The JMI Vox "AC-100/2" and "AC-100 MkII" Amplifier Head

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Vox introduced the second generation AC-100 amp head in the summer of 1965. The earlier
"80-100 Watt" version of the AC-100 head incorporated a 80 watt RMS, "cathode biased" power amp section that was prone to overheating. The redesigned 1965 version of the AC-100 utilized a more powerful 100 watt RMS "fixed bias" output stage that eliminated the thermal issues that plagued its predecessor.

The circuitry for the second generation AC-100 was detailed on a series of three schematics from Vox. The first of these schematics was labeled "OS/036 - 100 Watt Amplifier," dated May 30, 1965. This schematic was replaced a scant four weeks later by "OS/036 - AC 100/2," originally dated June 28, 1965 with revisions through December 1, 1965. A third schematic named "OS/167 AC100 MkII Amplifier Circuit" was introduced on July 17, 1967 with revisions through December 4, 1969.

In all, Vox produced about eighteen hundred fifty AC-100 "second generation" amp heads. Not all of these heads were produced by JMI. Royston Industries, the parent company of Vox, declared bankruptcy in early 1968, ending the JMI era. Some of the prior managers and engineers at JMI convinced the trustees at the receiver bank to allow them to reopen production under the "Vox Sound Equipment Ltd" (or VSEL) name. VSEL continued to build AC-100 amps through 1969 using a left over stock of JMI branded control panels and serial plates.

Simplistic Design
Like the original "80-100 Watt" AC-100 circuit from 1963, the updated AC-100/2 and AC-100 MkII were single channel amplifiers with only two inputs and three controls. The design goal of the AC-100 was an abundance of power, not the inclusion of effects such as vibrato or reverb.

This simplicity of design must have made the AC-100 a great money maker for Vox. While the thirty watt AC-30 circuit included three channels, six input jacks, six controls, seventy-three resistors, thirty-eight capacitors and nine tubes, the AC-100 circuit had only one channel, two input jacks, three controls, thirty-five resistors, nineteen capacitors and seven tubes. In November 1965, an AC-30 head retailed for £96 and an AC-100 head for £105.

Control Panels
Nearly all AC100/2 and AC100 MkII amplifiers had "etched and filled" gray control panels. However, at least one AC-100 MkII head had a black control panel.

Tube Complement
The preamp and tone control stage of the AC100/2 utilized a 12AU7 and a 12AX7 tube. A second 12AU7 powered the phase inverter. Four EL34 tubes drove the 100 watt amplifier stage.

The AC100 MkII substituted a 12AX7 for the 12AU7 in the AC100/2 phase inverter circuit. This tube swap gave the AC100 MkII a significant increase in gain.

Thin Edge vs Thick Edge Cabinets
About 125 of the earliest "80-100 Watt" AC-100 heads built in 1963 and 1964 had "thin edge" cabinets. The thin edge cabinet was constructed from lock jointed 3/8" thick baltic birch plywood covered in black basket weave vinyl. Some of these heads had brown Vox grill while others had black. These amps had a Vox handle but no plastic corners.

By the time the AC100/2 and AC100 MkII were introduced in 1965, Vox was already equipping AC-100 amp heads with a "thick edge" cabinet. These thick edge cabs were built from 3/4" thick baltic birch plywood covered in black basket weave vinyl (see comparison photo at left). Thick edged AC-100 cabinets normally had black Vox grill, plastic corners and a Vox handle.

Voltage Selector
A rotary voltage selector was mounted on the right side of the control panel. The rotary voltage selector allowed the AC100/2 and AC100 MkII to be manually adjusted to accept five different regional mains voltages: 115VAC, 160VAC, 205VAC, 225VAC and 245VAC at either 50hz or 60hz.

Detachable Power Cable
Unlike Vox AC-10, AC-30 and T.60 heads, the AC-100 did not provide storage space inside the head cabinet for the power cable. This necessitated the use of a detachable power cable. Vox made a rather curious choice of cabinet and cable connectors for this power cable, a four-pin XLR jack and plug.

While normally designed for low voltage audio connections, JMI included four pin XLR connectors on the power cables of the AC-100, AC-50, AC10SRT, 400 and 700 Series heads. It is unlikely that modern electrical standards would allow the use of an XLR connector in this fashion. Click here for wiring information.

Speaker Jacks and Warning Placard
The AC-50 and AC-100 amps were also the first amps from Vox to utilize a detachable speaker cable. On earlier Vox amp heads one end of the speaker cable was permanently attached to the amp chassis. As there was no room in the AC-100 head cabinet to store a permanently attached speaker cable, one that detached was the only option.

Rather than more conventional ¼" jacks, Vox chose to employ three-pin XLR connectors for the speaker connection. An XLR connector insured that the cable was securely locked in place prior to operation. It also made the Vox speaker cable unique, doubtlessly providing JMI a steady market for replacement speaker cables. Further, there was no chance of plugging the four-pin XLR mains cable into the three-pin XLR speaker jack.

An etched and filled aluminum plate near the speaker jacks of this AC-100/2 amplifier warned that a loud speaker enclosure must first be connected to the amplifier prior to switching on the amp.

Vox "AC-100/2" and "AC-100 MkII" Amplifiers Specifications
Years of production
1965 - 1969
Production Numbers
About 1850
JMI Vox Schematic Numbers
OS/036, dated 5/30/1965
OS/036, dated 6/28/65 and 12/1/65
OS/167, dated 7/17/67 and 12/4/69
RMS Power Rating
100 watts RMS
One with two inputs
Volume, Treble, Bass
Tube Complement - AC100/2
Two 12AU7, one 12AX7, four EL34
Tube Complement - AC100 MkII
One 12AU7, two 12AX7, four EL34
Power Supply Rectification
Thick Edge Cabinet Dimensions
19.75" wide, 7.5" tall, 11.5" deep



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Photos and editorial content courtesy Gary Hahlbeck, North Coast Music

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