"Aotobiasing" has several meanings. The oldest and simplest is biasing via a (usually bypassed) cathode resistor.
One can also adapt so-called fixed bias schemes to compensate automatically for output tube idle current drift. For a class-A circuit, this can be done very simply with a diff amp that compares the voltage across a small cathode resistor with a reference voltage. For class AB amps, it can get quite a bit more complex- two examples that come to mind are the digital scheme used by Berning in the BA-150 from the late '70s and a circuit shown in Jones's "Valve Amplifiers" which uses several opamps configured as diff amps and clippers. I adapted a system originally published by Chater in "Audio Amateur" for biasing a N-channel mosfet stage. It worked, but as with the previous circuits, it was complex and really offered no significant advantage over properly-executed conventional bias arrangements.
One can also adapt so-called fixed bias schemes to compensate automatically for output tube idle current drift. For a class-A circuit, this can be done very simply with a diff amp that compares the voltage across a small cathode resistor with a reference voltage. For class AB amps, it can get quite a bit more complex- two examples that come to mind are the digital scheme used by Berning in the BA-150 from the late '70s and a circuit shown in Jones's "Valve Amplifiers" which uses several opamps configured as diff amps and clippers. I adapted a system originally published by Chater in "Audio Amateur" for biasing a N-channel mosfet stage. It worked, but as with the previous circuits, it was complex and really offered no significant advantage over properly-executed conventional bias arrangements.
I experimented once with an op-amp-driving-a-PNP autobias/auto balancer once. SY's right, no significant advantage. Just adds to the complexity and just adds creedence to the saying, "The more complex the plumbing, the easier it is to stop up the drain" 
Maybe in stage gear where Xtreme punishment is the norm......
Maybe in stage gear where Xtreme punishment is the norm......
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