The AC-50 "Big Box" Super Twin represents the third and fourth evolutions of the AC-50 amplifier offered by JMI Vox. All four versions were developed sequentially within a two year period from 1964 to 1966.
The first AC-50 "Small Box" amps were produced for the Beatles. Often known as "thin edge" AC-50 amp, the head cabinets were constructed of 3/8" baltic birch. Additionally, they had red control panels, a single channel with dual inputs, a plug style voltage selector, and a tube rectifier. This is the rarest of all AC-50 amp heads. These heads were coupled to a modified AC-30 sized 2x12" speaker cabinet with a Midax horn. The speaker cabinet was not sufficiently deep to enclose the horn, so the magnet of the Midax protruded from a hole in the back of the cabinet.
The second version of the AC-50 amp head had a single channel with four inputs for instruments arranged in the shape of a diamond. Most of the amp heads produced in this series had a slightly larger "thick edged" cabinet constructed from 3/4 baltic birch. This model retained the tube rectifier. I believe that these all of these amps had red control panels. The larger and deeper speaker cabinet shown at left appeared at this time and was included with all future JMI AC-50 amps. The added depth allowed the Midax horn to be contained inside the enclosure.
In the third generation, officially called the AC50/2 on JMI Vox Schematic OS/053, Vox added a second channel and a "tall" cabinet to the head. Named the AC-50 Mk II, this model was still tube rectified and looked exactly like the pictures at left. The control panel was now gray, and a five position rotary voltage selector switch replaced the earlier "movable plug" connector.
The next incarnation of the JMI AC-50 is shown at left. Called the AC-50/4 Mk III, it appears that the only significant change was the replacement of the tube rectifier in the power supply with a diode bridge. This amplifier circuit is detailed on JMI Vox schematic OS/072. The AC-50/4 Mk III was the JMI Vox version of this amp produced in the largest numbers.
The final version of the JMI AC-50 increased the gain of the amp by replacing the 12AU7 i(ECC82) n the preamp with an 12AX7 (ECC83). This model was named the AC-50 Mk IV.