The Vox AC-50/4 Mk III "Big Box" Diode Rectified Head

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The Vox AC-50/4 Mk III was the second in a series of three
versions of the AC-50 "Big Box" amp head. The production of the three "Big Box" models at JMI spanned the period from late 1964 through 1967. The "Big Box" AC-50 models replaced the single channel "Small Box" AC-50 Mk I heads made in 1963 and 1964.

The Evolution of the "Big Box" AC-50 Models
The first of the "Big Box" heads was introduced in the latter part of 1964. It was depicted on JMI Vox Schematic #OS/053 and was called the "AC-50/4 Mk II." The circuit added a second channel while retaining the GZ-34 tube rectified power supply from the Mk I "Small Box" models. This dual channel chassis was also installed in a few of the Mk I "small box, thick edge" cabinets during the model transition.

The AC-50/4 Mk II was replaced in 1965 by the two channel, diode rectified "AC-50/4 Mk III," detailed on JMI Vox schematic #OS/072. Vox offered the AC-50/4 Mk III, the subject of this web page, through late 1966. Aside from a diode rectified power supply and the location of the bias adjustment pots for the output tubes, the Mk III was virtually identical to the AC-50/4. The AC-50 Mk III is probably the most common of all of the AC-50 models.

The AC-50/4 Mk III was as well replaced in 1966 with the dual channel "AC-50 Mk IV," detailed on JMI Vox schematic
#0S/163. The primary difference between the Mk III and Mk IV versions of the amp was a tube change in the first stage of the preamp. The Mk IV replaced the 12AU7 tube formerly used in the first position of the preamp with a 12AX7. The higher gain offered by the 12AX7 preamp tube allowed the Mk IV to be both louder and more easily overdriven than previous AC-50 models. This helped to change the reputation of the AC-50 from a "clean and chimey" amp to one better suited to the overdriven sounds popular in the later 1960s.

The "Big Box" Head Cabinet
The "Big (or Tall) Box" head cabinet used on Mk II, Mk III and MkIV AC-50 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 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, the rapid development of the Vox UL "hybrid" models during 1966 probably caused Vox to abort this plan.

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 III and Mk IV amp heads would typically have black Vox grill and use the later, "three field" Vox Amplifier serial plate, as shown at right. This serial plate was installed on JMI amps produced from 1965 to 1967. It is a fairly safe bet that an AC-50 amp head with this "three field" serial plate is one of the diode rectified AC-50 models.

Dual Channel Preamp Design
Each channel featured dual 1/4" input jacks and individual bass and treble tone controls. Both channels of the AC-50 MK III 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 III to be overdriven.

The circuitry for the first, or "Normal"  channel voiced for full range response. It also had lower gain and higher headroom, making it the better 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.

You can read more about the preamp design of the AC-50 Mk III amplifier in the "AC-50 Mk III "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 460-470 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 safely 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 III debuted a new solid state, diode rectified power supply, similar to the power supply used in the AC-100 amp head. This new power supply replaced the GZ-34 based tube rectified power supply used in the Mk I and Mk II AC-50 models.
The change from tube to solid state rectification on the AC-50 Mk III offered some advantages. Replacing a GZ-34 rectifier with four relatively inexpensive silicon diodes offered a significant cost savings to Vox. Diode bridges are highly dependable, reducing the potential for power supply failures and eliminating tube rectifier replacements. Removing the rectifier tube from the power supply also eliminated the heat it would have generated.

Due to their highly efficient nature, amps with silicon diode rectifiers tend to exhibit less power supply "sag" when driven hard. This tends to give diode rectified amps more clean audio head room, making the AC-50 MK III a better choice for use with bass guitar. However, it is this loss of power supply "sag" that causes the AC-50 Mk III to suffer as a guitar amplifier. The audio compression that is a natural byproduct of an amp with a tube rectified power supply is preferred by many guitarists.
The diode rectified power supply also incorporated a special time delay switch called a surgistor. This device acted as an automatic "standby" switch, allowing the tube filament heaters to warm the tubes for about twenty seconds prior to applying the 460 volt B+ supply to their plates, extending tube life. Read more about the surgistor in the AC-50 Mk III "under the hood web page.

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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