Master Volume in Dr.Z Prescription ES with Cathode Bias

[binito]

Hello,
I would like to install a master volume in a Dr.z Prescription ES.
It has power amplifier with 4 x EL84 with cathode bias.
I installed a PPIMV (lar mar) with A250k stereo pot and 2,2Meg resistors. It works well. But the amp does not saturate in the pre-amplifier.
All saturation occurs in the power-amp. Is there any way to install a master volume in cathode resistors to take of the saturation of the EL84?
Thanks in advance.

[Sowhat]

If I understand you right I think what you would use is a power soak. I kind of dislike the term PPIMV because it works for both "pre phase inverter master volume" and "post phase inverter master volume". With the description of the two 250K ganged pots I assume you mean a "post phase inverter master volume". With that arrangement you will only be able to saturate up to the phase inverter output. To saturate the amp, the big 4 power tubes you need to divide up the power output from the output stage and just let a portion get to the speakers. That is what a power soak does... I think?

[pdf64]

Sundown amps used this method, with what looks to be a high power 200 ohm rheostat, see http://bmamps.com/Schematics/Sundown/sundown_a50.pdf

I suggest a multiposition eg rotary switch be used to insert various values of power resistors between the ground return of the existing cathode bias resistor//cap, and ground.
The switch rating may not allow it to be actuated whilst the amp is on, ie passing full cathode current.
The volume should be whisper quiet by about 470 ohms.
But as it also cuts the power tube gain right down, there may still not be much overdrive, but my arrangement didn't bypass the added cathode resistance with a cap; if bypassed (as per the Sundown), there may be more overdrive.

Variable voltage reduction, using a MOSFET to lower the VB+ may be a better option.
But the most simple and perhaps effective way forward would be to add some gain for the pre-amp with your existing master volume, eg an external signal booster.

[drewl]

For cathode biased amps the type if master volume I usually install uses a dual pot to replace the two input grid resistors that go to ground on the output tubes.
They are usually two 220k or 470k resistors.