1975 AMPEG B15-N
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1975 Portaflex characteristics: Black painted steel chassis bolted onto the lid, aluminum front control panel, "blackline" 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 pots, metal polarity and power switches, 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 sticker on back panel left side, fuse, three prong power cord and outlet, Magnavox logo silkscreened on back panel right side, schematic, vinyl handles on top and sides of speaker cabinet, 4 chrome lid latches, vinyl covering, plastic "a" bullseye logo positioned on the silver grille cloth, 15 inch Eminence speaker, matching dolly with wheels.
Ampeg serial number dating is undocumented and very difficult. An example of 1975 B-15 serial numbers are 7172x and 7278x. For more precise dating cross reference the serial number with the amplifier's transformer, potentiometer, capacitor and speaker codes.