It would be interesting to know why you're doing anything with 25 volts. Perhaps another approach might yield an easier solution. So... why are you doing this?
For instance, if you don't crank the holy living crap out of an amp, a 25 volt transformer will work perfectly well in a 70 volt system. A 70 volt transformer will work perfectly well in a 25 volt system, but the wattages named at the taps will be wrong (lower than named)... you'll have to set them by ear.
A good answer is easier with a clear question giving the make and model of everything. "The biggest problem in communication is the illusion that it has taken place." -- G. “Bernie” Shaw
