in the chassis photos above. The earliest examples of such Vox amplifiers featuring "Top Boost in panel" appeared on AC-30/6 amplifiers in 1963.

To more fully understand the Top Boost tone control circuit, let's roll back the clock to look at the evolution of the tone control circuitry in Vox AC-30 amplifiers.

The Tone Control Vox introduced their first amplifier lineup in the late 1950's and early 1960's. As was common for amplifiers designed in this period, the original Vox AC-4, AC-10, AC-15 and AC-30 models were equipped with a primitive single control tone circuit to roll off treble response. This tone circuit wasn't located, as one might expect, in the preamp area. It was located in the power amp circuitry between the phase inverter and output tubes. Here is how it worked.

The signal coming from the preamp was directed to the phase inverter tube. The phase inverter (more properly called the "phase splitter") divided the output from the preamp into two signals 180 degrees out of phase with each other. These opposing signals were supplied to each half of a "push-pull" amplifier stage.

However, the treble frequencies coming from the opposing sides of the inverter circuit were first directed to the Tone control. The Tone control combined these treble frequencies, passively reducing treble output from the amp as the "out of phase" treble signals canceled each other out.

The Tone control affected all three channels of the AC-30/6.

The Quest For More Treble
Led by the UK instrumental group "The Shadows," many British guitarists were asking for more treble response from their AC-30/6 amplifiers. By this time, Fender included active treble and bass controls with most of their amplifiers, a feature not yet included on any Vox model. It was apparent that an active tone control circuit with separate bass and treble controls would need to be developed for the "Brilliant" channel of the AC-30/6.

The Gibson GA-70 Amplifier When designing circuitry for their products, guitar amplifier manufacturers often referred to books such as the RCA Receiving Tube Manual. The RCA Receiving Tube Manual published the electronic specifications for all of the tubes RCA produced in the era. It also offered schematics of sample tube circuits for developmental purposes. The RCA Receiving Tube Manual was a US publication but Mullard offered a similar book in the UK. Dick Denney doubtlessly consulted these books when designing the circuitry for Vox amplifiers. However, Vox would not develop their Top Boost tone circuitry from sample circuits found in the back pages of a tube manual nor was the Top Boost circuit a unique JMI design. The design of the Vox Top Boost circuit appears to have been lifted, part for part, from the 1950's era Gibson GA-70 amplifier (see photo at right). The Gibson GA-70 tone circuit included one 12AX7 tube. One half of the tube was used for gain, the second half was a cathode follower to power the tone control circuit. The GA-70 tone control circuit is shown in Figure 1 (lower left), the Vox Top Boost circuit in Figure 2 (lower right). Both circuits are identical.

Gibson GA-70 Amplifier (Photo courtesy www.tksmith.net)