Tube coating

[Valvehead]

Correct me if im wrong here-- i believe that there should be a shiny metallic coating on the top of a power tube (i dont know the theory behind it)--but every one ive seen has this "chrome" looking coating.

Look at this pic--this is a new set of GT 5881.

Or am i wrong ?

[tubeswell]

depends how they vacuum-seal the tubes at the factory. The method that leaves a silvery residue is some chemical (can't remember what) that they use to help evacuate the air before they seal the tube.

[Rob Mercure]

The material you are referring to is a "getter" and all tubes, as best I know, have one, not just power output tubes ("power" doen't only refer to output tubes - the rectifier is also a "power" tube). This material absorbs stray gasses left over from when the tube was evacuated or released from the elements when the tube is heated. When the tube leaks vacuum and the atmosphere gets inside the getter will turn milky white - when it's good it's either black or shiny or occasionaly brownish and not necessarily on the top of the tube.

Rob

[Enzo]

And your getter is right there in the photo on the righthand side of each tube. Many of them have it on the top, but the side is also perfectly legitimate.

[jrfrond]

The "getter flash" you describe could be anywhere in the tube, top, side, or near the bottom, depending on the design. On some tubes with octal bases, it could be buried below the base.

[Valvehead]

ok, thanks guys