why top of my 2a3 glass envelop change color?

Tube DIY Asylum

Do It Yourself (DIY) paradise for tube and SET project builders.

I recently finished a 6sl7 //, dc 2a3 amp, it has been operating very stable at 250V, 60 ma, +/- < 2%. The output tubes are RCA blackplate, used, different production date code, measured over 85% though. After about 30 hrs , there is some color change in both tubes, the very top of the envelope, used to be clear, now it looks tinted, it is kind of like charcoal, greyish in color. Both tubes so far measured the same as before. I had used both tubes in my Sun Audio RC circuit for few hundred hrs before the switch.
Any idea why?
Thank you
Wal

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Parts Connexion

Topic - why top of my 2a3 glass envelop change color? - bouncy ball 05:43:37 05/20/12 (7)

RE: why top of my 2a3 glass envelop change color? - drlowmu 06:26:28 05/21/12 (0)

RE: why top of my 2a3 glass envelop change color? - Chris O 16:06:16 05/20/12 (5)

never over 2.5 voltage. - bouncy ball 18:48:31 05/20/12 (4)

RE: never over 2.5 voltage. - Neff 19:00:41 05/20/12 (3)

will triple check it again. thanks (nt) - bouncy ball 19:06:11 05/20/12 (2)

RE: will triple check it again. thanks (nt) - Neff 15:12:15 05/21/12 (1)

Yes, big Ouch! It did sound marvelous before I "pronounce" it. - bouncy ball 18:45:46 05/21/12 (0)

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