There are several features of note which makes this amp different that the later B-15N revision. Some of the differences that distinguish the older models include:
- the 1960 B-15 pre-amp design is similar but quite different from that of the B-15N -- there is a volume control for each channel but only a single set of treble and bass controls,
- the tone circuits are built on the back of the pots -- the PEC tone modules are not present,
- the center input jack is stereo, linking to channel 1 and 2 -- for use with the Ampeg Peg pickup,
- this amp has the stereo input but also has separate treble and bass controls for each channel,
- the smaller resistors are all carbon composition,
- the red Astron coupling capacitors,
- the metal framework that holds the lamps in the N revision is not present,
- the eyelet board is black vulcanized paper -- like Fender used in the early amps,
- there is an extra pot next to the heater Hum balance pot on the back of the amp, the function is not clear.
The red 0.02uF Astron coupling capacitors later gave way to yellow Cornell Doublier ones. I believe that the black 0.5uF capacitors have a date code of the 28th week of 1959. I suspect that these are cathode bias caps for the first stage triodes.
The tag boards on the back of the pots are used as a platform to build the tone circuits. These were later replaced with the PEC modules.
The brown paper covered electrolytic capacitors are commonly found in Ampeg amplifiers. Look for a date code on them.
The early models used this single lamp socket. The lamp is a commonly used model 1847, Edison's birth year. The B-15N revisions had a more elaborate lamp holder that positioned the two lamps more above the chassis.
The square cement 250 Ω 10W resistor is part of the power tube's cathode bias circuit. This resistor runs Hot and the eyelet board is often scorched as a result. It helps to mount this resistor a mm or so above the board to help better dissipate the heat. This resistor can go out of spec with time, as does the Parallel 25uF 50V Capacitor. This will affect the bias of the power Tubes and the performance of the amp. It's a good idea to check these components when the amp is serviced.