AX84.com is a site with several beginner to intermediate tube amp projects. Each project has a "theory document" which explains how the amp you're about to build works. I've found these documents to be a good overview of tube amp basics.
Here's a link to one of them:
http://ax84.com/p1/P1_Theory_Document.zip
Here's a link to one of them:
http://ax84.com/p1/P1_Theory_Document.zip