JMI Vox AC-50 "Mk I" Two Input Single Channel Amplifier Head - 1964
"Washington DC Amp Head"

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In 1963, the year before the Beatles invaded America,
Vox started to investigate the development of high powered amplifier circuits. JMI had no model in the 80 watt power range of the Fender Showman, an amp that was 50 watts more powerful than anything offered by Vox. In response, Vox started development of both a 50 and a 100 watt guitar amplifier.

Vox decided that the major selling feature of these new amps should be their abundance of power, not the inclusion of effects or multiple channels. Neither the expansive Vib/Trem circuitry of the AC-15 and AC-30 or the troublesome phono cartridge reverb effect would be included in these new amps. The new amps would be simple, uncomplicated and loud. Leaving out the effects circuitry might actually lead to a cost savings in production over the more complicated AC-15 and AC-30 models. A higher power rating on the new amps could even justify a higher retail price.

Additionally, Vox would soon learn that the total output level of the AC-30 amplifiers used by the Beatles would be insufficient to cut through the sound of the screaming fans trying to hear the band. Larger amps were needed.

Lead Vox engineer Dick Denney recalls on page 47 of his book, "The Vox Story," that Triumph Electronics, an AC-30 chassis subcontractor for Vox, developed a 50 watt power amplifier circuit powered by two EL34 power tubes that might be suited for use in a new Vox amplifier. When combined with the "top boost" tone circuitry from the AC-30, the new AC-50 was born.

This first version of the AC-50 was a single channel amplifier with dual 1/4" instrument inputs and no effects. The copper control panel featured volume, treble and bass controls, an indicator lamp, a power switch, a fuse and a plug style voltage selector. The tube complement included a GZ34 tube rectfier, two fixed bias EL34 power tubes, two ECC83 (12AX7) and one ECC82 (12AU7) preamp tubes. The amps were capable of about 45 watts RMS.

Despite the addition of "top boost" circuitry, the AC-50 failed to achieve the brilliant top end response players loved in the Vox AC-15 and AC-30 amps. My experience with AC-50 heads leads me to suspect that the AC-50 simply has more bass response, effectively masking the treble. Vox added a Goodmans Midax horn to the guitar cabinets mated to the AC-50 in response to this treble defiency.

Dick Denney once told me that Vox tried to make their amp heads no larger than the size of a "lunch pail" whenever possible. This "small is better" influence was certainly at work in the case of the "small box, thin edge" AC-50 heads. The cabinet was designed to be no larger than necessary to enclose the chassis. The cabinet sides, front and back were made from 3/8" baltic birch. The cabinet had a Vox handle, a small Vox logo, four feet and two expanded aluminum vent grills. Additional venting was provided in the front grill cloth panel. No corners were supplied because the corners normally used on Vox cabinets were incompatible with the "thin edge" cabinet construction.

The serial plate was fastened to the bottom of the amp as there was insufficient room to mount it to the back.

The chassis was fastened to the bottom of the enclosure with machine screws and cage nuts. This model predates the use of a chassis slider board on the AC-50. Chassis slider boards were introduced on the Mark III version of the AC-50.

Three XLR jacks were mounted on the rear panel. The twin three pin XLR jacks on left were for speaker output. The four pin XLR jack on the right was used to supply AC mains voltage to the amplifier.

The Vox AC-50 single channel thin edge amp head became historically significant when George Harrison and John Lennon took delivery of the amp several weeks prior to the Beatles first appearances in America in 1964. Their new AC-50 heads powered modified Vox AC-30 sized speaker cabinets equipped with two Celestion Alnico Blue speakers and a Goodmans Midax horn. While not visible to the televsion cameras, these heads and cabs were played by the Beatles on the February 9, 16 and 23 1964 performances on the Ed Sullivan Show.

The Beatles also played a concert using these amps at the Washington Coliseum in Washington DC on February 11, 1964. This concert was filmed and has been subsequently released in DVD format by Apple.

Over the years, this first version of the AC-50 amp head and accompanying "small box" AC-50 speaker enclosure have come to be known as the Vox "AC-50 Washington DC Amp."


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Photos and editorial content courtesy Gary Hahlbeck, North Coast Music

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