JMI Vox UL 715 Guitar Amplifier
JMI Vox UL 710T Amplifier






Close up view of the illuminated panel lamps


Three button foot switch remotely controlled reverb, tremolo, and distortion


The UL amps had wooden slider rails rather than individual feet




Two views of the Vox crystal phono cartridge reverb pan



Features - UL715, UL710T
Output Power 15 watts
Tube Compliment 1 - ECC83, 2 - EL84
Channel One Two inputs,
Boost switch,
Volume,
Treble,
Middle,
Bass,
Speed
Depth
Channel Two Two inputs,
Boost switch,
Volume,
Treble,
Middle,
Bass,
Reverb
Speaker(s) Two Celestion 10" speakers
Size (Head) 22.5" W x 8.5" H x 11.25" D
Size (Speaker Cabinet) 32" W x 19" H x 12" D
Accessories Covers, foot pedal and chrome trolley.
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Introduced in 1966, the JMI Vox UL715 amplifier was the little brother to the Vox UL730 amp. Inexplicably, Vox also issued some of their UL 715 amp production with the name UL 710T.

Many of the concepts that led to the development of the UL715 amplifier in the UK actually originated at the Thomas Organ labs in the United States.

During the summer of 1965, Thomas Organ in California was working on new solid state Vox amp designs that they intended to produce for the American market. The US distribution contract inked between JMI Vox in the UK and Thomas Organ in the US included a provision that allowed Thomas to design and produce their own Vox products in the United States. Due to the high costs of shipping and customs, Thomas decided it would be more profitable to build their own Vox amps in the US rather than import them from the UK. 

In late October, 1965, JMI Vox lead amp engineer Dick Denney traveled from the UK to California to get a first hand look at the progress made toward the conversion from tube to solid state amp circuitry at the Thomas Vox development labs. 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." As the trip ended, Denney started to recognize some of the advantages of solid state guitar amps. Soon these new solid state Thomas Vox amp designs would begin to affect the development of new amplifiers at JMI.

Not yet willing to make the jump to a totally transistorized amplifier, Denney developed a revolutionary new hybrid amp design that would incorporate a modular solid state preamp section with a tube output amplifier. This hybrid concept would become the basis for the new Vox "UL Series" amp line introduced in 1966.

A 1965 Vox catalog announcing 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. I have not read any credible support to suggest that the initials "UL" stand for "Ultra Linear" as some have indicated, but I am open to view any evidence to that claim.

Fifteen, thirty, sixty, and one hundred twenty watt versions of these hybrid solid state/tube amps would be produced for both guitar (UL 700 Series) and for bass (UL 400 Series). JMI expected to replace the "AC Series" amps with these new UL models.

The UL715 featured a formed full face anodized aluminum front panel. A series of small lamps, located just above the diamonds in the front panel, illuminated the controls.

The preamp section of the UL715 was completely solid state, similar to the modular preamps found in the Viscount and Super Beatle amps Denney had seen at the Thomas Labs in California. Each of the dual channels in either amplifier had volume, treble, middle, and bass controls along with a "Boost" switch. Reverb was featured in the Normal channel, the vibrato channel included a foot switch operated "distortion" circuit and tremolo.

The tremolo circuit in the UL 715 utilized an opto-isolator. This device combined a small light bulb with a photo resistor in a small sealed cardboard tube. A circuit in the amp flashed the lamp in varying speeds and brightness as controlled by the tremolo depth and speed controls. The higher the tremolo speed control was set, the faster the lamp would flash. The higher the tremolo depth control was set, the brighter the lamp would flash. These pulses of light were directed into the photo resistor. This caused the photo resistor to apply resistance across the audio output of the channel, based on the intensity and pulse speed of the of the light coming from the lamp, creating tremolo. When the tremolo circuit fails in a 730, it is often the lamp in this opto-isolator that is to blame.

A single reverb control in the UL715 introduced reverb to the Normal chaanel and used the infamous Vox crystal phono cartridge reverb pan.

The reverb pan designed by JMI for the 715 used a single delay spring between two 1 volt output Sonotone 2T crystal phono cartridges. These cartridges were used as the reverb drive and receive transducers.

Tom Jennings, the president of JMI Vox, resented having to pay the $1 per amp licensing fee charged by Hammond Accutronics fee for the use of their patented reverb pan. The Vox crystal phono cartridge reverb pan just barely skirted the patents on the Accutronics unit.

About all one could say about the JMI reverb pan is it worked to a degree, but the tone and depth of the reverb was certainly lacking when compared to Fender amps of this era. Furthermore, this phono cartridge based reverb pan would easily slip into a howling acoustic feedback if the amp was played too loudly. There are no replacements available for defective Sonotone 2T cartridges.

A three button foot switch allowed remote actuation of the reverb, tremolo and distortion features in the UL715.

Like the AC-15 amp it was intended to replace, the output amplifier for the UL715 was powered by two cathode biased EL-84 tubes. One ECC83 tube was used as a phase inverter. The three tubes were easily exchangeable through the open rear panel of the head cabinet.

A rear panel mounted five position rotary voltage selector allowed the UL715 to be adjusted for the mains voltage in any country.

The open backed speaker cabinet designed for the UL715 had two 10" Celestion speakers and a swivel trolley. The UL715 head could accept an 8 or a 16 ohm speaker load.

The UL715 speaker cabinets featured a trapezoidally shaped injection molded plastic Vox logo with white letters. This was a departure from the gold letters used on Vox amp logos previously.

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. In fact, no mention of the UL700 or UL400 Series amps is made just one year later in the 1967 UK Vox catalog.

Price
The JMI 1966 price list addenda indicated that the UL715 head and matching speaker cabinet retailed for £160. By comparison, an AC-15 Twin retailed for £99 in 1965.

The UL 715 amp head and cabinet shown at left is part of the North Coast Music amp collection.




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