48 years later I finally cooked an amp!

[Bob Womack]

Well, it may only be a tube. From a post on another forum:

Well, this is a first for me. I was playing along merrily today, at bedroom volume, when my twenty-year-old JTM60 went HUMMMM... Click.. and silence. I know, I know, these are known to overheat and no-one likes them but this one has been great for me. I use it mostly at the studio.

Power light still on but I shut her down immediately and let her cool. The fuse is good. The trannies look pristine, no color or leakage. No smell and I didn't let the smoke out. I opened up the tube cage and checked the original 1996 Huntsville/Petrograd Svetlana EL34s that are set horizontally at the bottom of this chassis. No carbon trace on the pins. However, one of them had overheated to the point that the envelope melted (!) and deformed inward in an inverted bubble. The silver at the crown of the tube was a little oxidized away but there was no hint of singeing on the plates. Obviously zapped. I know: should have replaced the tubes long ago but I'm used to colder biased Traynors that use up a pair of EL-34s about once a century.

So:
a) Do I replace the tubes and try it out to check for function or does that just run the risk of immediatelly zapping another tube?
b) Or do I simply convey it forthwith and henceforth to the local amp tech without passing GO or collecting $200?

Got any idea of the cost if there is collateral damage? I figure about $85 bench fee for biasing if I remember right, plus tubes. Obviously there would be troubleshooting as well if we've zapped anything else. Can you help me keep from wasting money?

Thanks boonches!

Bob

[J Patrick]

...based on the several amps i have cooked in the last 48 years..if it’s cooked you smell it...

[H165]

Sounds like bias setting too hot - maybe a bias cap or bias pot failed.

[muscmp]

could be a cap or a tube. you could swap a known but not valuable tube, but i'd first get inside to see if i can see the problem--something burnt or cap with a pimple or blown. then, i'd put it on a bulb limiter and variac on it and bring it up slowly checking the voltages. that should tell you where the problem is.

the tdpri forum would be another resource for troubleshooting this.

clintj will pop in here soon and have a good recommendation also.

play music!

[clintj]

Thought I felt my ears burning, someone must have been talking about me. ;-)

At 20 years old, your JTM is due for servicing anyway. 15 to 20 years is the service life for the filter caps in tube amps, whether or not they "look fine". The electrolytic fluid in there dries out, which has a negative effect on how well they work and can lead to one failing and causing other damage if left unchecked. Old caps can also stress a power transformer, making it work harder. While he's in there, your tech can also check for any issues from the tube meltdown.

[Bob Womack]

Thanks, guys! I'm actually looking at this as an achievement! Who has done this for nearly a half-century and hasn't blown a tube amp?


Bob