So as the title suggests I plan on taking out the secondary coils and rewiring the two microwave transformers to build a 30-40W class AB guitar amp that I am designing (exact wattage is yet to be determined). One will act as a 400-0-400 power transformer, the other as the output. I know purchasing transformers wired to my specifications is much better, but I have these two transformers and I would rather spend 20-50 $ on copper vs. 100$ on transformers. I'm just looking to throw together my first tube amp from scratch with all the tubes I have on college-kid budget, I'll worry about perfecting sound later.
My question is what gauge of copper would suit each of the transformers, and what would be the best way of modifying the output tranny for better frequency response?