Ampx61 - I wonder if power filter cap replacements can have something to do with temperature increase.
Transformer with rectifier and filtering capacitors works much harder than the same transformer (and the same power delivered) on resistive load. Current is drawn in very short pulses of very large amplitude limited only by ESR of caps and transformer's internal impedance. High frequency content of these pulses is heating up the core while larger rms current is heating up the copper (rms value is much higher than average value for narrow pulses).
It might be possible that with older caps (that were a little dry) ESR was higher and amplitude of current pulses was lower (and a little longer) heating less copper and the core. Temperatures of transformer in order of 70 deg C are not unusual and can be even higher if better (higher temp) magnet wire is used. I'm not sure where Curie point is but suspect that much higher than that.
Transformer with rectifier and filtering capacitors works much harder than the same transformer (and the same power delivered) on resistive load. Current is drawn in very short pulses of very large amplitude limited only by ESR of caps and transformer's internal impedance. High frequency content of these pulses is heating up the core while larger rms current is heating up the copper (rms value is much higher than average value for narrow pulses).
It might be possible that with older caps (that were a little dry) ESR was higher and amplitude of current pulses was lower (and a little longer) heating less copper and the core. Temperatures of transformer in order of 70 deg C are not unusual and can be even higher if better (higher temp) magnet wire is used. I'm not sure where Curie point is but suspect that much higher than that.