[Arran]

Sometimes you can find replacement audio output transformers on fleabay, new or used, but Antique Electronic Supply also sells them, generic Push-Pull units as well as the Hammond brand ones, make sure you get one with the correct plate load impedance measured in Ohms, for 6K6 tubes, 6F6s and 6V6s are different. With regard to the audio output transformer failing, this era of Philco is known for this so I think it may be a corrosion issue, that or they skimped on the gauge of wire used. I have a chassis from a 30-180 and it has a replacement Hammond output transformer from the 1950s or 60s judging by the label. These sets used 6K6s in push pull, but wired them with a screen grid inversion setup, so I wondered if that may have something to do with the failures. However Philco was using the same setup as early as 1937 in their 37-640 and 37-650 sets, and those were not known for output transformer failures.
Regards
Arran