[ohohyodafarted]

Tom,
Even if no leaks were found, When you finally put the tube under vacuum and heat, this is the point where the leaks are created. The heat and physical vacuum stress, causes weak areas (usually in the glass) where micro cracks form at the juncture of the glass and the ultor ring.

It is an easy matter to find out if the envelope is leak free before evacuating. The leaks are created during the evacuation process. Even if you could test for a leak during evacuation during pump down, prior to pinch off, What good would it do? You could shut down, but you still have a new gun installed in a envelope that has a leak. I guess you might be able to salvage the gun and stem assembly by cutting it of and possibly re-using it on another tube. But we are better off finding a method of reliably sealing the envelope. Without a method of reliably sealing the envelope so that it doesn't leak during evacuation, attempting rebuilds of 15G's is pointless and we are just crossing our fingers that maybe 1 in 10 attempts might yield a good tube.

The one tube that RACS was successful with was a tube that did not have any leaks to begin with. It was pure luck that It did not develop any new leaks during the evacuation. It was just pure dumb luck. But it does demonstrate that it can be done under the right circumstances. RCA was rebuilding duds that were not leakers.

That's why we are hoping that a long, slow, low temperature pump down, combined with a special high temp epoxy that we have worked with, may help to lessen the possibility of causing new leaks in an envelope that is leak free prior to evacuation. It may even be able to seal micro fractures that exist in tubes that have a slow leak.

The epoxy would be applied in a manner similar to what RACS did with frit glass in France. However their attempts to seal with the frit were utter failures. IMHO frit was not the correct approach. Frit is designed to be placed between 2 precission ground glass surfaces. RACS thought it might work by slathering it on the outside of the tube. Frit was never meant to be used that way.

And there is no assurance that we would have any better luck with a low temp pump down and the special high temp epoxy. We will never know unless we are able to do some more experiments. Perhaps some day when Nick gets his crt rebuilding facility up and running.

As for me, until I am free of the responsibility of caring for my 102 year old mother, I rarely have any time for pursuing my hobby. I have care giving responsibilities 7 days/week, 365 days/year.