[Don Pearce[_3_]]

On Sun, 04 Sep 2011 11:24:20 -0500, John Byrns
wrote:

In article ,
(Don Pearce) wrote:

On Sun, 04 Sep 2011 00:16:25 -0400, Don
wrote:


If I had two KT88's (I wish!) I'd build the Williamson design, but
stabilize it with the "low frequency shelf" discussed here at length a
short while ago.
Cheers,
Roger

Regarding the "low frequency shelf" and high frequency
roll-off filters:
why are they usually after the voltage amp?
Why not a band-pass filter before the V1 tube?

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-Don

I think it is a sort of masochism - a craving for low and high
frequency instability. Large phase shifting mechanisms within a loop
are never a great idea. There should be ideally a single pole that
doesn't let up until beyond the unity gain frequency.

The use of a dominant pole to stabilize the feedback loop is transistor
thinking, not always practical in tube circuits.

Just good engineering practice and pretty much standard in op amps.
And the first op amps were valve, not transistor.

d