Compensating multi-stage feedback (was: RE: all tranny vca+ )

René Schmitz uzs159 at
Mon Jul 3 12:24:49 CEST 2000

At 18:44 02.07.00 -0700, Don Tillman wrote:
>   > Effects of bias changes changes with waveform transients
>   Good point, covered by the linearity point.
>I was also thinking of one particular effect here.  In a tube guitar
>amp, a sudden transient will cause the grid to conduct on the positive
>peak, charging the coupling capacitor, and changing the bias until the
>capacitor has had a chance to discharge.  Eric Pritchard claims in
>his tube emulation patents that this is an important part of the
>characteristic tube guitar amp sound.  I don't know if I believe that
>or not.  I know from personal experience that an extreme case of
>coupling cap charge sounds awful, the signal can actually cut off
>after a transient, but I can't say I've studied more subtle cases.

In the experiments that predated the tube VCA, I did a simple triode
amplifier.  The cathode resistor (which sets up the negative grid bias) was
decoupled with a large cap, the input coupled thru a cap and the grid
grounded thru a resistor. 

It was quite interesting what happened when I raised the amplitudes so 
that they got larger than the negative bias. 

While experimenting I wondered why I couldn't make the clipping harsher by
applying more amplitude. The grid started to conduct (it can act like the
plate) when it is driven positive against the cathode. So there is a
current flow for a fraction of the waveform, that shifts the bias towards
negative voltages, were obtainable gains decrease. The "conduction angle"
is very small, and such a vacuum "diode" isn't clipping very hard, due to
the bias shift the angle is kept to a minimum. The circuit has not a static
transfer characteristic, it depends on the past (due to the storage in the
caps) and present amplitude at the input. Of course the output was grossly
distorted, in a highly nonsymmetric fashion. Although I had set bias for
maximum linearity for the smaller input. Not what I expected from opamp
based circuits, where I generally use DC coupling.

To me it crys out for using the effect for amplitude stabilization in the
positive feedback loop of a filter. (It has been often used for this
purpose in tube oscillators.) I'll try it with tubes, but all of the said
should apply to FETs equally well. Oh, and it made a great distortion
device BTW. I think I'll replicate my testing circuit as a module.

I think too that this a part of the tube sound, since its very likely that
it happens in many tube circuits (like in my VT-VCA), since these are
(nearly) always AC coupled. 

uzs159 at


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