My JTM45 is simultaneously getting fireworks in one power tube and blowing the HT fuse. The amp is stable at idle and low volume, but when I turn up some I get a minute or two and then flash, pop.

First time it happened, I saw the white fireworks in one of my expensive Gold Lion KT66's. The fuse did NOT blow and I powered down immediately. I swapped the power tubes around in the sockets, powered up and the same tube again flashed in the other socket. I took just a couple seconds to determine the fireworks responded to signal/pick dynamics -- play soft, little flash, play hard, big flash -- which audible static/distortion also indicated.

I figured it's a bad tube, replaced with a set of Tung Sol 6L6-GC STR ($45), got 'em biased ($43), played 5 minutes and … flash, pop. I did the socket swapping as before with the same results, so I replaced the -- what appeared to be  -- offending 6L6 ($25), and the new one lasted 2 minutes … flash, pop.

The tube arcing appears all internal. I've examined the tube bases and amp sockets (both plug and solder sides) and they're pristine, no scorching at all. And there's never any smoke or smell.

I'm going to have the shop fix it -- for no reason at all I'm betting it's the OT -- and I'll pay through the nose I'm sure. Obviously I could have saved a lot of money had I not run with my initial erroneous "bad power tube" assumption. I'm eating power tubes like they're boxes of raisins! Doh!!!

I'd like to hear what you guys think the problem might be, both to learn something and keep the techs honest, so to speak.

Casey

This is a JTM b=Bluesbreaker, right, not a JTM45?

This was from April 2008? I think thats the record we have on 72sg,

If so, was the amp biased fine, accounting for the bias wiggle trem?

And, I guess, PI tube as well.  I think, I recall on occurrence of arc on 1 power tube, due to failed Phase inverter tube.

I guess it can be the OT. But not sure, since after that, the problem follows tube, not stay on socket.

if OT, ie 1 side failed, it'd stay on the side with the problem, I guess. Ie, when you swap the tubes around, you'd mess/damage  the other tube. But of course, if the HT fuse stops it in time, then no prob.

On the matter, I am not sure if you emailed me. Dont think you did? I would have advised, IF ANY SPARK in power tube, not to proceed with changing with other power tubes.

Failed or failing power tubes would usually be indicated by not so toneful sounds, and/or redplating.

If arcs, usually indicate some other problem.

It could still be the OT, I am not ruling that out. After all, how long can coconut shells last?

Thanks!

nik