Post-Phase Inverter Master Volume (PPIV MV) Installation in a Marshall amp

How to install a post-phase inverter master volume in a Marshall amp. This is a JCM800 Master Model 50W Mk II Lead amp that already has a pre-phase inverter master, but adding a MV post-PI allows more gain and fuller tone at much lower volumes. The procedure is the same for any similar amplifier, as the PPIV MV is installed between the output of the phase inverter and the power tube grids.
I offer this as a DIY kit on the GrangerAmp website, for those who want to add it to their amps, and who have enough electronics experience to do so safely. IMPORTANT DISCLAIMER! Voltages inside tube amps can severely injure of even kill. Do not attempt this or any other modifications if you are not sure what you are doing, or if you do not understand how to properly discharge any high voltages present inside the amp.