On Sat, 07 Jun 2008 01:29:31 GMT, Paul <paulguy@[EMAIL PROTECTED]
> wrote:
>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.
Combining your last post and this one, that's what I would have
expected.
> However, when I closed to global feedback loop, the stability was
>much worse.
Since you talk below about LF poles I presume you mean motor boating,
and not HF, when you says stability is worse.
> That's probably because I have quite a bit of overall
>negative feedback,
How much now, and before the PFB increase?
Also, I'm curious. What triodes are you using and how much gain are
you getting now?
> 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).
Well, except it isn't passing the signal through, unless you have so
much PFB it's still got gain down there.
> 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".
Hmm. Well, I'm no math whiz when it comes to poles but having the amp
trying to drive the OPT lower than it can pass seems backwards to me.
That will cause large voltage swings in the amp, as it tries to
'force' things through, and it often causes HF bursts. Speaking of
which, the HF bursts can cause motor boating too although I presume
you're not having those or else you'd see them.
Where is the LF it's 'amplifying' coming from? Strikes me that it
might still be B+ filtering and it's just more sensitive now that gain
is higher.
> 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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