We always used an RGP15-20 for any scan derived power supplies. It's a fast recovery 2000V 1.5A diode, and works perfectly in my CTC16, and cheap at a quarter for one diode, far cheaper than the NTE506.
http://www.fagorelectronica.com/imag...or/rgp1520.pdf
RCA specified this type in the later chassis(M134, CTC203) and once we figured out the root part number, we ordered a bunch and never looked back. great for boost diodes, and convergence diodes.
Conventional diodes like the 1N4007 and 1N5408 are just too slow, and tend to overheat.