USE GRID-BIAS

Radio Record, Volume I, Issue 25, 6 January 1928, Page 15

USE GRID-BIAS

BETTER FONE; SAVES BATTERIES Among home-made wireless receivers no fault is more common than failure to provide eftective grid biasing (with a "C" battery or by another method described below) for the valves in the audio frequency amplifiet. To operate effectively amplifier ¥alves must be so adjusted that the grid is at a potential lower by several volts than the fila- | ment, and the greater the pressure which is applied to the plate of the valve the greater must be this negative bias on the grid for effective operation. Proper grid bias makes three important improvements in the sct. It almost invariably increases the purity of reproduction, it increases signal strength, and it makes the high-tension battery last longer by reducing the amount of current the amplifying valves draw from it. Grid bias can be obtained in two ways. The average receiver is operated from an A battery, which gives a higher voltage than that needed by the valves, and the voltage from the battery is "broken down" to the correct value by means of rheostat. For instance, a three-cell lead accumulator, such as is used to operate the average five-volt filament valves, gives a pressure when fully charged of 6.6 volts. This means that a pressure of 1.6 volts must be absorbed in the filament rheostat. This pressure, or, as it is termed, voltage drop, across the rheestat can be used to bias the grid of the valve. Great care to see that connections are correctly made is necessary m employing it,

POSITION OF RHEOSTAT, The rheostat must be inserted in the terminal from the valve filament, which goes to the negative (marked -) terminal of the A battery. The lead which is taken from the secondary of the inter-valve iransformer to the filament Circuit of the set is then connected to the terminal of the rheostat, which is connected to the battery. Care must be taken that the connection is made to this terminal of the rheostat, and not the terminal connected to the filament.. For pressures of up to 40 volts of high" tension battery this bias will be sufficient, Jf more than a 40-volt B battery is used a special biasing, or C battery, should be included in the set. Small units, giving a total pressure of 4.5 volts, can be purchased as C batteries. To use these batteries the positive terminal (marked N) is connected to the negative terminal of the A battery and the filainent return from the transform--er is connected to one of the two nega--tive taps on the C battery. One of these tans is at three volts and the other 44 | volts are on the C battery, and the lead should he tried on each in turn to see_ where best results are obtained. |