[John Folsom]

There have been a number of discussions here at AK and elsewhere about rebuilding the 15GP22 CRT. There is an active effort under way to attempt this. This effort has been under way for YEARS, with small incremental progress followed by long periods with no progress. But now we are close....

There are a number of issues involved in attempting to rebuild a 15G. Here is an over-simplified overview.

First, you have to find a rebuilder who will even consider attempting it.

Then you need a stem. This is the piece of glass which the wires pass through, and which has the hollow evacuation tube. 15G stems have not been produced for probably 40+ years. We have had a limited number of stems custom-made to the original 15GP22 specifications.

Next there are issues with the heating and cooling cycles used to process the tube during evacuation. When new, the 15Gs were processed at a lower temperature than is normal for modern CRT manufacture, and the heat-up/cool-down times were much longer. We are having custom oven controlcams made to allow us to use a 6 hour cycle time, instead of the usual 3 hour cycle time to process the CRT.

Then there are issues with the equipment used to hold the CRT during processing, including special thermal shields to protect the stem during processing, and assorted pieces used to hold the CRT in the lathe while the old gun is removed and the new one is welded in place. Some custom made pieces are being fabricated to support this.

Then there are issues with the gun itself. No 15G guns exit. So when a CRT is processed for rebuilding, a "good" gun must be ready to install once the "bad" gun is removed. The CRT cannot remain exposed to air for a long time (at least rebuilders do not want them to do so). The "bad" gun is then sent off to have its filaments and cathodes cut out and new ones installed, transforming it into a "good" gun. Of course, the gun was never designed to have this done, so some percentage of guns will be broken during the process. This will be one of many on-going problems to solve.

Then once a CRT is processed, there is some considerable debate about whether the enormous frit (glass to metal) seals at the 15G ultor will survive the thermal stresses.

And then there is the issue of the "leaker" tubes. Most candidate 15G CRTs have some air in them due to leaks. It is unknown if the leaks can be plugged, and if they can, if the exposure to air will make the CRT unusable.

Most of these issues have been addressed, at least to the point that, with any luck, we are with several months of our first attempt to rebuild a non-leaker 15G CRT. We will keep you posted..... !