Tech experts - what could make an old tube amp cut in & out?

[Jefenator]

The other day I was doing a party with my old Sunn 200s. It was sounding great, then halfway through the first song it cut out completely. I quickly checked the speaker cable - that was OK. After a minute or so the sound came back for a minute or so (maybe weaker?), then it cut out again very abruptly. I finished the set through the PA and dashed home to get my LMII on the break. (Should have brought it for backup in the first place. )

I've used the Sunn a dozen times or so before this with no signs of trouble. Of course, I'll get it looked at by a pro before playing it out again. (Alas, that's going to be a while - I have some big bills that will be trumping tube amp maintenance. )

In the meantime, I'm just curious. What in your experiences might cause an old tube amp to cut out like that?

Thanks in advance.

[JimmyM]

any number of things from a tube that needs to be pushed back in the socket to the transformers getting ready to catch fire

if you have any spare tubes, try swapping them to see if one is bad. or take a pencil and tap each tube lightly once it's good and warmed up and try to listen for microphonics. that could cause it. also check and make sure your switches and jacks are tight and not crackly. could also be a cap going bad. lost of stuff in there.

doesn't sound too serious to me, though. you probably won't get hit with a $300 bill.

[Jefenator]

Thanks, JimmyM!

It is encouraging that it came back online for a minute. I'll try tinkering with the tubes & looking for something obvious. (Wouldn't have thought of that pencil tapping test...)

[T-Bird]

Hi.

While a tube problem can be intermittent, I've never encountered one in the about 25 years that I have been around tube equipment.

IME 9 times out of 10 a symptoms You described has been caused by a loose solder joint and/or oxidized contact somewhere.

Regards
Sam

[MAMMOTHvolume]

Originally Posted by T-Bird

Hi.

While a tube problem can be intermittent, I've never encountered one in the about 25 years that I have been around tube equipment.

IME 9 times out of 10 a symptoms You described has been caused by a loose solder joint and/or oxidized contact somewhere.

Regards
Sam

+1 not a valve or transformer problem, as they tend to be a bit more spectacular when they give up the ghost

[WingKL]

Beside a cold solder joint as mentioned before, it could also be a tube socket put that needs to be re-tensioned.