Tube flash

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?

Jaffeassc,

http://db.audioasylum.com/cgi/m.mpl?forum=tubes&n=225981&highlight=bad+EL34+Winged+C&r=

http://pages.prodigy.net/jimmcshane/

I'm not a CJ expert (in fact I've never owned one) but as I recall the CJ Premier 12 was originally designed around a 6550 tube, not an EL34. It also has a manual bias system where you turn a pot until a LED turns on. If I'm correct so far read on....

Amps of this type come with a bias arrangement that anticipates the use of certain tubes which would exclude others. For example many, if not most, will bias 6550's at around 50ma. But smaller tubes such as an EL34, E34L or 6l6 and 6l6 GC performs optimally at a much lower number, ranging from 25ma to 35ma (for example the latter for the E34L and the lower numbers for EL34s which would be around 30ma). This is general and amps/tubes will be different considering amps design and the extent that you select the bias and are willing to lower or increase bias to get the best tone, etc.

If your amp has a bias circuit which assumes 6550 type power tubes, as well as KT88s and KT90's, then you would be biasing your EL34's far to hot. This would cause early death, shorting, etc.

I do not know how you could circumvent this unless you could manually bias the tubes using a meter with real numbers or by getting CJ (or a good techie) to alter the bias arrangement so that when properly biased according to the LED your bias will match the requirements of smaller power tubes.

I hope that helps you a bit. I may be way off base.....:-)

PS

BTW when you read Jim McShane' post of AA (posted above by Jea48) be sure to read it in context, not just superficially by looking at the words 'bad' and the yellow highlighting. McShane has always spoken highly of SED EL34's and that has not changed, although he recently indicated he had recieved some negative comments on NEW SED 6550's which suggests a new quality control propblem.

A tube tester won't apply the full B+ voltage testing a tube. Even the top testers won't if the user doesn't know all the operating parameters of your amp.A lot of newer amps drive tubes a lot harder then they used to back in the tube only days. Vintage amps also drive tubes harder now do to the higher line(outlet)voltage than we had years back.That might explain why good NOS tubes may be failing sooner than they should be.If a tube does flash in a amp,I wouldn't want to reuse it even if it wasn't at fault,assuming a part in the amp failed causing the tube to flash.