JMI Vox AC-10 Super Reverb Unit Amp Chassis - A Look "Under the Hood"

JMI introduced a new 10 watt, single channel guitar amplifier design on JMI schematic "OS/026 AC10 Super Reverb Unit," dated January 27, 1964. While this circuit and chassis would be included in the Vox AC10SRT head, AC10 3x10 combo amp and the JMI Domino Super Reverb Piggy back amplifiers, it bore little resemblance to the original and namesake Vox AC10 dual channel chassis documented on JMI schematic "OS/008 AC/10 Amplifier No 3," dated September 9, 1960. The newly designed "AC10 Super Reverb Unit" and the original "AC/10 Amplifier No 3" circuits were produced concurrently by Vox in 1964 and 1965.

The preamp circuitry for the AC10 Super Reverb unit eliminated the odd-ball ECF82 and often microphonic EF86 preamp tubes from the original AC-10 design, replacing them with more traditional ECC83 (12AX7) and ECC82 (12AU7) tubes. The AC10 Super Reverb also incorporated the JMI Vox "phono cartridge reverb" system, a feature not included on the original AC10 design.

Like its bigger brother, the AC-15, the AC10 Super Reverb Unnit power amplifier section was powered by two EL84 power tubes in a Class A, cathode biased, no negative feedback amp circuit. A close comparison of the AC10 Super Reverb Unit schematic (OS/026) and AC15 schematic (OA/031) reveals that both models have similar electronic components and plate voltages in their power amp circuitry, suggesting that the AC10 Super Reverb Unit chassis is actually capable of producing about 15 watts RMS.

When installed in a Vox AC10SRT head, the AC10 Super Reverb Unit chassis most frequently incorporated a gray anodized control panel with five black control knobs. I have also seen an AC10SRT head that had a red anodized control panel and black control knobs. Vox AC10 3x10 and Domino Super Reverb Piggy Back amplifiers were not equipped with the traditional Vox anodized control panel. White control nomenclature was silk screened over a base coat of black paint on the top of the chassis. Amps with black control panels often had white control knobs.

Vox AC10SRT Tube Function
V1a - 12AX7 initial gain stage
V1b - 12AX7 vibrato oscillator and modulator
V2a- 12AU7 preamp gain recovery
V2b - 12AU7 reverb mixer
V3a - 12AU7 reverb send
V3b - 12AU7 reverb return
V4 - 12AU7 phase inverter
V5, V6 - EL84 output stage
V7 - EZ81 power supply rectifier


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Photos and editorial content courtesy Gary Hahlbeck, North Coast Music

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