1961 AMPEG B15-N
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An tilt back rod was added onto the back of the amp after mid year. In October the blue power light was discontinued and replaced with a light up plexiglass logo. This could be personalized with your name for an extra 50 cents.
Other 1961 Portaflex characteristics: Chrome chassis shock-mounted on to the lid, white silkscreened graphics, two channels with normal and bright input jacks on channel 1, black stove type knobs, three metal switches, two large black transformers, black tube cage protecting the tubes and capacitors, fuse, two prong power cord, schematic, one leather handle, Navy Random Flair vinyl covering, "optional" side port holes, chrome script logo, 4 chrome lid latches, 8 pin XLR speaker cable connector and jack, two styles of silver grille cloth, serial number sticker occasionally with tube chart, 15 inch Jensen speaker, matching dolly with wheels, protective vinyl cover.
The serial numbers are 6 digits in length with the following pattern: YMMNNN, where Y denotes the year, MM the month, and NNN the # unit produced that month. The # unit, however, may not have started with 001. Ampeg may very well have used 100 or 101 as the starting point during slow production periods. Nevertheless, for dating purposes, the serial number provides adequate information during this period. For example, an amp with serial number 404553 was made in 1964 (4), during April (04) and may have been the 553th amp made that month (553). For more precise dating cross reference the serial number with the amplifier's transformer, potentiometer, capacitor and speaker codes.