Vox AC30H2, AC30H2L & AC30HH 50th Anniversary AC-30 Amps (2007-08)
A Look "Under the Hood"

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AC30H2/AC30HH chassis and slider board
The limited edition "Heritage Series" AC30H2,
AC30H2L and AC30HH were manufactured in 2007 and 2008 under contract to Vox by the International Audio Group of Shenzen China.

EF86 Preamp Tube
By 1960, most guitar amplifier manufacturers were incorporating dual triode tubes such as the 12AX7, 12AT7 or 12AU7 in their preamp circuits. JMI lead engineer Dick Denney chose a different path. Denney included an EF86 pentode tube in the preamp circuits of the JMI Vox AC-2, AC-4, AC-6, AC-10, AC-15 and AC-30/4 amplifiers. Denney preferred the additional gain and rich harmonics offered by the EF86.

Shortly after the EF86 equipped AC30/4 combo amp was introduced in 1960, an unforeseen engineering problem arose. The EF86 tube proved to be susceptible to damage from excessive vibration. At the lower sound levels of an AC-4, AC-10 or AC-15 amplifier, this was not a large issue. In the AC-30/4, the increased sound levels produced by a 30 watt power amp powering two 12" speakers was often more than the EF86 could tolerate. As the EF86 started to break down, it often would became microphonic, creating acoustic feedback between the tube and speakers. The squeal of a failing EF86 tube would often accompany the sound of the guitar playing through an AC-30/4 amp. As a result, Vox replaced the AC30/4 combo amp with the AC-30/6 in late 1960. The circuitry of the AC-30/6 replaced the troublesome EF86 with a more conventional ECC83 (12AX7) tube.

When Vox made the decision to include the EF86 tube in their limited edition 50th Anniversary AC30H2 and AC30HH, they faced the same issue with microphonics that JMI attempted to address nearly fifty years earlier. The AC30H2 included three design innovations to minimize the issues caused by a microphonic EF86 preamp tube.

First, Vox moved the location of the preamp tubes from the outer to the inner chassis to further isolate the EF86 from the speakers (see photo at top of page). Next, they mounted the EF86 tube socket on four rubber grommets to minimize vibration transfer from the chassis (see photo above). Finally, the EF86 was jacketed with rubber heat shrink tubing to dampen the ability of the glass bulb to ring (see photo above).

Hand Wired on Printed Circuit Boards
In a 2007 catalog, Vox announced that the 50th anniversary "Heritage" amp series (AC15H1, AC15H1L, AC30H2, AC30H2L, and AC30HH) would have hand-wired circuitry.

In the past, Vox produced hand-wired AC-30 amps during the JMI era (1960 - 1967), the early VSEL era (1968-1969), the early Dallas era (1973-1977) and the AC30HW made for Vox by Marshall (2002). All of these amps featured point-to-point (PTP) hand-wired construction. PTP implies that all component leads are soldered directly to each other on a "tag" strip. Interconnecting wires are kept to a minimum. PTP amps have no printed circuit boards (PCB).

The circuitry for the Heritage series amps was hand-wired on turrets mounted to a PCB (see photo at right). While hand wired, the Heritage series amps did not utilize PTP construction. Circuit board traces made the connections between electronic components.
Hand Wired AC30H2/AC30HH Printed Circuit Board

12" Celestion Alnico Blue Speaker with Vox Sticker on Magnet Cover
Vox/Celestion G12 Alnico Blue Speaker
Originally designed as a radio speaker, the Celestion G12 played a major role in the tone of the Vox AC-15 and AC-30.

Early Vox AC-15 and AC-30 amps produced in 1959 and 1960 were equipped with the CT3757 version of the Celestion G12. The CT3757 was painted tan and did not include the pressed steel magnet cover featured in later production.

By 1961, Celestion produced a version of the G12 specifically for Vox. This G12 T530 was finished in blue paint and included a pressed steel magnet cover.

The Celestion G12 featured an Alnico magnet. Alnico is an alloy of aluminum, nickel and cobalt that creates a powerful yet compact magnet. The Celestion G12 also utilized the H1777 cone, originally produced in the UK by Pulsonics and more recently by Kurt Müller Ltd, another UK firm. The combination of an alnico magnet and the H1777 cone worked tonal magic in Vox amps.

The Chinese industrial conglomerate, Gold Peak, purchased control of Celestion in 1992. Speaker production was divided between the original Celestion factory in Ipswich UK and a new Chinese facility.

Now that Celestion was producing guitar speakers in China, it would not be necessary for the International Audio Group to import the 12" Blue Alnico from the UK. However, a critical component of the G12 Celestion Blue, the Müller H1777 cone, was produced exclusively in the UK. Rather than import the Müller H1777 cone from the UK, a decision was made to substitute a different cone into the Blue Alnico speakers built for the Vox AC30HW2. No one anticipated the furor this modified Blue Alnico would cause in online chat rooms. Consequentally, Vox included the Müller H1777 cone in all Celestion/Vox Blue Alnicos produced after 2009.

The AC30HW2 and AC30HH each have four fuses. The main fuse (FS4) is located inside the AC recepticle. Amps designed for 100 - 120 VAC operation utilize a T4AL main fuse. Amps designed for 220 - 240 VAC operation require a T2AL main fuse. The other three fuses are located on the lower printed circuit board. The chassis needs to be removed from the slider board to access these fuses. FS101 is a T500mAL fuse that protects the B+ circuit. FS102 is a T6.3AL fuse for the preamp and power tube filament heater windings. FS103 is a T5AL fuse for the GZ34 filament heater winding.


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Photos and editorial content courtesy Gary Hahlbeck, North Coast Music

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