JMI Vox AC-30 Super Twin Reverb Trapezoid Amp Head
A Look "Under the Hood"



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A peek inside the open back of a trapezoid AC-30 Super Twin Reverb amp head reveals an incredibly compact cabinet design that has very little wasted space. The upright portion of the chassis barely clears the vertical strips that secure the rear panel to the cabinet. The reverb pan, mounted to the bottom of the chassis slider board, barely clears the bottom of the cabinet. Despite the tight tolerances, JMI still was able to provide storage space behind the reverb pan for the cables and vibrato foot switch.

The AC-30 Trapezoid Super Twin chassis utilized the classic Vox amp chassis design. The base of the chassis was made of formed steel. In the amp pictured on this page, serial #04954, the chassis base supports a Woden power and output transformer, a Woden choke, a filter cap, a GZ34 tube rectifier and a quad of EL-84 power tubes. A vertical piece of aluminum, bent into the shape of an inverted "L," was bolted to the steel base and to the tops of the transformers to support the preamp tubes, preamp circuitry and the control panel sections of the amplifier. The entire chassis bolted to a "slider" board. Once the back panel was removed, the chassis could be easily slid out of the cabinet like a drawer for service.

This unique Vox chassis design yielded many benefits. The steel base section of the chassis provided the strength and rigidity to support the heavy power and output transformers. The output and rectifier tubes were mounted vertically to this steel chassis base, facilitating proper ventilation and providing an excellent heat sink for the tube sockets. Locating the hand wired preamp circuitry on the inside of the grounded vertical aluminum structure electronically isolated the sensitive, high gain preamp circuitry from stray electronic fields emanating from the transformers and power amplifier circuitry. The chassis was structurally rigid yet lightweight and not easily susceptible to damage.

This AC30/6 chassis had factory installed "integral Top Boost circuitry" in the Brilliant channel. The Top Boost treble and bass controls were located in the main control panel.

The addition of this integral Top Boost circuitry was a significant development for the AC-30. Those wishing to add Top Boost to earlier versions of the AC-30 were required to have an optional after market circuit installed by their dealer. The treble and bass controls would be not be located in the control panel, they would be inconveniently mounted in a rectangular cutout in the rear wooden panel of the amplifier.

The standard AC-30/6 control panel had to be reconfigured to incorporate the addition of the treble and bass controls. In this process, the two mounting screws that held the back to the upper rear edge of the chassis were relocated as well. The original distance between the upper chassis mounting screws on AC30/6 had been 13." This distance was reduced to 11 3/8" with the control panel modification to incorporate integral "Top Boost" controls.

The amp is powered by thirteen tubes. A GZ34 was used as the rectifier for the power supply. A quad of EL-84 were used in the power amp section. The Vib/Trem or "Vibravox" circuit required an ECC83 (12AX7) as a preamp, another ECC83 as an oscillator, and a ECC82 (12AU7) as a modulator. Five additional ECC83 tubes served the functions of preamplification, reverb drive, phase inversion and powering the tone controls.





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Photos and editorial content courtesy Gary Hahlbeck, North Coast Music


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