Steve’s guide to classic amp sounds 1: The Vox AC30 sound

The AC30 has a characterful clean sound, not at all hi-fi and quite ‘coloured’. Turned up loud it has a fat smooth overdrive. Wonderful and unique, but how does it do what it does?

The basic non-top-boost AC30, played through one of its non-vib/trem channels, is as simple a guitar amplifier as you can imagine. There is a gain stage (just the one), a volume control, and a power amp with a tone cut control. That’s it.

The absence of a tone stack, indeed of any tone controls at all in the preamp, means that the AC30 preamp’s one little gain stage is actually capable of pushing the power amp into overdrive. Tone stacks – especially the three-knob sort – 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. Good for gain, and really good for clarity and ‘chime’ and that lovely airy thing you get from such glorious simplicity in a valve amp.

The top boost section on the ‘brilliant’ channel (if you have treble and bass controls, then you have a top boost version), adds a gain stage and a further ‘cathode follower’ stage dropping the impedance and bringing up the current, before introducing its two tone controls. However it adds them after the basic AC30′s gain stage and volume control are allowed to overdrive these extra stages, making the preamp portion of the overdrive sound subject to the tone-shaping power of the bass and treble controls. Which is good – you want your tone stack after your distortion so you can shape the crunch the way you like it. The overall result on the top-boosted Brilliant channel is a still-simple preamp, though now with three stages and some more stuff in the signal chain, which puts out distinctly more gain than the basic preamp, and which can itself be overdriven internally.

Is that all there is to the AC30? Nope, there’s the power amp, which has several unique tone-shaping factors. But first, some philosophising…

I think it helps to understand the nature of old equipment if you first try to work out why its manufacturers did what they did in designing it, rather than looking at the fascinating but misleading business of how it was actually used by the wayward genuises of popular music in creating works of musical art. So what was in Dick Denny’s mind when he sat down in a smoke-filled room at Jennings Musical Instruments and sucked his pencil with a new amp in mind? Well one thing we can be sure of is that he was not thinking about the Brian May squeal, the Rory Gallagher howl, chimey Beatley strumming, or any of that. Above all he definitely was not thinking about overdrive, as he sat down to plan an amp that was to give some of the best overdrive sounds the world has ever grooved to. One thing we can be absolutely sure of is that he was thinking about the market as it was in the late 50s/early 60s, and doing so with some responsive accuracy, or his designs would not have sold.

If you’ve ever taken the back off an old AC30 you will have seen quite a bit of complexity inside there – and thus you might now be wondering how I can I describe it as an utterly, beautifully simple amplifier. The explanation is even simpler – the majority of the components in there are not employed in amplifying your guitar, but in creating the vibrato and tremelo options you probably never use. Tremelo – the regular variation in volume familiar from many early Sixties amp designs, especially blackface Fenders – involves an oscillator, and some way of applying the wobble it makes to the signal – ‘modulating’ the basic sound. Vibrato, which the AC30 also offers, is a much rarer beast and involves a regular variation in the pitch of the output. The same oscillator can be used for both vibrato and tremelo but a different and more complex modulation system is required for the variation in pitch that vibrato requires. That’s what most of the guts of the AC30 is doing – oscillating and modulating – and nowadays hardly anyone uses the options those circuits provide, especially the most complex one, vibrato.

The Jennings company’s first product had been a valve synthesiser called the Univox. It was either this, or the very similar Clavioline, that was used to produce the space-age melody line on ‘Telstar’. I happen to have one of these – a quite complex piece of valve technology. If you think of where the company was coming from when they began to expand their range it’s easier to understand the AC30′s design.

This was a synth manufacturer moving into the growing guitar market. Rock’n roll bands were forming all around them. There was a market for their piano-add-on space-age sound-generator, but a much bigger market was taking off, and it featured guitars – and those innovative bass guitars. These bands needed amplifying so the guitars could keep up with the drums – and Vox offered them a one-box solution. An amplifier with a section for bass, a section for a bright-sounding guitar, and an effects section to build on their brand’s reputation for spooky sputnicky sounds – each section with two inputs, and each with a volume knob to get the mix right. The AC30 was conceived as a band-in-a-box amp, but this was a synthesiser firm, and to build on that reputation the AC30 circuit was also designed as a guitar sound synthesiser, making the then-radical new vibrato and tremelo effects available to enhance the sound.

With all these things to think of, each of those channels needed to be as simple as possible, as every complexity designed into in the preamp channels was going to be multiplied by three.

So the simple guitar amplifier that sounds so wonderful in its simplicity is kept so simple in order to make room for three channels, and for some state-of-the-art sound-processing circuitry.

My reading of the development of this circuit is that the prime goal in designing the Vox amplification circuits was to keep the preamp down to one gain stage, which is only really possible with no tone control at all. Just a gain stage and a volume control to turn up nice and high.

Which brings us to the stage after the preamp, where Denny put his one tone control – the phase inverter, or PI, which splits the signal waveform into top and bottom halves for the push-pull power amp to drive into the two-part output transformer. He used a long-tailed pair PI, which has some inherent gain and good fidelity. It is possible to use a single triode for the PI (eg in the ‘concertina splitter’ that I discussed a month or two ago), but despite its economy this doesn’t give any gain. The AC30 needed that extra gain – but it is more economical to provide it in the PI, which all three input sections use, rather than trebling the effort by putting extra gain into each of those sections.

The PI splits the signal into two inverse halves. If you mix those halves together they will cancel each other out, the result being silence. Denny put a high-frequency filter across the two halves, and made it adjustable, so that if you turned it up you cancelled out the tops. Which is why the Vox’s ‘cut’ control works the ‘wrong way round’ for a treble control – the more you turn it up, the more tops it cuts out. It’s a very effective filter for the sharp highs of this amp’s brilliant channel – and it costs nothing in preamp gain. It is there because you can’t put tone controls in single gain stage preamps. Oh, and it just happens to provide exactly the top cut you need to smooth out an AC30′s overdriven sound, though nothing could have been further from its designer’s mind.

Now to the AC30 power amp. It uses EL84 valves, which were a UK design and make. Very important, that, in those days of import controls. To get the amp up to the 30 watt (ok, 27) that Vox felt (quite rightly) was necessary to keep up with rock’n roll drummers, you needed four of these slim, light, cheap British valves. And you needed to run them hot. Denny knew that you could drive these little beasts above their normal plate dissipation for extra watts, and he did so. Perhaps he had noticed that Leo Fender was doing the same to squeeze extra volume out of USA valves – more voltage, more current, more watts. He biased his quartet of El84s with a single resistor on the cathodes, and bypassed it with a fat capacitor to maximise gain. That cheap, simple method of biasing – ‘cathode bias’ – also provided a sweet compression that Fenders didn’t have when you opened them up – but once again, that was a completely unintentional byproduct of a market-driven design.

One more thing about this power amp. It has no negative feedback. Negative feedback is just what it sounds like – you take a part of the signal from late in the amp’s signal chain, often from the speaker output itself, and you feed it back into an earlier part of the amp where the wave is running a negative version of the signal you are using. It’s a bit like the two halves of the signal in the PI, so to avoid cancelling the signal out you cut down the fed back part to a fraction of the signal at the point where you’re feeding it back in. It cuts down the gain a bit, but it also flattens down the response, making it more linear and hi-fi. It helps control that demon of the high-gain amplifier, internal oscillation. Leo Fender always used it, probably because his amps without it were prone to oscillation and it helped avoid tiresome fiddling at the quality control stage. But the relatively low-gain design of Denny’s AC30, with its spacious chassis and good layout, avoided oscillation problems anyway, so Denny could dispense with negative feedback. Probably he had his one-stage preamps in mind, and avoided everything that might stop him wringing every last ounce of gain out of his amp.

Negative feedback doesn’t sound bad in itself – but when I sit nine of out ten guitarists down by the bench with a guitar in their hands and lift the negative feedback out of their amps, they want me to leave it out. The sound without it is a bit gainier, and a bit less ‘tight’, a bit more ‘natural’ and a bit less hi-fi. And that, my friends, is the final piece in the complex sonic jigsaw that makes the AC30 circuit the unique musical instrument that we all know it to be. Especially cranked.

Oh yes and alnico speakers. Jennings fitted a good pair of current-manufacture speakers to their amp, which seem to have been chosen for their high efficiency – giving more volume. The famous Celestion blue alnicos. As it happens, they also compress the sound quite nicely as they approach their relatively low power ceiling, putting the finishing touch to the the overdriven AC30 sound – a sound it was not designed to produce!

Do you have an AC30 with ceramic-magnet speakers, and are you thinking of buying it an expensive pair of Celestion Blues for its birthday? Here is a wee hint to end with – don’t change both speakers, just change one. The mix gives you the best of both worlds. Try it and see, and save yourself £120 or whatever it is these days.