Amperex tubes cutting out

[praga]

My lite II plays like a dream with JJs and old Telsa Hex El84s, but I've tried two different pairs of Amperex, from completely different pulls no less, and both either start making weird fizzy noise or fade out completely after about 5 or 10 minutes of very soft playing. Let 'em cool and I can repeat the routine. Is this what it sounds like when tubes are over-biased?

[lastwinj]

yes, or they are tired tubes.

germ