1979 AMPEG B15-S
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1979 was the last year of the Portaflex original run.
1979 Portaflex B15-S characteristics: Black painted steel chassis shock-mounted onto the lid, aluminum front control panel, black silkscreened graphics with rounded corners, two channels with normal and bright input jacks, round black D shaft knobs with silver tops including "response" knob, CTS potentiometers, white plastic standby and power switches, small round standby and power lights, two large black transformers, black tube cage protecting the tubes and capacitors, circuit board, fuse, three prong power cord and outlet, serial number sticker on back panel, white Magnavox logo silkscreened on back panel, impedance switch, 1/4 inch speaker cable and jack, 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 speaker.
Ampeg serial number dating is undocumented and very difficult. An example of 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.