After replacing each, and re-biasing all 4 corners. I find that the V3 (front right) power tube bias will jump from .40 vdc to about 3.25 vdc, (and hum) after 5 minutes or so of operation.
This is consistent, in that if I let it set for a few minutes, and power it back on, all 4 tubes come up to normal bias – then after about 5 minutes the V3 tube bias jumps.. the other tubes stay about the same.
It is not a gradual change, it jumps!
I have rolled the tubes around , even changed to a different quad of power tubes and the issue stays with the V3 position.
Any thoughts on what I might check next?