[old_tv_nut]

I got the new 5U4GBs from Tube Depot (new from JJ Elctronics, made in the Slovak Republic). They may have increased th B+ a few volts (hard to tell because of the loading effects of beam current) but have not fixed the brightness drift. Cathode voltage still drifts up about 10 volts during this long warmup.

Brightness still starts high and drifts lower slowly, and returns to too high after the set is off a while. Still wondering what can have this characteristic. Still a possibility of a resistor under the chassis where I can't get to it with the freeze spray?

Don't know - more head scratching is in order.

I tried running with the line voltage 5 volts higher to bring up the B+ some, but this doesn't seem to make a big difference in the drift.

I'm not running totally out of brightness control range, but have to set it fairly high after the set is warm. Running through a slide show, I just see the usual effects of less than 100% DC coupling.