AC50/2 - AC50/4 Mk III Control Panel

JMI Vox Foundation Bass Amplifier - 1963 - 1966

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Prior to 1962, a bass guitarist wishing to purchase a Vox
bass amplifier had only two choices, the AC-15 Bass or the AC-30 Bass. Both were combo amps that used circuitry that was originally designed for guitar, modified only slightly for bass. Either amp had an open backed speaker cabinet that tended to cancel bass response. Neither of these early Vox bass amps had the potential for full, rich bass tone. Recognizing the market potential for a true bass amp, Vox developed two models specifically designed for bass guitar; the T.60 and the Foundation Bass.

The Foundation Bass amplifier was the combination of a Vox AC-50 amp head and a compact 1x18" speaker enclosure. The speaker enclosure alone was also often referred to as the Foundation Bass.

The Foundation Bass speaker cabinet first appeared in the 1963 JMI Vox catalog featuring Cliff Richards and the Shadows on the cover. The first pairing of an AC-50 amp head with a Foundation Bass speaker enclosure is documented in a 1964 JMI Vox catalog featuring the Dezo Hoffman "Jumping Beatles" photo on the cover.

The Vox AC-50 Amp Head
The AC-50 amp head used to power the Foundation Bass amp went through five permutations in the years between 1963 and 1966. The original model is often referred to as the "small box, thin edge" or Mk I AC-50 head and was offered in late 1963 and early 1964. This earliest model had a dual input, single channel circuit and a copper control panel. It featured a a GZ34 rectifier tube, two fixed biased EL-34 power tubes, two ECC83 (12AX7) and one ECC82 (12AU7) preamp tubes. There were three controls: volume, bass and treble.

The second version of the single channel Mk I AC-50 head appeared in the spring of 1964 and increased the number of input jacks from

two to four, arranged into the shape of a diamond. In other respects, the features and cabinet were identical to the first version of the amp.

Lead JMI Vox engineer, Dick Denney, once told me that Vox tried to make their amp heads no larger than the size of a "lunch pail" whenever possible. That certainly was true in the case of the "small box, thin edge" AC-50 heads. The head cabinets were constructed of 3/8" Baltic birch plywood.

Next came the AC-50 MK II amp head, introduced in the spring of 1964. It featured a "small box, thick edge" enclosure. The sides of the slightly larger cabinet shell were 3/4" Baltic birch plywood. Electronically, it was identical to it's four input Mk I predecessor, a single channel amp with three controls, four inputs and a copper control panel. The output stage was fixed biased and the circuitry was tube rectified.

The fourth version of the AC-50 head, known as the AC50/2 or "Mk II" , was introduced in late 1964. It added a second channel (along with an additional ECC83 tube) and a larger cabinet, the outputs were again fixed biased. The gray control panel featured volume, bass and treble controls for each channel. The chassis was no longer bolted to the cabinet, it was fastened to a removable "slider board," similar to those found on the AC-15 and AC-30. Storage for cables was provided at the bottom rear of the head enclosure. This amp is often referred to as the tube rectified "big box" or "tall box" AC-50 head.

The fifth version of the AC-50 head, the AC50/4 Mk III, was released in 1965. The most notable change from the previous model was the replacement of the GZ34 tube rectfier with diode or "solid state" rectification in the power supply. The version of the Foundation Bass pictured on this web page uses this fifth version of the AC-50 head.

A minor circuit change to the AC-50 in 1966 created the AC-50 Mk IV. This model was not offered with the Foundation Bass cabinet.

The Vox 1 x 18" Foundation Bass Cabinet
With dimensions of 26" x 22" x 15 1/2," the Foundation Bass speaker enclosure was no larger than necessary to enclose an 18" speaker. The enclosure was constructed of 3/4" Baltic birch plywood.

Earlier versions of the Foundation Bass speaker enclosures featured brown Vox diamond grill. As Vox started the conversion from brown to black diamond grill on all their amps in 1964, the Foundation Bass switched to black diamond grill as well.

The earliest versions of the Foundation Bass cabinet were covered with the "smooth" vinyl Vox used in 1963 during the transition away from fawn vinyl. I am not aware that the Foundation Bass was ever available in fawn vinyl. By late 1963, the smooth vinyl Vox used in the transitional period from fawn was dropped in favor of black basket weave vinyl.

Vox used 18" speakers from three UK based manufacturers in the
Foundation Bass enclosure: Goodmans, Celestion and Fane. The most commonly used speaker found in the Foundation Bass was the 50 watt Goodmans Audiom 91 with a ferrite magnet, shown at right. By modern standards, the 50 watt RMS rating of this speaker would be considered to be puny. Some modern 18" speakers can handle 1000 watts RMS.
Rather than installing handles on the sides of the cabinet, Vox provided recessed four handle holes. Eight one pin corners were standard as was a large vertical "pie" logo.

Vox offered the Foundation Bass cabinet with or without casters. Vox also offered a small "bracelet" style trolley with casters for the Foundation Bass (see photo at right, thanks to Ihor Boyko for allowing the Vox Showroom to take this photo of his Foundation trolley).

Many 1960's British Invasion bands used the Foundation Bass amplifier. Bill Wyman from the Rolling Stones often used two Foundation Bass cabs powered by an AC-100 head. Vox promotional photos of the Dave Clark Five, Billy J Kramer and the Dakotas, Gerry and the Pacemakers, and Paul Revere and the Raiders featured Vox Foundation amps or cabs.

Vox retired the Foundation Bass amp in 1966 with the introduction of the UL Series bass amps, the UL430, UL460 and UL4120. In 1967, Jennings Vox restored the Vox Foundation Bass to their line, retaining the 1 x 18" speaker cabinet but replacing the tube AC-50 with a Thomas Organ Vox influenced 50 watt solid state head.

1964 Vox Promotional Photo of Gerry and the Pacemakers with two AC-50 "big box" amps and a Foundation Bass enclosure


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