1971 AMPEG B15-N
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1971 Portaflex characteristics: Black painted steel chassis bolted onto the lid, aluminum front control panel with "blueline" silkscreened graphics, two channels with normal and bright input jacks, black plastic tone adjustment rocker switches, round black D shaft knobs with silver tops, CTS potentiometers, metal polarity and power switches (with at least one black), small round red power light, two large black transformers, black tube cage protecting the tubes and capacitors, metal standby switch on back panel left side, serial number stamped into back panel top left, fuse, two prong power cord and outlet, white Linden logo silkscreened on back of the chassis, 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 CTS speaker, matching dolly with wheels, literature.
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.