The Vox 60th Anniversary AC30HW60 Limited Edition Amplifier (2017) Made in the United Kingdom
Vox Amplification contacted North
Coast Music in October 2015 expressing interest in purchasing a quantity of our vintage style cast aluminum Vox "egg" foot switch cases. I suspected that something big must be in the works, and it was.
Dave Clarke, the head of R&D at Vox, told me about plans to produce two special limited edition models based on the designs of the 1960 JMI AC-15/4 and the 1964 JMI AC-30/6 Top Boost amps. Not only would these amplifiers recreate the original JMI era point-to-point hand-wired circuitry, they would be made in the UK. These amps would be named the AC15HW60 and AC30HW60 and would be released in 2017 to commemorate the 60th anniversary of Vox.
AC30HW60 The Vox 60th Anniversary Limited Edition AC30HW60 was based on the circuitry of the 1964 Vox AC-30 Top Boost amplifier, as detailed on JMI schematics OS/065 and OS/010. The design included eleven tubes: five ECC83, one ECC82, four EL84 and one GZ34.
Like the AC-30/6, the AC30HW60 had six inputs and three channels: Vib/Trem, Normal, and Brilliant with "Top Boost." The Top Boost circuit added bass and treble controls to the Brilliant channel.
The AC-30/6 and the AC30HW60 both featured the complex "Vibravox" circuit in the Vib/Trem channel. Vibravox was unique in that it offered either tremolo or vibrato. While tremolo pulses the volume of the amp, vibrato varies the pitch of the signal. A two position panel mounted rotary switch on the AC30HW60 selected either tremolo or vibrato and a three position panel mounted rotary switch adjusted tremolo speed. A single button "egg" foot switch remotely turned the effect on and off.
The AC30/6 and the AC30HW60 power amp sections were both powered by a quad of cathode biased EL-84 power tubes running in parallel push-pull mode. The EL-84 is a highly efficient tube capable of producing 15 watts per push/pull pair at a relatively low circuit plate voltage of only about 340 volts.
The efficiency of the EL-84 also had a downside. EL-84 tubes were a bit more prone to distort due to their reduced "headroom." Simply stated, when pushed hard, the distortion level could creep up into the 7 percent area. This distortion was normally controlled by the incorporation of a circuit design called "negative feedback." Negative feedback sends a bit of the signal coming out of the amplifier back to the input of the amp. Negative feedback not only cleans up undesirable distortion, it also removes some of the pleasing even order harmonics from the amplifier output.
After listening tests, JMI Vox lead engineer Dick Denney decided he preferred the harmonically rich tone and the natural sounding overdrive of the AC-30 amp when negative feedback was not employed. Although unconventional in design for the period, eliminating the traditional negative feedback circuit in the power amp made another major contribution to Vox tone.
The open back AC30HW60 cabinet was constructed of 13 ply baltic birch plywood covered in smooth black vinyl. The cabinet also featured three leather handles and three plastic vents.
Celestion 12" G12 Silver Alnico Speaker Originally introduced in 1936 for use in radios, a modified version of the 15 watt G12 alnico speaker was produced by Celestion for use in sixties era JMI Vox AC-15/4, AC-30/4 and AC-30/6 amps. The earliest version of the Celestion/Vox G12 speaker was painted tan and had no magnet cover. G12 speakers produced for Vox from 1960 through 1964 were painted blue and had a stamped steel magnet cover. Later Vox G12 speakers were painted silver. All versions of the Vox/Celestion G12 used the H1777 cone, first supplied to Celestion by Pulsonics and later by Műller.
Celestion produced a UK made, 8 ohm G12 alnico speaker in "period correct" silver paint for the AC30HW60 utilizing the correct Műller H1777 cone. You can actually read the H1777 imprint on the cone of the speaker shown at left.
Egg Foot Switch A single button "egg" cast aluminum foot switch remotely enabled the Vibravox effect. Vox turned to North Coast Music to produce the "egg" foot switch cases from the original UK sand cast mold.
AC30HW2 Specifications
Country of Origin
United Kingdom
Tube Complement
5 -12AX7, 1 - 12AU7, 4 - EL84, 1 - GZ34
Circuit Construction
Point-to point hand-wired on turret strips
Cabinet Construction
13 Ply Baltic Birch Plywood
Inputs
Vib/Trem Input Jack (High and Low)
Normal Input Jack (High and Low)
Brilliant Input Jack (High and Low) Foot Switch Jack
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