Here's a great source for amp schematics:

AMPEG: http://www.schematicheaven.com/ampeg.htm

FENDER: http://www.schematicheaven.com/fender.htm

GIBSON: http://www.schematicheaven.com/gibson.htm

MARSHALL: http://www.schematicheaven.com/marshall.htm

VOX: http://www.schematicheaven.com/vox.htm

Dumble schematics are found at : http://ampgarage.com in the "Dumble Files" section.

Here's a ton of schematics, from amps to Theramin, stomp boxes, etc.
http://www.freeinfosociety.com/electronics/schempage.php?cat=1

These are for guitar amps:

http://www.ampwares.com/
http://www.thevintagesound.com/ffg/