I found this today. Zenith TO G500

[Electronic M]

Originally Posted by zombie1210

Dark as in it looks like carbon. Like it is burned. The cracks at the base are pretty obvious, as well.

That sounds like the getter in the normal state it should be in. When a new tube is made a vacuum pump pumps out most of the air/gas in the tube but not all. So they take a strip of a highly reactive metal (like magnesium) vaporize it with high power RF which distributes it as a blotch on the inside of the glass (the RF also bakes the other tube elements to drive out gas/contaminants from them) to maximize its reactive surface area. That blotch then eats up any air/gas molecules that contact it preventing harmful gas buildup inside of the tube.

In short that carbon look is SUPPOSED to be there and is a GOOD thing/sign.