[ohohyodafarted]

Actually if a metal cone crt is still under vacuum it is every bit as rebuildable as an all glass crt.

The trick is that the glass on the face of a metal crt is much thinner than the face of an all glass crt. As a result it is not as strong, and is prone to cracking during the rebuilding process due to heating and cooling to rapidly in the evacuation oven.

It has been determined that if you raise the temperature of the evacuation oven very slowly and cool it down very slowly, so that the stresses of expansion and contraction can equalize, the tube can be rebuilt successfully. The whole oven process will take about 6.5 hours if done slowly. The expansion coefficients of the metal and the glass are different and rapid heating and cooling will cause the face of the crt to crack.

Metal tube rebuilding was a very common occurance in the US back in the 50's. But back then the rebuilders would treat a metal tube just like an all glass tube. Some made it through the oven ok and some cracked. There was such an abundance of the metal tubes at the time it did not mater. They just threw the ones away that cracked and tried to rebuild the next one. Today, the tubes are too scarce to chance a rapid heat up and cool down, so Scotty uses a very long 6.5 hour cycle and has very good success. He also charges more for a metal tube because of that extra long cycle.