"The Emperor's New Amplifier" by Norman Koren uses servo bias, as it's called:
This is a hi-fi design, though. In a guitar amp, you have to consider how the bias servo will react to extreme overdriving of the power tubes. You don't want nasty pumping effects, or even radically wrong bias, when it's cranked.
I settled for a compromise: I used ordinary bias adjustment pots but regulated the voltage with a LM337. This doesn't necessarily lead to stability of the bias current, though, unless you regulate the plate and screen voltages too, which I did