Something I always wondered

[Eric H]

That's not paint, it's called a Getter and it help maintain a good vacuum inside the tube.

See this Wiki article:
https://en.wikipedia.org/wiki/Getter

There are some tubes that have some type of coating on the inside, like the 6V6 often has a gray coating over most of the inside surface, I don't know what the purpose of that was.

[dtvmcdonald]

Didn't some tubes have Aquadag on the inside, like CRTs? Presumably
for shielding.

[jr_tech]

Possibly to aid in heat transfer from the plate to the glass and beyond?

jr

[darklife]

Originally Posted by Eric H

There are some tubes that have some type of coating on the inside, like the 6V6 often has a gray coating over most of the inside surface, I don't know what the purpose of that was.

That is what I am talking about. Like the gray coating inside this tube for example...
https://www.tablix.org/~avian/blog/a...s/img_3788.jpg

I have a lot of early tubes, and like you mentioned a few 6V6 tubes that have that same gray coloring all around the tube so you can't see inside of it easily.
By the way I know what the getter and silver coatings look like, but yes this is different.