Ampeg SVT-CL OK on bench, fails with a bass.

[TimmyP1955]

"Stopped making sound during show." We replaced a bad 6550 with a good pull and all seemed well - biased right up, normal clean output on the bench, runs heavily into clipping with no issues. With a bass and a speaker (4 ohm Marshall) it sounded fine up to a point them it tripped. We installed a new sextet of 6550s - same issue.

We were unable to find any bad joints. Amp has the 220 Ohm resistors instead of the early 22 Ohm.

I seem to recall that a particular faulty cap can cause premature tripping, and that there may be a mod for the trip circuit - however this amp is from 2014, so the mod should not be relevant I'm thinking.

Ideas?

[nevetslab]

"Stopped making sound during show." We replaced a bad 6550 with a good pull and all seemed well - biased right up, normal clean output on the bench, runs heavily into clipping with no issues. With a bass and a speaker (4 ohm Marshall) it sounded fine up to a point them it tripped. We installed a new sextet of 6550s - same issue.

We were unable to find any bad joints. Amp has the 220 Ohm resistors instead of the early 22 Ohm.

I seem to recall that a particular faulty cap can cause premature tripping, and that there may be a mod for the trip circuit - however this amp is from 2014, so the mod should not be relevant I'm thinking.

Ideas?

That's kind of what I was looking for with this SVT-VR amp that was only exhibiting drop in output level driving an SVT 810 cabinet on stage, being more suspect of one of the power supply caps than a tube. I have had one of the series caps in the main power supply go bad intermittently, which would blow the 10A mains fuse. I'm not aware of any particular mod in the power supplies on these amps for that. I'll have a look at ESR on each of the caps, once I get it apart. I'd like to replace the culprit and not just shotgun the lot of them out of frustration of not finding the cause.

Not having had any success with stress-testing of our amp, which hasn't blown any fuses, I'm pulling it apart today to dig further. I would have thought the symptom heard on stage would have shown up this past week.