United States Patent and Trademark Office - Go here to do some patent research on tube and guitar amp patents. Try searching for Leo Fender, or similar.
Adam's Amps Attenuators - This is a nice site explaining how to make an attenuator. When you want that driven tube sound, but not the ear blowing volume, you use an attenuator to absorb most of the power.
Tube reviews - This information on various tubes comes from the Tube Store, the same as Antique Electronics. Some good info.
Duncan's Amps - This site has a ton of stuff, software, tube data sheets, software and more.
Tube Amplifier Debugging Page - The title says it all. Have a problem with your vintage amp? This is your resource. This page is designed for people who have an amp that worked and is acting up. It is not necessarily meant for debugging new, homebuilt, amps.
Star Grounding in Tube Amplifiers - This page explains how to properly ground your homebuilt amp. A must read for builders.
AX84 - This site is all about their own DIY amp. They have schematics, layout, and tons of pictures on how to build this small class A amp. This is your first stop if you want to build your first amp.