Oct 262017

By Mitch Colby

Over the last few years, this column has discussed a variety of collectible vintage amp brands and builders, but for the next few columns I’ll be going into further detail and discussing several of the most interesting and collectible vintage amp models individually.

My first subject is the Vox AC100. Vox was the U.K.’s leading company in amp sales from 1959 through the mid Sixties, and they offered everything from small four-watt combos to 100-watt heads and cabinets. Just about every British Invasion band used a Vox amp at one point in their career. Most guitarists are familiar with the AC30, which was and still is found in many amp collections, on numerous stages and at countless recording studios.

Vox developed the AC100 because bands in the Sixties needed amplifiers that were powerful enough to fill clubs and concert venues before powerful P.A. systems were developed and became the norm. Introduced in 1963, the AC100 featured a preamp that was similar to the lower powered AC50, but it offered only a single channel with volume, treble and bass controls. In the first of the two versions of the AC100, the power amp was a higher powered version of the cathode-biased AC30 circuit that produced about 80 watts of output and incorporated EL34 tubes instead of the smaller EL84 tubes used in the AC30. I don’t know of any other cathode-biased EL34 guitar amp (other than my own design, the Colby Mod Machine). The benefits are an interesting combination of attributes, as this first version of the AC100 has a very dynamic feel with some compression and harmonically rich tone. However, it is quite rare as Vox built less than 400 version 1 AC100 heads between 1963 and 1965.

When Vox introduced the second version of the AC100 in 1965, the output stage was converted to grid (fixed) bias. This bumped up the power output to 100 watts and also increased reliability. This second version of the AC100 was produced only until 1969 with an estimated total production of about 1,850 units. It is also a great sounding amp, but the tone and dynamics do differ from the first version in several ways.

A notable feature of the AC100 is the “very small box” design. Unlike the Marshall “small box” which is only narrower than a standard full-sized head, Vox “very small box” amps are short and narrow. The very first AC50s were housed in the same small box as the AC100 but soon changed to a taller cabinet. The earliest AC100s had thinner wood on the front and back. Although physically appealing, this cabinet was more difficult to manufacture and was soon eliminated.

AC100 amps have an XLR output for the speaker and an unusual four-pin XLR connector for the AC mains cord. This connector was not very robust and many guitarists that that used AC100s in band situations had their amps modified with hard-wired AC cords or IEC plugs replacing the XLR connector.

Like most other early Vox amps, the AC100’s transformers add rigidity to the chassis by being mounted to both a horizontal and vertical chassis. The earliest AC100 had an “L” bracket added to the top of the power and output transformers that affixed them to the horizontal chassis. In later AC100s and most other Vox amps, the transformers had an end bell with “feet” that attached directly to the horizontal chassis.

The tubes used in the AC100 are: one ECC82 (12AU7) for preamp gain stage; one ECC83 (12AX7) for gain stage and cathode follower tone driver; one ECC82 (12AU7) for driver/phase inverter; and four EL34 tubes for the power amp. As with most other Sixties British amps, the primary AC voltage tap labeled 115 volts (or 105/115 on early Marshall amps) shouldn’t be used without taking proper precautions. These amps ideally need 110 volts to operate properly, but power outlets in the USA typically provide an average of 120 volts or even higher. The solution is to use a Variac dialed down to provide 110 volts (although this is really best for temporary applications), get a voltage regulator like the BrownBox, or buy a 120-to-240-volt step up transformer and use the transformer’s original U.K. 245-volt setting. If you don’t, you run the risk of blowing up filter caps and other critical components in the amp’s circuit. Unfortunately, the AC100 version 1 is particularly prone to failure, but can be modified to provide more reliable performance.

A version 1 AC100 in prime working condition is a beautiful thing to behold sonically and visually, especially with its matching 4×12 cabinet and chrome stand.

Mitch Colby helped develop many Marshall and Vox amp designs and is the founder of Colby Amplification.

Little Big Amp: The Vox AC100
Source: Guitar Aficionado