One of the best series of tube amp videos on the internet is the series done by "Uncle Doug" on YouTube . There are 32 total videos on things like > "How tube amps work?", "Biasing", "Negative feedback","Scratch building a tube amp" and many more ... Link below ..
Uncle Doug's Tube Amp Bootcamp video series ...
Bob
Uncle Doug's Tube Amp Bootcamp video series ...
Bob