Not that I'm likely to ever be confronted with such a problem, but which is the "Not" procedure-
The fet conversion?
The nuvistor conversion?
Peter Drefahl's VF14 rejuvenation?
Incidentally, I have no experience regarding Peter's U47 service, but I purchased one of his power supplies for my C12A about 12 years ago. It has been extremely reliable and stable. I also sent that C12A to him for routine service a couple years ago and am quite pleased with his work.
Incidentally, could you point me to a source of comparison of the C12 and the C12A? I know they have the same capsules, and C12 is tubed by a glass tube while a C12A uses a 7586 nuvistor. Transformers? Basic circuit concepts? Power supply requirements? Production runs, etc.- inquiring minds want to know.