[Arran]

You can use a replacement without the extra tap, you may have to add an extra resistor ahead of the pi filter or use a larger value choke resistor but these sets are usually not that fussy. If the set is cooking output transformers then there has to be a circuit defect like a bad bias resistor a very leaky paper cap, I don't think it fried the second transformer because it was used. Just get another from another AA5 or 6 with a similar output tube 35L6, 50L6, 35C5, 50C5, it doesn't matter as long as it fits mechanically.
Regards
Arran