The only bad thing with those transformers from microwave furneces is BIG idle curent and comparetivly
big power with small sizes.It cold be used with this big power/smal size BUT only for short period of time.It overheats!!The primery coil has small amount of turns-and a big idle curent.To decreese idle curent we need to increese amount of turns at primary....AND WE LOOSE a POWER!!
Couldn't the idle current be addresses by doing what the microwave designers do, use a triac and a opto isolator, run the transformer only when transmitting. The transformer has a grounded secondary, use it in a half wave voltage doubler configeration.
People use microwave transformers. Personally, I would not.
First, they are bad for regulation. This is because they have high equivalent secondary resistance. If they get hot, it also means they are lossy. When they are lossy, it means they have bad no load to full load regulation.
Putting them in a full wave doubler makes them worse for regulation and heat, putting them in a half wave doubler would be terrible.
This isn't saying they can't be used. It just all depends on what your standards of acceptance are.