[Big Bad Bob]

On 03/22/11 04:40, Patrick Turner so witilly quipped:
If the amp OLGain is -3dB at say 20Hz and 8kHz, and this is not
unusual in many pentode or tetrode amps with pentode input stages then
the NFB will flatten everything out so you get 10Hz to 65kHz.

worthy of note: NFB also significantly improves THD, _especially_ if
you're using pentodes as pre-amps, since pentodes have non-linear
characteristics.

/me prefers triodes for pre-amps anyway - 12AX7 or 12AT7 preferred and
the triode pair within a single tube can give you combined circuit gains
as good as (or even better than) a tweeked up pentode pre-amp, with
better sound quality (theoretical mu gain of two 12AX7 triodes in series
would be around 10,000). Two 12AX7's (4 triodes) driving a beam pentode
P-P Class AB with plenty of NFB will typically give you outstanding THD
and IM behavior and a nice flat response, even if you use semi-crappy
output transformers, but the clipping power level at the edges of
frequency response might become a LOT lower [so you'll either need
adequate amplifier headroom or better OPTs]. Maybe that's why the
circuit config I just described is used so often.

That being said, using single-ended 'triode config' power output tubes
would likely improve the THD and IM that much more, maybe requiring less
NFB to achieve quality results (at the expense of output power and
efficiency).