The Essex Bass Amp - A Look "Under the Hood"

Thomas Organ invited Dick Denney, lead amp engineer at JMI Vox in the UK, to visit the California based Thomas Developmental Labs in October 1965. The purpose of the visit was to have Denney audition and critique the new US Vox amp designs under development at Thomas Organ. Denney evaluated a number of new US Vox models, including the very first amp that Thomas designed for the US market, the V-4 Essex bass.

The results were reported to JMI in a letter from Thomas Organ dated November 4, 1965. When discussing the V-4, Thomas explained to JMI that the Essex was "designed primarily to produce a self-contained bass amplifier to sell at a retail price lower than any other bass unit on the market." The letter also admitted that the undistorted acoustic output capability was limited in the bass region. You can read the entire contents of this letter on pages 142 - 146 of "The Vox Story," which is offered for sale at North Coast Music.

The picture at left shows the fifth and final version of the Essex, model V1043, with the back removed. The control panel and associated preamp is visible on the top of the amp. There were only two controls, Volume and "Tone X."

The "Tone X" circuit was designed by Thomas Organ engineer Sava Jacobsen. Jacobsen explained to the Vox Showroom that the Tone X

control made it possible to shift from a low-pass audio filter to high-pass audio filter with the twist of one knob. A adaptation of this "Tone X" circuit would later be the electronic basis for the Vox Wah-Wah pedal.

The V1042 and V1043 Essex bass models incorporated a foot switch into the "Tone X " circuit. This change required the addition of a "Pedal Control Amplifier" circuit (see photo at left) that was located on the inside right wall of the cabinet. Depressing the foot switch created an effect that was equivalent to advancing the Tone-X control to "10."

The power supply and power amp module (see photo at right) was mounted to the left inside of the cabinet. I am quite confident that this power amp module was originally designed for use in a Thomas Organ console and was later adapted for use in the Vox Essex amp. This is further suggested by the presence of an AC convenience outlet located on the amp chassis module. As the Essex has a closed back, there would be no access to use the outlet.

Powered by two germanium output transistors, the amp was capable of 30 to 35 watts RMS. Replacements for these output transistors are still available from NTE Semiconductors as part # NTE 121.

A shielded cable terminated with an RCA plug dropped down from the preamp to provide the audio signal to the power amp.

Thomas Organ installed a pair of 12T6 speakers from the Oxford Speaker Compamy of Chicago IL into the Essex Bass.

Assigned the Thomas Organ part number 33-5066-6, each Essex speaker was 8 ohms and was probably rated at about 20 - 25 watts RMS. The ferrite magnet structure appears to be in the 40 ounce range. The black Oxford 12T6 speaker shown at far left was installed in a V-4 Essex (the first version of the amp), and the gold Oxford 12T6 speaker at near left was installed in a V1043 (the final version of the amp).

The V-4 cabinet had a particle board baffle and back but plywood sides and bottom. Subsequent models were constructed solely of particle board.

A "G-Tuner" feature was added to the V1043 Essex Bass amp circuitry.

The "G-Tuner" was a basic tone generator circuit comprised of a variable inductor, a transistor, a capacitor and a few resistors. When operated, the circuit generated an organ like reference tone pitched to "G " in the musical scale. This was used to provide a reference frequency for tuning the G string on a bass guitar.

This circuit was mounted to the inside upper back panel of the Essex Bass. A slide switch, recessed into a round hole, enabled the tuning tone. A second small round hole in the back was located just below the switch. This provided an access point to insert a screw driver to adjust the frequency of the G-Tuner circuit should it drift from the desired frequency.

At left is a picture of the chrome plated tubular "push cart" or roller trolley that was included with the Essex Bass amp. The amplifier merely rested on the bars at the bottom of the cart, held in place by gravity alone.

The protective cover for the Essex Bass amp is pictured below.


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Vox V4A, V1041 & 1042 Essex Bass Amplifier Schematics - Free Download
Click here to download the original Thomas Organ schematic for the V4A, V1041 and V1042 Vox Essex Bass amplifier, courtesy of the Vox Showroom and North Coast Music.


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Photos and editorial content courtesy Gary Hahlbeck, North Coast Music

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