Cracked Tube Issue on the new ST-35?

[dustinsterk]

Hey Everyone,
Hopefully someone can assist. I ran my ST-35 without issues for about an hour. I was adjusting the volume when all of a sudden one of the EL84 tubes internally burst into flames.....scared the hell out of me.

Upon further inspection I found that the bottom of the tube appears to have been cracked as seen here (I took a few photos):
http://imgur.com/a/yMQS5

What would cause this? In full disclosure I did perform the initial bias and feedback (20.3k and 400 ohms from pin 3) as per the instructions. I did NOT double check the tube bias at idle (I figured I would do this after about an hour of burn in time).

Could it have been a bad tube from the start or did I push to hard and when they heated up it just finally ruptured at the crack?

Also one other fabrication note....that same socket hole is not perfectly centered as the others are. Could that cracked pin/glass area have touched the chassis causing some type of grounding issue? The tube pins appear not to touch anything but the socket itself..... but I have included another photo from above the amp to show the placement of the socket vs the drilled hole. Please note that the socket is actually below the chassis surface and not flush or poking out of the hole. When I 'push' in the tubes to the sockets the tube glass DOES touch the chassis (if that matters at all).
http://imgur.com/a/qvCRt

Thanks in advanced!
--Dustin

[Geek]

It happens.

New pair, check bias and rock and roll.

Cheers!

[Ty_Bower]

Also one other fabrication note....that same socket hole is not perfectly centered as the others are... When I 'push' in the tubes to the sockets the tube glass DOES touch the chassis (if that matters at all).
http://imgur.com/a/qvCRt

Not the ideal situation. Might be OK, might not. I can't really quite say. I suppose the chassis touching the tube might act as a heatsink, drawing the heat away from one spot while the rest of the envelope gets hot. Temperature differential might cause thermal stress in the tube and cause fractures in the glass. Or maybe nothing happens. I dunno.

The most conservative course of action would be to pull all the components off the chassis (or at least the circuit board), enlarge that hole with a curved rasp or Dremel tool, repaint, and reassemble. These amps typically come apart fairly easily due to the terminal strip connections, and you might be able to just touch up the paint job.

You might also be able to loosen the six bolts holding the board to the chassis, and wiggle the thing just enough to encourage the tube to clear the hole. Then re-tighten and check again for fit.

[WA4SWJ]

Hi Dustin,

Sorry to hear about the problem. It definitely looks like the tube lost vacuum due to the crack. When the getter turns white like I see it has in one of the photos, it's a sure sign that the vacuum has been lost and no vacuum will cause the fireworks.

Looks to me like you may have been a little too energetic when you inserted the tubes. Just be careful and don't force them and all should be well.

Good luck!! Tube amps are great fun and sound wonderful. It's worth the effort.

Regards,

[Shannon Parks]

It's happened to all of us. I did the same thing to a 6922 (in a nice matched pair) just a month ago, and I thought I was being pretty careful. I blamed the socket and swapped them out.

Shannon

[dustinsterk]

Thanks everyone,
I will just try being more careful and replacing the tube....if it happens again I will re-drill and repaint.

I must say that was my first firework show when it comes to tubes....I think I jumped a foot off the ground. LOL.

--Dustin

[TomMcNally]

JJ Tubes are noted for internal fireworks!

You can probably either file the socket holes carefully without
ruining the paint, or possibly enlarge the mounting holes a size up
and reposition the pc board to better center it.

... tom

[dustinsterk]

I am happy to report that after replacing the tube it has been running strong for over 2 hours with no issues.....thanks for the input everyone!

--Dustin