The JMI Vox AC-4 Chassis - A Look "Under the Hood"


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© 1998 - 2019 The Vox Showroom, all rights reserved. No use on online auctions, eBay or Reverb.
The original circuit for the AC-6, AC-2 and AC-4 was detailed on JMI schematic OS/006 dated 12/9/1959. A revised AC-4 circuit was released on JMI schematic OS/051, dated 9/9/1964.

The JMI Vox AC-4 chassis was formed from a folded piece of 16 gauge steel. The outside of the chassis, including the control panel area, was painted tan. Unlike all other Vox amps from this era, the AC-4 did not include a separate control panel. The control nomenclature was silk screened directly to the painted chassis steel.

Most of the electronic components of the JMI AC-4 circuit were hand-wired to either the tube sockets, the lugs of the rotary controls or to a grounded buss wire that extended the length of the chassis. The simplicity of the AC-4 circuit eliminated the need for an additional tag strip to interconnect the elements of the circuit.

AC-4 Preamp Circuit
The AC-4 preamp used one of Dick Denney's favorite tubes, the EF86. Denney's preference for the harmonically rich EF86 tube was apparent as he designed it into the preamp circuitry of the entire original Vox amp line (AC-2, AC-4, AC-6, AC-10, AC-15 and AC-30). Tendencies toward microphony in larger amplifiers forced a redesign of the AC-30 in 1961 to eliminate the EF86, but it was retained in the AC-2 through the AC-15 until Vox discontinued the models.

A single passive "Tone" control attenuated high frequency response by shunting treble frequencies through a .001uf capacitor to ground. The "Tone" control also included the power switch.

The vibrato (tremolo) oscillator circuit was powered by the 12AX7 tube. A 1 meg rotary "Speed" control adjusted the tremolo rate while a single button foot switch enabled and disabled the effect.

AC-4 Power Amp Circuit
The 3.5 watt AC-4 power amp circuit utilized one EL84 tube in a "single ended" power amp circuit. The output transformer was designed to accept a 3 ohm speaker.

AC-4 Power Supply Circuit
The power transformer used in this 1962-63 version of the AC-4 chassis featured primary (or mains) winding taps of 200VAC, 230VAC and 250VAC. JMI preselected and hard wired the appropriate mains voltage tap to the power supply based on the location of the dealer selling the amp. Later versions of the AC-4 included a revised power transformer with taps to accept either 115VAC or 230 VAC wall current. A "plug in" style voltage selector plug on the control panel allowed the end user to choose the appropriate mains voltage. The fuse was located inside the cap of this voltage selector plug.

The secondary of the power transformer included a center-tapped 250 VAC B+ supply winding and a 6.3 VAC filament heater winding. An EZ80 (6V4) full wave rectifier tube combined with two 32uf filter capacitors and a 1k 5 watt resistor to create a 270 VDC B+ power supply.

AC-4 Speaker
The 1962 AC-4 chassis shown on this page powered a factory installed 8" Elac speaker rated at 3 ohms.





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Photos and editorial content courtesy Gary Hahlbeck, North Coast Music


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