Andrea Ciuffoli, (Several OTL's and others).
Classic and Vintage Audio Circuits Page, (Classic schematics and links).
Gunter Wagner (A lot of classic microphone schematics).
Kiewa Valley (source of the Leak schematics).
Tube Lovers Anonymous, (A 6C33 OTL explained).
Williamson amplifier, (the famous Williamson circuit undressed).