A few articles to start:

http://www.du.edu/~etuttle/electron/elect27.htm

http://www.radio-electronics.com/info/data...asic_theory.php

http://www.oldtuberadio.com/tubes/theory.htm

http://www.paia.com/tubworks.htm

http://earlyradiohistory.us/1920vacu.htm

http://www.ee.umd.edu/~taylor/Electrons3.htm

http://www.tubecad.com/articles_2003/Groun...fier/index.html

Great links! That first one is one of my favorites. Heres a few more...

Fun With Tubes:
http://www.angelfire.com/electronic/funwithtubes/

Lots of books, including both the third and fourth edition of RDH!:
http://www.pmillett.com/tecnical_books_online.htm

And other interesting stuff if you poke around on the main site:
http://www.pmillett.com/

Elements of Radio Servicing, Marcus & Levy:
http://www.archive.org/details/Elements_Of_Radio_Servicing

John