Out of respect for Mesa Engineering I'm specifying the second-most restrictive license on this one. Later on I intend to make a similar EQ circuit that does not use the bulky inductors and is programmable, but will essentially have the same ability to shape tone the way this circuit does.
This EQ circuit design was originally developed in the early '70s by Randall Smith, according to Mesa Engineering's web site and a quote by Erik Kirkland of Guitar World. Refer to the site here (Viewed as of Jun 23, 2016): http://www.mesaboogie.com/pedals--related/electric-guitar-pedals/eq-pedals/boogie-graphic-eq.html
While the circuit design is Randall / Mesa IP, this schematic and simulation setup (the waveforms attached to this project) were all done by myself.
I used to look at the Mesa amps and wonder why they only used a 5-band EQ, but I realized that if it was indeed designed in the 70s and also used hand-wound inductors for the resonant circuits, anything more than 5 got way too big and expensive.
I don't own one of these amps, but if I could get my hands on one, I would do a thorough analyses of the inductors to find their ESR and EPC characteristics, and find out if the cores saturate easily (which would add some distortion to the sound) or if they are gapped to prevent saturation (which would increase the number of turns necessary and bump up the series resistance and parallel capacitance). Then I could calculate very accurate gyrator circuit elements to precisely replicate the original inductors using opamps.
I would very much like to do that, so if you know anyone who has one of these in SoCal please contact me!.
Meanwhile, I intend to make one with RM4 or RM6 inductor cores and AWG 42 wire (the same size used for guitar pickups), and see if I can get similar sound response. This is a long project and will take months due to my limited spare time, but if people show an interest it will motivate me to devote more effort to it on weekends ;-)