Peavey Classic VT tube problem

[tristanshout]

One of the 2 6L6GC tubes in my Peavey Classic VT guitar amp runs very hot when the amp is sitting idle. This is accompanied by a loud 60Hz hum. Kinda scares me. First thought: bad tubes (the amp's been gathering dust for years). The problem is still there though after installing new tubes.

Any ideas? Is this likely to be a simple or complicated problem to diagnose and fix? I have the schematics (from this site - thanks).

...Bill

[Enzo]

Then the first thing to do is check voltages while the amp hums.

I'd look first at pin 5 of each power tube - what bias voltage do you find there? I'd expect something like -50v. If one side - the hot one - is a lot lower, then you probably have a leaky coupling cap from the phase inverter.

[tristanshout]

thanks. i'll give that a try when i have some time to play with it. i'll let you know how it turns out.

[tristanshout]

Finally getting around to looking at this.

I'd look first at pin 5 of each power tube - what bias voltage do you find there?

For a few minutes after powering on, both were reading around -40V - -50V. Once it began to hum I read -40V on V1, 0V on V2, which was by then glowing pretty hot.

Leaky cap (IC23)?

...Bill

[Enzo]

Well, it is teh amp or the tubes. Try swapping places with the two tubes. Does the problem travel with the tube? Or does it remain in the same socket? And have you tried more than one set of tubes?

And another test - remove the tubes, and turn on the amp. Does that bias voltage still disappear? or does it remain without the tubes? A shorting tube could easily short the bias to ground.

[tristanshout]

Hi Enzo,

Thanks for your help so far and patience with the long turn around time.

Well, it is teh amp or the tubes. Try swapping places with the two tubes. Does the problem travel with the tube? Or does it remain in the same socket? And have you tried more than one set of tubes?

Yes, same problem with two different sets of tubes. Swapping tubes in sockets V1 and V2 results in the problem following the socket, not the tube. I think we can rule out the tubes as the problem.

And another test - remove the tubes, and turn on the amp. Does that bias voltage still disappear? or does it remain without the tubes? A shorting tube could easily short the bias to ground.

I see -40V on both sockets open circuit. I noticed my voltmeter loads the terminals pretty badly at lower ranges (it's a cheap analog volt meter), so I'm not entirely confident in the exact voltages. But pin 5 on both sockets reads the same voltage open circuit.

I noticed this time that the hum and tube getting hot were preceded by a sort of crackling sound - presumably some component failing after reaching operating temperature.

...Bill

[Jazz P Bass]

Peavey Classic VT

Check that interboard connector real good.
Also the 220K & 47K resistors.
It sounds like you are losing a connection.
Resolder, resolder, resolder.