In 1960, Oliver invented and patented the B-15, a bass combo amp with closed-back reflex cabinet, double-baffle porting system, and an innovative flip-top function. The B-15 was the first in Ampeg's Portaflex series, and after becoming the preferred studio amp of session musicians like James Jamerson and Chuck Rainey, went on to become the most-recorded bass amplifier in history. By 1963, the Portaflex series business had grown to 44% of Ampeg's amplifier sales.
