|The Vox AC-30/6, covered in fawn vinyl, represented the second generation of the AC-30 amplifier. It replaced the Vox AC-30/4 amplifier that had been introduced only one year earlier. Here is the story.
Vox amplifiers were introduced to the UK market in 1958. The original Vox line up included the AC-2 (4 watts) , AC-10 (10 watts) and the top of the line AC-15 (17 watts). However, as the English music scene grew from small pubs and coffee houses to auditoriums and gymnasiums, larger amps were needed. The arrival of the 60 watt Fender Twin amp in UK music stores convinced Vox that their 17 watt AC-15 amp was not loud enough.
Vox decided that the easiest solution to this problem was to use the AC-15 as the basis for a new larger amplifier. By adding a second speaker and doubling the number of EL84 output tubes, a new 34 watt amp would be created. The EF-86 pentode based AC-15 preamp would be retained for the new amp. When completed, this new four input 30 watt amp would be called the AC-30/4. It was introduced to the UK market in late 1959.
Shortly after the AC30/4 amp was introduced, an unforeseen engineering problem arose. The EF86 tube was susceptible to damage from excessive vibration. In the 17 watt AC-15, this was not a large issue. In the AC-30/4, the strong vibrations caused by a 34 watt power amp powering two 12" speakers was more than the EF86 tube could tolerate. As the EF86 would begin to fail, the tube often would became microphonic. Howling and riniging tones from the failing EF86 tube would accompany the the tone of the guitar playing through the amp.
Vox addressed this problem be redesigning the preamp of the AC-30. The troublesome EF-86 tube was replaced with a battery of ECC83 (12AX7) tubes. A third channel was also added. By the end of 1960, the AC-30/4 was being phased out in favor of the new AC-30/6.