Quote History Thanks for posting up the pics. Will copy here what I emailed you and add a bit more detail.
This EP was on the input side of the tube and was particularly stubborn. In my limited sample size, screen side EPs are the most common. Root cause is typically the aluminized backing layer behind the phosphor peeling or otherwise having a defect that then concentrates the field gradient. Input side EPs, be they on the photocathode or MCP input side, are a different animal and are caused by a microscopic and invisible defect that similarly concentrates the field gradient. On this tube, the aluminized layer and phosphor coating on the screen were burned away due to the EP. Could see thru the tube under illumination. Under magnification, I could see a tiny spec of "something" on the photocathode that was causing the EP. Never seen FOD inside a tube before though there's no reason why it couldn't happen. Potentially possible the debris got dislodged during the life of the tube and the tube left the factory with a clean bill of health.
The EP was contributing to a background glow that obscured fine detail in really low light conditions. Before being zapped, I could only see down to Group/Element 2/6 on the TS-4348 test set using the Gen 2 setting (approx. 7 line pairs per mm resolution) under low light conditions. After treatment, it improved to 3/3 (approx. 10lp/mm). A filmless L3 WP tube sees down to 3/6 (approx. 14lp/mm) tho on the somewhat dimmer Gen 3 setting. Not the most scientific method but at least there's something to back up the change. Comparing apples to apples, your tube has half the resolution of the L3 tube (34 SNR, 0.6 EBI) on the same Gen 3 low light setting. Not bad for Gen 2. Overall, to my eye, the low light view is far more pleasant and less distracting without the bright EP. The resultant blem, is unfortunately, much larger than I would have liked or would have hoped for. Had to zap the tube more than once to see any change. That didn't help keeping the blem to a minimum.
High light performance of the tube has been good and did not degrade. Its resolution is lower than a 64lp/mm tube. Detectable by eye with a side by side comparison. The Ekran datasheet for this tube seems to imply that the minimum resolution is 43lp/mm. On the high light setting, I can resolve down to 5/3 on the test set with the L3 tube (approx. 40lp/mm). This is the resolution of the entire optical system and will always be lower than the resolution of the tube. The 40lp/mm is in line with the theoretical resolution of a PVS-14, as a system. Your Gen 2 tube can resolve down to 4/6 (approx. 29lp/mm). With optical elements being identical, scaling the results against a 64lp/mm tube gives an approximation of relative resolution (more to it, not the best approximation). I'm getting about 45lp/mm for your tube, something that I'm inclined to believe.
One detail I have been unable to verify yet is whether or not your tube is autogated. I could not measure if it's autogated but the method used depends on the power supply being of the same type as used by American tubes, something that may not be true. In operation, the tube seems to be autogated, on two counts. The (very) high light resolution is not meaningfully degraded and is higher than the very high light resolution of a nongated Omni 4 tube. The Gen 2 photocathode also does not leave streaks when panning past bright lights, something I've only seen with a custom autogated Gen 2 tube. Inclined to say it may be gated.