KT-90 query

Tubes Asylum

Questions about tubes and gear that glows. FAQ

I use my pre-NATO KT90 in triode mode. The B+ is 460V and bias current is 45-50 mA (each tube) - that is under 1/2 of the rated disspisation.
However, one of the tubes will glow OCCASIONALLY - once every 2- 3 weeks. It's NOT the plate that glows, it's something behind the plate, and I suspect it's the screen grid. The glow is not red-hot, just slightly reddish. The bias current would have gone above 50 mA. This usually happens immediately after I turn on the amp. I could always hear something abnormal (like metal expanding) when I put my ear to the speaker.
On turning the amp off for about awhile, and power it up again, everything goes back to normal.
Since it always happens with the same tube, I presume that tube is a bit faulty - but not to the extent of being unusable.
My question is: what could have caused that? Perhaps Jim McShane has an answer.

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Topic - KT-90 query - ianm0 19:07:33 12/11/06 (0)

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