Bias

The standing direct-current condition of a valve amplifier's output stage, set so the valves idle at the correct current for the voltage they see.

Bias is adjusted either by a trim pot or by swapping a resistor, and the setting affects headroom, crossover distortion and tube life at once. Idle too cold and the amp gets a thin, gritty crossover artifact with a harsh edge; idle too hot and the valves run away, red-plate and die young. Adjustment needs the chassis open, so it is a job for someone comfortable working around several hundred volts.

Example: a freshly retubed amplifier is normally biased at the idle current its maker specifies, and many players then move it slightly for a softer or tighter feel.

Bias

See also