JMI Vox 4120 (UL4120) Bass Amplifier - 1966

Figure 1 - 12" T.1225 Celestion Speaker with a Ceramic Magnet
This speaker would evolve into the "Green Back"

Figure 2 - Close up view of the illuminated panel lamps

Figure 3 - Two wooden support rails replaced individual feet

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Thomas Organ Dumps Tubes for Solid State Design
A distribution contract finalized in mid 1965 between JMI, the parent company of Vox, and Thomas Organ, their North American distributor, allowed Thomas to design and produce their own Vox products for the U.S. and Canadian markets. As a first priority, Thomas Organ wanted to eliminate tubes from their amp circuits. The new Vox amps built by Thomas Organ would be transistorized.

JMI Vox lead engineer Dick Denney traveled from the UK to California in October 1965 to get a first hand look at the progress made toward the conversion from tube to solid state amp circuitry at Thomas Organ. Thomas made a compelling case for the advantages of solid state and modular amp design.

Thomas developed a solid state preamp module for guitar and a second for bass. Transistorization allowed Thomas to include a plethora of features and effects into their modular preamps. It would be both difficult and expensive to recreate such a complex preamp circuit using tubes.

Thomas also developed a trio of 35, 60 and 120 watt RMS solid state power amplifier modules that were dependable, lightweight and relatively inexpensive to produce. Thomas Organ combined these preamp and power amp modules to create five new amplifier models: the Viscount, Buckingham, Royal Guardsman, Super Beatle and Westminster Bass.

Not yet ready to take the leap into a fully solid state amp, Denney developed a revolutionary new hybrid amp design that would combine a modular solid state preamp section with a modular tube output amplifier stage. This modular hybrid concept would become the basis for the new Vox "UL Series" amp line introduced and produced exclusively in 1966.

Fifteen, thirty, sixty, and one hundred twenty watt versions of these hybrid solid state/tube amps would be produced under sub contract for Vox by Triumph Electronics, a UK based amplifier manufacturer. Models were produced for both guitar (UL 700 Series) and for bass (UL 400 Series). JMI hoped to replace the "AC Series" amps with these new UL models.

The UL Series only survived one year of production. JMI replaced them with a new series of solid state Vox amps in 1967. There was no further mention of the 4120 or any of the other Vox UL Series amps in UK JMI catalogs after 1966.

Underwriters Laboratory Approval
A 1965 Vox catalog announcing the development of the UL Series stated that "this amplifier was produced to meet certain special electrical
and fire safety regulations enforced in the United States and Scandinavian countries." This would suggest that the "UL Series" name likely refered to the potential for an Underwriters Labratory approval for the amp's circuitry.

UL4120 Amp Head
The introduction of the UL Series marked a major change in the appearance of Vox amplifier heads. Prior to the introduction of the Vox UL series, the front panels of Vox amp heads were covered with diamond grill cloth. A gold plated plastic Vox logo was mounted in the upper left corner of the grill. The controls and input jacks were located on the top of the cabinet.

The front of the 4120 head featured a formed panel made of anodized aluminum. The controls and input jacks were located "Fender style" at the bottom of the panel, facing the audience. The aluminum panel above the controls was etched in the diamond pattern of Vox grill. A series of five small lamps, located near the top of the front panel, illuminated the controls (See Figure 2).

The 4120 had eleven air vents, four rows of two on the top of the cabinet and three on the bottom. Two-pin plastic corners made their first appearance on Vox UL Series amps. Two Vox logo handles helped to distribute the 62 pound weight of the UL4120 head. Rather than using plastic feet, the UL Series amps had a pair of wooden skids (See Figure 3).

The two channel preamp section of the UL4120 was completely solid state. Although the amp featured "Bass" and "Normal" channels, the circuitry for both was identical. Each channel featured dual high impedance inputs, volume, treble, middle, and bass controls plus a "Top Boost" switch. Unlike the printed circuit board construction used by Thomas, the UL4120 preamp circuits were hand wired on tag strips.

The 120 watt power amp stage of the UL4120 was powered by a quad of KT88 tubes.

60 Cycle Hum
All 7120 and 4120 heads suffer from an inherent amount of 60 cycle hum. This subject is handled in depth in the "Under the Hood of the 4120" web page in the Vox Showroom.

UL4120 Speaker Enclosure
The closed backed 8 ohm speaker cabinet designed for the UL4120 had four 12" 25 watt T.1225 Celestion speakers (See Figure 1), two Goodmans Midax horns and a swivel trolley. The cabinet included eight two-pin corners but no handles. It also included a trapezoidally shaped plastic Vox logo with white letters.
Low Market Place Acceptance
In 1991, Dick Denney co-authored a book on the history of Vox called "The Vox Story." Denney reported on page 61 of his book that "although the (UL400 and UL700) amps were loud and reliable, they had a bland and unremarkable sound which failed to engage the interest of most of the top groups it had been undoubtedly aimed at." Failing to achieve significant sales numbers in the UK, the large majority of hybrid UL amps were sold by Vox distributors on the European continent.

The JMI 1966 price list addenda indicated that the UL4120 head and matching 4x12 speaker cabinet retailed for £280. By comparison, an AC-100 Super Deluxe head and matching 4x12 enclosure retailed for £252 in 1966.

Specifications - Vox UL4120
Output Power
120 watts RMS
Tube Complement
1 - ECL86, 1 - EL84, 4 - KT88
Channel One (Bass Channel)
Two inputs,
Top Boost switch,
Volume, Treble, Middle, Bass
Channel Two (Normal Channel)
Two inputs,
Top Boost switch,
Volume, Treble, Middle, Bass
Speaker Complement
Four Celestion 12" T.1225 speakers
Two Goodmans Midax Horns
Size (Head)
24.25" W x 9.5" H x 11.25" D (less skids)
62 pounds
Size and Weight (Speaker Cabinet Less Trolley)
41 3/4" W x 30" H x 13" D (less hardware)
120 pounds
Covers and trolley


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