This is one of Raphael’s favourite mods and can be seen on many of his own amps.
PPIMV is an abbreviation for Post Phase Inverter Master Volume. This means that we add a high quality master volume control pot that enables the phase splitter to be driven rather than the power valves.
It’s the closest you get to power valve distortion without blowing your ears off or spending a fortune on a decent power attenuator.
Any non-master volume amplifier with a push-pull output stage (please ask if you’re not sure). Most early amps by Marshall, Fender, Vox, Selmer, Hiwatt etc fall into this category.
You can also add a PPIMV to Master Volume amps to enable a blend of two types of distortion, creating gut wrenching overdrive tone (think EVH).
Any Plexi Marshall (anything pre JCM800 really), Fender Hotrod Deluxe, Bassman, Super, Princeton, Deluxe Reverb, Twin etc.