AC-30 Top Boost Control Panel

The AC-30/6 Top Boost Super Twin with Reverb, Closed Back Cabinet - 1964




Closeup of the reverb controls in an AC-30 Super Reverb Twin amp head


"Egg" Foot Switch for Vib/Trem Effect

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When Vox first produced the AC-30, it was only available as a self contained "combo" amp. In fact, the name AC-30 stands for "Amplifier Combination - 30 watts. With the amplifier and speakers combined into one case, the AC-30 tipped the scales at about 80 poumds. Fender had addressed this issue by converting a number of their combo amps to separate heads and cabs for ease in transport. JMI followed the trend by introducing the AC-30 head and a matching speaker cabinet in the 1962 Vox catalog. These new amps were called the AC-30 "Super Twins."

Vox also recognized that Fender amps offered separate treble and bass controls as a standard feature. Prior to 1964, if one wanted separate treble and bass controls on an AC-30, an optional Vox "Top Boost" kit needed to be retrofitted to the amplifier. The Top Boost circuit added treble and bass controls that were mounted rather inconveniently in a rectangular hole cut into the upper back panel of the amp. In response to customer demand, Vox revised their control panel in 1964 to relocate the treble and bass controls into the top panel.

The addition of reverb into many Fender amps placed additional pressure on Vox to include this feature into their amps. Since Fender incorporated the Hammond Accutronics style reverb circuit into their amps. one would think that Vox would follow their lead, but this was not to be the case.

Tom Jennings, the president of Vox, resented having to pay the $1 per amp licensing fee charged by Hammond Accutronics for the use of their patented reverb pan. Instead, he designed his own reverb pan, just barely skirting the patents on the Accutronics unit.

The reverb pan designed by JMI for the AC-30 Super Twin used two 1 volt output ACOS GR71 crystal phono cartridges for drive and receive transducers. A single delay spring was connected to the needle saddles of each cartridge.



The earlier addition of the Top Boost controls to the main control panel left no room to add reverb controls. The reverb controls were fitted into upper the back panel, as formerly done with the Top Boost controls. See photos at left.

The best that one could say about the JMI reverb pan is it worked to a degree, but the tone and depth of the reverb was certainly lacking when compared to Fender amps of this era. Furthermore, this phono cartridge based reverb pan would easily slip into a howling acoustic feedback if the amp was played too loudly. Even worse, replacement phono cartridges for this reverb pan have been unavailable for many years.

Not all AC-30 Super Twin amp heads incorporate either or both of the Top Boost or JMI reverb circuits. Some AC-30 Super Twin heads have non top boosted "Treble" or "Normal" circuitry while others may be top boosted with the treble and bass controls installed either in the back or top control panel.

The amp shown on this web page also features a 2x12 AC-30 cabinet with the rare closed back option. The cabinet is powered by two Celestion Alnico Blue speakers.

A single button "egg" foot switch remotely controlled the Vib/Trem circuitry of the AC-30.


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Photos and editorial content courtesy Gary Hahlbeck, North Coast Music


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