Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: g-man on April 25, 2014, 10:51:25 am
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The tube is probably fine.
This link has what I consider good info (https://www.tube-town.net/info/doc/dark-getter.html). A second source (http://tubesound.com/2008/09/30/brown-or-black-getter-flashing-does-not-mean-used/) has additional info and pics, but I think some of what they write could be misinterpreted.
Bottom-line, if the getter flash fades and disappears it means it is doing its job of recombining with stray gas molecules inside the tube. But a black getter is fine, and simply indicates a different process was used in the formation of the getter flash.
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You typically see black getter flashes on some Mullard and Philips (old stock) tubes. In some ways, it is considered the mark of a superior tube.
This is different from the "smoked glass" of some 50's tubes, or the solid black coating on modern Russian 6V6-alikes.
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Fluffy-white is bad.
Almost anything else is fine.
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Fluffy-white milky-white means the glass envelope is broken and gas (air) has entered the tube. Dead giveaway, do not pass go, end of tube. Not *all* tubes with broken glass envelopes exhibit the whiteness but those that do are trashed, done, no question about it. Save an octal base if you have a need for one, otherwise, into the trash.
If you are asking for the chemical explanation, there's no doubt it's a metal oxide of some sort. I tend to doubt it would be tungsten oxide; but I can't tell you for sure.
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I believe I know the answer to why fluffy-white is bad (WOx), , but could you explain why, please. (My guess could be wrong).
Look above at my post, follow the 2nd link. Scroll to the bottom picture. The tube on the right has lost its vacuum (glass is broken somewhere).
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What you say about filament composition is true; I don't know about the vapor pressure of WO2 vs pure W. However, I would ask, what is the difference between the tube as it is being made (fond memories of HS field trip to the RCA tube factory in Harrison, NJ circa 1970) and the tube after it has been evacuated and gettered? It's barium oxide, no? Gettering presumably turns the barium wool into barium oxide and *maybe* upon loss of vacuum, the oxide comes out of the barium and moves to the tungsten, but that makes little sense as we would expect barium to be more reactive (and thus hold the oxygen tighter) Maybe it exactly the opposite....the operation of the tube creates WOx and the loss of vacuum oxidizes whatever barium is left. Honestly don't know but that scenario seems more plausible. The formation of the white whatever it is, I think, is influenced by how quickly the tube envelope lost vacuum. It's an interesting question.
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The with fuzzy is usually associated with a crack in the tube. So as stated air is getting into the tube. The tungsten metal reacts with the oxygen in the air ...
Follow the links I posted and read the information. The getter flash itself is deposited barium vapor. The getter flash itself is what turns white. I presume then that barium oxide is white.
Nothing at all to do with the filament or tungsten.
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The getter in most glass tubes is a small cup or holder, containing a bit of a metal that reacts with oxygen strongly and absorbs it. (In most modern glass tubes, the getter metal is barium, which oxidizes VERY easily when it is pure.) When the tube is pumped out and sealed, the last step in processing is to "fire" the getter, producing a "getter flash" inside the tube envelope. That is the silvery patch you see on the inside of a glass tube. It is a guarantee that the tube has good vacuum. If the seal on the tube fails, the getter flash will turn white (because it turns into barium oxide). ( www.vacuumtubes.net (http://www.vacuumtubes.net) & Svetlana)