1964 AMPEG B15-N
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The 15 inch speaker changed to a CTS.
Other 1964 Portaflex characteristics: Chrome chassis shock-mounted onto the lid, white silkscreened graphics, two channels with normal and bright input jacks on channel 1, black stove type knobs, CTS potentiometers, three metal switches, two large black transformers, black tube cage protecting the tubes and capacitors, light up plexiglass logo, fuse, two prong power cord, schematic, one leather handle, 4 pin Switchcraft XLR speaker cable and jack, 4 chrome lid latches, chrome script logo, Blue Check vinyl covering, silver grille cloth, serial number sticker including tube chart, tilt back rod, matching dolly with wheels.
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 406123 was made in 1964 (4), during June (06) and was probably the 123rd amp made that month (123).
The serial number alone could denote April 1954 or April 1964. The key is the model and the cosmetics. For instance, the B-15 wasn't introduced until 1960. And an Ampeg amp from 1954 would not be covered in blue check vinyl whereas an amp from 1964 would have blue check covering. For more precise dating cross reference the serial number with the amplifier's transformer, potentiometer, capacitor and speaker codes.