Is the inside of this tube bad, or is it just typical greyish coating?

see pics, tube is an old sylvania 6X5GT. Tested OK. Seems to run OK.

That is called Vapor Deposition.

It is what happens when metal is heated and sublimates - those molecules are deposited on all surfaces randomly. On glass you see the grey coating. This is a very few molecules at a time landing on the glass and staying.

Sublimation is when a solid goes directly to a gas without an intermediate liquid state. When 'getters' are flashed, the shiny coating you see is a much heavier example of vapor-deposition.

Under controlled conditions, vapor deposition is used to make everything from nano-tubes to jewelry to electronic circuits. Neat stuff.

Is VD (no, not that V.D. deepee) a sign of aging or being past its prime? Or is it just normal wear and tear.

arledgsc wrote:Is VD (no, not that V.D. deepee) a sign of aging or being past its prime?  Or is it just normal wear and tear.  

It is not really anything but age and use. As long as the tube functions, there is no cause for alarm as the tube turns black. Note that the same thing happens with any filament-based electric device from incandescent lamps to fluorescent lamps - which have tiny heaters (filaments) at each end. They blacken from use as they age.

Looking at the bottom metal strip, there is clearly a coating of some kind, how is that possible?

jfine wrote:Looking at the bottom metal strip, there is clearly a coating of some kind, how is that possible?

Between decayed getter material, filament outgassing, plate outgassing and such, there is relatively a lot of material, all of which drops down to the bottom of the tube as it cools.

Jim McShane would know, he's the expert, forgot about him.

I heard my ears burning, arledgsc. Get a Bendix JAN 5852 (they go NOS anywhere from about $40 to $100) and turn your back on rectifier worries. Your downstream tubes, because of its very slow and gradual start-up, with thank you, too. Straight drop-in for any 6X5. Outlive your grandchildren. Wish they'd made output tubes. And correct me if I'm wrong: rectifiers either work or they don't, kinda like a light-bulb. In my experience, there's no decay in performance.