Vox AC15 twin

If you were a young guitarist in the early 1960s and couldn’t afford an iconic Vox AC30, a pretty good bet was the cheaper AC15 twin. Externally the AC30 and the AC15 twin had the same dimensions, looked pretty similar and both had two 12” speakers.

The more common AC15 comes in a smaller cabinet and has just one 12” speaker, but other than that the circuit is basically the same as an AC15.

The design of the AC15 is notable for a few reasons:

In the 50s and 60s many guitar amp designers would incorporate a circuit design known as “negative feedback”. Negative feedback sends a portion of the signal coming out of the amplifier back to the input of the power amp. The intention of this is to increase headroom and reduce distortion, but whilst doing so some of the harmonics in the signal are removed.

Dick Denney, the designer of the AC15 decided that he preferred the sound of the amp without negative feedback with its increased harmonics. He also liked the way the amp distorted when it’s overdriven.

Another design feature of this particular iteration of the AC15 that adds to the harmonically rich sound of the AC15 twin is the use of a single EF86 valve in the pre-amp section which is higher gain than the more commonly used ECC83 valves, of which there are already three. Later designs of the AC15 remove the EF86 valve.

In 1963, the year this amp was built, the AC15 twin shipped with the lesser quality Goodman Audiom 50 speakers which we have since replaced with modern Celestion alnico speakers which sound more like the classic recognisable Vox sound.

The sweet spot of this amp where there is just the right amount of crunchy distortion I find to be quite unique and beautiful and although to my ears it sounds different to the more common AC30 the sound is quite recognisable as the sound of the British guitar bands of the 60s.