Something I always wondered
 

Why are some tubes painted gray or black inside on the glass?
I never understood the reasoning behind this. Not to confuse with black plates, I realize black plates tend to handle more heat unless I am wrong about that.
But a lot of old tubes I have had would have half the top darked out so you couldn't see it glow or the internals.
Is there a purpose for this other than annoying me when I want to see them glow and look at their internal structure?
On this note.. I absolutely hate metal enclosed tubes.

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.

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

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

jr

BTW Some tubes were colored blue.

The capacitance between the inside and outside coatings of the CRT, is what filters the high voltage. The coatings serving as capacitor plates and the glass of the CRT as the insulator. This is why newer TVs with aquadag don't need doorknob capacitors to filter the high voltage. It's also why the aquadag needs to be grounded.

Wow, nice tidbit Max. Something I never knew....

Of course, I didn't answer the actual question. The grey coating inside of tubes is RF shielding, it's used on power tubes which were designed to be used at RF frequencies. Some audio tubes like the 6V6GT have it because they were dual rates for RF use too.

I agree it does tend to make tubes a bit uglier, but even the famous GEC KT66 has it

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.

Neat. Guess that must be the answer then and that makes sense. I can't help but think I remember seeing a few tubes that were not RF related that had that coating too though :scratch2:

Wouldn't the coating have to be grounded then? I have never seen a ground connection to the paint on the inside of a glass 6V6... the metal versions *did* tie the metal shell to pin one so that it could be grounded, but the glass versions list pin one as "nc". :scratch2:

jr