Scratchy pot fix for VVR

[mooreamps]

I draw up these thoughts in responce to e-mails I've received from people who expressed their concerns on my opinion how I would have addressed this issue. This circuit is un-tested, since I don't use it myself, and I have no plans at this time to power scale any of my pre-amps. But for those who do, or for those who wish a clearer understanding of circuit theroy, it is presented for educational purposes.

The image on the left is the current solution, which there is nothing wrong with.

-g

[tubeswell]

Thanks Gary

So what does the diode do? (do something to the voltage at the cathode??)

[mooreamps]

The diode clamps the bias to 0.5 volts, regardless of plate voltage or plate current, down to 100 ua. This should keep the grid from flowing current when the plate voltage is reduced to the lower levels, hense abaiting the grid current from flowing into the guitar pots.

-g

[tubeswell]

So its only in the first VA stage them?

[mooreamps]

That is my understanding. The remaining gain stages would be isolated by the plate coupling caps and any series resistors. However, if I were to be power scaling the pre-amp, I would set this up for all pre-amp gain stages, not just the first one. Perhaps this point was over looked as well.

-g

[coco]

Did anyone ever try the diode fix for scratchy pots?

To rid your scratchy pot from low voltages with a B+ scaled amp, try this:

SCRATCHY POT LAYOUT - VRM INPUT MOD