Fixed bias

In a fixed-bias output stage a separate negative supply holds the output tubes' grids below their near-grounded cathodes, setting the idle current directly. "Fixed" means the bias voltage doesn't move with the signal — not that it can't be adjusted; many of these amps carry a bias-set trimmer. It is the counterpart to cathode bias, where the tube sets its own operating point.

Designers chose it to get more out of a given pair of tubes. Because the cathodes sit near ground rather than lifted by a resistor, the tubes see more of the B+ rail and can be run closer to class B, so a fixed-bias stage makes more power, holds a firmer low end, and stays clean further up than the same tubes cathode-biased. The price is the extra bias supply and the need to set — and occasionally reset — the idle current when tubes are changed.

That is the sound of the bigger amps in the archive: tighter, louder, and clean until pushed genuinely hard, where the earlier softness of a cathode-biased tweed is gone. Every higher-power circuit here — the tweed Super and Bassman, the blackface and British lead amps — is fixed-bias.