Vox AC30C2 and AC30C2X Amplifier
"A Look Under the Hood"

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The web site MusicRadar.com ran an international reader's poll in 2013 to determine which was the best valve combo guitar amp. The Vox AC30C2 took the number one position, easily topping the
poll with 16% of all votes cast. It defeated amps from manufacturers such as Mesa Boogie, Marshall, Orange, Peavey and Fender. Let's take a look under the hood at what makes the AC30C2 tick.

AC30C2 Features
The AC30C2 was a two channel amplifier powered by three 12AX7 and four EL84 tubes. It included spring reverb, tremolo, an effects loop and output jacks for extension speakers.

The AC30C2 steel chassis had a "C" shaped cross section. It bolted to the amplifier cabinet with four large machine screws.

While some earlier versions of the AC-30 had tube sockets mounted directly to the main PC board, the tube sockets for the AC30C2 were fastened to the chassis. This allowed the chassis to act as a heat sink for the tube sockets.

B+ Power Supply
The original design for the JMI AC-30 B+ power supply included a GZ-34 full wave rectifier tube and a choke. The GZ-34 tube converted AC voltage voltage from the power transformer to rippled DC. The choke worked in conjunction with two 16 uf 450 volt capacitors to eliminate the ripple in the B+ supply.

A bit of Vox mojo was inadvertently created by the GZ-34 rectifier tube. The GZ-34 rectifier tube had the tendency to exhibit a momentary "sag" or dip in power supply voltage when an AC-30 was pushed toward the limit. These drops in voltage compress the audio output of the amp. Many feel that the audio compression created by an overdriven AC-30 is an essential component of Vox tone.

Vox hoped to design the AC30C2 so that the street price of the amp might be less than $1000. Such pricing would make the amp available to many who could not previously afford an AC-30. Vox determined that through modern design the GZ-34 rectifier tube and the choke could be eliminated from the AC30C2 power supply. These would be replaced with less costly components that would not affect performance.

The GZ-34 tube was replaced with a bridge of four IN4007 silicon diodes. As diodes are highly efficient devices, they would not create the audio compression caused by the voltage "sag" of the GZ-34. However, replacing the GZ-34 rectifier tube with diodes offered a number of benefits to the AC-30. Diodes generate no heat and have a much longer service life than a GZ-34. Additionally, the electrical efficiency gained from the diodes helped the AC30C2 amp to produce close to 40 watts of RMS output power.

A ladder of additional power supply filter capacitors in the AC30C2 power supply replaced the choke.

For those wanting an AC-30 that included a choke and a GZ-34 rectifier, Vox offered the hand wired AC30HW2 and AC30HW2X amplifiiers.

DC Filament Heaters
The wires running between the 6.3 VAC filament tap on the power transformer and the tube sockets are typically twisted together. Twisting the filament wires cancels the electrical and magnetic fields emanating from the wires and reduces 60 hz hum in the audio output. However, the connections to the tube sockets on the AC30C2 were completed by copper traces on a circuit board. This makes a twisted pair filament connection between tubes difficult to accomplish.

Clever engineering solved this problem on the AC30C2. A 4 amp bridge rectifier and two 4700 uf smoothing caps were used to convert the filament voltage from 6.3 VAC to 6.3 VDC. The change to a DC filament supply eliminated the potential for filament induced hum in the audio output.

All Tube Signal Path
The AC30C2 featured an all tube signal path.

The signal from the "Normal" input jacks was preamplified by one half of V1 (12AX7). The signal from the "Top Boost" inputs was preamplified by the other half of V1. In a slight departure from the original circuit, the value of the plate resistor for the "Normal" channel was 100k rather than 220k. The 100k plate resistor allowed the "Normal" channel to produce more gain. The "Top Boost" channel retained a 220k plate resistor.

The classic "Top Boost" circuit (Bass and Treble controls) was powered by V2 (12AX7). A third 12AX7, labeled V3, served as the phase inverter.

Four EL84 output tubes (V4-V7) created the classic 30 watt Class A, NFB Vox AC-30 output stage.

Solid State Reverb and Tremolo Circuits
Both the reverb and tremolo circuits in the AC30C2 were solid state. The reverb send signal was powered by three solid state opamps. A fourth solid state opamp operated the reverb return circuit. The AC30C2 tremolo circuit included a MOSFET transistor.

Tone Cut Control
The AC30C2 included the classic Vox "Tone Cut" tone control. The "Tone Cut" control was linked to both channels and was located in the power amp circuit between the phase inverter and output tubes. Here is how it worked.

The signal from the preamp is directed to the phase inverter. The output from the phase inverter is split into two signals. One of these signals is 180 degrees out of phase with the other. When "out of phase" signals are combined, they cancel each other out. The "Tone Cut" control works by combining the high frequency signals from one side of the phase inverter with the other.

The AC30C2 has three fuses. F4 is the main fuse and is located inside the AC recepticle. The 120 VAC version of the AC30C2 requires a T2AL/250V, the 240 volt version requires a T1.25AL/250V. Two fuses are located on the main circuit board. F1 requires a T500mAL/250v fuse. This fuse protects the B+ (262V) circuit. F2 requires a T6.3AL fuse. This fuse protects the heater filament (6.3V) circuit.

There is also a fusible link, F3, which is soldered to the main circuit board. It is not replaceable by the end user.


The VOX Showroom!

Photos and editorial content courtesy Gary Hahlbeck, North Coast Music

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