Is cathode bias the same thing as "self" bias?

[Steve Conner]

It is a valid point. Cathode/self bias doesn't work very well with Class-AB amps. Every excursion into Class-B causes the bias voltage to pump around. Nevertheless guitar amps are commonly built that way.

Hence the reason for the Chuck H Ruby Mod mod that puts a zener across the cathode resistor, you can think of it as a halfway house between fixed and cathode bias.

[jmaf]

Hence the reason for the Chuck H Ruby Mod mod that puts a zener across the cathode resistor, you can think of it as a halfway house between fixed and cathode bias.

That's interesting. When the tube approaches cutoff the cathode resistor voltage is at a maximum, the zener doesn't allow it beyond a rated voltage...

Just found this thread: http://music-electronics-forum.com/t17903/

Thanks Chuck H. And Steve Conner for citing it. One more piece of knowledge gained over morning coffee.

[kleuck]

It is a valid point. Cathode/self bias doesn't work very well with Class-AB amps. Every excursion into Class-B causes the bias voltage to pump around. Nevertheless guitar amps are commonly built that way.

Hence the reason for the Chuck H Ruby Mod mod that puts a zener across the cathode resistor, you can think of it as a halfway house between fixed and cathode bias.

Another way to minimize this issue is to use separate cathode resistors, and huge caps.
Almost the sound of a fixed-bias, little less power of course (just like a "regular" cathode biased amp), but no need to use matched tubes, and very little compression.
The Paul's trick if i remember well is aimed against the "18 watts fiziness", witch can also be related to the OT's impedance (see Zobel and conjunctive filters)