Do you have 115 or 230V mains voltage? If you have 115V, you can use a 115/
15V power transformer with a primary winding connected to mains and a
secondary winding connected in series in counterphase (so that the voltage
that feeds the amp would be 115V-15V=100V) with the amp power iron's
primary. A nice method of stepping voltage down

Oh, and the 15V winding should be rated at least at approx. 1,5A (or more).