1973 AMPEG B15-N
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The silkscreened graphics were now "blackline".
Other 1973 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, potentiometers, metal polarity and power switches, small round red power light, two large black transformers, black tube cage protecting the tubes and capacitors, metal standby switch on back panel, fuse, power cord and outlet, serial number, 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, plastic "a" bullseye logo positioned on the silver grille cloth, 15 inch CTS speaker, matching dolly with wheels, literature.
Ampeg serial number dating is undocumented and very difficult. Some examples of 1973 serial numbers are 2501x and 2914x. For more precise dating cross reference the serial number with the amplifier's transformer, potentiometer, capacitor and speaker codes.