Shannon's site is called DIYTUBE. He offers extremely high quality PCB's for modifying Dyna and Eico amps. And they have some generic tubes sections with discussions of tube circuitry and such. I only mention it on ART because it is pretty much dedicated to the four amps mentioned. I use his version of Dyna's ST 35 EL 84 PP amp and boy is that amp sweet. And very cheap to build.