[Amps] Another GassyTube Question

[Scott Manthe]

Hi to the List,
I just bought an Ameritron AL-80A, with what I was told was a NOS Eimac 
3-500Z (with an 89 date code. I inspected the tube before putting it in the 
amp and it looked very nice- the glass was completely clear, with no brown 
deposits on the inside, so I have no reason to disbelieve the fellow from 
whom I bough the amp. Knowing this was an older tube, before applying RF to 
the amp, I let it run for a few hours in an effort to degas the tube. I've 
since run it a few times and it seems to work well- I'm getting more than 
700 watts output on all bands with 45-50 watts drive. However, when tuning 
it up on 15 meters yesterday, I noticed a blueish glow on the inside of the 
tube towards the plate choke. The blueish glow is definitely on the glass 
inside the tube and only on the side of the tube toward the plate choke. Is 
this something I need to be concerned about? Should I pick up another, 
newer, 3-500z(g)?

I almost hate to post this to the group, because it seems like something I 
should already know, but I have little experience with large envelope glass 
tubes- all my amps have been fed by small tubes, like the 811A or 572B.

Any help with this would be much appreciated!

73 to all,
Scott, N9AA