[Russtafarian]

When one tube in a pair of push-pull power tubes goes into thermal run-away, which tube caused it?  The one that got too hot or the other one?

[Steve]

When one tube in a pair of push-pull power tubes goes into thermal run-away, which tube caused it?  The one that got too hot or the other one?

Could be several causes.

1) The hot output tube itself.

2) A driver (or previous tube if the FR of the driver is high enough).

3) The circuit design itself can cause an oscillation in the tens of khz or higher range. Improperly designed would not
allow for tolerances, thus an oscillation could develop. Normally it would include both, but not necessarily.

4) Signal grounding scheme. A ground is not necessarily a ground at RF frequencies. And if two
circuits have a common path, or close enough physically, enough feedback at RF frequencies could occur for oscillation.

Cheers.

[Ericus Rex]

This article might answer your question:

http://www.ramlabs-musicreference.com/virtues.html

[Steve]

I believe the OP is interested in one output tube becoming unduly hot, even cherry plate.
 
Under point 1, output tube itself, a hot tube/plate red, could also be caused by leaky grid, or gas.

Roger's article would not be applicable as it refers to the tolerances of tubes and why matching is important. I suppose
one could mis adjust individual biases, but a meter of some sort to measure cathode current is necessary,
so mis adjustment would be rare.

Cheers.

[Ericus Rex]

It's been a while since I read that article but I do believe it does mention the uneven distribution of current through two PP output tubes.  While Roger's gist was about proper tube matching a bias-circuit failure could cause the same phenomenon, which I think is what the OP was asking about.

[Russtafarian]

The problem here is the OP's inability to properly state the question.    I'll take another run at it.  When one of the two power tubes in a mono push-pull amp starts glowing bright orange, it probably means one of the tubes is malfunctioning.  Is it a malfunction of the non-glowing tube that causes the other tube to glow?  Or is it the glowing tube that is malfunctioning?

I know there can potentially be other issues with the amp that could cause this, but given my experience with these amps, I'm 90% sure i'ts a failing tube.  I just want to make sure I identify the right tube for replacement.

Thanks for the help.

Russ

[Freo-1]

The majority of the time, the time that is glowing red/orange IS the problem child.  The old T/S 6550's were notorious for being gassy, which would cause them to turn cherry red.   
 
The replacement tube needs to be somewhat close to the mate to prevent a mis-match, which I think is what you are asking.   Does the amp has individual bias pots for each output tube?  If so, then that makes it easier.  If there is just one bias pot for the pair, then it may be more prudent to replace both with a matched pair.
 
Does that help?

[Russtafarian]

Yes, that helps.  And yes I'm getting a matched pair of replacement tubes.  Thanks.

[Ericus Rex]

Swap the two tubes.  If the same tube goes red then it's the tube.  If the other tube goes red then it's your amp.

[Steve]

The problem here is the OP's inability to properly state the question.    I'll take another run at it.  When one of the two power tubes in a mono push-pull amp starts glowing bright orange, it probably means one of the tubes is malfunctioning.  Is it a malfunction of the non-glowing tube that causes the other tube to glow?  Or is it the glowing tube that is malfunctioning?

I know there can potentially be other issues with the amp that could cause this, but given my experience with these amps, I'm 90% sure i'ts a failing tube.  I just want to make sure I identify the right tube for replacement.

Thanks for the help.

Russ

To explain further, If the output tubes are each cathode biased, it is either the hot output tube at fault, almost always, or another
problem causing an rf oscillation, as mentioned above.  If fixed biased, individually biased, then the bias circuit could be at fault.
Also, if an older component, possible, leaky coupling capacitor, causing a bias shift. Been a long time since having this problem though.
Swapping output tubes, as someone mentioned above, is a good idea.

Cheers and sorry about multiple posts with causes.