This week, we will talk about one of the most iconic and well-known guitar amps, the vintage Vox AC30. By listening to Brian May (Queen) or The Edge (U2), you can quickly identify the sound of the Vox AC30, which has turned into a very recognisable signature sound for many artists. John Lennon used a Vox AC30 for the first half of his career, along with the rest of the Beatles.
Vox Amplification begins with Dick Denney, a young amplifier designer who began working for England’s JMI Corporation in 1957. Dick was a guitarist himself and was focused on designing an amplifier that could offer more volume and sustain, something guitarists at that time were looking for. And so the AC1/15 was born, which later became the AC15.
With the rise of Rock ‘n’ Roll in the early 60s, Dick and the Vox crew saw the need for more power from their amplifier and thereby doubled the power of the AC15. Denney also saw it fit to expand the dimensions of the amplifier’s cabinet and add an additional speaker. The resulting amplifier was dubbed the AC30/4 Twin.
Let’s start with the input section, where you can have up to 6 inputs, divided into 3 different channels. Normal inputs, brilliant inputs and optionally separate vibrato/tremolo inputs.
To the right of the input section, we generally have the vibrato/tremolo parameters. In older units, they are Speed and Switch controls. In newer versions, they only kept the tremolo effect with its Speed and Depth parameters. Next are the volume controls for each of the 3 channels (vibrato/tremolo, normal and brilliant). Newer versions also have a separate reverb section, which we won’t cover in this article. The AC30 comes with or without a Top Boost option, which can be recognized by Bass/Treble knobs. And then finally, we have the tone control.
Design and Sound
The AC30 sound can be described as clean, yet with a lot of character. The sound is also quite colored and turning it up creates a fat smooth overdrive.
Assuming everyone reading this understands the vibrato/tremolo effect, we will skip this section. Basically there are 3 main stages in the amplifier design: a single gain stage, a volume control and a power amp with a tone cut control.
The first preamp stage doesn’t have any tone stacks. Tone stacks tend to suck out loads of gain and usually have at least one extra gain stage to drive them. With none of that in the way in the basic AC30’s signal chain, there are a minimal number of components and stages between your guitar and the power amp. This helps create enough gain and on top of that adds clarity and an airy sound to the amplifier.
The volume control determines the sensitivity of the preamp section. This control blends in more gain by turning it clockwise. Then, before going into the power amp, there is a tone control section. The signal first goes into a phase inverter, which divides the signal into two signals 180 degrees out of phase with each other. These opposing signals are supplied to each half of a “push-pull” amplifier stage. By using a high-frequency filter across the two signals, and making it adjustable, you cancel out the highs the more you turn it up. This is why the Vox’s ‘cut’ control works the ‘wrong way round’ for a treble control – the more you turn it up, the more highs it cuts out.
Now we have come to the AC30 power amp. In order to get the 30 watt Vox was aiming for, they needed four slim, light, British valves. For this, they settled on 4 EL84 valves. To get extra watts, these valves can be run above their normal plate dissipation. Vox decided to bias the four El84s with a single resistor on the cathodes, and bypassed it with a big capacitor to maximize the gain (this is called a ‘cathode bias’).
The speakers that have been chosen for the AC30 are the 2×12 Celestion Alnico Blue Speakers. They are highly efficient speakers, giving extra volume. Not only that, they also compress the sound quite nicely as they approach their relatively low power ceiling, putting the finishing touch to the AC30’s overdriven sound.
One thing to note is that the AC30 has no negative feedback. In order to avoid internal oscillation, a lot of amp designers would use negative feedback in their designs. But the relatively low-gain design of the AC30, with its spacious chassis and good layout, avoided oscillation problems, and therefor no negative feedback was ever implemented.
Brilliant channel and Top Boost
As we previously mentioned, the AC30 has a separate brilliant channel. This has been achieved by adding a gain stage and a ‘cathode follower’ stage, thereby dropping the impedance and bringing up the current. Originally spotted in the GA-70 & GA-77, Vox decided to add Bass/Treble controls to this channel, and named this option “Top Boost”.
The brilliant channel with optional top boost is placed after the volume control section and before the phase inverter. This means you can shape the sound after the distortion.
At Abbey Road Institute Amsterdam, we are very fortunate to own an early model vintage Vox AC30, one without Top Boost. And to make this article a little more fun, we have created another interactive widget that allows you to listen to the actual amplifier.
A big thanks to Mathijs Rosenbrand for playing his Gibson Les Paul Standard for us. We recorded the amp using a Shure SM57, going through SSL preamps and Focusrite converters.
To hear the effect, flick the on/off switch on the unit below. You can use the patch cord to try the different channels. At the bottom right corner, you can click the “Too cool” button to see how you can combine multiple channels and get a nice blend going.
Besides changing the inputs, you can also change the tone control to hear what it can do. Have fun!