[Boatanchors] tube question

[bonddaleena at aol.com]

Hi. I have what may be a 'dumb question', but I'd like to be able to learn more about vacuum tubes.

I have several pieces of HP test equipment, but the one that sees the most use, is my HP 606A. Lately, the Modulation and RF output pots have become flaky. I have had this jewel a LONG time, and it has been moved a LOT. I was tired of looking at it's ugly cover, so this week I pulled it apart and did a restoration. While it was apart, I tested all the tubes. Man, there's a bunch in there!
Anyway, I found one tube with a 'short', but what I am asking concerns the result of  'Leakage' testing.

I have several tube testers including a nice Weston Transconductance tester. However, my Sencore TC-142 gets the most work. I realize this is pretty much a 'go - no go' tester, but here's my question:

Testing several of the tubes, the 'emissions' test was fine, but when switching to 'leakage', some of the tubes showed very high leakage, initially. However, the longer I left the tube in the tester, the leakage came down to an acceptable level.
When testing a NOS spare of the same #, switching from 'emission' to 'leakage', immediately, shows ZERO leakage.

Is this a situation where the grid is contaminated?

The generator is a remarkable piece of Engineering. I could NOT imagine what it would cost in today's $ to build a tool like this, castings and all!

As with all my tube equipment, I installed two, small muffin fans to move the air around......


thank you in advance!

ron
N4UE