This week I have been building a turret version of the ROG Flipster. It is an FET-circuit designed to emulate the Ampeg SB-12 Portaflex. These amps are still highly sought after for use in studio recordings and can be heard on a lot of classics, especially a lot of the old Motown recordings.
I was under the impression it was based on the B-15, hence my mistake in the name. The design is courtesy of http://www.runoffgroove.com/flipster.html