1958 - 1959 JMI AC-15 Amplifier
Originally Called the Vox AC-1
"A Look Under the Hood"




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The amplifier that would later be known as AC-15 was originally introduced as the "A.C.1" by Jennings Musical Industries in 1958.

As Vox increased their amp line with new models rated at 6, 10 and 30 watts, Vox devised a new model designation scheme. By early 1959, JMI included the RMS power rating of the amp in the model name. As a result, the A.C.1 became the "AC-15." The AC-15 was soon joined by new Vox amplifiers named the AC-6, AC-10 and AC-30.

Chassis
A rectangular steel pan served as the chassis. The power transformer, output transformer, choke, filter cap, tube sockets and three sealed cans that concealed a number of vibrato circuit components were mounted to the outside of the chassis pan. The chassis was finished in a blue-gray "hammertone" finish.

The high gain preamp circuitry was located inside the chassis pan. This location shielded the preamp circuit from stray electronic fields emanating from the transformers.

The mounting location of the transformers caused an unbalanced distribution of chassis weight. Vox mounted the cabinet carrying handle off center to compensate for this imbalance.

A black anodized control panel featured gold lettering.

The Vibravox Circuit
The first generation AC-15 included a Normal and a Vibrato (or Vibravox) channel. Vibravox was offered on the AC-15, the AC-30 and as a stand alone effects unit that could be used with any amplifier.

In an interview with Guitar Player magazine, Dick Denney admitted that the complex Vox Vibravox circuit was "reverse engineered" from the design of a Wurlitzer home console organ. Ironically, Vox was so protective of the Vibravox circuit design that they initially encapsulated some key electronic components of the circuit into a set of three sealed modules. These modules are similar in appearance to a typical chassis mount filter capacitor found in higher power amplifiers (see photos above).

Jennings did everything in their power to discourage prying eyes from opening one of these sealed modules. The 1958 Jennings schematic for the Vibravox circuit states:

"Important Note: - The hermetically sealed cans No. 1, 2 & 3 must not be opened to the air. In the event of a suspected failure the can should be returned to the factory for a matched replacement."

The owner of the amp shown on this page indicated that he opened the hermetically sealed Vibravox modules to replace several defective internal capacitors. He successfully restored proper function to the vibrato circuit.

Two 12AX7 and one 12AU7 tubes were used in the Vibravox circuit. Both halves of the first 12AX7 were used as preamp gain stages. One half of the second 12AX7 served as the vibrato oscillator, the second half acted as the vibrato circuit phase inverter. The 12AU7 was the vibrato modulator.

Vox used black nuts on the input jacks for the Normal channel and white nuts on the input jacks for the Vibrato channel. This convention was followed on most future Vox amps that included the Vibravox circuit.

Power Supply
The power transformer used in the first generation AC-15 power supply had primary (input) taps at 110 VAC, 130 VAC, 200 VAC, 225 VAC and 240 VAC. The input voltage was selectable using a "plug" style voltage selector on the control panel.

The secondary side of the transformer had a 350 VAC B+ tap and a 6.3 VAC filament tap.

A 5Z4 full-wave rectifier tube provided primary DC rectification. The rippled DC from the 5Z4 was smoothed by a dual section 16 uf, 450 volt capacitor and a 10 henry choke in a "capacitive pi" filter circuit.

Tone Control
This first version of the Vox AC-15 included a single "Tone" control for both channels. Typically, tone controls are located within the preamp circuitry of the amplifier. This was not the case with the Vox AC-15. The "Tone" control was located in the power amp circuit between the phase inverter and output tubes. Here is how it worked.

The signal from the preamp is directed to the phase inverter. The output from the phase inverter is split into two signals. One of these signals is 180 degrees out of phase with the other. When "out of phase" signals are combined, they cancel each other out. The "Tone" control works by combining the high frequency signals from one side of the phase inverter with the other.

In later Vox amps, the "Tone" control would be renamed the "Cut" control.

My Thanks
Much gratitude is extended to Mike Handley who graciously dissembled his 1959 AC-15 to take the photos featured on this page.



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Photos and editorial content courtesy Gary Hahlbeck, North Coast Music


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