JMI Vox AC-30 with "Top Boost in Panel"
The Story of Top Boost
"A Look Under the Hood"






2_Footer
© 1998 - 2017 The Vox Showroom, all rights reserved. No use on online auctions, eBay or Reverb.

Many feel that the design of the AC-30 was not complete until JMI moved the controls for the "Top Boost" tone control circuit from the upper back of the cabinet to the main control panel, as shown
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)


The Mystery of the Grounded Bass Control
While extremely simple in design, the tone controls of the Gibson GA-70 were strangely interactive. Advancing the GA-70 bass control also affected the tonality of the mid and high frequencies. The interaction between the tone controls was caused by an unusual ground connection on the GA-70 bass control. While a more conventional tone circuit design might either leave off the ground or install a resistor between the bass pot and ground, Gibson directly grounded one of the legs of the bass control potentiometer. As all "top boosted" Vox amplifiers share the GA-70 tone circuitry, they exhibit the same peculiar tone control interaction.
The fact that this unconventionally grounded bass control was included in the Top Boost circuit provides additional evidence that Vox copied the electronic design of the Gibson GA-70 tone controls.

A debate has arisen regarding the grounded bass potentiometer in the GA-70. An excellent and highly detailed article written by Glen Lambert and published at www.blueguitar.org suggested that the grounded bass control was a design error by Gibson that has been perpetuated in thousands of Vox "top boosted" amplifiers, including current production. In his extensively researched book entitled "Vox Amplifiers - The JMI Years," author Jim Elyea offered an opposing opinion. He suspects that the bass potentiometer was intentionally grounded as a part of the GA-70 circuit design.

Retrofitting the Top Boost Circuit to the AC-30/6
When first introduced in 1961, the Vox Top Boost circuit was offered as an optional kit that could be added to any AC-30/6 amp. The installation would frequently be handled by the factory.
The entire Top Boost circuit was mounted to a compact "L" shaped steel bracket that fastened to the flange just below the center of the control panel and to the bottom center chassis mounting hole (see photos at left). The words "Bass" and "Treble" were engraved into a rear facing control panel. The tone controls were exposed behind a rectangular access hole cut into the upper rear cabinet panel.

Vox described the AC-30 Top Boost kit in a 1965 catalog where it stated, "A conversion unit to give you the ultimate top boost (over 30db at 10 kilocycles). Incorporates separated bass boost for new wonderful split sound and pseudo stereo effects."

Moving the Top Boost Controls to the Main Control Panel
Problems relating to the after market installation of the optional Top Boost circuit soon developed. JMI employed expert technicians that would professionally secure the Top Boost bracket and make the proper connections. JMI also stocked the vinyl covered back panels that included the rear control panel access. However, some customers hoped to save money by making the Top Boost kit a "do it yourself" project. The results of such installations were often less than desirable.
Furthermore, the placement of the optional Top Boost circuit with rear facing tone controls looked like an afterthought, which indeed it was. Additionally, the location of the tone controls on the back panel made them inconvenient to adjust. It no longer would be adequate for JMI to offer Top Boost only as an optional kit, it deserved a proper presentation. This problem was corrected in 1963 when Vox introduced a new AC-30/6 model with the Top Boost controls located on the main control panel.

The original AC-30/6 control panel and chassis (Figure 4) would now need to be reconfigured to make room for two more controls and an additional ECC83 tube. By crowding the jacks and controls closer to each other, room was created for the Top Boost bass
and treble controls (Figure 3). Reducing the distance between the five preamp tubes in the earlier AC-30/6 chassis (Figure 4) provided room to add the additional tube to power the Top Boost circuit (Figure 3). The inside of the new AC-30/6 Top Boost chassis would be stamped with the words "Top Boost" in red paint (see second photo from the top of the page).

The pair of brass mounting screws that secured the upper back of the cabinet against the control panel also needed to be relocated on the AC30/6 Top Boost chassis. The distance between the upper mounting screws on the original AC30/6 chassis had been 13." This distance was reduced to 11 3/8" after the control panel modification to incorporate integral "Top Boost" controls.

The earliest versions of the AC-30/6 Top Boost chassis were produced in 1963 with dark red control panels. As JMI shifted away from brown to black grill fabric in 1964, the control panel color went from dark red to gray to better complement the new grill.

The control panel layout of the JMI AC-30/6 Top Boost amplifier survived the VSEL, Birch-Stolec,
Dallas, Rose-Morris and Korg ownership periods of Vox, a great testimony that JMI had "gotten it right." The eight control "Top Boost" panel was finally retired in 2004 with the introduction of the two channel Vox AC30CC2 amplifier.

AC-30/6 Treble Model
JMI introduced the AC-30/6 Treble, a specialized version of the original six control AC-30/6, in 1964. Often known as the AC-30/6T, this model included a few minor circuitry changes that accentuated treble response. While an improvement over a standard AC-30/6, the AC-30/6T lacked individual bass and treble controls and was not capable of the "glass shattering" treble response of an AC-30/6 Top Boost amp. I have included a brief mention of the AC-30/6 Treble amplifier for the sake of clarity.

The Vox AC-30/6 "Treble" amplifier added a 220pf treble bypass capacitor to the volume controls of the Normal, Brilliant and Vibrato channels. It also changed the values of four capacitors in the phase inverter circuit and another for the tone control to passively accentuate treble response. An AC-30/6 Treble model had a red "T" stamped on the lower right corner of the chassis (photo at left).
Credits
The Vox Showroom wishes to thank the following for their contributions to this page.
  • Jeff Benske from the Top Shelf Guitar Shop for allowing the Vox Showroom to photograph the chassis of his Vox AC-30TB.
  • Jill Smith from TK Smith Custom Guitars (www.tksmith.net) for the photo of the Gibson GA-70 amplifier
  • Gibson Guitars for making the Gibson GA-70 schematic available in their web site.






North Coast Music offers a large selection of replacement parts for the AC-30/6 amplifier.
Many of these items are manufactured exclusively by North Coast Music under license to Vox.




2_Footer

.................



The VOX Showroom!


Photos and editorial content courtesy Gary Hahlbeck, North Coast Music


Any and all material presented herein is protected by Copyright.
© 1998 - 2017 The Vox Showroom and North Coast Music, all rights reserved

The images and editorial content in this web site may not be copied or reproduced
in online auction sites such as eBay and Craig's List. Sellers may provide a link
to the Vox Showroom web site if they wish to refer to this copyrighted material.


URL: http://www.voxshowroom.com/uk/amp/ac30_tb_hood.html