Re: 'PC Speaker' Tube Amp

On Sun, 08 Jun 2008 02:21:43 GMT, Paul <paulguy@[EMAIL PROTECTED]
> wrote:

>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....

I'm surprised more B+ filtering didn't help but the cap in series with
your PFB R breaks the PFB at low frequency.

Dynaco did that in their ST35 but they were DC coupled and I thought
your AC coupling would be enough.

>  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!

That's what I do. Sim, sim, sim.

>
>-Paul