The Vox AC-50/4 Mk II "Big Box" Dual Channel, Tube Rectified Head

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The first generation JMI Vox AC-50 Mk I amp heads were
minimalistic in design. Vox decided that the major selling feature for these first AC-50 heads should be an abundance of power, not the inclusion of effects or multiple channels. Neither the complex Vib/Trem circuitry of the AC-30 or the troublesome JMI phono cartridge reverb system would be included in these new amps. These amps would be simple, uncomplicated and loud.

Following this design philosophy, the first AC-50 head, the "Thin Edge" AC-50 Mk I, was designed with just one channel and two inputs. John Lennon and George Harrison of the Beatles played their guitars through the two input AC-50 Mk I in their February 1964 appearances on the Ed Sullivan Show and at their first US concert at the Washington Colliseum.

Vox soon realized that two inputs were not enough. I suspect that many potential AC-50 customers felt that an amp in this price range and power output should have more than two inputs. To further complicate matters, it was a common practice in the UK for a beginning band to pool their money to purchase one good quality amplifier and have all the members play through it. The single channel, two input AC-50 Mk I could not address this issue.

In response, Vox introduced a new four input version of the AC-50 Mk I "Thin Edge" amp in mid 1964. Although the amp now

had four inputs, it still retained the original single channel design. While an improvement over the two input model, the AC-50 really needed a second channel.

Vox added that second channel to the AC-50 as a part of an update to the model in late 1964. The new AC-50 Mk II amp featured a "Normal" and a "Brilliant" channel, a gray control panel and a new "Big Box" head cabinet constructed from 3/4" thick baltic birch plywood. The circuitry for this new amp was detailed on JMI Vox schematic OS/053.

Vox also installed a few dual channel AC-50 Mk II chassis into the Mk I "Thick Edge, Small Box" head cabinet during the transition period from the Mk I to the Mk II models.

The "Big Box" Head Cabinet
The "Big (or Tall) Box" head cabinet used on AC-50 Mk II amps offered a number of benefits. Most obviously, it provided a storage area in the rear of the amp for the power and speaker cables. The "Big Box" cabinet also allowed the chassis to be mounted to a "slider board" for ease in service. To service the chassis, one only needed to remove six back screws, remove the rear panel and slide the chassis out of the cabinet like a drawer. By comparison, the chassis was bolted to the bottom of the small box AC-50 cabinets.

JMI even considered the addition of a reverb circuit to the AC-50 "Big Box" head. This circuit, detailed on JMI schematic OS/75, would resemble the reverb installation on the Vox AC-30SRT head. Two controls would be mounted in a hole in the back panel and a reverb pan would be suspended from the bottom of the slider board. However, these plans were dropped. Vox decided to concentrate on developing new models rather than updating the old.

AC-50 "Big Box" Slider Board

A common fallacy surrounding the "Big Box" amp cabinet is that it's larger size provided the chassis with better ventilation than the "Small Box" AC-50 cabinet. This is not the case. While the "Big Box" cabinet is indeed larger, the slider board provided a partition that made the chassis compartment of the "Big Box" cabinet the same size as the "Small Box" cabinet.

The AC-50 slider board had four oval shaped vent holes located near the outer edges of the board. Unfortunately, these vent holes significantly weakened the slider board at the point where it slips into the cabinet rails. This weakening has caused many of these slider boards to snap under the weight of the amp chassis. Should this occur, the chassis and control panel drops inside the cabinet. North Coast Music offers a replacement AC-50 slider board to correct this problem.

The AC 50 Mk II amp head would typically use the "two field" Vox Amplifier serial plate, as shown at right. This serial plate was installed on JMI amps produced from 1960 through late 1964. It is a fairly safe bet that an AC-50 "Big Box" amp head with this two field serial plate is one of the tube rectified AC-50 models.

Dual Channel Preamp Design
Each channel featured dual 1/4" input jacks and individual bass and treble tone controls. The circuitry for the first, or "Normal"  channel voiced for full range response. It also had lower gain and higher headroom, making it the preferred channel for bass guitar. The second, or "Brilliant" channel had higher gain and was voiced to gradually roll off frequencies below 680 hz, making the tone bright and chimey.

Both channels of the AC-50 Mk II amplifier used a low gain 12AU7 preamp tube in the first stage of preamplification. This low gain design kept the audio signal clean, minimizing the ability of the AC-50 Mk II to be overdriven.

You can read more about the preamp design of the AC-50 Mk II amplifier in the "AC-50 Mk II "Under the Hood" web page.

Power Amplifier
The power amp featured two EL-34 output tubes running in fixed bias mode, producing about 45 watts RMS output power.

It can be a challenge to install modern EL-34 output tubes in the AC-50. Many modern EL-34 tubes are too tall to fit in the clearance between the chassis and top of the cabinet.

To extract the maximum audio power from a pair of EL-34s, Vox pushed the plate voltage to 480 volts. This was still within the "safe operating range" for the Mullard and Brimar tubes Vox originally installed at the factory in these amplifiers. However, many modern EL-34 tubes will not tolerate plate voltages that exceed 400 to 425 volts. Installing EL-34 tubes that are not rated to operate at the 460+  plate voltage of an AC-50 can create premature failures.

Dual Amphenol three-pin XLR "round top" jacks were provided in a recessed rear panel for speaker connection. A speaker impedance selector allowed the choice between 8 or 16 ohm total speaker loads.

To connect to a single 16 ohm cabinet, the selector plug should be in the 16 ohm position and the speaker system could be plugged into either jack. If either one 8 ohm or two 16 ohm cabinets were

connected, the selector plug should be moved to the 8 ohm position. The AC-50 does not accept speaker loads less than 8 ohms, so pairs of 8 ohm speakers should never be connected to the amp.

A red "Warning" placard located above the speaker jacks advises that the amplifier should never be operated without a loud speaker connected.

Power Supply
The AC-50 Mk II retained the GZ-34 tube rectified power supply from the AC-50 Mk I models. This tube rectified power supply exhibited a sag in voltage when driven hard. The audio compression that is a natural byproduct of this momentary voltage sag in the power supply is preferred by many guitarists.

A rotary voltage selector switch allowed the amp to accept local mains voltages of 115, 160, 205, 220, or 245 volts. This feature made it possible to accomodate the AC power standard in most countries.


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Photos and editorial content courtesy Gary Hahlbeck, North Coast Music

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