Bogen Sound Systems by David Bogen, power and PA tube amps made in 1950s/60s. Used by guitarists. Bogen is now a mega audio company.

Observations on dating Bogen  amps

Bogen schematics

http://www.freeinfosociety.com/electronics/schempage.php?cat=1David Bogen amplifier head

Images Bogen GA20 courtesy of Pat Maguire:

Bogen amp Bogen amplifiers valves David Bogen Co Inc New York Made in USA amplifier