Fixed cathode bias?

[Hamfam47]

I am currently designing a 120 watt valve amp for a Leslie using KT88s. I have looked at the idea of a fixed cathode bias as described at 300w-5-bias-stabilizer . Has anyone tried something like this? If so, I would be interested to know how well it worked. Thanks for any replies. Glen Lewis

[Steve Conner]

Looks like it would work fine, but why not just use fixed bias? If there's any essential difference between this circuit and fixed bias, other than the power wasted by the volt drop across the transistors, then I can't see it.

[Hamfam47]

I guess that the main advantage to this fixed cathode bias is that it is self-adjusting, so that once it is set up correctly it will not need adjusting if there is any drift in the supply voltage or as the valves and/or resistors age

[MWJB]

Well really most fixed bias amps aren't set so hot/cold that drift in wall AC is a problem. The resistors are likely to last much longer than the tubes, so rebiasing may be an issue when subbing new/different KT88s.

A cheaper, simpler alternative is to use 1ohm cathode resistors, run a wire from these to pin jack sockets on the chassis & have an externally adjustable bias pot. Then even if you do need to adjust the bias, it only takes a minute.