[Rick Lee]

I only know about guitar amp transformers, but they pretty much last forever if treated properly. That means the impedance or load has to always be correct. A tube shorting out can kill it. As long as it's running properly, it should not be putting out much heat, which means it could be covered in dirt, dust and debris and still run fine. But have a mismatch in impedance, and it's gonna end badly. Just had this happen on a 1979 Marshall 100w head I have ridden hard and put away wet. A bad tube is what killed it. Otherwise, the power tranny was going to last forever.