[Boatanchors] 539 tube tester problem

[Yongsurk Lee, HL1FB]

This is HL1FB/WW6N in Seoul, Korea. I have been repairing tube equipments
here in Korea for many years as a hobby. I successfully repaired about 200
TV-7s, TV-2s and some Hickok tube testers.
I had a Hickok 539A and a 539B which I could not find the exact cause of
the problem. When I test tubes like 6L6, the test values increase as time
goes by. Right after turn on it may indicates 6000 but after 30 minutes it
goes up to 8000, then 9000.....
I tried 50VAC and 10k ohm in series and applied between plate and cathode
at different ranges. They all indicate correct values with no increase with
time. Grid signal voltage is contant with time.
Do you have some idea what can be problem?