Go to ABSE for a basic circuit which provides the error correction
functions in a vacuum tube amp. The main amp consists of a 12AT7
Diff Amp input driving a pair of 2A3's. The 2A3's are intentionally
over biased so that they are running well into Class AB & into B.

The side amp compares a fraction of the output with the input & then
amplifies that to a level sufficient to correct the non-linearities in
the
output waveform. The correcting waveform in this case is inserted
in series with the main output.

This circuit is for demonstration only & is not optimized.

I was able to recover three of Patricks attempts at error correcting
from the usenet-replayer, thanx to Watt Sun's information. I have to
report to all that John Byrne's comments reference these circuits is
absolutely correct. They are all NFB.

Cheers, John Stewart