|The 1960 JMI Vox AC-30/6 represents the second generation of the traditionally styled 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 helped to convince JMI that a more powerful Vox amp would be needed.
Vox decided that the easiest solution to this problem was to use the popular 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 and control section was designed into the new amp without changes.
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 used in the preamp area of the amp proved to be 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 ringing tones from the failing EF86 tube would accompany 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 phased out in favor of the new AC-30/6.
The Beatles first recordings in the Abbey Road studio used a fawn AC-30/6 similar to the amp seen at left.
The early version of the AC-30/6 was covered in a tan or "fawn" vinyl that was as thin as wallpaper. The amp also had three leather handles, stamped brass vents, brown diamond pattern Vox grill, and a gold plated VOX logo.
One of the three channels in the AC-30/6 included Vib/Trem, an effect that combined variable tremolo (pulsed volume) with variable vibrato (wavering pitch). A cast aluminum remote foot switch canceled the effect.
Each of the three channels in the AC-30/6 had an individual volume control.
A single tone control rolled off the treble on all three channels simultaneously. Individual treble and bass controls were not included in the stock version of the AC-30/6. An optional factory installed, rear panel mounted "Top Boost" circuit became available in 1961. This circuit added a treble and bass control to the "Brilliant" channel.
Fawn Vox AC-30 Bass amplifiers had the early "thin edge" cabinet. While the top and speaker baffle of the amp are 3/4" baltic birch plywood, the sides and bottom are 1/2" baltic birch. The back panels are 3/8" thick. The back edges of the cabinet were not rounded (see photo, lower left).
The amp also featured two 12 "Celestion Alnico Blue speakers. The Celestion Alnico speakers in the earliest production of the AC-30/6 might not have a magnet cover and might be tan, rather than blue. See picture below.