Explanation of Self-Biasing/Cathode Biasing

Have a look at Randall Aiken's web site.

Under "Tech Info" there's lots of good stuff including an explanation of biasing.

Aiken amps

 

The problem with cathode bias IS the notion that it's self biasing. Sure it will generate it's own bias voltage, but that doesn't mean it's the right voltage! You can have over/under biased conditions just like fixed bias. This happens alot when you run into designs saying you can swap tube types without changing the bias resistor. Alot of times in those cases the bias is set to a medium point that will work for the different tubes the amp claims you can use. What you could wind up with is something like seeing 6V6's running cold class A/B, EL34's running about "right", and 6L6's running really hot. They might all sound cool, but one might sound great if the amp was biased up for just that tube. PaulC

 

That's correct. The more current that flows, the greater the bias voltage, which in turn reduces the current, so it is self-stabilising... for SMALL variations.

It is 'self biasing' provided the tube is within the right range to begin with - a slightly hotter-running tube will bias itself a little cooler, and vice versa. This means that if you're replacing tubes with the same type, you don't need to rebias.

But as Paul says, the same cathode resistance will not cope with wide variations, eg very out-of-spec tubes, or different tube types entirely.

For each resistance value and tube type, there will be a stable operating point, so by changing the resistor, you can easily rebias the amp.