[ohohyodafarted]

"I would like to ask someone who is more knowledgeable than me, how a tube can still have enough vacuum for the heaters not to burn open, but yet have enough gas inside to fail to work....Shouldn't the getther be able to deal with the gas? "

The kind of leak we see in must gassy crt's is not a catastrophic leak, but one that took perhaps 50 years to get to the point where there are enough air molecules in the tube to interfere with the flow of electrons.

Through our experiments with our Helium leak detector, we have terermined that the leaks almost always are caused by a microscopic defect somewhere on the weld, where the front and rear sections of the envelope have been weleded together. We also discovered that something as simple as oil from you fingers can temproarly seal the incredibly small leak.

If left long enough, (maybe another 50 years) it is possible that eventually enough air will infiltrate the tube to enable the burning out of the filiments.

There is no way that we have yet discovered to repair a gassy tube, except to rebuild it. And unless you locate the leak and repair it, the likelyhood of the rebuild working is very low.

You have to remember that the higher the vacuum inside the tube is (10 -6 torr is where you need to be for a tube to function) the stronger vacuum is that is trying to suck the air into the tube. As the vacuum inside the tube degrades (say down to 10 -4 torr) the vacuum inside the tube is pulling less strong on the air outside the tube. So eventually you get to a point where the vacuum inside the tube is not pulling hard enough to suck air into the tube through the microscopic leak in the weld and you reach an equibrilium and no more air enters the tube. At that point you are left with a tube that has too much air inside it to function, however there is still a significent vacuum inside the tube. Enough to still make it implode if struck.

In our experiments with the 15GP22 I probably removed the gun assemblies from at least a dozen tubes. Every one still had a huge amount of vacuum inside, but still the vacuum was nowhere good enough for the tube to function.

Getter flashes are only intended to clean up the remaining air molecules left inside the tube after you reach 10 -6 torr. That would be 1 molecule in 1,000,000. 10 -6 torr is 1/1,000,000 of an atmosphere.
A very good standard grade laboratory vacuum pump, like a Welch 1402, is capable of pulling a vacuum of .010 torr. A vacuum good enough to make a crt work needs to be 10,000 times better than a mechanical pump is capable of. To get a vacuum good enough to make a crt work, you need a diffusion pump or a turbo pump and the tube has to be elevated in temperature to around 600F to excite the gas molecules inside the tube to a point were they can be traped and pulled out of the tube by the diffusion or turbo pump over a period of several hours.

So given the kind of vacuum needed to make a crt work, if the leak took 50 years to polute the tube, you can see how incridebly small the actual leak is.

We are talking about a leak in a weld that is likely not much larger in size than an atom of Ozygen or Nitrogen. We use helium as the trace gas in the helium leak detector process, because the helium atom is many times smaller than an atom of Oxygen or Nitrogen (which is what composes the majority of gas in our atmosphere) and because the He atom is so small it can more easily get sucked through the microscopic leak so we can find the location of the actual leak.

Ideally the best course to fix a leaky tube would be, cut the old gun off, locate the leak on the weld using a Helium leak detector, figure a way to fix the leak (that is the big issue), then retest the tube with the He leak detector to make sure you fixed the leak, and lastly to install a new gun assembly and evacuate the tube and seal it off.

FYI we have already tried re-welding, and that only created more leaks than it fixed. We are presently thinking about either nickel plating over the weld, or using frit glass. But the project has come to a screeching hault because the only rebuilders left are located in France.