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.
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 under sub contract to 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 expected to replace the "AC Series" amps with these new UL models.
The UL 730 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 UL730 was completely solid state, similar to the modular preamp found in the Viscount and Super Beatle amps Denney had seen at the Thomas Labs in California. Unlike the solid state Thomas amps, the preamp of the UL 730 was hand wired on tag strips. Thomas solid state preamp circuits were assembled on a printed circuit board.
Each of the dual channels in the UL 730 had volume, treble, middle, and bass controls along with a "Boost" switch. Reverb was included in the Normal channel only. The Vibrato channel included a foot switch operated "distortion" circuit and tremolo.
The tremolo circuit in the UL 730 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. This created tremolo. When the tremolo circuit fails in a 730, it is often the lamp in this opto-isolator that is to blame.
The 730 reverb circuit used the infamous Vox crystal phono cartridge reverb pan.
The reverb pan designed by JMI for the 730 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.
A three button foot switch allowed remote actuation of the reverb, tremolo and distortion features in the UL730.
Like the AC-30 amp it was intended to replace, the output amplifier for the UL730 was powered by four EL-84 tubes in a cathode biased, no inverse feedback circuit. An ECC83 tube was used as a phase inverter. All five tubes were easily exchangeable through the open rear panel of the head cabinet. Unlike the AC-30 it was designed to replace, the 730 used two diode bridges rather than a GZ34 rectifier tube to provide DC operating voltages to the amplifier.
A rear panel mounted five position rotary voltage selector allowed the UL730 and UL430 to be adjusted for the mains voltage in any country. A speaker impedance selector, also mounted on the rear panel of the amp head, allowed the UL 730 to accept either an 8 or a 16 ohm speaker load.
The open backed 16 ohm speaker cabinet designed for the UL 730 had two 12" 15 watt "Silver Alnico" Celestion speakers and a swivel trolley. The speaker cabinet featured a trapezoidally shaped injection molded plastic Vox logo with white letters, a departure from the gold letters previously used on Vox amp logos.
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.
The UL 730 amp head and cabinet shown at left, serial # 3022, is part of the North Coast Music amp collection.
The JMI 1966 price list addenda indicated that the UL730 head and matching 2x12 speaker cabinet retailed for £190. By comparison, a top boosted AC-30 head with reverb plus a matching 2x12 enclosure retailed for £169 in 1966.
The UL 730 George Harrison Auction Amp
In December 2011 a famous UK auction house announced that they were auctioning George Harrison's original UL 730, serial # 3020. The auction house indicated that they expected the amp to fetch close to $100,000. However, the sale of the amp was withdrawn several days prior to the public auction date. A spokesperson from the auction house indicated that the amp’s current owner made the decision not to sell.
While the Vox Showroom takes no position in the discussion, some questions had arisen regarding the validity of the provenance connecting this amp to George Harrison and the Beatles. Some have suggested that the "JMI Artist Promotional Stock" sticker inside the amp tying it to the Beatles was counterfeit. Others have pointed to chalk marks found on the inside left side of the auction UL 730 speaker cabinet that did not totally correspond with chalk marks visible in an photo of George's amp taken in the Abbey Road Studio in 1966 (seen at lower left and close up below).
Auction photo - 2011
Abbey Road Photo - 1966
However, some interesting facts about the 730 were brought up from the research for the auction. Sources indicated that just a few more than one hundred UL 730 amps were produced by Vox. Seventy-six of those amps were returned to Vox by customers in even exchange for solid state models introduced by the company in 1967. The seventy-six UL 730 amps that were returned were reportedly destoyed by Vox. Knowledgable sources contacted by the auction house believed that no more than twenty-six UL 730 amps are still in existance today.
In 1997, I interviewed Dick Denney regarding the UL Series amps. He told me that Vox did indeed take many, if not most of these amps back from customers in exchange for the all solid Vox state amps introduced in 1967, confirming the statements above.
The UL 730 is certainly one of the rarest of all Vox amps.