Basic tube amp question - Why do some tubes have metal sleev

[luther]

Probably a really basic question, *but when I open up the back of my Marshall, *only one of four preamp tubes have a silver metal "sleeve" *around it. *On my Mesa they all have these sleeves. *What are these things? *Why does the Marshall only have one?

Luther

[bluenote]

your marshall had them at one time as well....they are basically for protection....ive heard someone say once that they help keep out interference......i dont think they do anything really

[luther]

hmmm, *I bought this Marshall Head new, *and pretty sure it only had the one sleeve, *but it has been in the hands of a tech once.

Is it possible Marshall thinks one particular preamp tube would need it and the others don't?

Luther

[Kremlin]

They're meant to block interference, but it isn't needed on all tubes, depending on the circuit, some tubes need them more than others. In my Mark IV, I think only one tube has it.