This has been bugging me for some time so I just have to ask. The tubes in my amp have a shiny silver/grey coating on the inside of the tube just on the top. What is that? Reason I ask is 3 of my tubes have that in a pretty uniform manner, but the 4th has hardly any at all and I was wondering if that tube may have a problem, maybe not now but on down the line someday?
Stupid Question Time Again
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They are called getter, http://en.wikipedia.org/wiki/Getter
Bigger vacuum tubes have them on top and sides like the old Tung Sol 6550s.
Problem is, no one seems to be consistent. In that very article, there is a contradiction between whether the getter is the silver metal ring or whether it's the deposited "flash" on the inside of the tube (see the labeled photo in the article). ;-)
I typically call the deposited material on the inside of the tube the getter "flash," while the square, circular, or other antenna looking device is the "getter" itself. Whether that's correct or not, I don't know - just as with the Wikipedia article you cite, there are varying opinions over which is which.
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My understanding is they are both getters. The antenna thingy has different shapes, halo, horseshoe, D. I believe they complement each other in their purpose.
Look at the side getters on these Tung Sol 6550s. They have the flash getter and the antenna on both sides.
These are vintage Tung Sol 6550s were made in the USA. The reissues from Russia don't have these antennas on the top sides. Guess which sounds better?
Just an FYI, but the deposition material is placed on the antenna thingies - that's what "flashes" the chemical to the inside of the tube when it's shot with the induction heating at the factory. I'm going to stick with calling the antenna thingies the "getters" and the chemical on the inside of the tube the "getter flash." I've been doing it for years and no one's objected. It bothers me when I see articles that describe just the opposite, though.