In order to avoid internal oscillation, a lot of amp designers would use negative feedback in their designs. One thing to note is that the AC30 has no negative feedback. 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. They are highly efficient speakers, giving extra volume. The speakers that have been chosen for the AC30 are the 2×12 Celestion Alnico Blue Speakers. 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’). To get extra watts, these valves can be run above their normal plate dissipation. In order to get the 30 watt Vox was aiming for, they needed four slim, light, British valves. 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. 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. These opposing signals are supplied to each half of a “push-pull” amplifier stage. The signal first goes into a phase inverter, which divides the signal into two signals 180 degrees out of phase with each other. Then, before going into the power amp, there is a tone control section. This control blends in more gain by turning it clockwise. The volume control determines the sensitivity of the preamp section. This helps create enough gain and on top of that adds clarity and an airy sound to the amplifier. 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. Tone stacks tend to suck out loads of gain and usually have at least one extra gain stage to drive them. The first preamp stage doesn’t have any tone stacks. 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 sound is also quite colored and turning it up creates a fat smooth overdrive.Īssuming everyone reading this understands the vibrato/tremolo effect, we will skip this section. The AC30 sound can be described as clean, yet with a lot of character. And then finally, we have the tone control. The AC30 comes with or without a Top Boost option, which can be recognized by Bass/Treble knobs. Newer versions also have a separate reverb section, which we won’t cover in this article. Next are the volume controls for each of the 3 channels (vibrato/tremolo, normal and brilliant). In newer versions, they only kept the tremolo effect with its Speed and Depth parameters. In older units, they are Speed and Switch controls. To the right of the input section, we generally have the vibrato/tremolo parameters. Normal inputs, brilliant inputs and optionally separate vibrato/tremolo inputs. Let’s start with the input section, where you can have up to 6 inputs, divided into 3 different channels. The resulting amplifier was dubbed the AC30/4 Twin. Denney also saw it fit to expand the dimensions of the amplifier’s cabinet and add an additional speaker. 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. And so the AC1/15 was born, which later became the AC15. 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. Vox Amplification begins with Dick Denney, a young amplifier designer who began working for England’s JMI Corporation in 1957. Go to the gadget at the bottom of this page, to listen to the actual Vox AC30. John Lennon used a Vox AC30 for the first half of his career, along with the rest of the Beatles. 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. This week, we will talk about one of the most iconic and well-known guitar amps, the vintage Vox AC30.
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