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Hot spots on 6V6 plates

Posted by Alpha Al on June 8, 2017 at 10:19:40:

This is a Fender Deluxe Reverb amp with new TungSol 6V6GT's.
In the photo, the red spots look worse than they actually are because of the time exposure. It's a dull red glow.
The tubes are within their dissipation limits. Plate/screen voltage is around 450V, and in the photo they are pulling 24mA each. Negative
bias is -40V.
I have to drop plate current down to 12 mA to make the hot spots disappear.
It looks to me like the electron beam is being concentrated on this one area of the plates. The tubes test normal.
Any ideas?