Electronics > Repair

Copland Tube amp burns channel fuse

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Hi all,
I've got a Copland CTA-402 tube amplifier that burns F202 after 5-10 minutes of being on.
It doesn't require a signal to lose its cool and this started out of the blue as far as I can tell.
I resoldered the tube sockets just in case but I think the problem may lie elsewhere.
I don't know anything about tube circuits so I can troubleshoot in depth.
I know that the bias voltage on the 10ohm resistor keeps rising way above 400mV and the tube starts glowing hotter until the fuse is open.
The tube continues glowing hot so I have to switch the unit off.
I'm not sure how I can test the tube and if the actual tube is at fault here or I'm lucky enough to have anything else going bad in there.
These are bit hard to find so I can't just try a new tube at the moment.
Any ideas would be very welcome!
There's a full schematic here:
and here:

Wallace Gasiewicz:
The schematic shows a push pull amp
First just check the ohm values of the resistors.
Switch the output tubes and see if the same thing happens
See if the same tube gets hot or its the same tube socket.

Resistors seem fine. Measured most of them.
Swapped the output tube from the R channel with the one from the good L channel.
I monitored the bias voltage on the previously good channel with the suspected tube and watched rising rapidly as it did on the "bad" channel.
So yes it must be the output tube..
What do I do now?!
I need to buy all four of them or I just replace the one that causes the issue?
EDIT: I think my question is stupid; no way these tubes are matched so I probably need to buy just the offending one..!

Wallace Gasiewicz:
Try to get a tube manufactured at about the same time from the same manufacturer if you can.
I think this is an example of a really bad "grid leak" when the tube warms up something on the grid contacts another grid.
Did you try "banging" the tube with something like a piece of wood when hot?

find  a tube tester


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