1971 AMPEG B15-S
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1971 Portaflex B15-S characteristics: Black painted steel chassis shock-mounted onto the lid, aluminum front control panel, "blackline" silkscreened graphics, two channels with normal and bright input jacks, round black D shaft knobs with silver tops, CTS pots, black metal standby polarity and power switches, small round standby and power lights, two large black transformers, black tube cage protecting the tubes and capacitors, fuse, two prong power cord and outlet, serial number stamped into back panel bottom left, white Linden logo silkscreened on back of the chassis, impedance switch, schematic, vinyl handles on top and sides of speaker cabinet, XLR speaker cable and jack, 4 chrome lid latches, vinyl covering, metal "a" bullseye logo positioned on the silver grille cloth, 15 inch Long Engineering speaker, tilt back rod, matching dolly with wheels.
Ampeg serial number dating is undocumented and very difficult. Some examples of 1971 serial numbers are 10254x and 10590x. For more precise dating cross reference the serial number with the amplifier's transformer, potentiometer, capacitor and speaker codes.