The Vox Supreme, Defiant, Conqueror and Virtuoso Preamp Modules
"Under the Hood"
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The Transition from Tube to Solid State Amp Design Begins at JMI - 1966
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When JMI Vox introduced the hybrid "UL Series" amplifiers in 1966, they followed a trend started a year earlier by Thomas Organ, the US manufacturer of Vox gear.
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Thomas Organ was a proponent 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. Thomas Organ then 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 Super Beatle, Royal Guardsman, Buckingham, Viscount, Sovereign, Westminster and Scorpion amplifiers were all constructed using combinations of these preamp and power amp modules.
Much of the design of the British UL models was inspired by Thomas in America. 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 truly 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 new 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 developed their new purely solid state models.
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The Arrival of Modular Solid State Amp Design at JMI - 1967
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JMI probably realized that they were taking a bit of a risk when 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 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
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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 circuit board, JMI designed their preamp with indiviual circuit board modules. The new JMI solid state amps had separate circuit boards for the Normal channel, the Brilliant channel, Tremolo, Reverb and Mixer. You can see the layout of these boards in the photo on the top of the 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.
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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.
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As in the Thomas preamps, reverb was assignable either to the Normal or Brilliant channel via a rotary selector switch on the lower control panel.
The reverb pan designed by JMI for these amps used a single delay spring between two 1 volt output Sonotone 2T crystal phono cartridges. These cartridges are long out of production and nearly impossible to replace if defective. They 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. The reverb pan was suspended by two rubber bands which often dried up and broke with age.
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.
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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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Photos and editorial content courtesy Gary Hahlbeck, North Coast Music
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URL: http://www.voxshowroom.com/uk/amp/SSpreamp_hood.html
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