[TV MAN]

Does anyone know why some tubes have a blackish coating inside on the glass? I have seen this on a lot of output tubes, such as the 50L6GT & 6V6G/GT. Some rectifiers also. Is this some type of shielding?

I am also curious about the "getters" and the coating around the getter in the glass. Some are D getters, some are round, some have silver coating, some bronzish color. I know what the getter does, just wonder what it is, etc. I have also seen some output tubes 6BQ5, 6LQ6, ...that look 'burned' on the glass, kind of like an impression of the plate and supports, like there may have been some kind of flash inside at some time, is usually a darkish color. They usually test fine. I havent read anywhere in any of my old books where it explains this. Just curious about this.

[morzh]

No. This coating is the remnants of the getter. Getter is the tablet they put in to absorb the remnants of gasses after the tube is evacuated and sealed. Then it is heated and the getter absorbs gasses and deposits on the envelope creating that coating.

[Radioroslyn]

Hummmm Tubes of color! Brownish color on the inside of the glass is an indication that it was run hard and put away wet. Don't see it much in receiving tube but in power tube like rectifiers, horizontal output, and RF output type transmitting tubes.
White on the inside indicates there's air inside. This tube is bad.
Terry

[Joe Rossi]

I have seen some older tubes with a 'battleship grey' glass that is very symmetrical. Was there a reason for coating tubes in that way, or just to make 'em look pretty?

[TA Forbes]

If your tube-tester has a "Life Test" feature, this is a good way to see if the tube you have with "deposits" on the interior of the glass envelope is on the way to being bad.

[TA Forbes]

Which is totally different than finding a "bad" woman.

[morzh]

Is there a tester that could test a woman and tell that she is on her way to being bad?
I would certainly pay more for it than I did for my Hickok 600A.

[TA Forbes]

Yes, or better yet a naughty one.

[morzh]

Oh.....I can feel naughty....it is there all the time. But bad is unpredictable.

[Eric T]

A "good" woman can be pretty darned naughty. Don't ask me how I know.

[TV MAN]

Bad women - know what you mean! I'll do an Ebay search and see if there is any tester for that If they can make dogs talk ... well, you know...

Here is a shot of some tubes like I am talking about. I have seen quite a few like this, but most seem to test ok. These are sweep tubes from a color tv.
   

[Mondial]

The darkened glass around the plate structure is due to electron bombardment. Extra electrons which are not attracted to the plate hit the glass and eventually darken it. It indicates the tube has had many hours of use, but does not necessarily indicate the tube is weak or bad.

The getter spot is on the side under the 6DK6 marking. The edges of the deposit are beginning to lighten, which shows that some gas has been liberated and absorbed. When the spot becomes completely light gray or white, then the getter material has absorbed all the gas it can and the tube is bad.