Lynch Box Power Tube Redplates

Lynch box has 3 Tungsol 12ax7 preamp tubes and 2 EH 6L6 power tubes in it.
Power tube closest to phase inverter red plates once in a while, and man can I hear it thru the speaker when it does. Other power tube is fine. Bias set at 30mv. Should I try new tubes? Are the Tungsol preamp tubes too high gain? Or something else going down?

Thanks.

First, swap the two 6L6s. If now, the one that used to be closest to the PI redplates in the other socket, then it's the tube. If the one closest to the PI redplates then, start looking for a bad screen grid resistor ( 1k 5 watt)
as the first cause. Of course, try different tubes. Another thing you may try is a 5751 in the PI position. Another possibility is (a) leaky coupling cap (s)
between the PI and the output tube(s).

Of course, I flip flopped the tubes and the amp worked fine. I guess I will have to just wait it out and see what happens. Thanks.

.....the waiting is the hardest part.....