GRID BIAS

Radio Record, Volume I, Issue 37, 30 March 1928, Page 13

GRID BIAS

A POINT which is not always realised when considering the setting of grid bias for the various valves within — a receiver is that one to three dry cells will provide ample bias for the | first audio stage. Assuming that sufficient demands are being made on the set to warrant biasing the grid of the-last valve to 9 volts, the peak value of the A.C, signal voltage swing will be 18 volts. Since we have a right to expect at feast 25 times voltage amplification from any modern stage of audio-fre-quency amplification, be it resistance coupled or trausformer coupled, this 18 yolts swing on the grid of the power valve would be produced by 18/25, or, say, 0.7 volt on the grid of the preceding valve. To accommodate this 0.7 volt swing a bias of only 0.35 yolt is necessary, always providing that the particular valve employed does not pass grid current until the grid bias has been raised to a potential positive in respect to the negative end of the filament. Reversing this argument, the grid UE ECE EE EE EE Ba

pee Habba sibselbbilibiiba bias produced by a single cell, i-e., 1.5 volts, is only fully employed when a voltage swing of 90 is deemed desirable on the grid of the output stage. The small bias normally requiréd could quite well be obtained in many cases by inserting resistance {fixed or variable) in the negative lead of the valve filament circuit, and connecting the grid to negative LT. Whether this method will work depends upon the characteristics of the valve filament. For instance, many filaments rated at 2 yolts will function well at 1.6 volts, leaving 0.4 volt available for biasing, resistance drop in battery leads, ete. With four or sixvort a large surplus voltage is ayailable, When only just sufficient bias is used the maximum performance of the valve will be obtained at an appreciably lower H.T. voltage than would otherwise be necessary, ‘thus economising in batteries,