UL7120 Amp Head - Serial #1023

JMI Vox 7120 (UL7120) Guitar 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 - Three button 7120 foot switch
Controlled Reverb, Tremolo and Distortion



Figure 4 - Two wooden support rails replaced individual feet



Figure 5 - Phono cartridge reverb



Figure 6 - John Lennon of the Beatles and a Vox UL7120
Munich Germany, 1966



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The Quest for More Power
The quest for more powerful amps at JMI started when the UK instrumental band the "Shadows" asked Vox lead engineer Dick Denney for a larger and more powerful amp than the AC-15. This led to the development of the thirty watt AC-30 in 1960. Vox introduced the fifty watt AC-50 and the one hundred watt AC-100 Super Deluxe amps in early 1964 in response to the Beatles' needs for yet louder amps.

JMI introduced an even more powerful amp in 1966. Rated conservatively at 120 watts RMS, the Vox 7120 (UL7120) was used by the Beatles in Germany, Japan and the Philippines during their final world tour in 1966. It featured a radical new design that blended solid state circuitry with tubes. Some of the inspiration for the unique circuitry of this British Vox amp came from Thomas Organ in America.

Solid State for the United States
A newly signed distribution contract finalized in the summer of 1965 between JMI and Thomas Organ allowed Thomas to design and produce their own Vox products for the North American market. 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. Denney had the opportunity to audition the solid state replacement for the AC-30, the "Viscount." He also tried out the solid state replacement for the AC-100, the "Super Beatle." Soon these new solid state Thomas Vox amp designs would begin to affect the development of new amplifiers at JMI.

A Modular Hybrid
Thomas Organ 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. Some of the new solid state Thomas Vox amps included a four button foot switch that would allow remote control of the effects. It would be both difficult and expensive to recreate such a complex preamp circuit using tubes.

Thomas also developed a series of modular 35, 60 and 120 watt RMS solid state power amplifiers that proved to be 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.

While the solid state preamp sections designed by Thomas Organ appealed to Denney, he had reservations about transistorized power amplifiers. Vox was still dealing with the aftermath of the failure of countless germanium output transistors in their T.60 bass amps.

Not yet ready to take the leap into another 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.

Even as the first deliveries of Vox UL amplifiers were arriving at retailers in mid 1966, JMI was well into the development of a line of fully solid state amps. Announced in a JMI Vox price list in April 1967, these amps would be similar in design to the solid state Vox models offered by Thomas Organ.

The arrival of the new, all solid state Vox amps signaled the end of production for the UL Series. There was no further mention of the 7120 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.

UL7120 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 Vox logo was mounted in the upper left corner of the grill. The controls and input jacks were located towards the rear of the top of the cabinet.

The front of the 7120 head featured a formed panel made of anodized aluminum. The controls and input jacks were located at the bottom of the panel, facing the audience. The area above the control panel 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). An etched Vox logo graced the upper left corner of the front panel.

"Two pin" plastic corners made their first appearance on the UL Series amps. The 7120 had eleven air vents, four rows of two on the top of the cabinet and three on the bottom. Two Vox logo handles helped to distribute the 62 pound weight of the 7120 head. Rather than using plastic feet, the UL Series amps had a pair of wooden skids (See Figure 4).

The preamp section of the 7120 was completely solid state. Unlike the solid state printed circuit board construction used by Thomas, the preamp of the 7120 was hand wired on tag strips. JMI included this same hand wired, solid state preamp circuit in their 715, 730 and 760 Vox amplifiers.

The 7120 head had two channels, Normal and Vibrato. Each channel had two inputs. Both channels had volume, treble, middle, and bass controls along with a "Boost" switch. Distortion (fuzz) and optical Tremolo were included in the Vibrato channel. Both channels shared "crystal phono cartridge" reverb (See Figure 5).

A three button foot pedal, not unlike those developed by Thomas Organ, allowed remote actuation of the reverb, tremolo and distortion features in the 7120 (See Figure 3).

Unlike the EL34 powered AC-100 amp it was intended to replace, the output stage for the 4120 and 7120 was powered by four KT88 tubes.

The 7120 rear panel included a mains voltage selector, a "pill" type speaker impedance selector and twin ¼" speaker jacks.

UL7120 Speaker Enclosure
The open backed 8 ohm speaker cabinet designed for the UL7120 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. The speaker cabinet also included a trapezoidally shaped injection molded plastic Vox logo with white letters, a departure from the gold letters previously used on Vox amp logos.

60 Cycle Hum
All 7120 and 4120 heads seem to have a small amount of 60 cycle hum. This subject is handled in depth in the "Under the Hood of the 7120" web page in the Vox Showroom.

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 (UL700 and UL400) 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.

I interviewed Dick Denney in 1997 regarding the UL Series amps. He told me that Vox took many, if not most of these UL amps back from unhappy customers in exchange for the all solid Vox state amps introduced in 1967.

Price
The JMI 1966 price list addenda indicated that the UL7120 head and matching 4x12 speaker cabinet retailed for £300. By comparison, an AC-100 Super Deluxe head and matching 4x12 enclosure retailed for £252 in 1966.




Specifications - Vox UL7120
Output Power
120 watts RMS
Tube Complement
1 - ECL86, 1 - EL84, 4 - KT88
Channel One (Vibrato Channel)
Two inputs,
Boost switch,
Volume, Treble, Middle, Bass
Tremolo Speed and Depth

Distortion
Reverb
Channel Two (Normal Channel)
Two inputs,
Boost switch,
Volume, Treble, Middle, Bass
Reverb
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
Accessories
Covers, foot pedal and trolley





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