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John L Stewart John L Stewart is offline
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Location: Toronto
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[quote=Patrick Turner;928239]On Mar 18, 9:55*pm, flipper wrote:[color=blue][i]
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

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Hey Patrick, I won't comment on your most recent except to day that Flipper is correct in his post that it is important to attack distortion in the hilevel stages first. If need be, find gain somewhere else, a low level stage perhaps if there is a shortfall.

The short equations I gave predict the worst case of +ve FB since we assume the triode gain to be simply equal to mu while bootstrapped. In reality, while bootstrapped the triode gain is somewhere between the grounded cathode value A1 & mu. So the +ve FB is always less than predicted.

In the McIntosh the local NFB in the OP stage is 15-20 db, depending on loading, Etc. So lots of room for a couple of db +ve FB.

The rp of the sample circuit given turns out to be about 10K since the cathode current in each triode is 4.3 mA. That translates to a worst case +ve FB of 2.74.

The equations can be further reduced as follows-

A1 = ( mu*Rl ) / (rp + Rl ) and

A2 = mu

Gain change is A2 / A1

So Gain change becomes ( mu ) / ( mu*Rl ) / (rp + Rl )

That reduces to Gain Change

A2 /A1 = (rp + Rl ) / Rl

Very easy to apply to various conditions.

I made some real measurements on working systems 8 or 10 years ago but can't find the results. As I recall, all less than 2 db I think. When I get some time, or if there is rain or snow I will make some measurements. I've got a Circlotron, the 6AS7/6080 Amp & two different twin coupled amps I built for publication back in the day.

Yesterday was 8K on the cross country skis, then 45K on the bicycle. Not bad for an old fart.

Cheers, John