[Big Bad Bob]

On 02/25/16 17:21, John L Stewart so wittily quipped:
NTL an interesting topology as anyone who has tried it will agree.
Patrick manages to get about 95% of the signal at the plates of the
output tubes all the way back to their grids in the form of NFB. If one
looks carefully we can see an anode follower.

Without the MU followers as plate loads one is lucky to get ½ the signal
all the way round the loop. In Patrick’s amp the result is the output
tubes look very much like very stiff triodes driving the OPT. That will
help the LF response very much. But not so much the HF since the signal
still has to get past the OPT leakage inductance & winding capacities.

biggest problem with high NFB from downstream of the output transformer
is the phase shifting at the edges of frequency response. Careful
choice of component values (and possible filtering/peaking reactive
components) might be needed to avoid the inevitable oscillation that
would take place without it.

but yeah, if you can get away with it, it would be like a typical op amp
with very high open loop gain, and relatively low 'stage gain'. The end
result is very low distortion.