What can you tell about a power tube's condition when visually inspecting it?

[leydenjar]

I've read of people looking into power tubes and getting a sense of how much use they have had by some visually recognizable indication.

Is this true?

If so what are they looking for/at?

thanks,
leydenjar

[Justin Thomas]

I'm curious about this too. I don't claim to have any electrical "seer" powers, but there are a few things I can tell.

1. Lost vacuum - getter went white.
2. How much ABUSE they've had - flakes of white cathode material, glass splinters, and metal shrapnel rolling around inside. I'd guess more from bad handling.
3. "Shadows" on the glass envelope by holes in the plates & getters that have turned brown or black - tube's probably spent some time "on."

I've had several Ampegs and two Fenders with original or at least very old and well-used power (and preamplifier) tubes, and only two of those have been seriously "bad." Most of them show signs of heavy use and still work (and SOUND) fine! If there are any tricks, I'd like to know!

Justin

[Chuck H]

The only tubes that MIGHT demonstrate their age visually are those with heat sensitive paint. Having used Groove Tubes a bit, I've seen them darken with age though they've never been over heated. As a result it's easy to recognize old Groove Tubes. There are other manufacturers, some older ones too, that did this as well. But it's spotty at best. Some power tubes show tell tale browning on the glass near plate vents. But I've even found this to be spotty as far as recognizing a tubes useage or suitibility. And the only way to test a tubes sound is to listen to it!!! Anyone who claims they can look at a tube and tell if it will sound good or bad in a guitar amp is either blowing smoke up their own, or someone elses, arse, or they are utterly confused by a previous successful, or unsuccessful experience and are just plain wrong. It can work out very differently on mulriple occasions.

[R.G.]

I'm guessing now, but I guess that only the gross problems show - things like "if the glass is cracked or broken, it's probably not going to sound good." The items Justin mentions are good things to note, but except for things that make the tube completely non-functional, it's hard to say they're age indicators in terms of the sound.

Cathode emission is one indicator, but it's invisible. That's why the old drugstore tube testers were emission testers. This takes into account heater aging/temperature and oxide poisoning/exhaustion. Grid poisoning with cathode oxides is much harder to find and impossible to see. I don't know of any plate poisoning or aging mechanisms other than melting, but possibly they exist. Dark spots of plate metal deposited on the glass behind the plate are indicators that the plate ran hot enough to boil off some metal, but I think this corresponds to the plate operating temps more than hours of use.

All that being said, humans do have a remarkable ability to pull correlations out of very noisy data. The thing that comes to mind is chicken sexing. No, not what you think. Girl chickens are the economically valuable kind for eggs. Boy chickens are useful primarily for soup. So the poultry industry wants to know early on which chicks are girls and which are boys. The problem is, there is no reliable external difference at the age they're separated. Still, reliable experts can achieve over 90% accuracy on about a thousand chicks per hour for a full work day. Amazing, to me at least. See http://cogprints.org/3255/1/chicken.pdf

[leydenjar]

I guess a curve tracer is the best tool for "seeing" what a tube actually does under various operating conditions.

[Jazz P Bass]

And still it cannot be determined 'when' the tube will fail.
Ageing & loss of performance can be observed by montoring the output voltage.
(as compared to a known standard /ie: a good tube)

[Alex R]

If they've been redplating for a while, a kind of stain develops on the plate where the red patch was, quite marked and easy to spot, usually on and around one of the inside folds of metal.

Getter marks on the inside of the glass will usually be silvery-black, and can appear outside holes in the plate in good tubes. White=no vacuum, as has been said. If a tube has been arcing you will somtimes see a trace of a black deposit on the glass separate from the getter marks.

[Prairie Dawg]

If the tube has a lot of hours on it sometimes the edges of the getter flash will turn sort of a dirty brown.

[Gtr_tech]

Output tubes with high hours will usually show "erosion" of the getter flash. What was a defined shiny patch will turn brown at the edges and if you hold it up to a light, you will be able to see thru it.

http://i.ebayimg.com/t/Vintage-BOGEN-Sylvania-7868-Vacuum-Tube-USA-Top-Halo-Getter-/00/s/MTAyNFg2ODI=/$T2eC16F,!zUE9s38-PG+BQGcQgpNo!~~60_35.JPG