The JMI Vox AC-100 "80-100 Watt" Amplifier Head

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The AC-100 "80-100 Watt" amp head played a crucial role in the process that led Vox to become
a world wide leader in guitar amplification equipment. Paul McCartney received one of the first production AC-100 amps just before the Beatles invaded America in February 1964.

Vox started to investigate the development of high powered amplifier circuits in early 1963. JMI had no model in the 80 watt power range of the Fender Showman, an amp 50 watts more powerful than anything offered by Vox at the time. In response, Vox started design work on new 50 and 100 watt amp heads.

Additionally, JMI must have been aware that the maximum output of the Vox AC-30 amplifiers used by the Beatles was no match to the sound of their screaming fans. Might it be possible that the Beatles could jump ship from Vox to Fender's louder amplifiers? JMI knew that more powerful amps were needed soon.

Vox decided that the major selling feature of these new amps should be their abundance of power, not the inclusion of multiple channels, reverb or the complex Vibravox circuit from the AC-15 and AC-30. Despite their simplicity, Vox hoped that a higher output power rating would be sufficient to justify a higher retail price. The goals were set to make the new amps loud, clean, simple to build and profitable.

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 eighty watt AC-100 "80-100 Watt" circuit had only one channel, two input jacks, three controls, thirty-five resistors, nineteen capacitors and seven tubes. In February 1964, an AC-30 head retailed for £85 and an AC-100 head for £105.

The "80-100 Watt Amplifier" Circuit
At best estimate, Vox produced about twenty-two hundred AC-100 amps between 1963 and 1969. The original AC-100 circuit design was documented on JMI factory schematic "OS/036 80-100 Watt Amplifier," dated 9/9/63. JMI further revised the AC-100 circuit several times in 1965 and once again in 1967.

About three hundred fifty AC-100 heads were produced with the original 1963 era "OS/036 80-100 Watt Amplifier" circuit, including the amp featured on this page.

Like the AC-30, the "gold standard" of Vox tone, the "80-100 Watt" AC-100 amp head had a cathode biased output stage. In further homage to the AC-30, the AC-100 did not incorporate negative feedback (NFB). Negative feedback fed a small portion of the output signal back into the preamp to reduce distortion. While the AC-30 utilized four EL84 output tubes to produce ~30 watts, the AC-100 produced ~80 watts from four EL34 tubes.

JMI developed the first generation AC-50 and AC-100 heads concurrently in 1963. The preamp circuits of these early AC-50 and AC-100 amps were nearly identical. They even shared the same anodized "copper" control panel.

Thin Edge and Thick Edge Cabinets
While JMI produced about 350 AC-100 heads with the "80-100" circuit, only about the first 125 of these amps had a "thin edge" cabinet. 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. All of these amps had a copper control panel, Vox logo handle, removable plug style voltage selctor and no corners.

The balance of AC-100 "80-100" production had a thick edge cabinet constructed of 3/4" lock jointed baltic birch plywood covered in black basket weave vinyl. Most had plastic corners, black Vox grill, a Vox logo handle, a rotary voltage selector and either a copper, black or gray control panel. See photo at left.

Phil "Fang" Volk of Paul Revere and the Raiders once told me that his early AC-100 amplifier suffered failure when wax from an overheated power transformer oozed out of the front of the amplifier. Such heat related problems were not uncommon with the "80-100" AC-100 head. Some have suggested that the smaller "thin edge" head cabinet restricted airflow to the chassis and caused this problem. However, this should not be the case as the inside dimensions and ventilation grills of the "thin edge" and "thick edge" AC-100 head cabinets were identical. The real culprit was likely the additional heat created by the cathode biased output stage used in the AC-100 "80-100" amp. This overheating problem was for the most part eliminated by the 1965 redesign of the AC-100 which changed it from a cathode biased to a fixed bias output stage.

The Beatle Connection
The cathode biased, thin edge AC-100 with brown Vox grill has special historical significance as it was the amp model used by Paul McCartney when the Beatles arrived in America in February 1964. When the Beatles returned to tour America later in 1964, John Lennon and George Harrison were also playing their guitars through cathode biased, thin edge AC-100 heads with black Vox grill. In addition to the connection with the Beatles, the AC-100 "80-100" is loved for its touch sensitive response and uncompressed tonality.

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 previous Vox amp heads, one end of the speaker cable was permanently attached the amp chassis.

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 engraved plate near the speaker jacks of this thin edge AC-100 "80-100" head warned that a loud speaker enclosure must first be connected to the amplifier prior to switching on the amp.

"BASS" Flag
Vox installed a "BASS" model flag in the lower right corner of the grill of Paul McCartney's first "80-100" AC-100 head. To my knowledge, these were not normally installed on AC-100 heads.

A Look "Under the Hood"
Please click on the following links for a close up look at the AC-100 "80-100 Watt" chassis.

AC-100 "80-100 Watt Amplifier" Specifications
Years of production
1963 - 1965
Production Numbers
Less than 400
JMI Vox Schematic Number
OS/036, dated 9/9/1963
RMS Power Rating
~80 watts RMS
One with two inputs
Volume, Treble, Bass
Tube Complement
Two 12AU7, one 12AX7, four EL34
Power Supply Rectification
Thin Edge Cabinet Dimensions
19" wide, 7" tall, 11.5" deep
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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