisfaction on the local station ? A.: It is very doubtful. In all probability you would need ayn audio stage. A valve of course could not: be used. It would overload too readily. 4. Is the double-grid valve equal in volume, tone etc. to an ordinary valve with 90 volts on the plate, and is its life as long? : A.: The volume would be approximately the same, but the tone would be thinned in the lower registers. The life is equal to that of any other valve. Can Phones be Used? UITE a few statibns come in, not quite strongly enough for me to get the call-sign. Can I use head phones? The set is 2 3-valve screen grid factory-byilt,-"T.T." (Dalefield). A.; If it is an A.C. set, the ripple would probably be too strong for use with phones. -It is doubtful to give any definité decision. Your best plan would be to borrow a pair of phones and try it out for yourself. You could not do any harm, providing the positive ter-

minal of the phones is connected to the positive terminal of the set. "Remote Control. I USE my loudspeaker 30 feet away from my set, and wish to switch off my set from this point. How can this be done? asks "H.M.D." (Palmerston North). A.: Get a filament break jack and connect the A-+-- by means of a 80ft. wire to one of the filament springs, and take the other spring to the positive terminal of your set. Break all other connections between A-+ and thé set. Upon pulling out the plug the set will be cut off autoriatically, Keep the two wires close together and use the coarsest wire possible; 18 D.C.C. would be excellent; twin lightning flex would also be good. Round-the-World Two. AM building the Round-the-World Two and wish to know the following, asks "S.P." (Wellington). 1. Would a .00025 variable condenser do for a .0002? A.: Yes, but it will alter your tuning slightly. It will be better if you can put a .001 condenser in series. 2. Which is the primary and which is the secondary coil? A.: There is no primary; the set is one of the tapped secondary class. 8. Is the Amperite necessary? A.: If using five-volt valves. 4. Is the neutralisation condenser necessary ? A.: It would hardly have been put in if it were not. Its function is to sharpen tuning. . 5. Would two-volt valves be suitable? -yYes, but 4, 5, and 6 are better. . mene re aorernen arse cen anes cemuunesumimisuhctial TS answer SS

Reception Weakening. "L H.S. (Kaikohe).-I have found that * I can get no other station except Wellington, but that at very good strength. The agent has inspected the set and says that if we can get Wellington we,must be able to get other stations, and that their weakness is due probably to atmospheric conditions. A.: It is most difficult to advise you differently from a professional man who has seen your set. As you state the problem, it appears as if there is somer thing wrong, probably the valves are weakening. We should advise you to have them tested. If they are in order, we shpuld recommend that your set be overhauled by a competent service man. It is possible that a by-pass condenser has broken down.: ; Shorting Valves. Js there any danger of burning out valves should a short-circuit occur between the two flexible leads in the speakex? asks "Aerial" (Temuka). .: No. 2. Would a shield of thin zinc be sufficient to screen a frame aerial from the induction .from a nearby power line?-No. Volt-Meter Reading from an Eliminator. P J.W. (Dunedin) asks if he should * get a voltmeter reading between the negative and any of the positive tappings in the B eliminator.-Yes, providing the meter has a high enough resistance. 2. How can I get the correct voltage on the plate of the detector valve when using a B eliminator? _A.: You will have to use a very sensitive meter with a higher resistance (1000 ohms per volt). The negative terminal of the meter is connected with B- and the positive terminal to the plate of the detector socket. -3. How can I test the winding of an ordinary transformer. . Connect B-+- of the transformer ‘to a + positive tapping of the eliminator and the. negative of the meter to the negative of the eliminator. Touch the positive terminal of the meter to the "P" terminal of the transformer, and you should get a reading slightly smaller than that given from the positive to negative of the eliminator. To test the secondary, turn the transformer round and make the connections in the same way. The voltge drop under these conditions will be slighly greater than for the primary. have made up a 4-valve screen-grid receiver and can get nothing on the first three valves, and only a local station on the fourth. A.: You need far more turns on the secondary coils, and it seems that there are other faults in your receiver which could only be checked up by an examination. Get .an experienced man to go over your circuit with you. Short-wave Receiver. you will find enclosed the circuit diagram of a short-wave set, Is it g§uitable, and can you give me aljl details for construction? writes "A.W." (Port Chalmers). A. This is a tall order and would take some time. We cannot undertake to give layouts. Build up "Round the World" Two, if necessary leaving off the audio valve. The problem of winding coils on valve base formers was dealt with in last year’s "Radio Listeners’ Guide."

Overloading. OW can I-prevent overloading? writes "J.K." (Upper Hutt). A.: There are very many methods, other than removing the aerial. The best, no doubt, is to build a local station Singlevalve receiver, of the anode-bend type described by "Megohm" some time ago and connect this with the audio amplifier of the receiver, Crystal detector, likewise, can be coupled up with your amplifier, for the aerial terminal can be taken in to the grid terminal of any of the radio valves or detector. For those who dabble in construction, we should advise biasing the radio valves. Circuit Wanted. " A NXIOUS" (Rongomai) asks for details of a 4-valve all-electric screengrid short-wave receiver-using valve base plug in coils. A.: In the 1930 Listeners’ Guide we are bringing out a set of this description using only three valves, which we think is quite adequate for all purposes, An extra audio stage could of course be added. The guide should be out in about three months, and until then we cannot give an},details. Accumulator Problem. WHEN my accumulator gets below 1200 it ceases to function. I charge it to 1250 or 1300, but it soon runs down. The plates are well covered with the liquid, states "R.E.H." (Kohu Kohn). 1: What would’ be the correct rate to charge the battery? A.: Charge at about 2 amps. to 1250 or 1800 8.G. Do not let the battery get as low as 1200, 2.; What would be the effect of a too high rate of charging? A.: The plates will suffer, and shorten the life of the accumulator. 3: What would be the effect of charging after the reading is 1800?

\ A.: The accumulator will froth badly, ut if the rate of charging is low enough, o damage will result. Have you tried ) washing out the battery with pure rajn water? Quite likely sediment has collected in the bottom and is short-circuiting some of the plates. . Screen-grid Short-wave Receiver. T WISH to build a S.G. S.W. receiver, and have a .000125 condensér and another with two moving plates, and three fixed. Would the latter be about the same capacity?-"F.H." (Huntly). A.: It is difficult to state the capacity of this condenser as it very much depends on the size of the plates. If the two look about the same, then it can be taken for all practical purposes that they are. 2. What number of turns and sized wire are needed: for valve base coils with .000125 condenser, using .00025 for reaction, Spi The sizes given in last year's . "Radio Listeners’ Guide" will be approximately correct. Allow a turn or two extra.on the reaction coil for the smaller capacity. fe : Internal Short. "B C.A." (Alexandra) states that he '*. has a popular three-valve set, which has given satisfaction, until now it appears that when the filament current, is tutned fully on signals fade away. The fuse between the A and B has blown out and been replaced by wire. He now finds that the set has stopped working altogether. A.: Have you tried the detector and audjo by placing your finger on the gridleak? ou should get a loud squawk if these are functioning. It appears that the screen grid valve has shorted internally ; the elements were evidently very close to one another and when the electrons sereen reached full strength when the fila- . Isent was turned full on, it was so close ~, to the plate or sereening grid that this latter became clouded and the valve ceas‘ed to function. It now appears that the elements are touching. Give the valve a tap and see if this makes any difference. The fuse between the "A" and "B" .should have been replaced by a very fine strip of good tinfoil. This makes splenlid fuse, and safeguards the valves. The valyes may have been blown out, owing to the absence of the fuse. A Noisy Set. [MMEDIATELY I switch on my set there is a surging or grating noise such as would be caused by a loose valve. have had a new detector and tried a new valve.in each of the 222 sockets, ‘"M.B.S." (Palmerston North). A.: Your question is rather too meagre to permit of full exlanation. Did the set always make ‘the noise? Why did you get a new detector? Have you seen the local agent? Unless you supply this information, we cannot help you. Resistances and Voltage Drop. [ PROPOSDHD changing the valves in my ¢ receiver, and wish to know the effects m the readjustments. The ‘-valves each taking .1 of an amp, are wired in series, and are controlled by a 2-ohm resistance, They would require 7/10ths amp. Would the 3-ohm resistance still be all right? A.: You have made a mistake somewhere. If the seven valves are wired in series the total consumption will be only .l of an amp, whereas the total voltage will be 42, — 2. If I leave the valve wired as at present, would a 20-ohm rheostat be suitable on the first valve, wired separately ?-Yes. What valve resistance should I use for the other six in series?---They would have to have 3/5ths of an amp, and I Yr ran’t get this value in resistances anywhere, asks "Valve," Hastings. A.: This question is most confusing. You state the six are in series, and yet ‘equire 3/5 of an amp. and state that you cannot get this value in resistance, We have presumed that they are wired in yarallel, which is the normal method of yiring, in which case a 3-ohm resistance

would be ample. You will probably have difficulty in securing a résistance of this value, so wé advise you to get two 6ohm rheostats and wire them in parallel. You can get these latter from Harringtons (Wellington), who are selling them off at present. You will have to be quick, however, as they do not propose to carry a stock, as they aré rapidly going out of ate, .