1976 AMPEG B15-N
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During the year the silkscreening graphics design changed to black with rounded corners. The black tone adjustment rocker switches became white plastic. The power cord added a third prong for grounding, so the polarity switch was discontinued. The front switches were now standby and power, made of white plastic. The hardwired speaker cable now had a angled 1/4 inch jack to match the new speaker input jack.
Other 1976 Portaflex characteristics: Black painted steel chassis bolted onto the lid, aluminum front control panel, two channels with normal and bright input jacks, round black D shaft knobs with silver tops, CTS potentiometers, small round red power light, two large black transformers, black tube cage protecting the tubes and capacitors, serial number stickers, 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. Some examples of 1976 serial numbers are 7476x and 12737x. For more precise dating cross reference the serial number with the amplifier's transformer, potentiometer, capacitor and speaker codes.