Can PPIMV work with Cathode biasing?

[sepulchre]

I have a friend who is building a Plexi style amp. (no, it's not me in disguise). He wants to use a PPIMV, and he wants to employ a switch that changes from fixed to cathode biasing. My thought is that a PPIMV will not work properly (if at all) when in cathode bias mode. A PPIMV works by controlling the bias voltage, right? So switching to cathode bias would negate that effect? Am I totally missing something here? Or is my friend (who I cannot convince that there may be an issue here) running down a road to trouble?

[martin manning]

No problem. The MV is not controlling bias voltage- the pots are variable voltage dividers that reduce the signal level at the grids. They are referenced to the DC bias voltage, which in the case of cathode bias, is ground.

[roberto]

Well, you have to take care that not only you have to switch in a resistor and a cap on the cathode of the power tubes, but you have also to switch the grids (so also the MVPPI) from a negative voltage up to ground reference.

[sepulchre]

Thanks, guys!