Cutting Current Consumption in the Portable

Radio Record, Volume III, Issue 20, 29 November 1929, Page 12

Cutting Current Consumption in the Portable

THE small size of the "B" batteries ‘ which can be accommodated in a portable receiver renders the problem of reducing the drain on them one of importance. An apparently small reduction in consumption will frequently mean quite a pronounced increase in the life of these small batteries, owing to the fact that the impracticability of rapid depolarisation makes them uneconomical for any considerable dis-. charge rate. Not much can be done in the direction of reducing the consumption of the last stage, since the successful operation of a speaker is dependent upon a fair current through the power valve. However, one can at least experiment with a slight increase in bias over that normally used, and it will not infrequently be found that the consumption of the stage can be reduced to 5 or 6 milliamps without unduly affect-' ing the quality of the output. It is possible, by using a push-pull final stage, to reduce the steady eurrent to about 1 milliamp, this increasing auto‘matically to accommodate loud pasgages. The valves are biased down to the centre of the curved portion of their characteristic curves, the pushpull arrangement still permitting operation without distortion. However, the adjustment must be rather precise and an output transformer or choke is practically a necessity, 80 that not every constructor will feel disposed to use two valves in this manner. The first audio stage, if one is emPioyed, may also be overbiased a trifle, though not so much as to introduce distortion. With 120 volts B supply most valves can be biased to at least 4% volts negative without any perceptible effect on results. The commonly-used grid rectifier necessarily receives a positive bias. Something can be done, however, by reducing the "B" voltage applied to the plate of this valve, either by tapping lower down the battery, or, befter still, by interposing a resistance of = rs

about 50,000 or 60,000 ohms in the feed line to this particular valve. The high-frequency yalves, if of the ordinary three-electrode type, may receive a negative bias of 3 or 44 volts; stop short of sacrificing amplification, however. he screen-grid valve now more usually used for high-frequency amplification is normally used without any fiegative bias, under which conditions it draws about 4 milliamps. Its high amplification factor renders it unsuited to the application of too high a negative bias, but if a 14-volt flashlight or other cell is provided. to bias this valve, a distinctly worth-while reduction in current from 4 milliamps to 1.5 milliamps per valve will be effected, without any loss of amplification. If these precautions are observed, it | is usually a fairly easy matter to keep the current consumption of a fouryalve portable under 10 milliamps, a current which most of the small batteries suitable for portable work are capable of readily providing. In constant use, a life of about three months may be expected from these smallersized batteries with the drain mentioned, so that, considering how cheaply the batteries may be bought, the cost of the current cannot be deemed excessive.