:-)
It's been an eye-opener for me.
You hear rumours about variability in Russian tubes but I haven't seen it directly myself before.
The bad tube clearly has a perfect vacuum, the heater takes exactly 3A as expected, there are no internal shorts measured and it's perfectly happy at 4kV with no flashovers. I don't have a HiPot tester but I'd bet it passes.
...and yet, it's absolutely useless!
Lesson learned.
