Cathode bias — self-bias — develops the output tube's operating point from its own current: a resistor in the cathode lifts it above ground, and a bypass cap across it keeps that voltage steady for signal. There is no separate bias supply and nothing to set; the tube self-corrects as it warms and ages. It is the simplest way to bias an output stage.
It was the tweed-era default for small and medium amps, and it runs the tubes richer — closer to class A — than fixed bias does. That costs efficiency: a few watts are dropped across the cathode resistor and the stage makes less clean power. In return the rail and the bias move together under a hard transient, so the amp compresses and "browns out" musically as it is driven.
The result is the softer, spongier, earlier breakup that people mean by the tweed voice — sag and compression built into how the output stage biases itself. The Champ, Deluxe, and Princeton here are all cathode-biased; the larger circuits switch to fixed bias for headroom.