I make small vacuum tubes for a living. I am a production engineer in a glass tube line. We produce about 50,000 tubes per year. Our vacuum requirements far exceed anything ever required for television tube evacuation so I am quite intimately familiar with processes related to tube production and evacuation. I also worked indirectly with Universal Video, a Minneapolis color tube rebuilder, long since extinct.

My comment is, yes, the "frit" material is a glassy material that, in later tubes, long after the 15EGP22, was used to seal the faceplate to the funnel. This is STILL the preferred method for big tubes. Smaller tubes like the 5 inch monochrome tubes can be blow molded in one piece, with neck and tube entry chamfer molded in. But big tubes generally need to have the funnel molded, the neck added, and then the mask installed. Since the mid 1950's, the lighthouse method of patterning the phosphor dots via a light source at the gun position, through the shadow mask, to the photoresist on the face plate. This faceplate was then forever wedded to this funnel. Once the phosphors, and the metallization over the phosphor (aluminum) were depositied, the faceplate was fused to the funnel with the frit. After this process, the gun was installed into the other end in place of the light source.

Then the entire thing was evacuated to somewhere in the range of 1E-7 Torr, baked to temperatures of about 300C or so, induction fire of the gun and getter, and sealed. Then, of course, a run in process was carried out, including activation of the cathode.

Replacing the seal on a 15 EGP22 with frit migh not be a walk in the park, however. The nature of the mating joints where the adhesive or frit went was not very forgiving in terms of texture, area, degree of parallelism and many other "trade secrets".

While the phosphor on these old tubes does deteriorate, I would submit that a simple cathode replacement might make most tubes quite acceptable in performance. There once was a fellow in Pennsylvania who replaced the filament/cathode in 3KP4 tubes for Pilot Candid's. I had one once and it performed beautifully. Of course this was monochrome. He had worked out the details of removing the gun, replacing the filament assembly, adding a new tip-off tabulation to the wafer, resealing the entire assembly to the funnel and the easiest part, evacuation, baking and sealing.

Were this possible with the 15EGP22, I would think you could evacuate it at a temperature that is safe for the bonding adhesive used on the faceplate. Then you could leak test it with a helium mass spectrometer. Any leaks on the faceplate to funnel seal could be sealed indefinitely with Torr Seal, a sealing adhesive made especially for sealing UHV systems to vacuum levels of 2E-10 Torr! Then the tube could be baked on its dedicated vacuum system for a few WEEKS at a temperature well below the safe temperature for the adhesive. Remember, we don't have to make thousands of these - about 20 or so would do. Thus, for a fee, you could pump on one for weeks or more. Remember, the high temperature of bake out is simply a way to speed up the offgassing process. If the tube, once vented, is kept in a clean dry air atmosphere, no in-diffusion of water into the glass will occur. Thus you only need to bake away the water that forms a few monolayers or so on the surface of the glass and gun parts. This will easily take place in a week or so, even at room temperature. Again, the heat only serves to speed up the process. Once the vacuum gauge reads in the range of 1E-8 Torr or so, you can close off the pump valve and watch to see how fast the tube pressure rises to 1E-6 Torr. From this you can calculate how long it would take to reach excess pressure for tube life (rate of rise test). Please use a cryo pump or turbomolecular pump, not a diffusion pump, to evacuate the tube. Diffusion pumps invariably leave oils behind. An Ion Pump is even better but these typically have slow pumping speeds.

Thus I see no real issues for someone good with glasswork to do this task. It will just take some time.

Sorry for the long post but it's not a short process!

Vacuumguy