Burman to 1984 http://www.burmanamps.fsnet.co.uk/reviews/pro501.html

http://guitargeek.com/rigview/542/

http://archive.ampage.org/threads/2/gagd/069936/Donald_Brosnacs_The_Amp_Book-1.html

Burnam schematics

http://www.freeinfosociety.com/electronics/schempage.php?cat=1

Burman schematics