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Originally Posted by andy
I thought the whole problem with the 15GP22 was that it didn't use a frit seal. I thought they attached a metal ring to the bell and another to the screen, then welded the two metal pieces together after the screen was coated with phosphor. Could a bead of frit be added around the two metal to glass seals to seal and strenghten the CRT?
Does anyone know how much air it takes to ruin a CRT?
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Yes, that problem is characteristic of all vacuum technology, in general. But, the 15PG22 is a bit of an exception because it so happens that there's a lot of iron inside this tube. 15GP22 would have been an outgassing nightmare for RCA during tube manufacture. For a rebuild there should be much less water vapor to pump out - if the tube is not initially up to air and the rebuilder is clean and pre-baked all parts in dry nitrogen gas prior to installation into the old 15GP22. The more residual gas that's removed the less electron scattering occurs from the gun assembly when cathodes reach emission temperature. Less scattering effectively means somewhat lower emission temperatures are required to create a space charge around the cathode.
The real question to ask is:
How much residual gas pressure does it take to ruin a CRT, i.e. make the tube too dim to watch?
Well, it seems to me that the question depends on both electron gun efficiency and ultor voltage field intensity accelerating the electrons towards the phosphor screen. Bright pictures require more energetic electrons, and more of them, with high kinetic energy to produce more photons as a result of slamming into the phosphor dots at high velocity, hence giving us a brighter picture. As pressure inside a 15GP22 rises to 10-6 torr the rate at which residual oxygen molecules inside the tube strikes the phosphor dot plate rises to 3.6 x 10 to the 11th power (molecules per square centimeter). That's a hell of a lot of garbage in the way of the electron stream trying to hit a group of phosphor dots while being scanned at horizontal line rates! As the pressure inside the tube gets higher towards the 10-5 torr range the picture begins to noticeably lose brightness. Above 10-4 torr the picture starts to get really dim. At higher pressure less than 10-3 torr you can expect the tube to arc over inside - especially when the ultor sits above 20KV. Now, the getter should be turning white too! Also, you don't have to be at room air pressure to internally arc the tube.
Let’s put pressure into perspective: a new two-stage Edwards EM8 rotary pump (pretty common in most labs) after an oil change should reach 10-3 torr after 24 hours of continuous outgassing. To reach lower pressures requires either a diffusion pump or turbo molecular pump on the system to reach up to 10-6 torr to 10-7 torr range or maybe less (if a turbo pump). To get pressures lower takes either ION pumps or cryogenic pumps to reach the 10-9 range. Passive chemi-absorption techniques - hence the use of a 'getter' inside vacuum tubes can get things down to less 10-9 if the tubes seals are really good and keep it there.
I recall one interesting incident as a graduate student; I once cranked the voltage up on an electron detector in a mass spectrometer quite high one day - just to see what would happen. Even at less than 10-9 torr I was able to strip enough electrons off a detector plate once it reached 40KV to create a massive arc inside the vacuum chamber! Ka-Boom!!! The SS chamber didn't implode - just made a heck of a loud crack ...I jumped, probably 5 feet and ducker under a table. Yet, it always amazes me how modern large CRTs can work at 35KV or higher and not cause severe ionization inside a tube over time - pressures must be very low and kept very stable inside these tubes - a definite a challenge for old tubes! ...of yes, I forgot to mention the 40KV supply I used could deliver over 100 milliamps of current ...several orders of magnitude higher than a TV chassis!!!
Tom