Thermionic emission varies with the square of the temperature. An unregulated power supply for the filaments and/or substantial changes in ambient temperature will alter the filament temperature and therefore the cathode temperature. The cure is negative feedback in the amplifier signal circuit which works to stabalize gain. One of the many negative consequences of not using feedback is that the circuit quiescent operating point will drift around forever. Where stability is critical, a regulated dc power supply to heat the filaments and supply the electrode voltages and an enclosure to stabalize temperature can help so long as the tube does not overheat. If the tubes are not power output tubes, tube shields help too.
How are they drifting? I had a couple of 6BQ6s where the plate current would not settle down, but kept creeping higher and right into red plate territory. That turned out to be due to excessive gas. A tube tester confirmed that, even if the blue glow wasn't yet visible.
If that's what's happening here, suspect gassy tubes. If it's just a case of the plate current not staying put, it's probably normal, and they should settle down after running 'em awhile. That could take a few hours to a few days.
Personally,I wouldn't leave it running unsupervised, at least until it calms down, just in case anything 'funny' happens.
But,it's your call.
Ray, my 6lr6 tubes took 10hrs to stabilize and they wanted a hefty - bias, I had to add a couple 6.3 volt windings to get the neg bias low enough. One tube wanted -65 so I put it aside.... I'm running 160V on G2, just the draw of the zener stack. I also use it to power the nixie tube. I run 40ma through the output tubes..
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