Vox, AC-30/6 Top Boost, 1966


AC-30/6 Top Boost
The Story Behind


About the Amp (from Wikipedia):

The Vox AC30 is a guitar amplifier manufactured by Vox. It was introduced in 1958 to meet the growing demand for louder amplifiers. Characterised by its "jangly" high-end sound it has become widely recognised by British musicians and others.

The Vox AC30 was originally introduced in 1958 as “big brother” for the fifteen watt (15 W) AC15 model at Hank Marvin's request because the AC 15 was not loud enough with the screaming fans at Cliff Richard concerts., Vox's original flagship amplifier. The AC15 was powered by a pair of EL84 tubes, an EF86-driven "Normal" channel, an ECC83-driven "Vib-Trem" channel, and rectified by an EZ81. The Original first generation AC30, or AC30/4, had only a single 12 inch Goodmans 60 watt speaker in a "TV Front" cabinet, as opposed to the later, conventional twin 12” speaker configuration. The AC30/4 sported two channels with two inputs – hence the “4” in the model name, and had a single tone control in it as well. The amplifier used a GZ34 tube rectifier, three ECC83s (12AX7) for the Normal channel and the tremolo/vibrato oscillator/modulator circuits, and one ECC81 (12AT7) phase inverter, and it had EL34 tubes in the power amplifier circuit.[2]

This first generation of AC30s were housed in "TV-front" cabinets, much like the early to mid 50s tweed Fender amps. However, the amps sported a thin white covering ("Rexine") with a small printed diamond pattern and larger diamond pattern grill cloth. The EL34 powered AC30 was short lived, and a new AC30 version appeared in late 1959. This second generation AC30/4 was powered by a quartet of EL84 (6BQ5) power tubes, making it truly a doubling of the AC15 circuit, and it also carried over the AC15's preamplifier, which included the EF86 pentode in its "Normal" channel. However, this preamplifier, coupled with the AC30's increased output over the AC-15, revealed a reliability issue. The high gain EF86 tube was susceptible to microphonics, or even failure, when exposed to the increased vibration present in this uprated amp. Vox initially offered a 1X12" version and subsequently introduced the 2X12" AC30 Twin, which solved the volume problem at larger venues. The first AC30 Twins used Goodmans Audiom 60’s 15 Watt Speakers, followed by Celestion G12 Alnico speakers.

By 1960, Vox had forsaken the more conservative TV-front look for the now legendary cabinet design that has remained largely unchanged since 1960. The new cabinets featured a different covering known as fawn Rexine, which was a sort of beige leather-cloth with a subtle printed grain. The front baffle was now divided by a thin gold-toned strip with the upper valence covered in fawn Rexine, and the lower grille covered in brown diamond cloth. Ventilation was provided by three small brass vents on the top of the cabinet, and the TV-front's single suitcase type handle was replaced with three leather straps.

To cure the unreliability issues Vox introduced the AC30/6 in 1961 and redesigned the troublesome EF86 tube circuit, replacing it with an ECC83 (12AX7). The AC30/6 was now an amp with three channels, each channel having two inputs.

About this time, the "Top Boost" (or "Brilliance") feature became available as Vox's optional addition of a rear panel-mounted circuit that introduced an extra gain stage and tone controls for bass and treble (as opposed to the single "tone" control of earlier AC30s). The unit became so popular that its features were soon incorporated in newer AC30/6 models, and the controls moved from the rear panel to the control panel. Vox AC30/6 amplifiers from around 1963 had already implemented the top boost, and therefore had 3 tone controls. People began to refer to these amplifiers as AC30TBs.[3] Later on, Vox also offered additional versions of the AC30 unit. In addition to the "Normal" version without the Top Boost, and the Top Boost version (which was a Normal version with the "Brilliance" unit added), Vox, with slight circuit modifications, created two more versions that were “voiced” in Brilliant (Treble), and Bass styles. Of all the different models that came around many consider the AC30 "Super Twin" to be the ultimate AC30, with a "trapezoid" shaped head and separate speaker mounted on a trolly (Vox Story, Peterson & Denny 1993) and Vox showroom web site.

By the middle 60's, the various combo amp versions of the AC-30 started to consolidate into one model.

Cosmetically, brown Vox grill cloth was being phased out in favour of black. The predominant control panel color was now grey. Three injection moulded, black plastic Vox logo handles and three metal Vox logo air vents were mounted on the top of the amplifier. The amps were covered in "basket weave" Vinhyde, and eight "one-pin" corners were standard. A new rotary voltage selector switch replaced the former "removable plug" style system. The chassis was completely hand wired.

Perhaps the most important change was moving the Bass and Treble controls of the rear mounted "Top Boost" circuit to the top control panel. The "Top Boost" circuit would no longer be an optional, added-on circuit, it would be a standard feature on AC-30 amps. As a result, the Bass and Treble controls would no longer protrude from a rectangular hole in the vinyl covered upper back panel of the amplifier.

The AC-30/6 Twin had three channels, Brilliant, Normal, and Vib-Trem. The Bass and Treble controls only affected the Brilliant channel. The Cut control affected all three channels.

The Vib-Trem channel offered a choice between vibrato (wavering pitch) and tremolo (wavering amplitude). A rotary, three position Speed switch adjusted the the rate of modulation. An egg shaped single button foot switch operated the effect.

Two 8 ohm 12" Alnico Silver Celestion G12 speakers were standard. The typical Celestion from this era had wire connecting lugs mounted to a card board tag strip that was riveted to the speaker frame.

This Vox, AC-30/6 Top Boost is in perfect working condition and produces this special sound you cannot find on new amps!


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