Servo-controlled fixed bias

Please,

As I understand it, a common way of doing this is:

Detect cathode current with low-value resistor and op-amp.
Clip the output of the op-amp to make it's AC component symmetrical.
Smooth the result with a filter having a time period of several
seconds.
Amplify and/or invert this as necessary and use the output voltage
to bias the grid.

It seems to makes sense to use the bias current of one master valve
as a reference for the others, in multi-valve output stages.

The parts of the circuit I am particularly interested in are the
clipping and the details of how one valve is used as a reference for
the rest. The purpose of the clipping, BTW, is to stop the servo
from responding to asymmetrical current peaks as the valve itself
approaches clipping. Otherwise the bias would drift in response to
extended AB operation.

Anyone with experience, circuits, thoughts? A better description of
how it works? I would appreciate a discussion.

cheers, Ian