Vox AC15C1, AC15C1X and AC15CH Amplifiers
"A Look Under the Hood"
2010 - 2016



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Introduced in 2010, the AC15C1 replaced the AC15CC1 offered by Vox from 2005 to 2009. The AC15C1 was a dual channel amp, the AC15CC1 was a single channel design.

AC15C1 Features
The AC15C1 was powered by three 12AX7 and two EL84 tubes. It included spring reverb, tremolo and output jacks for extension speakers.

Chassis
The AC15C1 steel chassis had a "C" shaped cross section. The top, bottom and inside of the chassis were painted in semi-gloss black paint. The control panel nomenclature was silk screened directly to the top of the chassis. The chassis bolted to the amplifier cabinet with four large machine screws.

While an earlier version of the AC-15 had tube sockets that were mounted directly to the main PC board, the tube sockets for the AC15C1 were fastened to the chassis. This allowed the chassis to act as a heat sink for the tube sockets.

Printed Circuit Board Design
Most of the AC15C1 circuitry was designed onto one of two dual sided printed circuit boards (PCB).

The larger of these two boards was the "Main PCB." The Main PCB included the controls plus the Top Boost, reverb and tremolo circuitry. The Main PCB was mounted vertically against the back of the chassis.

The "Tube PCB" was located on the inside bottom of the chassis. The Tube PCB was mounted to the five tube sockets and included most of the power supply components.

Four additional small printed circuit boards mounted the input jacks, output jacks, power switch and power indicator LED.

Power Transformer
Vox produced four versions of the AC15C1 chassis, each equipped with a power transformer that accommodated regional differences in mains AC voltage. The following chart details the part numbers of those power transformers.

Transformer P/N
Country
Mains Voltage
530000002128
Europe
230 VAC
530000002129
Japan
100 VAC
530000002130
United States
120 VAC
530000002131
Australia
240 VAC

While the primary winding of the AC15C1 power transformer varied to accommodate regional mains voltages, all four of the transformers listed above had identical secondary windings. In additional to the B+ (high voltage) and a filament heater winding, the AC15C1 power transformer included an additional secondary winding to supply the solid state reverb and tremolo circuits.

B+ Power Supply
The original design for the JMI AC-15 B+ power supply included a EZ81 full wave rectifier tube and a choke. The EZ81 tube converted AC voltage from the B+ winding on 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 EZ81 rectifier tube. The EZ81 rectifier tube had the tendency to exhibit a momentary "sag" or dip in power supply voltage when an AC-15 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-15 is an essential component of Vox tone.

Vox hoped to design the AC15C1 so that the street price of the amp might be less than $650. Such pricing would make the amp available to many who could not previously afford an AC-15. Vox determined that through modern design the EZ81 rectifier tube and the choke could be eliminated from the AC15C1 power supply. These would be replaced with less costly components that would not affect performance.

The EZ81 tube for the B+ power supply 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 EZ81. However, replacing the EZ81 rectifier tube with diodes offered a number of benefits to the AC-15. Diodes generate no heat and have a much longer service life than a EZ81. Additionally, the electrical efficiency gained from the diodes helped the AC15C1 amp to produce close to 20 watts of RMS output power.

A ladder of additional power supply filter capacitors in the AC15C1 power supply replaced the choke.

For those wanting an AC-15 that included a choke and a EZ81 rectifier, Vox offered the hand wired AC15HW1 and AC15HW1X amplifiers.

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 eminating from the wires and reduces 60 hz hum in the audio output. However, the connections to the tube sockets on the AC15C1 passed through copper traces on a circuit board, making a twisted pair filament connection impossible.

Clever engineering solved this problem on the AC15C1. A KBU 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 AC15C1 featured an all tube signal path.

The signal from the "Top Boost" input jacks was preamplified by one half of V1a (12AX7). The signal from the "Normal" input was preamplified by the other half of V1 (12AX7b). A 100k plate resistor was utilized in both the Top Boost and Normal channels.

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

Two EL84 output tubes (V4-V5) created the classic 15 watt Class A, NFB Vox AC-15 output stage.

Solid State Reverb and Tremolo Circuits
Both the reverb and tremolo circuits in the AC15C2 were solid state. The reverb drive signal was powered by three and a half NJM2147 op amps. A pair of NJM 2147 op amps powered the signal returning from the three spring Belton 3EB3C1B reverb pan.

Tone Cut Control and Master Volume Control
The AC15C1 included the classic Vox "Tone Cut" tone control. The "Tone Cut" control was linked to both channels and was located 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 Master Volume control was also located between the phase inverter tube and the output tubes.

Fuses
The AC15C1 has three fuses. F4 is the main fuse and is located inside the AC recepticle. The 120 VAC version of the AC15C1 requires a T1.25AL/250V, the 240 volt version requires a T630mAL/250V. Two fuses are located on the main circuit board. F1 requires a T250mAL/250v fuse to protect the B+ circuit. F2 requires a T6.3AL fuse to protect the heater filament (6.3V) circuit.

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

Reactive Attenuator
The AC15CH head added a new feature, a reactive power attenuator, to the AC15C1 chassis. Often called a "power soak," it was installed between the output transformer and the speaker jacks. A rotary selector on the attenuator allowed the total output power to be reduced from 15 watts RMS to either 1.5 or .6 watts RMS. The AC15CH power attenuator circuit is pictured at left.


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