Tube flash

Try lightly tapping the tube with a pencil eraser while it is on the tester, to make sure the tube doesn't have a marginal short circuit.

Also, it's possible that something else failed in the amp that caused the tube to flash. For instance if a coupling capacitor connected to the grid broke down, it might have put a large positive voltage on the grid, causing the tube to flash.

Regards,
-- Al

I think I see where Stevecham is going with his question - Amperex (Holland) small signal tubes will 'flash' when first powered up, and it's totally normal with those tubes.

On the other hand, I've had power output tubes 'flash' on me, blow a fuse, and test fine afterwards (all EL34's for what it's worth) but I've learned to think of my tube tester as a rough way to weed out obviously failed tubes, not a final arbiter of good tube vs. bad tube. A lot of my tubes have tested well, but were noisy/fuzzy or microphonic or had less output than a tube used in the opposite channel that tested exactly the same.

Have you seen those old Upscale Audio ads that say "we don't use your amp as a tube tester!"? Don't use yours as one! If an output tube flashed, don't risk putting it back in your amp, regardless of what your tester is telling you.

Just my opinion, of course. Good luck!

Do tubes come already gettered?
Would the flash be the last oxygen in the tube being consumed?

If this happened on a tube in-service, I'd remove it from the circuit.

don't use your amp as a tube tester?! how COULD you?

Thanks everyone very much for your comments. It was a power tube, coincidentally an EL34, as mentioned by Steveaustin. I did try the tapping test and saw no flicker in the testing lamp, and no, I did not put the tube back in the Amp after I tested it. I’ve replaced the tube and fuse and have 10-15 hours on the amp since replacing. So it does not appear to be a problem inside the amp, at least a problem that manifesteds itself right away.

Just to take it to the next level though, this is the fourth of eight EL34 Winged C tubes that has blown in the amps (they’re monoblocks) in the last two years. The tubes started blowing within the first 200 hours of usage. The tubes still have less than 1000 hours of service and all test in the good range on my tester. The tubes were purchased from a very reputable dealer. The tubes were all matched. The amps are Conrad Johnson Premier 12’s. The tubes have blown in both amps. I’ve taken the amps back to Conrad Johnson twice for testing (I live in the area). Each time they say no problems with the amps, just a bad tube.

Anyone have any ideas?