by sean.boyer » Fri Jun 08, 2012 3:12 pm
I did an MV mod to my Bassman 50 which had been converted to Blackface specs.
It was a pre-phase inverter mod. Just a coupling cap from the PI feed to a 1M pot, then to the input of the PI. Pretty simple.
To expand on what Malcolm said, a lot of the greatness of the "turned up" sound doesn't just comes from the preamp. It's a combination of preamp over drive, PI overdrive, power amp overdrive, speaker drive and cabinet/room resonance, etc.
I found with my Bassman that it was worth while to have the MV though, as you do get a good amount of nice sounding dirt from the preamp (especially since for my dirty channel I had added the unused triode section in a Marshall Plexi type configuration in series with the first preamp section to drive the gain up). The way I had it configured, I had split where the two channels meet for the PI input, and had the MV pot only affect the first (dirty) channel.
In conclusion, I'm not really sure how much of a benefit the MV control would be for you on an otherwise stock Bassman, but it wouldn't hurt.
(PS - I had removed the second channels input jacks and that's where I put my MV pot, as well as a mid pot.)
Hope that helps