[ohohyodafarted]

It is always possible that you have a leak somwhere at the glass/metal union, however the work that John Folsom and I have done with leak testing suggests that the more likely point of leakage is the weld that joins the front and rear sections of the crt. In that case it is not likely that frit glass would be of much use in sealing the leak. Frit glass is used to seal the leaks on the glass/metal or glass/glass joints.

There is also the possibility that your 21AX has a leak on one of the leads at the gun end, but a weld leak is more likely. By the time they started making 21AX crt's, the technique of bonding glass to metal cone crt components, was very well perfected. It is likely that there was a small weld leak in the tube before RCA rebuilt it. RCA probably saw silver getters on the dud, and figured the dud was air tight, and so they went ahead and rebuilt it. The leak is so small that even today your getters are still silver, but it only takes a very tiny leak to let enough air into a crt to destroy it. After RCA rebuilt your tube it likely worked fine for quite a while until enough air got in to prevent it from working.

John Folsom and I are in the process of getting our used helium leak detector up and running which will allow us to pinpoint where the vacuum leaks are. Then we will need to proceed with a process to repair weld leaks. Progress has been slow, so don't hold your breath. Hopefully some day we will be able to repair the leakers and have them rebuilt.