The Vox AC-50 Mk I Speaker Enclosure - 1964

After Vox completed the AC-50 Mk I amp head, speaker systems that could benefit
from the higher RMS power output of the new amplifier were needed. The existing Vox Foundation 1x18" and T.60 bass guitar enclosures were suitable to the 50 watt output of the new head. However, the standard thirty watt AC-30 guitar enclosure was not a good match for the AC-50.

The engineers at Vox discovered that the circuitry of the new AC-50 Mk I head did not have the capability to produce the brilliant treble response found in the AC-30. In comparison to the AC-30, the AC-50 sounded somewhat dark. Vox recognized that their signature "top boost" tone also needed to be achieved with the AC-50 head.

One solution to this problem could be to develop a guitar enclosure that would address the lack of treble in the AC-50 head. Time was of the essence. The Beatles needed this more powerful amp to overcome the sound of their screaming fans.

To this end, Vox adapted the standard AC-30 speaker enclosure for use with the AC-50 head. A horn would be added to the enclosure to boost the treble response. After listening tests were completed, the Goodmans Midax horn was selected. It is important to note the Midax is a mid frequency, not a high frequency horn.

A 2 uf oil filled capacitor was wired in series with the horn as a mid/high pass crossover.

The location of the horn near the top of the speaker board required that Vox cover the entire front panel with grill cloth, a departure from the traditional AC-30 vinyl and grill cloth "split-front" cosmetic treatment.

When it was discovered that the Midax horn was too deep for the slender AC-30 cabinet, a hole was cut into the rear panel, allowing the horn to protrude about 1/2" outside the rear of the enclosure (see picture below). Vox considered this modified AC-30 enclosure to be a short term fix to address the immediate needs of the Beatles. Vox planned to design a larger AC-50 Mk II enclosure sometime in the future that would fully enclose the horn.

Vox installed the same 15 watt Celestion Alnico Blue speaker into Mk I AC-50 enclosure as used in the AC-30. The 15 watt power rating of these speakers was somewhat underrated for this amplifier. This was another issue that Vox decided that they would address at a later date in the Mk II production model of the AC-50 speaker system.

Unlike the open backed AC-30 speaker cabinet which had separate upper and lower back panels, the AC-50 Mk I speaker enclosure had a one piece back. A large rectangular hole in this one piece back made the AC-50 Mk I enclosure open backed as well.

Some have claimed that the rectangular hole in the back of the AC-50 Mk I was specially designed as a tuned port to increase bass response, but in my opinion that seems fairly unlikely. First, the size of the opening between the upper and lower back panels on the AC-30 has nearly the same number of square inches as the rectangular opening in the AC-50 Mk I cabinet. Secondly, as Vox installed a horn to boost the high frequency output of the cab, boosting the cabinet's bass response with a tuned port would be counterproductive. Finally, based on the size of the rectangular hole in the back and and the internal volume of the cabinet, the AC-50 Mk I cabinet would be tuned to about 105 hz, an unusually high cabinet tuning frequency. The way to boost bass response in AC-30 sized cabinets is to install a closed back. This avoids the "front to rear" phase cancellation of bass reponse common to open back cabinets.

Prior to the introduction of the AC-50 Mk I enclosure, logos were mounted to the upper left corner of the split vinyl/grill front panel of Vox amps. Mounting the gold plated logo on the black vinyl portion of the front panel provided the Vox name a high level of legibility in photographs and on television.

The AC50 Mk I cabinet departed from the signature split-front Vox cabinet cosmetics. It had a full face grill. In an age dominated by black and white photography and televisions, there was a concern that the Vox logo would be difficult to read, especially against the angular diamonds of the grill cloth. The brown fabric and gold plated logo would appear as shades of gray.

To solve this problem, JMI mounted the -V-O-X- logo on top of a rectangular plywood base that was covered in smooth black vinyl. This logo foreshadowed the horizontal Vox nameplates later used on Thomas Vox amp heads.


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Photos and editorial content courtesy Gary Hahlbeck, North Coast Music

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