On Tue, 10 May 2005 09:29:44 GMT, Patrick Turner
wrote:

I had one failure of a sovtek EL84 a year ago.
But that was a rare failure.
But what is the muddy blackening that is seen in EL84 tubes, both sovtek and
NOS?

Sorry, I don't know. Always assumed that it came from the hot bits,
on the theory that anything from the plates would be drawn to the
cathode by their charge.

Mullards are famous for being muddy brand spanking new. I hope
some metalurgist will see this and shine some light.

Chris Hornbeck