growing bias

[bluemonk]

In my diy amp after a while one of the two old Philips 6V6 begins to grow up in bias measurement... I turn off the amp when it goes over 50mV (or mA)!
It doesn't seem to glow red but my tester says it wants burn. The other 6V6 stays steady. Swapping the tubes shows that the issue is in the tube not in the circuit (IMHO).
I used that tube for some time in another cathode biased project amp and didn't notice anything strange.
Ok most likely that tube is bad but... why that growing bias?

[Gtr_tech]

I grow my bias in a little secret room in the basement
But seriously folks.....sounds like the tube is at fault. I'd bet its been bounced around enough to knock the control grid out of alignment. Just get some new 6V's and you'll be hooked.

[Steve Conner]

It's thermal runaway, usually caused by gas in the tube, contamination of the grid with cathode material, etc. Fixed bias circuits are more vulnerable to it than cathode bias. So yes, get some new tubes.