Vox Amplifiers and Effects

Vox AC30 History

The AC30 came about due directly to demand from late 1950's group The Shadows, who were originally ardent endorsers of the Vox AC15. As the rock n' roll craze exploded in Britain and The Shadows and other groups began playing larger venues, it quickly became apparent that the AC15 could not provide the hall-filling volume they required. Coincidentally the first Fender "Twin-Amp" became available about this time in England, offering 60 Watts of clean output with two 30-Watt Jensen Loudspeakers. The Shadows had tried it and were mightily impressed by the volume, but the tone was not as rich as what they were accustomed to from the AC15. The hefty price tag and 110 Volt operation were major deterrents as well. Naturally, this lead them to ask Dick Denney at Vox, "Why not a "Twin" AC15"? It had the characteristic sound they wanted, they just need more of it.

For more on the AC15.

Essential Elements of Vox Tone

Dick Denney looked into the matter, examining the elements responsible for the distinctive sound of his original amplifiers, and determining the best way to preserve their tonal qualities while increasing output. He decided that there were three main factors that required his attention. First was the use of EL84's as power output valves. EL34's were considered at one point due to their heftier power output, but they did not possess the ringing chime that was so essential to the Vox sound. EL84's were designed to produce high power from a small package, at low voltage levels - in other words, they have a high power-gain coefficient. This does not come free of charge, the penalty being a fairly high inherent distortion level of some 5-7 percent at about 15 watts output for a push-pull pair. This normally would have been reduced to the hi-fi standard of the day using a technique called negative feedback, where a connection is made from the output of the amplifier back to a point close to its input circuitry via a resistor. This compares the output waveform to that of the input and absorbs gain from the amplifier to make them similar. As a result, harmonics that are in the output that are not also in the input are suppressed. Denny had tried negative feedback techniques in previous circuits however, and found them distasteful from a musical standpoint. As an accomplished musician himself, he preferred more harmonics rather than less. His choice of EL84's was guided by this preference. Another problem was that amplifiers that use negative feedback often have a harsh and sudden onset of distortion, whereas Vox's AC15 circuit was distinguished by its gentle and progressive clipping. The last essential ingredient of this sound was the use of a technique known as "cathode biasing", otherwise known as "self -" or "auto-biasing." Bias is a term given to controlling voltage applied to the output valves of an amplifier to keep the current conducted through them within prescribed limits, outside of which they would fail within a short time. One way of applying it is called "fixed bias" and is simply a connection of an externally generated negative voltage to the control grids of the valves. The use of the valve current itself to generate the bias voltage gives rise to the term "self-" or "auto-bias", while the placing of a resistor between the cathodes of the output valves and earth results in the term "cathode-bias". They all amount to the same thing, but the important part is that, where EL84's are concerned, cathode-bias sounds superior to fixed-bias, particularly in instances where negative feedback loops are not used.

A Legend is Born

After determining his three essential elements Denney came to the conclusion that in order to preserve the characteristically ringing, harmonically complex Vox tone while at the same time making a much louder amp, was to use four EL84's in the power section. A larger power transformer provided the extra current for this array via a heavy-duty GZ34 rectifier valve, and a single 30-Watt output transformer matched the valves to an 8 or 16 ohm load. An amplifier using this design approach was featured in JMI 1958 publicity as the AC30, but had never found a market. It used a single Goodmans Audiom 80 speaker unit but was more suitable as a bass reproducer. The EL84 circuit had not succeeded in overcoming its limitations as a guitar unit, and the model was unsuccessful. The Shadows vital contribution to Vox's fortunes lay in their ready acceptance of the increase in size and weight implied by the use of two 12-inch speakers, for when two lighter duty Goodmans speakers were tried, the problems vanished, and Vox's most successful model was born. The AC30 model number was transferred with the new model being distinguished from the old as the "AC30 Twin." Or to be more precise, AC30/4, denoting a four-input model. It wore the same light-tan finish, latticed cloth, and fittings as the AC15, but sported a new one-piece molded god emblem on a margin that extended across the top quarter of the front panel. Tom Jennings had been busy during Vox's first production year not only in organizing the many details of labor and supply involved in electronic musical products, but also in evolving the styling and finish of Vox amplifiers. This could be a painful process, with many samples tried and found wanting in the search for the ideal. Jennings desperately wanted Vox to be the electric-musical brand leader. Launching the largest, loudest British amplifier was a step towards this now attainable goal, and he was not prepared to take chances with how it looked.

The reputation of the AC30, and of Vox as the premier British musical equipment manufacturer, was sealed in 1960 when The Shadows recorded a # 1 hit with their new amplifiers, entitled "Apache". More hit singles followed shortly thereafter, and as a showcase for Vox amplifiers it could not have been better. Admirer's and imitators of The Shadows took notice of their reliance on the Vox AC30, and a legend was born.

* Excerpts taken from The Vox Story, by David Petersen and Dick Denney. Copyright 1993.