| The AC-50 "Big Box" amplifier head was offered in two versions. The earlier of these versions was officially called the "AC50/2" as depicted on JMI Vox Schematic #OS/053. The later version was called the "AC-50/4 Mk III," detailed on JMI Vox schematic #OS/072. The AC50/2 had a GZ34 tube rectifier while the AC-50/4 Mk III replaced the tube rectifier with a diode bridge. Aside from the means of power supply rectification, the circuitry in the two amps is very similar.
Unlike the earlier single channel versions of the AC-50, both "Big Box" head models had two channels. The first, or "Normal" channel was based on the normal channel from the AC-30 amplifier. The second, or "Brilliant" channel was based on the Vox "Top Boost" circuits used in AC-15 and AC-30 amps.
The power amp featured two EL-34 power tubes running in "Class AB" mode, producing 50 watts RMS output power. The earlier, single channel models of the AC-50 had "Class A," or cathode biased power amp sections. This design was dropped in favor of Class AB due to heat and tube life issues.
A rotary voltage selector switch allowed the amp to accept input voltages of 115, 160, 205, 220, or 245 volts. This feature made it possible for the AC-50 to accomodate the AC power standard in most countries.
Dual speaker jacks included a speaker impedance selector to choose between 8 or 16 ohm total speaker loads. It is important that the speaker load connected to the AC-50 be 8 ohms or greater. Using speaker loads below 8 ohms will likely lead to premature damage to the EL-34 tubes and output transformer.
A red placard above the speaker jacks warns against operating the amplifier without first connecting it to a speaker cabinet. Failure to follow this warning will cause a nearly instant failure of the output transformer.
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