Tube Question-Help

The silver is called the "getter", which I believe is Magnesium. It is fired after the tube is sealed to burn off any Oxygen remaining in the tube envelope. There is a little metal halo in the tube where the magnesium was before it was fired. After the firing, the magnesium leaves this silver area which has no further use and is of no concern. Don

I agree that what you are seeing is the getter, but not that
once it is fired it has no futher use. I am in R&D in vacume tubes ( very high voltage X-Ray tubes ) and the getter keeps working through the tubes life. As you use the tube it will
burn metal and imperfections off the plates and form gasses
that the getter sucks up like a spunge for lack of a better way of simply putting it. As the getter is used up it will fade. Your tube may test good but is on its way out. Rember you can have a tube that test good and does not preform well. If there is any question as to the tube's integrity
go buy what you hear not how the tube test like. For this reason I don't buy used tubes. Hope this helps. RG Knaak