The old adage holds that imitation is the sincerest form of flattery, but it’s also an easy gauge of any vintage amp’s “classic” status within the realm of great amps designed in the past 60 years. A quick tally of contemporary amps inspired by the AC30 — which includes great models from boutique makers such as Matchless, Dr Z, Bad Cat, TopHat, 65 Amps, Trainwreck, Bruno, Komet, and several others — shows us just how inspirational Vox’s 50-year-old circuit has been, and how secure its status as one of just a handful of seminal blueprints.
Like so many other great guitar and amp companies, Vox’s roots are pre-rock and roll, but the eventual boom in this new musical genre would help to secure its success. Tom Jennings, himself an accomplished accordionist, built electronic organs in the 1940s and started his Jennings Organ Company in 1951 to sell portable organs. He founded Vox amps’ original manufacturer, Jennings Musical Instruments (JMI) in 1957 with the specific intention of cashing in on the British rock and roll boom, and while the AC30 wasn’t Vox’s first model, it was the amp that really put the name on a bigger map.
The AC30 is a descendent of Vox’s first successful amp of the late 1950s, the AC15. While many guitar amps of the day were derived from circuits published by tube manufacturers for general amplifier applications in radios and gramophones, Dick Denney, the electronics wizard who designed the AC15 for Jennings, approached the job with the guitar’s own sonic performance in mind, adapted these circuits accordingly, and the results speak for themselves. This amp’s cathode biasing and lack of negative feedback give plenty of sparkling, slightly unchained chime to the guitar’s upper-mids and highs, thanks to the way they encourage second harmonics as artifacts of distortion. This offers a flattering tonality at low and middle volume levels, and a creamy overdrive when cranked (note that these design elements just aren’t suitable in larger amps, so you don’t see them in 50W or 100W models). Also, in letting the output stage run “open loop” – that is, free from any negative feedback-derived damping – Denney’s design offered a rich, touch-sensitive, and therefore very playable performance when pushed. These amps’ tube rectifiers also add to the touch-sensitive, slightly compressed playing feel that so many players love. While both the AC15 and AC30 are often sited as “classic class A amps”, it is probably more accurate to sum up their sound as the result of these features: their cathode biasing, lack of negative feedback, tube rectifier, and easily overdriven EL84 output tubes.
With rock and roll’s continued growth, a more powerful amp was needed, and by the late ’50s the popular British group The Shadows (featuring Hank Marvin on guitar) came knocking to ask Jennings and Denney to build them a more powerful amplifier. In late 1959 Denney and Jennings simply doubled the 2xEL84 output stage of the AC15 to give birth to the 4xEL84 AC30 – a more versatile and more rocking amp all-round, which is actually capable of putting out more than 35W in good condition. (Note that AC15s and early AC30/4 combos both used the lesser-seen EF86 preamp pentode preamp tube, but this was soon dropped in the AC30 for the more plentiful and reliable ECC83/12AX7 type. Several contemporary makers, however, have rediscovered the juicy EF86 and currently use it in their own designs.)
The AC30 originally carried a simple, single treble-bleed Tone control, plus a Cut (or “High Cut”) control that rounded off the harsher highs at the output stage. The archetypal AC30 was born a couple years after its introduction, however, when the powerful, interactive Top Boost EQ circuit was added in 1961. This at first came as a back-to-factory retrofit modification, which included adding the extra preamp tube required and mounting the Top Boost’s Treble and Bass controls on a plate bolted to the amp’s back panel. In 1964 the AC30 Top Boost model came direct from the Vox factory as an upgrade option, with the extra knobs mounted right on the main control panel. Wherever you find it, this Top Boost EQ stage adds the extra sparkle and high-end content that bands of the day were looking for to help them cut through the mix, and is a big part of that classic Vox shimmer and chime. Another big ingredient in the classic AC30 stew is the Celestion G12 alnico speaker, originally re-branded with a Vox label and now re-released by Celestion as the Alnico Blue. This great speaker is sweet and multi-dimensional when played clean, but aggressive and harmonically rich when driven hard. It’s highly efficient 100 dB rating also helps to make a 2x12” AC30 combo a low louder than its size and conservative wattage rating would make it appear.
The AC30 has been played by everyone from The Beatles and The Who, to Yank-janglers such as REM and Tom Petty and the Heartbreakers, to heavy rockers from Brian May right up to Dave Grohl of the Foofighters. It continues, in both its original and reissue formats, to be one of the most popular tube amps in the history of the electric guitar.
Taken from Gibson Tone Tips
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