1979 AMPEG B15-N
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1979 was the last year of the Portaflex original run.
1979 Portaflex characteristics: Black painted steel chassis bolted onto the lid, aluminum front control panel, black silkscreened graphics with rounded corners, two channels with normal and bright input jacks, white plastic tone adjustment rocker switches, round black D shaft knobs with silver tops, CTS pots, white plastic standby and power switches, small round red power light, two large black transformers, black tube cage protecting the tubes and capacitors, serial number sticker on back panel left side, fuse, three prong power cord and outlet, white Magnavox logo silkscreened on back panel, 1/4 inch speaker cable and jack, schematic, vinyl handles on top and sides of speaker cabinet, serial number sticker inside 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.
Ampeg serial number dating is undocumented and very difficult. An example of a 1979 B-15 serial number is 15853x. For more precise dating cross reference the serial number with the amplifier's transformer, potentiometer, capacitor and speaker codes.