1972 AMPEG B15-N
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During the year the "blueline" silkscreened graphics changed to "blackline". The serial number was now a sticker on the chassis and/or near the speaker, or stamped into the chassis' back left side.
Other 1972 Portaflex characteristics: Black painted steel chassis bolted onto the lid with aluminum front control panel, two channels with normal and bright input jacks, black plastic tone adjustment rocker switches, round black D shaft knobs with silver tops, CTS potentiometers, metal polarity and power switches (usually at least one black), 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, fuse, two prong power cord and outlet, white Linden or Magnavox logo silkscreened on back panel right side, schematic, vinyl handles on top and sides of speaker cabinet, XLR speaker cable and jack, 4 chrome lid latches, vinyl covering, metal "a" bullseye logo positioned on the silver grille cloth, 15 inch CTS or optional Altec 421 speaker, matching dolly with wheels, protective vinyl amp cover, literature.
Ampeg serial number dating is undocumented and very difficult. Some examples of 1972 B-15 serial numbers are 10568x, 11030x, 1333x, 1735x and 1751x. For more precise dating cross reference the serial number with the amplifier's transformer, potentiometer, capacitor and speaker codes.