Well, I've done a fair amount of guitar amp repair work in the past as part of a business that I used to be part of, and I have a pretty large library of amp schematics to use as reference for what I want to build. I'm pretty familiar with the common circuits in these old amps, it's when you start doing customized circuits that things get a bit more difficult. Building a copy of an older amp such as a Fender or Marshall isn't very difficult, but it gets expensive due to the cost of the transformers, that's one of the reasons that I rebuild these old amps. The expensive parts are already there. :-)