Ground probs w/ standard master vol?

[Namar]

Hey all.

I just threw in a ptp board in my '94 reissue. It works fine and sounds great. Throatier and thicker than the circuit it had before, although the two are pretty close. Looks way cool though, and will be alot easier to monkey with in the future.

I had a standard master, 1 meg pot between the tone pot and the cap headed to the phase inverter, it worked fine.

But, when I hooked it up with the ptp board, with the master vol on 0-1, it buzzes, like it does for a moment when you hook up a cable initially, I'm assuming that's a ground loop or shorting kind of sound. I've tried two different 1 meg pots here, they both do the same thing.

The odd part, when you turn the master vol past 1, it goes away. Even more strange, is that the presence does the same thing, but opposite, in that it buzzes in all its range except for the highest settings, between 9-10.

While probing around with my multimeter, I noticed that the buzzing would stop when I checked the terminal post which holds the wire from the master vol wire (tone control w/o the mstr) and the cap leading to the phase inverter.

This led me to trace along and compare the old circuit board and the new ptp, to see if anything was different, and sure enough, the circuit board has a ground lead coming off that terminal with little cap that I assume is a 220pf ceramic disc.

Is this ground a necessity to install a standard master volume? Is 220pf a good value here? The amp seems fine throughout it's range without this ground, what is the master volume doing to mess everything up like this? Anyone know or have any guesses? It must be something of importance, or there wouldn't be a ground lead on the pcb for this.

Thanks.

[flemingmras]

When you look at the master volume pot from the back with the terminals facing up, make sure that the leftmost terminal is grounded. Same with the Presence pot.

Jon