I've run into a couple of cases where gassy tubes also had unusually high transconductance, but can't say for sure that such a relationship exists. What I can say is that tubes that measure way above their expected transconductance range often go boom IMLE.. I now usually toss them as suspect..
I will mention that I have had a couple of JJ KT88 that measured well above normal transconductance, and they failed spectacularly and catastrophically within hours of being put into use. The collateral damage took days to repair, and the odor of burned parts weeks to dissipate from my listening room..
Maybe try a little lower temperature than 177C if you want to keep the phenolic base on the KT88 from bubbling.. I might go to about 150C Max. - but usually just cook at 120C but then leave them in for a good 5-7 hours.
For valves without bases, I go with a higher temperature.
I also try cooking gassy valves more than once for best result - ie. cook -> let cool -> cook again. For some reason this seems to be more effective.
At atmospheric pressure. But at very low pressure in a thermionic tube it becomes ionized, and ionized gasses are good conductors. That is the principle of operation behind gas-filled rectifiers.
The problem with gas in a vacuum tube is that their ionization potentials (usually of air) are high enough to cause massive positive ion bombardment of the cathode, which will destroy it in short order. That assumes that the plate current doesn't run away first, or the cathode gets poisoned by oxygen, or the tube suffers a massive seal failure and goes atmospheric
Ok. My experience was from freeze drying cambers. We have a short circuit of 230 VAC at 1 mbar or below if the distance is too low. But in this case we don't have ionized gas
A good tube should be in the 1x10-7 torr or lower range. 10-7 to 10-8 is probably pretty typical for regular receiving tubes. This is equivalent to .000000133 millibars or less, if my math is correct
If the tube gets gassy enough you will see a purple to pinkish glow BETWEEN the internal elements, which is a bad sign. However, a tube can get gassy enough to affect performance without showing any gas glow. Blue glow on the interior of the plate and on the interior surface of the glass is not uncommon and is NOT due to gas. It is caused by random electrons striking the surface and causing fluorescence. In fact, a tube that is gassy enough won't make the glass fluoresce because too many electrons are lost from collisions with gas atoms before they strike the envelope.
In vacuum science, the average distance an electron can travel before striking an atom of gas is called the mean free path. In a good tube the mean free path is measured in KILOMETERS! Still, there are so many electrons travelling between the cathode and plate that there are millions of collisions per second in a hard tube (i.e. one with a very good vacuum that has a mean free path of 10+ kilometers). You can imagine how many collisions must happen in a gassy tube. Amazing stuff when you think about it.
EDIT: I'm pulling this info from memory - it's been a while since I have read the textbooks. If I got something wrong, please feel free to correct me
If the tube gets gassy enough you will see a purple to pinkish glow BETWEEN the internal elements, which is a bad sign. However, a tube can get gassy enough to affect performance without showing any gas glow. Blue glow on the interior of the plate and on the interior surface of the glass is not uncommon and is NOT due to gas. It is caused by random electrons striking the surface and causing fluorescence. In fact, a tube that is gassy enough won't make the glass fluoresce because too many electrons are lost from collisions with gas atoms before they strike the envelope.
In vacuum science, the average distance an electron can travel before striking an atom of gas is called the mean free path. In a good tube the mean free path is measured in KILOMETERS! Still, there are so many electrons travelling between the cathode and plate that there are millions of collisions per second in a hard tube (i.e. one with a very good vacuum that has a mean free path of 10+ kilometers). You can imagine how many collisions must happen in a gassy tube. Amazing stuff when you think about it.
EDIT: I'm pulling this info from memory - it's been a while since I have read the textbooks. If I got something wrong, please feel free to correct me
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What The Gimp said. In the heyday of tube making, and probably in factories still operating today, the diffusion pump with a cryo cap did the heavy work. With the best hydrocarbon oils and a chevron baffled trap chilled with liquid nitrogen they could pull the pumping manifold down to the 10-9 torr range. With modern silicone or polyphenyl ether oils a diffusion pump can do 10-10 torr with just a water cooled cold cap, and 10-11 with a cryo cap. Nowandays, turbomolecular pumps are used to get to such levels in semiconductor manufacturing mainly due to lower energy costs and easier cleanliness - since they dont have to worry about backstreaming oil like a diffusion pump they dont need cryo traps. Even higher vacuums can he had from ion pumps and such, but they are only used for research purposes due to low pumping speed.
Vacuums at such levels can be measured with thermocouple gauges, penning gauges, and ion gauges. At ultra-high vacuum levels (beyond what even tubes operate at) it gets very difficult to measure the pressure with ion gauges since there is so little gas left. I'm don't recall what techniques are used, but it's not really relevant to this discussion anyway High and ultra-high vacuum technology is a whole science in itself.
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