There are several possibilities of implementing a master volume with an AC30. As the AC30 has no Clean / Lead distinction, you have to turn up the volume to get overdriven or distorted sounds. And an AC30 gets really loud.
Therefore, a master volume is really recommended.
I tried 3 different implementations of circuitry.
Option 1:
Taklen from the Vox AC30 CC series.
Pro: Simple to implement
Con: It is difficult to control the volume especially in the lower range.
Option 2:
In front of the phase inverter:
Pro: simple circuit
Con: phase inverter contributes only marginally to tone and distortion. Channel volume has to be turned up to get distortion. This can be an advantage as well if the amp is used primarily for clean tones.
Option 3: (implemented)
Original Vox Master Volume Mod (Post Phase Inverter Master Volume, PPIMV)
Pro: Sounds very good even at low settings, easy to control.
Con: Sound depends on synchronisation of the stereo pot, circuit more complex.Klang abhängig vom Gleichlauf des Tandem-Potis, schaltung etwas aufwendiger.
The two parallel 2.2M resistors (R27 und R 28) are for security reasons, if the wiper looses contact.