I have a Univox U65RN that breaks up real nice when cranked (especially for SS), but it's pretty loud for playing around the house. I'd like to add a master volume to the amp. As far as I can tell, there are 2 places in the circuit where this may be possible.

I installed a basic volume pot to ground (from the collector of the 2nd 2SC536F to the base of the 2SB405GN). It worked like it should for a couple weeks, then the amp developed a loud buzz with no guitar signal coming through. When I removed the volume pot, the amp was back to normal. I expect the potentiometer failed and was dumping all the signal to ground. There wasn't much voltage going to the pot, and it was rated at 1/2watt (I believe). but I guess it couldn't handle the current. Does this make sense?

I think the transformer that is after Q3 (the 2SB405GN in the main part of the circuit) splits the phase before the output transistors. Could something similar to the PPIMV mod that is popular with many tube amps be used here?

Here's the schematic http://vintageunivox.com/pics/schematics/u65rn.jpg