Getting a 1978 65 Reverb Head ready to gig for a local guitarist, got it cleaned up and biased, but noticed the Master Volume acts like an On/Off switch - completely on at 0 - 9, competely off at 10. Checking the pot, I see it's a later replacement, 1M audio taper. According to the schematic, it should be a 10K linear taper.
Before I replace it, did MM ever use a 1M audio pot as a Master Volume control? If so, I'll recheck the wiring and pot itself to see if either is damaged.