I am building a 2x 813 amp and am looking for a filament transformer. I need some coaching on a couple options since I cannot seem to locate a proper 10V 10A transformer. 1) Use a 13v @ 20A transformer with a Variac. 2) Rewind a microwave oven transformer secondary with 10 turns of #12 THHN. Thanks for any info, Steve, KK7UV