Early Electronic Television
As an alternative to rebuilding 12AP4s, Dennis Choinski located a number of 9 and 12 inch 40 degree radar tubes, with soft glass, in England. He purchased one and had Clinton Electronics re-phosphor it and rebuild it with a new gun. The project was a success, and cost about $300. Following up on Dennis's leads, we procured 15 of the 12 inch tubes. A poll was taken of owners of sets with 12AP4s, and there was enough interest in making "12AP4 substitutes" to order the radar tubes from England. They were shipped to Clinton, and will be re-phosphored and rebuilt with new guns. These substitute tubes will cost just under $500, including the cost of shipping the radar tubes from the UK, and will work nicely in sets using 12AP4s. Both the rebuilt 12AP4s and the substitute 12AP4s will have thin aluminized phosphors, which will be much brighter than the originals, and won't be subject to ion burns.
We also ordered two 21 cm versions of the same radar tube for 9AP4 substitutes.
Clinton has finished rebuilding the first of these tubes. We have put it in our RCA TRK-12. It produces an excellent, bright, sharp picture; the best I've ever seen on a pre-1945 set. The only electrical change required is to connect the filament lines in the power supply chassis to 6.3 volts. Here are photos:
A 12AP4 (left) and the substitute. Notice that the bulb is somewhat shorter, as is the neck