Start by downloading and reading these chapters:Originally Posted by perez
http://www.diyguitarist.com/Misc/J_Darr.htm
Here is a great book that explains exactly how a guitar amp works and what all the parts do:
http://www.amazon.com/Guitar-Amplifi...3&sr=1-1-spell
There is a chapter on building your own tube amp, a project the author calls the Two Stroke.
Some info on guitar amp speaker cabinets:
http://www.colomar.com/Shavano/copy_cabinets.html
http://www.colomar.com/Shavano/stereocab.html
Here is the mother load:
http://www.tubebooks.org/technical_books_online.htm
There is enough information to keep an advanced electrical engineer busy for decades.