The Correct Reading of Eliminator Voltage

Radio Record, Volume I, Issue 26, 13 January 1928, Page 3

The Correct Reading of Eliminator Voltage

Valuable Explanation and Data on Valves and Socket Powers

By

M.I.R.

E.

Radio listeners who appreciate the value of technical efficiency in maintaining the tonal qualities of their sets will value the simplicity and accuracy of the explanation here given by M.I.R.E. of the factors which contribute to the correct handling of their valves, B. battery eliminators, etc. This article is so comprehensive and valuable as to be worthy of clipping and reperusing.

sq: ONTINUING the discus- | sion of "B battery elim. inators," or "socket powers,’ it is proposed to explain a rongh and ready method of testing the voltage output of the eliminator, without the use of a voltmeter. As has been explained, a yoltmeter is quite unreliable when used in this connection, owing to the fact that the average voltmeter uses more actual current in showing a voltage reading than is taken by several valves, and in the majority of cases the readings given wil) be entirely misleading. The inethod of yoltage check most easily appied is that of finding at what bias on "C"? battery voltage on the grid of the power valve, for instance, the signals. hecome scratchy. Valves are always sold now-a-days with a pamphiet | shawitlg their characteristics, and if this.is examined it will be found that. either a "characteristic curve" is given to show the performance of the valve. under various conditions, or else thereis a list of plate voltages with the cor-, responding recommended bias or rid voltages to be used. . CHARACTERISTIC CURVES. In the case of the characteristic curve a horizontal and a vertical line will be shown, aud either one or several ‘curves will be drawn, according to the voltage or voltages on the plate at which the curve or curves have been taken. These will commence on the left-hand side of the vertical line, and mount in a curve, and then, after becoming relatively straiglit, after crossing the yertical line, either finish or bend over before finishing. The horizontal line represents C battery voltage, the readines along the horizontal line to the left being volts negative on the grid, while readings to the right represent positive grid volts from the C battery. The

vertical line shows the amount of current passing through the valve, | this current being zero at the point where the vertical line joins the horizontal one. Iixamining an actual valve curve, therefore, it will be seen that in the case of a typical general purpose amplifying valye the curve leaves the base line at abont 6 to 9 volts negative bias, and slowly bends until at about 4 to 5 yolts negative, it assumes a relatively straight line until it cuts the vertical line at the point corresponding to somewhere between 5 and milliamperes current flow and_ zero volts on the grid. It is noticed, of course, that as the grid becomes less negative the current in milliamperes through the valve increases, and the rate that the latter increases relative to a decrease in grid volts defines the efiicienecy or performance of the valve. It should be noted in passing that the amount of current passing through the valve is no indication of performance or amplification be_cause this latter is dependent solely on the amount of variation of current occasioned by the voltage variation on the grid due to incom: ing signals, In other words, because one valve passes 5 milliamperes with 3 volts negative bias and another of milliamperes under the same conditions it does not necessarily mean that the latter is twice as good a valve or even a better valve. If, however, a voltage variation

-of two on the grid causes the plate current to vary one milliampere in the case of one valye and two milliamperes in the case of another, then this latter valve is twice as good as an amplifier, As a matter of fact, the average performance of a good amplifying valve is one milliampere variation for each one volt change on the grid. ‘his is termed a "slope’’ of one. Valves of somewhat inferior performance are as low as point five, while others are as high as one point five. ‘The latter are invariably power valves, while the former are cither obsolete or else definitely designed for such purposes as resistance capacity amplification, and can therefore not be said to be inferior when ‘used as they ate intended. | TESTING THE VOLTAGES. To return to the checking of "socket power" voltages it will be seen at once. that providing a curve of the performence of the valve in question is available, sufficient excessive bias may be applied in order to cut off the current passing through the valve to such an extent as to cause easily-detected seratchiness of signals. It is somewhat difficult to give any definite guides as to values owing to the large numbers of yalve types available on the mar‘ket. As a rough guide, however, it may be stated. that the purity of signals should be unaffected with a rise of 265 per cent. above the bias recommended for the particular plate voltage in use. Above this value distortion will set in. This check is contingent on only medi(Continued below table.)

(Continued from above.) um strength signals being used for the test. It should be realised also that these figures do not apply exactly to batteries, because as the bias is increased and there is.less drain from tlic socket power, the voltage will tend to tise. This will mot be the case with batteries unless run-down dry cells are in use, Lefore passing to other considerations, ‘it should be noted that the scratchiness of signals with excessive negative bias is due to the current passing through the valve being cut off to such an extent as to not permit the full characteristics of the signal to pass the valve. In other words, instead of pass‘ing current of a value equal to that half-way along the straight portion of the curve the valve is working with characteristics represented by that portion of the curve at the bottom bend, In. passing it is interesting to note that this latter position of ad. justment is where rectification or detection"? takes place best. SOCKET POWER ELIMINATORS. Socket powers or battery eliminators are now appearing ou the market in fiumerous forms, ‘The term "Socket Power’ is being more generally applied to ontfits which supply both filament and plate (A and B) power and

perhaps grid bias (C) in addition, The term "Battery Eliminator’ has so far been applied principally to the outfits which give B powet only. This is dune to the fact that there are really very few straightout A eliminators as yet available. ‘The majority amount to a "Trickle charger’ and A_ battery combination. In at least one type the change-over from charge to discharge is done automatically when the receiver is switched on and off and to all intents and purposes therefore it is a "Battery Eliminatory" although a bet-" ter term would be "Battery tronble eliminator." Seeing that the drawback to a battery is the necessity to charge it, the eliminator part of the equation may be ausited as correct. The American term of ‘socket power" will* be geen to be an apt one because the power comes from the power socket via the battery. So far as A battery eliminators are concerned, using the term in its strict sense, the only really successful ones are those used in power amplification vhere the use of alternating current in the filament is inconsequent or else those supplying a limited number of dull emitter valves and consuming 8 small amount of current. (Usually insufficient to run more than 3 or 4 valves). Where the filaments of the valves are wired in series so that the same current passes through several yalves in succession, considerable, if vot total success is obtainable at an economical cost of eliminator outfit but unfortunately standard radio sets are not wired with their filaments in series. A change-over in wiring from parallel to series is invariably impracticable because of complications caused throngh the grid-returns having been designed by the set manufacturers to so to certain points to cause bias voltages to be picked up and these are dependent on parallel wiring. Summed up the position at present is that eliminators requiring ‘series filament wiring are higlily recommended where the receiver is series-wired to suit. Otherwise best leit alone where the receiver is a multi-valve factory built machine. As has been stressed frequently in this column on previous occasions, there is so much capital invested in radio apparatus that revolutionary imptovements which would result in too rapid obsolescence of equipment are out of the question

The introduction of apparatus de« signed to entirely eliminate batteries is inevitable, but even though new receivers may be introduced having: features rendering battery elimination a simpler proposition, there are so many millions of high-ly-efficient multi-valve receivers of orthodox design in vse to-day that there will always be apparatus of "socket power’? desig: available. THE THREE PRINCIPLES OF © SOCKET POWER. It is of definite interest to discuss ‘the three main divisions of principles involved in the design of socket powfers. . The basis of operation depends on the conversio of current of am alternating nature so far as direction of flow is concertied into one which flows continuously and smoothly .in a single direction. Rectifiers must therefore be employed which. reverse the direction of flow of one pulsation, thus causing two pulsations in the same direction. These pulsations are ‘ironed out" into a smooth flow by passing the current into condensers of electrical reservoirs, from which the receiver draws its ~ power. . Smoothing is carried out by standard methods in all tpyes of apparatus, but rectifying is not. The two main methods are electrical or thermionic i.ec., by means of valves or by. chemica means. ‘The latter method may be carried out by solids or liquids, and the liqnid is at present more widely in use than the former because the principles of operation of liquid rectifiers have been in use a long time and a good deal is. known about them. Drv chemical rectification has only just been applied, and undouktedly it will eventually supplant the former. Thermionic rectification by means of gaseous valves has been in use for many years and a terv reliable tech nique has been built. up round it, A wonderful, if not incredible, lengtif of life is obtainable from the bulbs. The prospective purchaser need not hesitate to purchase apparatus employing any of these principles, providing the chemical rectifier is made up and sold under a well-known brand. Amateur-made chemical rectifiers should be avoided at all costs or a ruined accumulator will be the | price to. be paid. Bull rectifiers are less prone to mis« handling and there is little damage likely to be sustained by the battery, but unless the outfit is designed cor rectly the life the bulvs will be short, aud tliese are not exactly inexpensiye items, Any rectifier, whether it be in a charger or socket power, will give complete satisfaction if it falls under the heading of the apyaratus described in this article and is made up by a recognised firm.