Hi there, I'm in the process of building a Fender Princeton Reverb (AA1164) clone and am interested in Mercury Magnetics transformers. I also want to have a universal voltage option as I may be using this overseas. Their standard blackface PT is called #125P1B; one of the 'universal' options is called the FBFPP-M/U, 50V bias tap, lower B+ 325-0-325. Does anyone know the voltage of the former or of a standard Princeton PT? I'd love to hear thoughts on the pros/cons of having a transformer with lower B+. Another universal voltage option is the FBFPP-S/DP, 50V bias tap, 360-0-360 unloaded B+. They mentioned that this one has 'unloaded AC open circuit' with 'more current capability' than the #125P1B. Can someone please briefly explain what 'unloaded B+' and 'open circuit' means in practical terms? Higher voltages and ...? More clean headroom? Better suited for different tubes? etc.? I've been successful at building a few amps from directions, layouts and schematics, however my understanding of electronics is relatively basic. Thankfully these forums have been a great help so far! Any insights would be greatly appreciated.