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How much vacuum in vacuum tubes?
QUESTION: At work today, one of my coworkers was explaining that he thought
vacuum tubes made before 1950 often had a pretty poor vacuum in them,
something like 1E-5 Torr, while "modern" tubes often are closer to
1E-8 Torr.
Do any of you actually KNOW if this is at all true, or whether there
is anything one can say about variations in vacuum in vacuum tubes in
general?
Were there certain tube types that were pumped out better or worse
than others? Or was it completely random?
ANSWER: The vacuum in all these tubes is very high. Infact there is no air
in them. They heat the tube to just under the point where the glass
will start to go too soft. Then the air is sucked out with a high
vacuum pump. Then they allow in some argon gas, and revacuum it
again. The inlet of the glass envelope is then sealed, and tube is
allowed to cool down.
Accross the heater is a thin magnesium wire. When the heater is
first fired up, the magnesium wire burns up, thus burning out any
air molecules that are left. This is the reason for the silver
surface you see on part of the glass of the tube. Whatever
atmosphere is left in the tube, it is in the parts per million.
If the vacuum is not high enough the tube will never work right. It
is a very high vacuum. Even tubes made in the 1920's were of very
high vacuum. The basic technique of doing this is about the same as
back in the mid to late 1800's when they were learning the consept
of vacuum tube effects.
Actually, the "getter" - the silver spot on most vacuum tubes isn't from
magnesium but from some other active metal or combination of metals. They
also aren't usually across the heater but in a compound contained in a ring
or rectangular shaped structure which can be seed under the silver spot.
A powerful RF source is then used to heat the ring or whatever and decompose
the compound forming the spot. I suppose some might have used magnesium
in the manner you describe but I doubt it was commmon. When magnesium burns,
it forms a white powder of magnesium oxide. The silver spot is pure metal.
I agree that even vacuum tubes made in the early part of the 20th Century
had better vacuums than 10E-5 Torr though I don't know precisely what it
was.
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