[John Byrns]

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.

--
Regards,

John Byrns

Surf my web pages at, http://fmamradios.com/