[Boatanchors] tube question

[bonddaleena at aol.com]

Hi, I think I knew the answer to this question, but old age must be catching up to me!

I am restoring an older tube-type VHF (FM) receiver. The kind that covers both VHF low and high. It was made by Sonar and during clean up, I ran the tubes through my Sencore tube tester. (I have several testers, but this is my 'go to' tester.

I found 2 'bad' tubes, which I need help understanding. The tubes are a 6BE8, which is a High Mu Triode - Sharp Cutoff Pentode.
The other is a 6KE8 which is also a dual device (Medium Mu - Sharp Cutoff Pentode).
Anyway, when testing the tubes, one section of both tubes tests fine for emission (I know how that part works) and NO Leakage.
However, when I set the tester to test the other 1/2 of each tube, the emission is fine, but the 'grid leakage' starts off low (not zero) and slowly climbs to the extreme right of the meter.

I'm trying to understand 'what' is being tested and how 1/2 of each tube can be OK, and the other 1/2 'leaky'.
It's NOT my tester, because I found a new 6KE8 in my stash, and both 1/2s test fine.......

Excuse my ignorance....

ron
N4UE