1977 AMPEG B15-N
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The silkscreening design was now black with rounded corners. The tone adjustment rocker switches were white plastic. There was no polarity switch. The standby and power switches were white plastic. The hardwired speaker cable had a 1/4 inch jack to match the speaker input jack.
Other 1977 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 pots, small round red power light, two large black transformers, black tube cage protecting the tubes and capacitors, serial number stickers on back panel left side, fuse, three prong power cord and outlet, 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, serial number sticker on speaker input jack panel, 15 inch speaker, matching dolly with wheels.
1977 Ampeg serial number dating is undocumented and very difficult. For more precise dating cross reference the serial number with the amplifier's transformer, potentiometer, capacitor and speaker codes.