JMI Vox AC-50 Single Channel Two Input Amplifier Chassis
"Washington DC Amp Head"
A Look Under the Hood

The Vox AC-50 Mk I dual input, single channel amp head was introduced by JMI Vox in late 1963. A pair of these amps accompanied the Beatles in their first trip to America in 1964.

The AC-50 Mk I chassis utilized the classic Vox amp chassis design. The base of the chassis was made of formed steel, supporting the transformers and power tubes. A vertical piece of aluminum, bent into the shape of an inverted "L," was bolted to the steel base and to the tops of the transformers to support the preamp and control panel sections of the amplifier.

The steel base of the chassis supported the power supply and output amp sections of the amplifier. The use of steel in the chassis base not only provided strength but also electronically isolated the high voltage (and hum producing) section of the power supply and output amp from rest of the amplifier. The power transformer and output transformers were located at opposite ends of the chassis base to provide proper balance. "L" brackets mounted to the top of the power and output transformers strengthened and supported the vertical aluminum portion of the chassis. The base of the chassis bolted directly to the bottom of the amp cabinet.

The vertical aluminum section of the chassis enclosed the preamp circuitry and secured the control panel. Aluminum is even less likely than steel to pick up hum and oscillations from the power supply and output amp section of the amp. This served to further electronically shield the preamp from the power supply.

This unique Vox chassis design yielded many benefits. The steel base section of the chassis provided the strength and rigidity to support the heavy power and output transformers. The output and rectifier tubes were mounted vertically to this steel chassis base, facilitating proper ventilation and providing an excellent heat sink for the tube sockets. Locating the hand wired preamp circuitry on the inside of the grounded vertical aluminum structure electronically isolated the sensitive, high gain preamp circuitry from stray electronic fields emanating from the transformers and power anplifier circuitry. The chassis was structurally rigid and not easily susceptible to damage.

The First Preamp Gain Stage -Figure 1
Input Jacks - Two plastic ¼" input jacks, manufactured by either Cliff or Rean of London, were mounted to the control panel to connect an instrument to the AC-50 Mk I. These jacks included an internal grounding contact to silence an unused jack.

Input Resistors R1 and R2 - The signal side of each jack was connected to the grid of the first tube (V1) with a pair of 68k "grid stopper" resistors (R1 and R2). These resistors interacted with the internal capacitance of the tube to create a RC filter that blocked frequencies in excess of 20khz from entering the circuit. This insured that the amplifier would not pick up radio transmissions and protected the amp from parasitic oscillations. A 1 meg grid load resistor (R3) was installed between the signal and ground connections on input jack 2.

12AU7 Preamp Tube - While the first preamp gain stage of the AC-15 and AC-30 utilized a 12AX7 (ECC83) tube, the AC-50 used a 12AU7 (ECC82). The 12AX7 was a high gain tube with an amplification factor of 100. The 12AU7 used in the AC-50 was a low gain tube with an amplification factor of just 17. It appears that Vox chose the 12AU7 for the first stage of the AC-50 preamp to minimize gain and to reduce "overdrive." This low gain 12AU7 preamp tube helped to make the AC-50 a "clean" amp.

Coupling Capacitor C2 - A .022uf coupling capacitor was used between the plate (output) of the 12AU7 tube and the 470k volume control (RV1). This .022uf capacitor affected the "voice" of the AC-50 Mk I amplifier by gradually rolling off bass frequencies below 64 hz. As the fundamental frequency of the low E on

Figure 1 - First Preamp Gain Stage
a guitar is about 82 hz and the fundamental frequency of the low E on a bass guitar is about 41 hz, the choice of a 64 hz bass roll off for the AC-50 Mk I made it better suited for guitar rather than bass. Had Vox subsituted a .1uf rather than a .022uf capacitor for C2, the low frequency roll off would have been significantly lower, improving the audio response of the AC-50 Mk I for bass guitar.

Treble Bypass Capacitor C4 - A 100 pf capacitor (C4 in Figure 1) was installed between the signal and wiper terminals of RV1, the volume control pot. This capacitor allowed treble frequencies to bypass the level adjustment of the volume control. Stated another way, treble was not affected by adjustments to the volume control, the treble frequencies always remained at the maximum level. At lower volume levels, this offered the perception of increased treble output. However, as the amp was turned up, the percieved amount of treble would diminish in relation to the rest of the audio signal.

Figure 2 - Second Gain Stage and "Top Boost" Tone Control Circuitry

The Second Preamp Gain Stage - Figure 2 - The signal coming from the first preamp gain stage arrived at the grid of V2a, a 12AX7 (ECC83) which acted as a second preamp gain stage. The signal from V2a was then sent to V2b to power the Top Boost tone circuit.

Top Boost Tone Control Circuit - Figure 2 - The AC-50 Mk I tone control circuit was identical to the tone control circuit in a "Top Boosted" AC-30. The design for the Top Boost circuit was found on JMI schematic OS/010, shown at left in Figure 2. The circuit is powered by V2b.

Cathode Follower
Tube V2b was not configured as a traditional gain stage. V2b was configured as a "cathode follower." While a traditional tube gain stage generates increased output voltage, a cathode follower generates increased current. As tone control circuitry is hungry for current, a cathode follower circuit is ideal for this application.

Most amplifier manufacturers placed their tone control circuitry between the preamp gain stages. This design was especially prevalent on 1960s era Fender amplifiers. The "Top Boost" tone control circuit used in Vox amplifiers was placed after the preamp gain stages.

Phase Inverter (Splitter) Figure 3 - The phase inverter (or splitter) circuit followed the preamp and tone control stages.

The purpose of the phase inverter circuit is to convert the single audio signal coming from the preamp and tone controls into two individual signals that are out of phase with each other. Each of these out of phase audio signals will supply one of the two EL-34 power tubes in the output amplifier stage.

The phase inverter circuit was powered by V3, a 12AX7 (ECC83) tube. The signal from the Top Boost tone control circuit was supplied to the grid of V3a via C10, a .01 uf coupling capacitor. The out of phase signals exited the inverter circuit through C13 and C14, a pair of .01 uf coupling capacitors.

Figure 3 - Phase Inverter (Phase Splitter) Circuitry

Figure 4 - Power Supply Circuitry

DC Power Supply - Figure 4 - The AC-50 Mk I DC power supply included a power (mains) transformer, a GZ34 rectifier tube, several dual pole filter (smoothing) capacitors, a choke, a neon bulb and several dropping resistors.

Power (Mains) Transformer - The winding for the primary side of the power transformer included five taps to accommodate the mains (wall socket) voltages in various countries.

The secondary side of the transformer had four windings. One winding provided 6.3 VAC/2A for the heaters on the preamp tubes. A second provided 6.3 VAC/3A for the EL34 tube heaters. A third winding supplied 5 VAC for GZ34 heater. The fourth winding provided 380 VAC for the B+ power source.

GZ34 Rectifier Tube - The GZ-34 was a full wave rectifier tube that converted the 380 VAC from the secondary of the power transformer to about 450 volts of pulsed DC. The GZ-34 rectifier tube had a natural tendency to exhibit a slight "sag" in output voltage when the amp was pushed toward the limit. These momentary drops in voltage caused a bit of audio compression to occur in the output of the AC-50 Mk 1. Many feel that this audio compression is an essential component of Vox tone.

B+ Power Supply - The B+ power supply provided two sources of clean DC voltage to power the preamp and power amp stages of the AC-50 Mk 1. The design of the B+ power supply included a 8 henry choke straddled by two 16 uf, 450 volt smoothing capacitors (C16-C17). Positioning the choke between two filter capacitors created a "capacitive pi" filter circuit, a superior, noise-free design.

Negative Bias Supply - The negative bias supply provided the -32 to -35 bias voltage required by the EL34 output tubes. The bias voltage was adjusted by a variable resistor (RV4), located on the bottom of the chassis. A pair of 8 uf, 150 volt smoothing capacitors (C14-C15) removed the AC ripple in the supply.

The negative bias supply also included a small neon lamp that glowed when the amplifier was turned on. This bulb served two purposes. First, it reminded a technician servicing the amplifier that the power supply was "live" to avoid the potential for electrical shocks. Secondly it protected the negative bias circuit from failure caused by overvoltage. Should the negative bias supply voltage rise to unacceptable levels, the neon bulb would go into "breakdown mode," allowing the excess voltage to pass through the neon lamp to ground.

Power Amplifier - Figure 5
The AC-50 Mk I and the AC-100 Mk I were the first commercially successful Vox amplifiers to incorporate EL34 tubes in the output stage. The EL34 was the "big brother" to the EL-84, the output tube Vox designed into the output stages of the AC-4, AC-10, AC-15 and AC-30.

The EL84 Output Tube
The EL84 was a highly efficient tube capable of producing about 8 watts each with a relatively low plate voltage of 320 volts. Vox lead engineer Dick Denney prefered the EL84 for its brilliant, chimey tone and for the harmonically rich response it produced when overdriven.

In 1963, the success of the Beatles showed Vox that they needed to develop amplifiers that were more powerful than the AC-30, leading to the introduction of the AC-50 and AC-100. Vox was faced with a similar situation in 1960 when the Shadows needed amps more that were more powerful than the AC-15, leading to the birth of the AC-30.

Figure 5 - Power Amplifier

When creating the AC-30, Vox simply added two additional EL84 tubes and a larger power supply to the design of the AC-15. Vox could have added even more EL84 tubes to the AC-30 to create the AC-50 Mk I and AC-100 Mk I. However, this approach posed several problems. It would require an oversized power supply, multiply the possibility of tube failures and generate an excessive amount of heat. An AC-50 powered by just two EL34 power tubes in the output stage would more efficiently develop a fifty watt RMS output.

The EL34 Output Tube
While a practical choice to power 50 or 100 watt guitar amplifiers, the EL34 had some issues. The additional output offered by the EL34 increased the bass response of the amplifier, effectively masking some of the treble response . As a result, the AC-50 Mk I did not offer the brilliant tone characteristic of Vox amps powered by EL84 tubes.

Vox compensated for this lack of treble response by adding a Goodmans Midax horn to guitar speaker cabinets packaged with the AC-50.

Additionally, many guitarists had grown fond of the sweet overdriven tones of Vox amps equipped with EL84 power amp stages. The EL34 offered significantly higher head room and lower distortion figures than the EL84. As a result, the AC-50 Mk I stayed much "cleaner" when driven hard. While a plus for bass guitar and organ, this cleaner response was a disappointment to some guitarists.

"Pedal to the Metal" Design
In order to squeeze every last watt out of the AC-50 Mk I output stage, Vox designed the AC-50 HT (or plate) power supply circuit, shown in Figure 4, to produce nearly 500 VDC. This pushed the very limit of the safe operating voltage range for 1960's era EL34 output tubes. This high plate voltage may cause problems for those installing modern EL34 tubes in an AC-50 Mk I. The specifications of many modern EL34 tubes call for a maximum plate voltage of 450 volts. For this reason, it is not unusual for modern EL34 tubes to fail prematurely in an AC-50 Mk I.

This HT voltage problem gets even worse due to a design error in the AC-50 Mk I power (mains) transformer. JMI was under the mistaken impression that the US line (mains) voltage was 110 to 115 VAC and ordered power transformers designed to this specification. In reality, US line voltage is more commonly between 120 and 122 VAC. This 5% miscalculation in the US line voltage caused the HT power supply to jump from nearly 500 VDC to 525+ VDC. This was well past the safe operating range for most EL-34 tubes, be they vintage or new.


The VOX Showroom!

Photos and editorial content courtesy Gary Hahlbeck, North Coast Music

Any and all material presented herein is protected by Copyright.
© 1998 - 2023 The Vox Showroom and North Coast Music, all rights reserved

The images and editorial content in this web site may not be copied or reproduced
in online auction sites such as eBay, Reverb and Craig's List. Sellers may provide a link
to the Vox Showroom web site if they wish to refer to this copyrighted material.