Questions and Answers

Radio Record, Volume II, Issue 38, 5 April 1929, Page 12

Questions and Answers

Filament Emission. "FLOW is it possible for a radio valve to light. brilliantly and yet to be ‘dead’ for use in a radio set?" ANSWER: There are a number of eauses for the refusal of a valve to function while its filament still glows with normal brilliancy. One of these is the short-circuiting (or the making contact not called for in the design of the valve) of two internal elements: as between the plate and the grid or the grid and the filament. In the latter ease tl. filament usually is destroyed immediately the current is turned on; in the former the valve will refuse to function, yet the filament will probably last its normal life. The most prevalent valve ailment, however, sets in after a valve has been in service for some time, usually a year. At this time a marked weakening of signal strength inabilit to get remote (DX) statiow.s wth the same

ease as when the set was new, and perhaps some degree of distortion, may take place. The cause of this is the burning down of the filament to a point where the amount of the "electrons" it gives off, when heated to complete the "plate" or "B" battery circuit, is considerably lessened, with a resultant drop in the efficiency of operation. As the valve depends upon the filament’s discharge of electrons for its operation, the manufacturers coat the filaments wit) an "element" known as thorium, which aids in giving off electrons for the successful operation of the valve. After the valve’s normal service the thorium is fairly burnt off, thus decreasing the number of electrons flowing from the filament to the plate. It

is this weakening of the filament’s operating efficiency which causes the unsatisfactory performance of the set. At times this condition will come upon a practically new valve-a fault known as paralysis. To correct this and also wenkening in old valves the process known as rejuvenation is employed. This was dealt with recently in these columns. Range of a Receiver. "WHAT is the distance range of a four-valve receiver?" ANSWER: This is a question often addressed to "Observer," so that it will be of general interest to all readers. No one can say just what distance a certain receiver can cover. There are so many controlling factors that enter into tue matter that it is practically impossible for any one to state with certainty the normal range of the receiver. Weather conditions have a good deal to do with distant reception; and so have the condition e the batteries, the valves, the densitivity, and the tuning ability of the receiver, the quality and condition of the antenna structure, and the efficiency and condition of the ground connection. Furthermore, there are certain areas in the country known as "dead spots,’ in which reception of anything but powerful local stations is extremely difficult even with the most elaborate receivers. Another controlling and very important factor is the operator. He is responsible to a great extent for the results obtained. Keeping the set and the accessories at maximum efficiency, a good knowledge of the tuning temperament of the receiver, and patience must eventually reward the operator with the desired "DX," as reception of the distant stations is called. What is an A.C. Valve? "TOES the A.C. valve overate entirely on alternating current?’ ANSWER: No. Alternating current is used only to light the filament, or the heater element which heats the filnment, depending on the design of the valve. As the modern A.C. valve requires anywhere from one and a-half to fifteen volts of alternating current to heat the filament, it is necessary to "step down" the usual 220 volts of the house supply by means of a step-down transformer. However, to supply the "plate" voltage or ‘"B" current, direct current obtained from "B" socket power unit or from batteries must be used. Because receivers to-day use plate voltage ranging as high as 400 volts, it is necessary that the 220 volts obtained from the house be first "steppedup" to the required voltage by means of a "step-up" transformer. After this the increased voltage must be rectified from tke alternsting to Girect enzrent, as the receiver cannot function with alternating current on the plates of the valves. After rectifying the increased current we find there is still a noticeable hum present in the speaker, due to the characteristics of the alternating current. ‘This hum is "ironedout" by an apparatus known as f "filter’-a combination of condensers and choke coils. The entire combination of the step-up transformer, rectifier (usually a valye) and filter system

-- goes to make up the B voltage supply unit, without which no radio receiver can operate. In the case of the modern A.G. set it is really an integral part of the receiver and is usually so considered. Care of L.T. Battery. "WV HEN I first took up wireless we used to be told that it was absolutel, fatal to short a low-tension battery, and great care was taken to prevent this. But recently I have noticed a tendency among my friends to think that accidental shorting of a battery will not do it any harm at all, and I have, indeed, heared this stated as a fact. What really happens when this occurs, and what is the damage done to the battery, if any?" ANSWER: Sudden over-discharging is likely to lead to a good many accumulator troubles, though certain modern accumulators are far more "hardy" than their predecessors. Amongst other things, shorting often sives rise to the buckling of a plate and to the loosening or shedding of the active material in the plates. (This latter is due to excessive sulphation, which is accompanied by a certain amount of expansion of the active material in the plates, and if this expansion is greater than the containing grid can cope with, when the sulphate is reduced by the next charge on the accumulator a certain unavoidable loosening and shedding takes place. The over-discharge due to a dead shortcircuit or to a short through quite a low resistance is likely to give rise to buckling of the plates and frequent over-discharge may quite easily cause a reversal of the polarity of the plates in one or more of the cells, especially if they are partially discharged to begin with. Many Battery Leads. "War is the advantage of having a lot of different ‘B’ plus terminals? In my last set I only had two and it was perfectly satisfactory, but in the new one which I am thinking of building there are no less than four. Are they really necessary ?"’ ANSWER: The idea of having a large number of B plus tappings on any set enables each particular valve to have exactly that B voltage which it requires. For very selective and high-quality reception it is frequently necessary to adjust the B battery positive and the grid-bias potentials very accurately in order to obtain the maximum results, and it is impossible to do this for separate valves unless each has a separate high-tension supply. It is for this reason that the B is duplicated or triplicated on many sets, and we think that in general you will find that it is a refinement that is well Concernirg the Moving Ccil Speaker. " C.W." (Tirau) writes: Having * just about completed the moving coil speaker, described in your issues of March 1 and 8, I would like you to advise me on the tollowive points. Where ecn I obtain .5-1 output transformer? A.: Ferranti manufactures one and this may be obtained from their agents or direct from the master agents, A. D. Riley, Ltid., Wiggins Buildings, Wellington. (2) How is this transformer connected, and what size wire is the best to run from th: set to the transformer and the transformer to the moving coil?

A.: The feeds from the set are connected to the two input terminals of the, transformer, and the output terminals are connected to the moving coil. Wir ranging from 30 to 18 swg. is quite suitable. (3) Can this speaker be run from an accumulator simultaneously with the set? A.:Yest, but the total draw (approximately 1.6 amperes) would be rather heavy, so that if possible a charger should be counected to the battery while it is in operation, i.e., an accumulator should be floated between the charger and the speaker. (4) Are Philips 609 and 605 valves in the two last stages satisfactory? . A.: No, at least use 603 in the last stage, 609 or 605 would, however, be quite in order if placed in the second last audio stage. (5) Will it be satisfactory made up in a cabinet 12in. square? A.:"Yes, if the back of the cabinet, is left open or covered with a gauze. ~~ larger baffle board would be preferaby as some of the lower notes will be lost when the baffle is restricted in size. -- eres xseawres 1%

Following are a few additional notes on this speaker. ‘The speaker coil is designed for the 210 type of valve, which has an impedance of 5000 ohms, and an amplification factor of 8. Where a valve of lower impedance is used, eg., 603 (1650 ohms), fewer turns will be required on the moving coil. In this latter

It is not necessary to open the window to let in radio waves; they , pass through stone, brick, wood, and glass. But, on the other hand, metal, such as structural steel, deflects them and often causes freaks of reception, especially in cities.

f SSS Se Se See te SEES Se) Dont plug an A.O. radio device into a@ D.O. socket-that is, unless you want to buy a new one. oer en

~ ~~, .-~"t i* °°» » .~Os~ case when working with a 25-1 transformer from 80 to 90 turns on the moying coil would be sufficient. If any constructor wishes to make his ‘speaker to suit the parts he has on hand, if he will write in stating the transformer ratio, the valve to be used in the last stage, and the anode yoltage applicable to it, details will be furnished him whereby he can construct his speaker to match his components. Difficult to Neutralise. mf AM unable to neutralise my Browning Drake, which has been made strictly to specifications. The turning on of the light switches causes the set to burst into oscillation, which can be stopped only by turning the rheostat controlling the radio frequency valve to a position where music is inaudible. The batteries are O.K."’-"Muggins" (Gisborne). A.: See the reference to "A" battery wiring in screened sets dealt with by "Pontode" this week. Detector Voltage Too High. My D-yalye neutrouyne has not been going too well for some time. A crackling noise is noticeable in | the speaker, and on two different pairs of ‘phones. On turning un the rheostat the noise increases. The voltage on the two radio frequency’ and two audio frequency yalves is 90, and I have been using this on the deteeto~ Can you help me to rem edy the trouble? asks "C.E.H." (Ngaio). ‘A. : Cheek over all A" battery connections, and apply the test to .ae transformers as described in Our Beginner Corner some time asoy Reduce the detector voltage to about 30 volts, as a high detector voltage will quite likely cause trouble of this ‘nature. Oscillation Trouble. HAVE a home-made all wave 3-valve receiver, writes "M.C.W.": (Auckland), the second stage of which works unsatisfactorily, especially on the short-wave. When tuning in on the long waves the trouble is vey slight, except on very loud signals, when it is inclined to mushiness. I am using good components, and have shunted a .001 mfd. capacity condenser across the primary of the second transformer. This has stopped a high pitched whistle when using. the 8 valves. Tone and clarity on 2 valves are almost .perfect. My "B" batteries are good, but should I use the heavy duty type? The set delivers ample volume, and 4YA comes in on the speaker at good strength, 2YA fills the house on 2 valves, but both 1YA and 2YA_ become slightly distorted when using the 3 valves. Would the eliminator described in the "Record" March 238, 1928, be suitable for an all-wave set? A.: Try + meg. grid.leak .in series with the grid leads of both the L.F. valves. Space the transformers correctly, well apart if possible, so that the axis of

one is at right angles to the axis of the other, or use choke impedance coupling for the last stage, as described in the "Radio Record." The failure of the set to handle volume may be attributed to very many causes, the most probable being that the last valye cannot handle the output. PM4 is used, whereas it would be advisable to try PM254. Then, again, the speaker may be unable to handle the output, for a good speaker is required to deliver the output of 8 valves from the local station. An eliminator would be unsatisfactory for short-wave work. Heavy duty batteries would be more satisfactory. Dynamic Cone Speakers. "C H." (Wellington) requests informa~e tion on the following points to guide him in purchasing a moving coil loudspeaker : (1) Which is the best type? _A.: Those operated directly from the lighting mains will repay amply the extra expenditure. (2). What would such a speaker cost per week.to run, say if it were being used for 15 hours? A.: As it consumes only 20 watts per hour, this would be 1.8 pence at Wellington rate,

(Sots etetnd th 2b Seid set Se SSS St Ee Sissi _ (8) Operated from an A battery, what is the amount of current used? A.: 6 of an ampere. (4) What is the best make; and where can I obtain same. A.: Good makes have been frequently advertised in the ‘‘Radio Record." (5) Do you consider that the extra cost is warranted over the average speaker? A.: Providing a good amplifier can be used in conjunction with the speaker, yes. (6) Will they deliver the same volume as 4 small cone speaker on the same output? A.: No, they are not as sensitive, but repay in quality. Push-Pull Amplification. "aN C.K." (Geraldine), writes: I wish to * build’ an audio amplifier to use with an electric pick-up so that I can use at least two speakers at fair volume. Which of the two amplifiers described in the "R.R." would you advise me to try, Penotde’s push-pull or Megohm’s amplifier, with super components. If either of these do not happen to be suitable, could you advise me as to the most suitable type? A.: If from 90-185 volts are available, the push-pull amplifier described by "Pentode" would admirably suit the purpose. A better amplifier would be one

working directly from the electric mains, but this would be both difficult and dangerous to construct, unless an electrician could do the constructional work. The Beverage Aerial. N.B." (Hekianga) asks regarding -\+ the Beverage aerial described in our issue of March 22, "Would the 600 yards of aerial (18 times the length of an ordinary aerial) would affect the tuning. A.: Under the conditions described in that issue the tuning was not affected. Providing the aerials are directional, tuning is quite sharp, and the whole band covered by the receiver can be tuned in with ease. (2) Are the cross wires supporting the aerial insulated or connected to earth? A.: They should be insulated, otherwise the effect of the aerial would be lost. (3) Could you tell the wavelength of 272M, Gisborne? A.: 260 metres, with power 60 watts.

os 33S SS Some questions propounded by various writers form the basis of @ special article by "Pentode"’ on page 28, to which these correspondents are referred.