[Arran]

Sometimes output transformers fail for no apparent reason at all, although the one thing that could do it is the output tube drawing too much plate current perhaps. Many 1939-42 Philcos are notorious for having bad output transformers, they almost always develop an open primary. I don't believe that an output filter cap is to blame, I actually have a few Rogers built sets that use output filter caps of about that size, presumably it was because they were designed for 25 cycle operation.
Regards
Arran