The Vox Dynamic Bass, Foundation Bass and
Super Foundation Bass Solid State Preamp Modules (1967-72)
A Look "Under the Hood"



The introduction of a series of new British made Vox guitar and bass amps in 1967 demonstrated the influence exerted on JMI by Thomas Organ, the manufacturer of Vox products for North America. JMI now moved away from hand-wired tube amps in favor of modular, solid-state circuits built on printed circuit boards.

The Impact of Thomas Organ at JMI (1965-66)
When Beatlemania blew the roof off at JMI in 1964, the UK Vox amp line was made up of a series of hand wired tube amps whose circuitry shared little in common. Each model had a unique circuit design that was hand wired on chassis mounted tag strips, a highly inefficient and labor intensive process.

Prior to 1967, JMI had produced only one purely solid-state amplifier design. It was a 30 watt hand wired circuit that was included in two models, the T.60 bass amp and the LW30 guitar amp. These amps were prone to frequent failure due to the fragile nature of the germanium power transistors in their output stage. It wasn't until Thomas Organ established a successful record of producing transistorized Vox amps in America that JMI was willing to once again attempt production of solid-state amplifiers.

Thomas Organ was a proponent of the efficiency of solid-state modular amp construction using printed circuit boards. Using this philosophy, Thomas designed a three channel transistorized modular preamp chassis for guitar and a second two channel transistorized modular preamp chassis for bass in the fall of 1965. Thomas Organ also developed 30, 60 and 120 watt modular solid state power amps that would connect interchangeably with either the guitar or bass preamp via a nine pin connector. The 60 and 120 watt modular solid-state amp modules incorporated silicon (rather than germanium) output transistors with large heat sinks, making them both durable and dependable. The Super Beatle, Royal Guardsman, Buckingham, Viscount, Sovereign, Westminster and Scorpion amplifiers were all constructed using combinations of these preamp and power amp modules. The manufacturing efficiencies demonstrated by Thomas Organ would soon have an impact at JMI.

The Vox "UL Series" Reintroduces Solid-State Preamp Circuits and Incorporates Modular Construction at JMI (1966)
JMI decided to develop a "hybrid" UL Series amp line that combined a solid-state preamp with a tube powered output stage, bridging the gap between tubes and transistors. Much of the design and features of the British UL Series models was inspired by Thomas in America.

The hybrid portion of the JMI Vox "UL Series" included models for guitar and bass. JMI designed a solid state two channel preamp circuit for guitar and a second two channel preamp circuit for bass. These preamp circuits were combined with a 15 watt, 30 watt, 60 watt or 120 watt tube power amp to create four guitar amps (UL 715, UL 730, UL 760, and UL 7120) and three bass amps (UL 430, UL 460 and UL 4120).

While being conceptually similar to the modular US Vox amps, the UL models were quite different in construction. The UL amps were not totally modular as the preamp and power amp circuits were assembled on one chassis. The UL amps featured point to point, hand wired construction while Thomas amps were built on printed circuit boards. The UL amps were "hybrids," having a solid state preamp and tube power amp. The comparable US Vox amps were 100% solid state.

The UL amps were the stepping stones between the all tube amplifiers JMI produced through 1965 and the modular, solid state designs Vox introduced in 1967. As such, the UL amps were only offered in the UK in 1966 while JMI quietly developed their new purely solid state models.


The Arrival of Modular Solid State Amp Design at JMI - 1967

JMI probably realized that they were taking a bit of a risk if they dropped most of their tube models in favor of new Thomas Organ inspired solid state amplifiers. The recent 700 and 400 amp series had been poorly received. Jim Marshall had already started making inroads in the market place by attracting Eric Clapton to play his amps instead of Vox.

JMI produced an elaborate new 1967 Vox amp catalog to introduce these new amplifiers. They also produced an eight page public relations pamphlet entitled "Why We Use New Silicon Transistors in Vox Solid State Amplifiers."

It isn't too hard to spot how much influence Thomas Organ was now exerting over amplifier design at JMI. The control panel

graphics, layout, features and even the shape of the control knobs of the new JMI solid state amp models came from the Thomas play book. Rather than give their new amps model numbers (like AC-30) as they had in the past, JMI gave the new amps names (Supreme, Defiant, etc), as also done by Thomas (Super Beatle, Royal Guardsman, etc). Internally, the new JMI amps used individual preamp and power amp modules, as also done at Thomas.

JMI identified an area where they saw the potential for improvement on the Thomas preamp design. While Thomas designed all of their preamp facilities and effects onto one large printed circuit board, JMI designed their preamp with individual printed circuit board modules. Separate circuit boards were developed for the Normal channel, the Brilliant channel, the Tone-X channel, Tremolo, Reverb and Mixer. The bass preamp incorporated just three of these circuit boards: the Brilliant, Tone-X and Mixer modules. You can see the layout of these boards in the photo of the laughably empty bass preamp chassis on the top of this page.

Constructing an amp with individual circuit board modules has several advantages. JMI could use one preamp chassis for all models in the range, installing only the applicable modules for a given model. As an example, bass guitar models would need the two channel preamp boards and the mixer, but would not need the reverb or tremolo boards. A second benefit is ease in service. Individual circuit modules help to narrow down the source of an electronic fault.

JMI also revised the Thomas Organ circuits for the Normal and Brilliant channels. In the Thomas preamp design, the Normal channel had Tremolo, assignable Reverb and foot switchable Distortion, or "fuzz" tone. The Brilliant channel of Thomas Vox amps had assignable Reverb and MRB, or "Mid Range Boost." JMI moved the Distortion circuit from the Normal to the Brilliant channel where it could be combined with MRB for additional tonal effects. The iconic George Harrison guitar solo in the Beatles' song "Fixing a Hole" was recorded using this combination.

JMI didn't stop there. The Distortion effect in the JMI solid state preamp featured three germanium transistors, reminiscent of the circuitry found in the Vox - Sola Sound "Tone Bender" fuzz tone pedal (see picture at right). When used in a "fuzz" tone circuit, germanium transistors offered a smoother and more tube like distorted tone that even allowed chordal use. JMI further improved the distortion circuit by adding a variable control to adjust the amount of fuzz introduced into the tone.


A multi-conductor cable with an eight pin octal plug was hard wired to the JMI modular preamp. It connected to a receptical located in the power amp/power supply module.

This cable carried DC supply voltages from the power supply to the preamp. It also sent the audio signal from the preamp to the power amp.




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