Questions and Answers

Radio Record, Volume II, Issue 29, 1 February 1929, Page 10

Questions and Answers

"IN BRIEF. T.8. (Nelson). asks for the details of a five-valve receiver that will bring in the American stations. A powerful set will be described in our "Listeners’ Guide," which will be out in the course of a couple of months or so. No one, however, can guarantee a set to bring in the American stations. Valves Die Off. M* receiving set is a five-valve set, factory made. When I plug in the valves light up, and the programme comes through good, but then it fades away, until in about half a minute or so it fades out completely and the valves die out. There is no other disturbance, it just dies out. The A, or wet, battery shows good readings, also the B and © batteries. I have examined and found nothing wrong with the connections of them. The wiring also appears to be good. I have tested with a new valve on all sockets, and still the same results. It is the same with headphones, as I thought the plug of the loudspeaker might be wrong. After it has been standing a day the programme comes good for, say, half a minute; if I plug in again in ten minutes or so it just lights up, and very small portion of the programme comes through, and dies away, about ten or 15 seconds.-J.O. (Nelson). ANSWHR.-The fact that the valves light up to die off a few minutes later seems to suggest that there is a fault somewhere in the filament wiring of the set, probably between the A battery and the valves. This would account for the reception dying out as.it does, The test should be that described in the "Beginners Corner" and in "Construction" some time ago: ie., by using a pair of telephones and a dry battery. Jake the instrument and test out @very piece of wire and every component: lying in this circuit. First examine the. A battery terminals to make certain that they are not corroded. From here follow the A battery end and one end at the binding post. If a good strong click is not heard it can be ‘taken for granted: that corrosion has taken place in the lead between the A battery and the set. Likewise, test thé other wires in the filament circuit. Test very carefully resistances, if they are in the circuit, and the rheostat. The rheostat quite frequently gives trouble ‘of this description, for after

being in use some time a rheostat, other than the very. finest quality, makes a faulty contact, and the slight-. est jar, or even a great fluctuation in current, may cause the contact to be broken, Examine this carefully, then, to see that the connections are sound, clean, and solid. Any loose parts should be tightened up. If the rheostat is at all doubtful it can be tested by the ‘phone method with the moving arm in different positions. Test also the valve sockets by this method, although it appears that the trouble lies in‘-a lead common to these rather than to the sockets themselves. General Points. "TN Trouble," Miramar, writes asking several questions, which are answered as under :- 1. Is the use of a condenser across the primary terminals of a transformer illegal? ANSWER: Certainly not. In fact, some transformer makers inelude them in their transformer, viz., Ferranti. However, such a condenser can be easily placed across the primary of any transformer and it is advisable to do so. 2. Is there any difference in the efficiency of an aluminium or brass condenser ? ANSWER: Yes, The brass condenser is slightly more efficient than aluminium, and in consequence is higher in price. The aluminium, being lighter is particularly useful for light receivers of the portable type. The difference between the two is very slight. 8. How would fixed condensers be placed in the audio side of the receiver and what capacity is best? ANSWHEHR: There are several places in the audio side where condensers may be placed :- (a) Across the primary of the audio transformer, Here a .0001 condenser is quite suitable; (b) Between the terminals of the telephones. Here the value should be approximately 2mfds. (ce) Across the B battery. (ad) Between the B positive and the earth. An advantage can be secured by placing a radio frequency: choke hetween the first audio transformer and

the regeneration coil of the: detector circuit. If this alteration is made, a condenser of .0001. should be placed between the coil side of the RFC and the A negative. 4, Do. these condensers cut down the signal strength, and do they _ improve the tone? The improvement in tone is worth any slight reduction in the signal strength. 5. What is the best way of toning down loud signals? Is it by insert-’ ing a variable condenser on the. aerial before it reaches the set?. If so, what would be the capacity of the condenser, and how would the strength of the signals suffer when the plates of the condenser were together? Would the condenser be used in a parallel or in series? ANSWER: Yes. The best method of controlling volume is to place a mi(get variable condenser in series with the aerial lead. The capacity should be .0001. 6. Is an amperite as good as a rheostat for the audio?’ If so, why? ANSWER: Yes, except with an occasional valve, amperites could be used to advantage. These are always connected in series with the valve filaments and the battery. Care should be taken in selecting that the correct amperite for the particular valve. is obtained, A chart has been prepared and may be obtained from any dealer who stocks amperites. Howeyer, other resistance may be used. 7, What value of rheostat should be used on a 4-volt valve in the audio stage? ANSWER: The correspondent here has not made his question clear. Does he wish to use a 4-volt valve with a 6-volt battery, or does he want +f to control volume? If it is the former, use a 300hm rheostat; if of the _ ter, 2 smaller capaeay w fixed resistance would do equally. well. 8. Could a 6-volt detector valve be used with a 4-volt audio if a separate A battery is used, and the two negatives connected? ANSWER: By using two batteries, the correspondent is looking for trouble, as well as expense. The uses of rheostats or fixed resistances is far to be preierred when valves: of different capacities are being used. 9. What is the fuction of a R.F.C.? Can it be used on the short waves if it is specified in the detector of the long wave circuit, or must it he subtracted ? ANSWER: A choke coil is a coil of great’ reactance or imnedence, whose purpose is to limit the flow of alternating or pulsating current of certain frequencies .through part of a circuit in which the choke is placed. The high frequency choke must oppose the. passage of Nigh frequencies. but must not choke back the audio frequencies. A choke should be used in the detector circuit of a short wave receiver. Power ‘Interference. I AM considering building a 8-valve receiver, and would be pleased to have your advice on these questions. I am using a 2-valve set (O.V. 1), but results are not as good as I would like. I require a set to give ‘phone work. from the local, and from the Welling‘ton station, Quality is the main consideration, and although the set has been improved by carrying out ‘your

suggestions, I would like it better still. I am only half a mile from 1YA, and on the car line, and so "troubled by power noises, ‘1. Would you advise Browning Drake or Hammerlund Roberts circuit (1.V.1.) in the circumstances? 2. Would the.use of a 22g: copper eabinet clear up reception*if earthed? (This apart from the screening. of the R.F stage.) ‘8. Would the use of silver plated wire for the wiring of.a set, in place of .the usual tinned bus’ wire, improve .a set by lowering the resis-: tance? . I am a plater by trade, and it would mean practically no extra cost to soften and give a good coating to some copper wire of .a suitable gauge. I. understand radio currents flow more. on the outside of a conductor, and if. this is so, the plated, wire would be much the same as solid silver wire. "Switch," of the R.R.;" seems .to like: the Browning Drake, but I have gained the impression that it might not be selective enough so close to 1YA. ANSWER.-(1) Both circuits are more or less suitable, but the correspondent would be well advised to con- sult the local manufacturers of the Hammarlund, Roberts receiver, Messrs. Johns, Limited, for further information on the use of this circuit locally. (2) Yes, screening always helps to reduce noise. The RF stage could well be. screened. (3) Silver-plated wire would certainly make a slight difference, but for anyoné" other than the correspondent, who, is in the trade, the experiment would hardly be worth the expense. Regarding selectivity, the correspondent will have great difficulty to get any set that will cut out 1YA within half.a.mile. The use of a wave trap may help to solve the difficulty. .The use of a counterpoise instead of an earth may also help to reduce noise, while care should be taken with regard to the direction of the aerial. It should be at right angles to the car lines: Four-Volt Valves. N the ‘Notes and Comments" in this week’s "Record" (dated January 18) ‘I notice a paragraph whereby a listener has been advised to use-a four-volt -valve in his radio frequency stage of a. "Browning Drake receiver. This was done by "Switch," who was. surprised at the results obtained from "distant" stations, My receiver is a four-valye Browning Drake, using one stage of radio frequency. I use a six-volt battery for A supply, and a wet B battery of 112 volts. ‘fhe valves I use are dull emit‘ters; viz., detector PM5, radio frequency PM6, audio amplifiers PM6, and PM256. ~ Will you kindly advise whether this is the correct combination for my particular machine? _ Also, let me know what type of fourvolt tube to employ in R.F. stage, as indicated by "Switch," and whether this should manifest the vast improvement in sensitivity claimed by him. I use a Bradley stat on R.F. tube, and séparate rheostats for detector and amplifier. I have logged about 20 "outside" stations (American and Japs.), in additién to the usual Aussie and N.Z. stations. ‘The aerial I employ is an inverted L, with two well-insulated " masts, each.62 feet’high. Locality-on a hill, and clear of all screening effect, -and’wires: Whatever? The situation,’ in fact; is little short of ideal,-but in

~ comparison with some of the big outside logs of northern listeners, my results are not what I think satisfactory. I have the Pierce earth system. Congratulations on the very fine information contained in the "Record." (Bluff). ANSWER.-When "gqwitch" spoke of a four-valye in the radio frequency stage of the Browning Drake, a valve of the UX199 type was meant. Numerous correspondents have written in stating that they have had difficulty to neutralise their Browning Drake receivers, particularly in the case of the 2RF Browning Drake. A case of this was cited last week. The UX199 type of valve, Le, the type having a high impedance, is most suitble for the first stage, because it may easily: neutralised, and will not spoil reception of a distant station by oscillation. By UX199, atype of valve

"t is meant, and not one particular make. All good valve makers turn out valves of this type, and the correspondent could ascertain from any reliable dealer the type of valve in the particular make he fancies that approaches nearest to UX199. It is not because the valve is four-volts that it is more sensitive; it is implied that the UX199 happens to be a _ four-volt valve, but other valves of this type need not necessarily be four-volts, i.e., Osram’ DEH610 is a six-volt valve, but equally suitable for the position. Of course if a four-volt valve is to be used with a six-volt battery, a resistance should be placed in the circuit. This need not necessarily be a variable rheostat.. The resistance of the Ampertite type is suitable. Judging from the correspondent’s log bis receiver is in very good order. PM5 is a high impedance valve, with a high amplification factor, and is therefore suitable for 2 H.F. stage.

SS p44 £4 sty FOUR GOLDEN RULES. 1. The cheapest way is to use specified components, and no others. 2. Never buy spare parts from a friend. Friendship is too valuable to risk on the difference between a cheap and a new article. 3. Read all the information you can before beginning to build. 4. Avoid second-hand batteries like the plague.