High voltage on the cathodes of pre-amp tubes?

[dude]

I have a friends Princeton reverb I'm trying to fix for him. It's a AA1164 w/5U4GB.  Opened up I saw the PT had melted wires all glued together and smelt like burnt toasted, I got a new PT, before installing I checked the amp out and was surprised to find it was still working. All the voltages are there down the rail (no sound) but the cathodes on first two preamp tubes had high voltage, 380v.

I was amazed a fuse didn't blow but I saw the ground wire had lifted on these cathodes grounding point and that wire had 380V on it, hanging in the air...?  I assume the rush of voltage lifted the ground and for some reason didn't blow the fuse? Something went?

What would cause high voltage on the cathode, pin 8? Of the first two preamp tubes?   Since the voltage is on the cathodes of the first 12AX7 and the reverb 12AT7, could the reverb transformer be causing it?

al

[sluckey]

What would cause high voltage on the cathode, pin 8? Of the first two preamp tubes?

That open ground wire is causing that voltage reading on the cathode. Connect the ground wire and the cathodes will get happy.

[dude]

What would cause high voltage on the cathode, pin 8? Of the first two preamp tubes?

That open ground wire is causing that voltage reading on the cathode. Connect the ground wire and the cathodes will get happy.

OK, I am limited in my knowledge could you briefly explain why a lifted ground on a cathode's grounding point like this can give high voltage? I got to learn from this. I've been beating my brains out trying to figure this out.

Thanks,

al



 

[sluckey]

You have a three element series circuit between B+ and ground. The complete circuit is like this... Start at B+, thru the plate resistor, then thru the plate to the cathode (inside the tube), then thru the cathode resistor to ground. When everything is connected, the circuit is complete and current can flow, and a little voltage drops across the little cathode resistor, more voltage drops across the tube, and a lot of voltage drops across the big plate resistor. But the circuit is broken and no current can flow, so there is no voltage drop across any of the three elements. Since there is no current flow, there will be no voltage drop across the plate resistor and full B+ voltage will appear at the plate. Full B+ will also appear at the cathode, and full B+ will also appear on the other side of the cathode resistor (the dangling wire).

Connect the wire back to ground to restore current flow thru the circuit and everthing will be happy. (Unless you have other issues).

That open ground wire did not melt your PT. But the amp will definitely produce no sound until you reconnect that wire.

[dude]

Thanks, I re-connecting the ground and sound, amp sounds good.

Guess I bought a PT for stock, I probably should put it in anyway as the old one smells cooked.

The hole where the wires come out has black goo that has harden, looks like something melted at one time.

Is this common in old Pt's, the goo and smells like it's burnt? But it still works, replace or keep?

al
 

[sluckey]

It's not uncommon for a PT to get hot enough to soften tar so that it leaks out. And it will have a burnt smell. The PT may continue to work for years, especially if not worked hard. I would leave it in the amp and play it cautiously, noting how hot it gets. Just stash the new PT for a spare in case it goes downhill.

[dude]

Thank you sluckey

I appreciate your time

al