Re: 'PC Speaker' Tube Amp

On Sat, 07 Jun 2008 16:30:34 GMT, Paul <paulguy@[EMAIL PROTECTED]
> wrote:

......snip!..............
>   I think you are correct about the B+ being the problem.... I think
>the supply is adding "zero" behaviour at the low frequencies, and the
>larger A (open loop gain) is making things worse. BA must never have a
>value more than 1 when its phase is 180 degrees, so I have a number of
>options... 
>  1- regulate the B+ (eliminates power supply "zero" behaviour,
>reduces phase ****ft) .... a zener maybe?
>  2- lower the amp gain (A) - no local +FB - keeps BA from getting
>close to 1, but increases distortion
>  3- put a pole in the feedback network, thus ****fting the offending
>phases the other way.
>  

the results:
   1- no effect
   2- trying not to do that
   3- marginal effect, required fairly large film caps (40 uF) and a
complex RC network

the solution: put a capacitor IN SERIES with the resistor that couples
the two cathodes together. Set the cutoff freq. of the RC section
(Xc=R) to about 10-20 Hz. Low frequency stability (global) vastly
improved, plus I can now put even more + feedback to increase global
loop gain.

  Why does it work?   You got me there....  That's one of the great
things about trial and error, you sometimes come up with solutions
that the math would just confuse me. Within the single local positive
feedback loop, I have added and extra zero to the local loop, which
should make things worse. But it has helped the global loop, and given
it an extra margin of stability. The only hint..... is that this added
RC section has the highest frequency cutoff of all the zeros in the
entire global loop, so it would be a "dominant" zero.
   I think I'd need to run a circuit simulator or something like
MATLAB's "simulink" to model this. It's way over my head!

-Paul