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Patrick Turner Patrick Turner is offline
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Default Determination of Maximum Positive Feedback in Bootstrapped Driver

On Mar 18, 9:55*pm, flipper wrote:
On Thu, 17 Mar 2011 22:27:28 -0700 (PDT), Patrick Turner





wrote:
On Mar 18, 3:05*am, flipper wrote:
On Thu, 17 Mar 2011 05:19:15 -0700 (PDT), Patrick Turner


wrote:


snip


In RDH4, PFB is mentioned and a schematic is given where a PFB loop
between two cascaded small signal voltage triodes is boosted 12dB and
the same amount of global NFB is used. The effect of PB when the
signal is below say 10Vrms is to increase open loop sensitivty without
much increase in THD or reduction of bandwidth


PFB is PFB and has the attendant increase in distortion and reduced
bandwidth regardless of the 'voltage level'.


But RDH4 didn't make your simple generalisation or come to the same
conclusion about PFB.


Yes they did and it isn't 'my conclusion'. It's a fact you'll find in
every electronics textbook on the subject.

And I quote RDH4 page 355: "Each 6dB increase in gain due to positive
feedback will double the distortion in this stage ; for example 24dB
increase in gain will increase the distortion in this stage by 16
times."


Page 355 is concerned about different operating condition to those
mentioned on Page 353 which I quoted.

I remain correct about what I said, and I selected the example omn
page 353 because its content is relevant to John Stewart's OP about a
power amp including gain stages and power tubes.

I cannot find time to answer your further concerns about what I said,
maybe your right, maybe your'e wrong.

I know people who have reverse phase connected UL taps on OPTs to
increase OP tube gain to more than double OP tube gain. The THD was
reduced and Rout reduced and bandwidth remained OK and stability fine
because they knew how to do it all, unlike most who'd end up with an
oscillator.

The man who told me about it was Neville Thiele, of the famous and
very well educated Thiele and Small gang who worked out all those
incomprehensible equations for speaker enclosure design. Theile was 75
yo when I phoned him up one evening in about 1995 for a casual chat
about something else which puzzled me.
He'd also written some fine articles in Electronics Australia, one
being about the PP amp in a deluxe model of Kreisler TV sets which
were made in Oz. The tube amp had CFB OPT and a pair of 6BM8, but
with no PFB except in the paraphase inverter. Unless careful Nyquist
parameters are considered, PVFB is a nightmare.
But Neville knew all about Nyquist. For what he did at home for fun,
he could get away with using PFB which I would not bother with - too
much trouble, and lowering THD with PVFB or PCFB may not improve the
sound much.

In McIntosh MC275 amps, the 12BH7 has bootstrapped load resistors and
there is an amount of PVFB which is substantial, but not huge, but
which allows the production of high drive voltages at lower THD than
could otherwise be easily achieved.

I again rest my case with the jury....

Delete the rest of whatever you said.....

Patrick Turner.