Non-Marshall PPIMV

[wozt]

The pot for the KF PPMV mod replaces the bias feed (grid load) resistors so you pick a pot value close to that resistor value in your amp. Other PPMVs effect the circuit differently. That's why there seems to be such a wide variance in pot values for the different types of volume controls.

Do the KF PPMV mod to your amp like this:

Can this arrangement be used with fixed bias amp, ie taking the ground lead on the schematic to the bias voltage?

[Chuck H]

Can this arrangement be used with fixed bias amp, ie taking the ground lead on the schematic to the bias voltage?

Assuming you're explaining the difference between a cathode biased amp, which has the grid loads connected to ground, and a fixed bias amp, which has the grid loads connected to a bias supply. Yes. The KF PPIMV (and the modified version in this post) work with fixed bias circuits that have the grid loads connected to a bias supply. But just to be clear, you would not be

taking the ground lead on the schematic to the bias voltage

That reads as if a ground would be connected to the bias voltage. I don't think that's what you meant but a mistake about this could be very bad.

[wozt]

The two inner lugs of the pots are connected and on the cathode biased version shown, they go to a ground point by a wire.
So for a fixed bias version does this same wire just connect to the -bias supply instead of a ground point?

I'm just trying to work out a fixed / cathode bias switching system and thinking can 1 pole throw that wire to the -bias voltage
or a ground point.

Thanks for your time.

[Chuck H]

You can do it the same with the pots as with the fixed resistors. It doesn't even matter if you use the "power tube grids on the wipers" circuit or the "PI decoupling caps to the wipers" circuit. All you'll be doing is switching one lead from ground to bias supply and vice versa. You still need another pole on the switch to short the cathode bias circuit to ground though. But you would need that anyway. So you need a DPDT switch (rated 3A or better).

[wozt]

Many thanks Chuck. The decoupling caps on the wipers just sits easy with me so I'll go that route.
When in cathode bias mode a max ccw turn on the pot sends the signal to ground, what happens in fixed bias mode, does the signal get shunted (where to?) or just attenuated by the resistance?

[Chuck H]

In the fixed bias mode any AC on the grid would be shunted through the bias supplies resistance and filter capacitors. Any small signal remaining will probably be phase cancelled by bringing the two PI outputs together. It's conceivable that you may not be able to dial in full silence. Hopefully you're not employing the amp for a "no sound at all" situation