Valves: an unbiased look at bias

摘要

I read with interest both Alex Staicu's query [see HFN Aug '16] and Ken Kessler's reply on the subject of fixed versus 'automatic' bias for valve output stages. Whether the differences are clearly audible with normal music or speech is open for debate, but I can assure you that there are indeed significant differences in the performance of the two arrangements. 'Bias' is the nominally fixed, negative voltage arranged to appear between the control grid and cathode of each valve. A small bias voltage allows for significant standing (direct) current in the valves, which as a consequence never turn off completely - even at peaks of the music waveform. This is so-called Class A operation, and generally yields the lowest distortion. It also results in large heat losses, meaning higher electricity bills and lower output power for a given valve life.