BATTERY ECONOMY

Evening Post, Volume CVIII, Issue 154, 27 December 1924, Page 21

BATTERY ECONOMY

THE PLATE CIRCUIT.

There is much confusion in some wireless amateurs' minds regarding the voltage of B batteries—largely as tha result of carelessly written advice. It is not uncommon, to find articles which advise using 40 volts or so for a detector valve. A very common defect in published accounts is an arrangement which places the same. B battery voltage on all the plates, whatever their valves are used for. Now, the B battery imposes one of Ihp regular expenses t>i operating ■;a wireless receiver, and if it is wastefully used it will run away with enough money to provide an additional stage of amplification. For economy in working, the B battery voltage should be adjusted as exactly as possible to the duty of the valves, and certainly it'should not be higher than is needed. The first thing to realise is that high plate battery voltage does not of itself increase ohe volume of sound produced, as will be shown later. The energy drawn from tha battery, however, does increase with the voltage; but more rapidly than- in direct proportion. By doubling the voltage on tiie plate of a valve; the current is more than doubled. This means that the energy drawn from the battery is more than quadrupled—and with it, thecost. The fact is clear plough when it is 6een that more than twice the current is flowing and twice the number of cells is being used; that is to say, one has to buy a double battery, and it will not last half as long as a single battery. There are few tubes which will not act perfectly well as detectors with less than 20 volts. The detector will, it is true, give rather more volume with a higher voltage, but in general about 20 volts—that is, the output of what is generally called a 22i volt battery—is ample for efficient working.. The voltage may, if desired, be cut considerably lower. *

Amplifier valves need a higher voltage, for two reasons. One is that the higher the voltage, up to a pertain point, the greater is the amplification attainable j and the other is that a very low voltage does nof give faithful reproduction of the signals. To get the best results from an amplifier, the grid of the valve should always be negative to the filament; and for this condition to be fulfilled along with that of true and strong amplification, the plate potential should be at least 40 volts for most standard valves. This is ample for radio-frequency amplification, unless more than.' two stages are used, whioh is not usual. It is also sufficient, in many cases, for a single stage of audio-frequency amplification, if there is no amplification before the detector. The plate voltage of the audio-frequency amplifier should, however, be increased—and the negative grid bias as well—if the output o£ sound is really strong. • That higher plate voltage does not result in a corresponding increase in the output of the valve is a point which is seldom appreciated. Actually it does increase the current output, in a higher de- i g«e. even.jthan..the: voltage. But this is npfc the.useful output. . Theplate circuit carries . a continuous current, whether signals are being received or not, and it is this;.which.is increased,or decreasedby changing the plate voltage. But when signals are being received, the amperage (quantity) of the plate current is varied. The useful work in the receivers (or, in the_ case of a multi-valve set, in> the radio-frequency or audiofrequency transformers) is all expressed in these variations of output. Take two imaginary plate circuits in which, the voltages are respectively 40 and 80, and the plate currents are 1 milliampere and 2£ milliamperes.' Suppose that in each case the to* tal change of-current caused by the signals is £ milliampere; .then, other things being equal, the energy output of the two valves will be precisely the same, though one will be using five times as much B battery power as the other (two and a half times the current at twice the voltage). The position will not really bo quite so bad as that, because "other things" are not quite equal, but there is still bo great a discepancy between battery power and useful work as to mdi- I oats great waste with the higher voltage. . The real function of the'higher voltages is to enable the valve to perform I more work when it is given stronger input; that is, roughly speaking, to provide it with a current big enough to carry large changes of amperage. Obviously a. current with an average value of 1 milliampere cannot be .varied over so wide a, range as one of' 2 milliamperes; the latter will have roughly twico the power-producing ability. But its production of the power depends not on itself, but on the intensity of the inEut. The plate voltage should therefore c adjusted to deal with the input, with, any further modification that may be needed to secure faithful amplification.