My recommendation (as someone who did exactly this when I got hold of an even rarer STA100 and one transformer was duff): get Will to wind you a pair of matching output transformers for this, don't mess about with second hand ones (even if it's possible to get them - you could wait a very long time). Will's will be 100% the correct Radford spec and good for another 50 years. I found the price for the STA100 ones very fair indeed.