UA OX catching amp on fire

[British1011]

Hey guys I have a customer who has had two amps catch on fire while using the UA ox box.  One was a twin reverb the other a mesa mark iib running at 60w.  I didn't see the twin and don't know what was repaired but it did actually catch fire. The mesa just smokes.  He said he had it on 8ohms running an 8ohm speaker.  I'm not real familiar with these things... Anyone have any ideas? Both amps were recently service.  Fresh cap job and tubes and we're running great before using the ox.  The mesa has a burnt resistor on the slave pot which seems odd because it wasn't even in use. I couldn't tell where the smoke was coming from exactly but it was around the filter caps and I don't see anything burnt in that area. 

[d95err]

 Attenuators and load boxes allow you to turn the amp up as much as you want. Not all amps can handle that.

[Latole]

Bias of Output Power Tubes of these amps could be too hot.
Too many people replace Output Power Tubes without checking bias.
Too many handy man ( not qualified amp tech) work on amp bussiness; job are not well done

Or / and use the wrong output impedance load.

[pdf64]

There’s maybe a dozen different TR models, some with quite significant variations to the general theme. And that’s before mods and dodgy repairs / maintenance are considered. Sorry to go on, I’m just pointing out that the descriptor ‘Twin Reverb’, whilst much better than nothing, doesn’t narrow things down to a single model / schematic.

Regarding the Mesa, has anyone verified what its OT primary impedance is (when loaded as intended)?
The burnt resistor noted may be a red herring, perhaps from a previous incident, as without anything plugged into the slave output, no current could flow through it. https://el34world.com/charts/Schematics/files/Mesa_boogie/Boogie_mkiib.pdf

Whatever, if a valve amp (especially one that’s intended for a lower load impedance than that being used) is heavily overdriven into a higher impedance load, its screen grid current draw may rise to damaging levels.
In that scenario, it’s beneficial for the amp’s screen grid supply resistors to overheat to the point of fusing, so as to avoid the valves shorting and further, potentially significant collateral damage, resulting.   

[Latole]

Attenuators and load boxes allow you to turn the amp up as much as you want. Not all amps can handle that.

And all attenuator or load boxes are good quality, some may harm the amp

[British1011]

After talking to him more about what happened it turns out that he was running the mesa for 10 hrs a day in the studio full bore for a whole week. I told him when I looked over it before that his caps were old and should be replaced even though they werent leaking at the time they would soon but he wanted to wait. He just cooked this thing.  Bias was actually a bit cold. I'm pretty positive it has more to do with how much abuse both amps were put threw than the ox although I'm sure it just added more stress to it.