On Sat, 07 Jun 2008 00:34:07 GMT, Paul <paulguy@[EMAIL PROTECTED]
> wrote:
.....snip!.........
>
> It would start to oscillate even when I had the global feedback
>disconnected. It is AC coupled. The amount of positive feedback that
>would start it oscillating was a resistor about 1000 ohms (instead of
>the 3300 I actually used). I assume that it is due to the B+ not being
>sufficiently filtered, and the 2 front stages would feed back through
>the B+. The output tubes run off a different supply, there is little
>effect from them.
> Since the frequency of oscillation is around 2 Hz, my guess is that
>I can simply reduce the value of the coupling capacitor (0.68 uF into
>a 220K grid resistor) between the two stages of the positive feedback
>loop. The -3db point of those values works out to about 1Hz. That's
>really TOO low.
> There's probably an "effective" zero or two in the B+ (like a
>large capacitor in series with the plate resistors, and common to both
>plate circuits), and they are likely causing stability issues with the
>feedback. I don't usually consider the stability, zero's, and Nyquist
>Criteria at the low end, and here it's bitten me! One zero is OK, but
>two or more is trouble!
> By reducing the 0.68uF to 0.1 or 0.2uF, I should be able to make a
>"dominant zero" just like you use a dominant pole at high frequencies.
>The poor thing is still gutted open on my bench, so maybe I'll see
>what effect that has.
>
Interesting..... Within the local positive feedback loop, using a
dominant zero (smaller coupling capacitor) allowed me to put a LOT
more positive feedback before motorboating ocurred.
However, when I closed to global feedback loop, the stability was
much worse. That's probably because I have quite a bit of overall
negative feedback, and the phase ****fts from the zero's (coupling
capacitors) were made worse by lowering the one stage's coupling
capacitor. If I'm not mistaken, the output transformers primary
inductance is effectively a "zero" (an "s" on the bottom of the
transfer function ?, or inductors in parallel act like caps in
series). So you'd have 3 zero's in the feedback loop, even if you
discount the power supply. That works out to almost 270 degrees of
phase ****ft... if you have any degree of gain at those low
frequencies, the thing is going to oscillate.
By decreasing a coupling capacitor, the phase ****ft will get worse
at a given low frequency, and make stability worse (for the same
amount of feedback).
So by decreasing that capacitor in the positive feedback loop, the
local loop became more stable, but the global loop became more
UNstable!
Since the transformer is pretty well a constant, overall low
frequency stability would improve by increasing the values of coupling
capacitors, until the transformer "dominates" the "zero's". That's
probably not a good thing with a SET topology, since the primary
inductance isn't very linear as well....
-Paul
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