The Vox 7120 Head - An "Under the Hood" Look at the Lower Chassis
Solid State Preamp and Phase Inverter Circuits (1966)

Vox 7120 Serial #1023

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The modular, solid state preamp circuit from the Vox 7120 was also shared with the Vox 715, 730 and 760.

Solid State Preamp Circuit
A newly signed distribution contract finalized in the summer of 1965 between JMI and Thomas Organ allowed Thomas to design and produce their own Vox products for the North American market. As a first priority, Thomas Organ wanted to eliminate tubes from their amps. The new Vox amps built by Thomas Organ would be transistorized.

JMI Vox lead engineer Dick Denney traveled from the UK to California in October 1965 to get a first hand look at the progress made toward the conversion from tube to solid state amp circuitry at Thomas. Denney witnessed how transistorization allowed Thomas to add a plethora of features to their solid state preamps. Transistor circuits cost less to build and could be more dependable than their tube counterparts.

Thomas made a compelling case to their UK partners for the advantages of modular solid state amp design. Introduced by JMI in 1966, the 7120 featured a fully featured modular solid state preamp circuit. Thomas Organ had begun to influence amp design at JMI.

Hand Wired Tag Strip Construction
The transistorized circuitry for the Vox UL Series amps, including the 7120, was hand wired on five tag strips. This was unusual as most solid state circuitry is assembled on printed circuit boards.

Optical Tremolo
Many amp manufacturers used the term "vibrato" to describe an effect more correctly known as tremolo. Vibrato is frequency modulation while tremolo is amplitude modulation. The "Vibrato" channel of the Vox 7120 offered tremolo, not vibrato.

The 7120 tremolo circuit utilized an opto-isolator. The opto-isolator was comprised of a small light bulb and a light dependent resistor (LDR) in a sealed can (see photo at right). When dark, an LDR has electrical high resistance, but in the presence of light, the resistance of the LDR drops to nearly zero.

The tremolo drive circuit was located on the left rear tag strip (see rear chassis photo above). The tremolo drive circuit caused the lamp to flash inside the opto isolator. Advancing the Speed control increased the speed of the flashes. Advancing the Depth control increased the brightness of the flashes. The flashing lamp in the opto-isolator caused the resistance of the LDR to vary over a wide range. When these changes in resistance were applied to the preamp signal, tremolo was created.

Phono Cartridge Reverb
Electronically simulated "spring" reverb was commonly available in guitar amplifiers by 1966. Hammond Organ had developed their "Type 4" reverb tank several years earlier and was licensing the technology to guitar amp manufacturers. Fender was incorporating the Hammond Type 4 reverb system into their Princeton Reverb, Deluxe Reverb and Twin Reverb amplifiers, among others.

Hammond required that a manufacturer pay a one dollar licensing fee for each unit produced using the Type 4 reverb system. Most manufacturers were content to license the Hammond patent.

Tom Jennings, the president of JMI Vox, resented having to pay the dollar per amp licensing fee charged by Hammond. Instead, JMI designed their own reverb delay line that would narrowly bypass Hammond's patents.

The 7120 reverb pan used a single delay spring between a pair of one volt output Sonotone 2T crystal phono cartridges. These cartridges were used as the reverb drive and receive transducers. The reverb pan was suspended inside the chassis base on two rubber bands.

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. Further, the pan was prone to feedback at higher volumes and the rubber bands suspending the pan frequently broke.

Fifty years later, replacement Sonotone 2T crystal phono cartridges are nearly impossible to source.

The chart success of "Satisfaction" by the Rolling Stones in 1965 convinced every guitarist that they needed a fuzz tone. The UL 700 Series amps, including the 7120, were the first Vox amps produced in the UK that included a distortion effect, or "fuzz tone" as a feature.

The 7120 fuzz effect used two circuits to generate the effect. Vox used a pair of back to back OA200 diodes as "clippers" (see schematic at right). These diodes created distortion by chopping a portion of the peaks from the audio signal.

While introducing distortion, the diodes reduced the level of the signal to an unusable point. Vox designed a gain recovery circuit using two transistors (TR3 and TR4) to increase the level of the fuzz tone to an acceptable level.

A DPST switch mechanism in the 7120 three button foot pedal completed the connections necessary inside the amp to turn on the distortion or "fuzz tone" circuit. The 7120 offered no provision to adjust the intensity of the distortion, it was controlled solely by a foot switch in the foot pedal.

Floating Paraphrase Phase Inverter
All amplifiers using a "push-pull" circuit design need to have a phase inverter circuit. The phase inverter converts the audio signal from the preamp into two equal but opposite waveforms. Each waveform feeds one side of the push-pull output amplifier. The output tubes then connect to the primary side of the output transformer where the original and inverted signals emerge as a single amplified signal.

The 4120 and 7120 utilized two tubes, an EL84 (V2) and the pentode section of an ECL86 (V1b) in a "floating paraphrase phase inverter" circuit. To ensure that the phase inverter is producing two equal but opposing waveforms, a level control (VR13, shown at left) is located on the left rear tag strip to adjust and match the level of the EL84 to the ECL82.

Foot Switch
A three button, cast aluminum foot pedal was included with the Vox 7120. It included three foot switches to remotely control the reverb, tremolo and distortion circuits. The pedal cable was terminated with a six pin DIN plug to connect to a jack mounted in the rear panel of the amplifier.

The foot switch cable had six conductors and a ground shield. The shield must be grounded to ensure quiet operation.

The tremolo circuit uses a SPST foot switch and connects to pins 3 and 4 of the DIN plug. The reverb circuit also uses an SPST switch and connects to pins 1 and 4 of the DIN plug. The distortion circuit requires a SPDT or a DPDT foot switch. Two circuits must be completed to energize the distortion circuit. Pins 2 and 6 power the gain recovery circuit and pins 4 and 5 introduce the diode clippers to the audio signal.

The 7120 foot pedal is electronically incompatible with the three button pedals included with solid state Vox Conqueror, Defiant and Supreme amps.


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Photos and editorial content courtesy Gary Hahlbeck, North Coast Music

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