Copland Tube amp burns channel fuse

[belzrebuth]

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:
https://www.drtube.com/schematics/copland/cta402.gif
and here:
https://www.drtube.com/schematics/copland/cta402-service-manual.pdf

[Wallace Gasiewicz]

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

[belzrebuth]

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?

[jonpaul]

find  a tube tester

[Audiorepair]

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..!

You are correct.
The schematics show there is a bias pot for each output tube, so you DO NOT need matched tubes.
You simply adjust each tubes bias current to be the same.

[NOON Design]

Check for leaking coupling caps, could be passing DC and pushing the output tubes into heavy conduction.

[elecdonia]

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?!

Yes, you have one faulty output tube.

Under normal operating conditions the plate of an audio power tube never gets hot enough to glow. Guitar players refer to this phenomena as “red plating.” It happens frequently in guitar amplifiers because the tubes are pushed so hard (often driven into clipping).

“Red-plating” is a very common failure mode for audio power tubes. It results from gas inside the tube. When the tube warms up then these gas molecules develop a positive charge. They are attracted to the control grid, causing it to become more positive. This increases the cathode current, causing the tube to get hotter. As the temperature increases more gas molecules become ionized. This creates a “thermal runaway” condition inside the tube. Fortunately your amplifier has cathode fuses which prevent the faulty tube from getting hot enough for its glass envelope to melt, crack, and break, or (worse) become a dead short from cathode to plate which could damage other component parts.

According to the service manual the output tubes (valves) are EL34.
Fortunately the EL34 is in current production by several manufacturers.
It is used in many electric guitar amplifiers, for example most Marshall brand guitar amps use them.
I recommend purchasing “new production” EL34 rather than searching for (expensive) “new old stock” tubes. 

Sweetwater is reliable USA vendor (there are several such vendors who specialize in providing tubes for guitar amplifiers)
     https://www.sweetwater.com/c1070--EL34_Guitar_Amp_Tubes

Another option to get your amplifier working again quickly would be to purchase a single EL34 from a local music store. Most music shops who cater to guitar players do carry stocks of replacement tubes.

Ultimately you might wish to get two sets of “matched pairs” of EL34. Each channel (left, right) should have its own matched pair. It is not necessary to get a “matched set” of 4 tubes where they are all matched to each other.

Follow the procedure in the service manual to adjust the bias controls for the specified voltage drop across the cathode resistors. Note that a tube which can be biased to the proper cathode current and where the cathode current remains stable for 10 to 30 minutes without changing is a “good” tube. Tubes with stable cathode current are in no danger of sudden failure. It isn’t necessary to replace such tubes in a regular basis. They could last for many years.

[elecdonia]

find  a tube tester

I’m not opposed to using a tube tester (I own several). However it isn’t necessary for every tube amplifier owner to have access to a tube tester. In my experience tube testers have limited capabilities for evaluating audio power tubes. The plate voltages and cathode currents supplied by tube testers are much smaller than the actual operating conditions when the tube is in the amplifier.

People who repair guitar amplifiers often use “bias master” devices which insert between each power tube and its socket. These permit accurate measurement of cathode current and control grid bias voltage under actual operating conditions.
     https://www.amplifiedparts.com/products/bias-master-tad-tube-tester-4-probes

The OP is fortunate to own a tube amplifier with individual bias controls and cathode resistors for each output tube. Their amplifier contains a built-in “bias master” feature, with the exception of requiring an external multimeter to measure cathode current.