I posted on the Guitar forum, but figured this may be a good place as well as I'm looking for advice from a Tech:

http://forums.musicplayer.com/ubbthreads.php/ubb/showflat/Number/1982534/page/1#Post1982534

Dan, it looks like the lead master pot is the last adjustment and the summed point for all circuits before the phase inverter/power tubes. Assuming that's the pot you mean, there's a component right underneih that pot that looks like a full wave rectifier, but I don't know what it is. It is, however, connected to ground and the pot, so at first blush that's where I'd start looking.

If you mean the actual 'lead' pot, same deal. It has 2 destinations: the reverb grid/cathode feeds(constant), and again one of those rectifier-type components that connects to, and depends on the pot's wiper position. Check both the reverb end and the wiper of the lead pot with a scope as you turn it up with signal, and see which leg is causing the problem(provided it's not later in the circuitry). FWIW, have you done quiescent voltage checks at related points to make sure they're within spec? Good luck.

One other thought. I didn't spend a lot of time on the diagram, but the lead pot may be an adjustment to apply a later tube bias that lowers the volume with respect to the clean signal. This will allow distortion while not compromising the overall volume. Might be what they intended. Try turning the master down a little, and see if the lead pot doesn't react differently.

Best, Paul

It's the lead pot, not the Lead Master. The box on the schematic you refer to is an optical switch - when the footswitch conducts to 2 of the terminals, the other two conduct. The one on the lead master pot connects one end of the pot to ground when engaged, thereby activating teh lead master pot. When disengaged, that end of the pot is open and so the same signal appears at both ends of the pot and it does nothing.

On the other one, it connects the wiper side of the lead pot to ground, thereby sending no signal from the lead pot into the next preamp stage. I've tested both of those and they are working properly.

I'm getting ready to start poking around now, I was just trying to narrow it down and get a plan from the schematics first. My best guess is eitehr a capacitor is leaking stray DC to one end of the lead pot and once you get to a certain point it affects the next preamp stage, or there's a bad ground connections somewhere - maybe at the other end of the lead pot.

I'm going to start by checking for appropriate DC voltages and make sure there is none present at the wiper of the lead pot.

Thanks

UPDATE: I opened it up and it ended up being really simple. The lead pot was twisted and the hot end of the pot was touching the case of the reverb pot. I bent the pin back so it wasn't touching and it works fine now. Whew!

. A little training, and you probably saved yourself a hundred bucks(or MUCH more) in bench time. They say that 95% of all amp problems are mechanical. The % just went up to 96 .

Best, Paul