JMI Vox UL 430 Bass Amplifier

Close up view of the illuminated panel lamps

The UL amps had two wooden support rails rather than individual feet

Specifications - UL 430 Bass Amp
Output Power 30 watts RMS
Tube Compliment 1 - ECC83, 4 - EL84
Channel One
"Bass Channel"
Two inputs
Boost switch
No other effects
Channel Two
"Normal Channel"
Two inputs
Boost switch
No other effects
Speaker(s) 2 x 12" Celestions
Size (Head) 22.5" W x 8.5" H x 11.25" D
Size (cabinet size, less hardware) 32" W x 21" H x 11.75" D
Accessories Covers, chrome trolley
© 1996 - 2023 The Vox Showroom, all rights reserved. No use on online auctions, eBay or Reverb.

Jennings Musical Instruments, the UK manufacturer of Vox, decided to refresh their amp line by introducing a new series of guitar and bass amplifiers in 1966. Known as the UL Series, with "400 Series" models for bass and "700 Series" models for guitar, these new amps were intended to replace the "AC" series lineup that had propelled Vox gear to the top of the pops.

The two smallest models of the UL Series, the 705 and 710, were all tube designs, derivatives of their predecessors, the AC-4 and AC-10SRT. Vox completed the UL Series with new 15, 30, 60 and 120 watt RMS hybrid designs that featured a modular solid state preamp with a tube output stage. The modular design concept and much of the circuitry of these unique new British Vox amps was actually inspired by developmental work completed at the Thomas Vox Labs in Sepulveda California.

History and Development of the UL Series
Vox products arrived on the American shores in late 1964 after a two-sided distribution contract was inked between Jennings Musical Instruments in the UK and Thomas Organ of Sepulveda, California. Thomas Organ imported JMI products as the sole US distributor of Vox while JMI imported console organs as the sole UK distributor of Thomas Organ products.

In less than a year, Thomas Organ renegotiated the terms of the Vox distribution agreement for the US. 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. Thomas also wished to further reduce costs through a complete redesign of the Vox amp line. As a first priority, tube designs would be dropped in favor of transistors. A second priority was the development of a standardized preamp/control section that could be combined with modular power amps of various RMS power outputs to facilitate rapid product development and offer greater economy in production.

Thomas Organ hired a brilliant electronics engineer, Sava Jacobsen, to lead this solid state conversion and standardization project. Jacobsen had recently worked on a similar project for Fender in 1964. His work at Thomas led to the development of a solid state standardized preamp/control section that would find its way into the new US Vox Viscount, Buckingham, Royal Guardsman, Westminster and Super Beatle amplifiers.

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 the advantages of modular amplifier construction using standardized solid state preamp and power amp sections. The new Thomas Vox amp designs would begin to influence the development of new amplifiers at JMI.

JMI was not yet willing to make the jump to a totally transistorized amplifier due to the reliability issues JMI had with the output stage of the solid state T.60 bass amp. JMI was not confident in the dependability of transistors in the higher current power amp stages of an amp, but felt comfortable using transistors in the preamp section. JMI developed a revolutionary new hybrid amp design that would incorporate a modular solid state preamp section with a tube output amplifier. This solid state preamp/tube output stage 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 under sub contract to Vox by Triumph Electronics, a UK based amplifier manufacturer. At first JMI expected to replace all of the "AC Series" amps with these new UL models but the popular demand for the AC-30 and AC50 models was sufficient to continue their production.

The UL 430 Bass Amp
The preamp section of the UL 430 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. The preamp circuitry featured hand wired, point to point construction on tag strips, unlike the printed circuit board construction used on US Vox amps. The same modular preamp circuitry used in the UL 430 was also incorporated into the UL 460 (60 watt) and UL 4120 (120 watt) bass amps. Each of the dual channels had volume, treble, middle and bass controls along with a "Boost" switch. The first, or "Bass" channel, was electronically identical to the second, or "Normal" channel. A formed full face anodized aluminum front panel was illuminated by a series of small lamps, located just above the diamonds in the front panel (see picture at left).

Like it's predecessor, the AC-30, and similar to the UL 730, the power amp section of the UL 430 was powered by four cathode biased EL-84 power tubes. Unlike the AC-30 and the UL 730, the UL 430 power amp circuitry included a "negative feedback loop." A negative feedback loop returns a bit of the signal coming out of the output transformer of the amplifier back to the input of the power amp. This circuit design not only reduces power amp distortion, it also removes some of the harmonics in the output. The addition of a negative feedback loop in the UL 430 made for a cleaner sounding bass amp.

Unlike the AC-30, the UL 430 used two diode bridges rather than a GZ34 rectifier tube to provide DC operating voltages to the amplifier. An ECC83 twin triode tube served as a phase inverter.

A rear panel mounted five position rotary voltage selector allowed the UL 430 to be adjusted for the mains voltage in any country. Twin speaker out jacks were provided along with an 8 - 16 ohm speaker selector switch. The removable power cord used a four conductor XLR connector on the chassis end.

The UL 430 speaker cabinet featured two 12" Celestion speakers in a sealed back enclosure. The trapezoidally shaped injection molded plastic Vox logo had white letters, a departure from the gold letters normally found 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 1967, Jennings Musical Instruments introduced a series of new modular amps that were totally solid state. These new models replaced the UL Series. In fact, no mention of the UL700 or UL400 Series amps is made just one year later in the 1967 UK Vox catalog.


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