1959 AC15 Control Panel



1959 JMI Vox 1x12" AC-15 Amplifier - Serial # 3745
(Originally Named the A.C.1)



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After producing a line of electronic organs and keyboards in the early
to mid 1950s, Tom Jennings, the president of Jennings Musical Industries (JMI), wanted to enter the guitar amplifier marketplace. JMI designed their first prototype guitar amplifier in 1956. After listening tests, Jennings recognized that the amp did not have the special tonal character that would make it stand out in the market.

Word had gotten back to Jennings about a 15 watt guitar amplifier designed and built by a local musician, Dick Denney. Denney was invited to the JMI offices in Dartford Kent to demonstrate his 15 watt home made guitar amp. Jennings was so impressed with the sound of the amp that it led to an agreement with Denney. Jennings purchased the design of Denney's amp and hired Denney to work for JMI as a design engineer. Much like the day that Lennon met McCartney at the church fete in Woolton, this 1957 meeting between Jennings and Denney had great historical significance in the world of music. This was the day that the Vox amplifier line was born.

The first production version of Denney's amplifier was offered for sale in 1958. Originally given the name AC-1, it's rich and harmonically complex tone became the sonic blueprint for most Vox amps to come. The dual channel amp shown on this page was built by JMI in 1959 and still features the first generation circuit of the original Denney prototype design.

The Recipe for Vox Tone
The heart of AC-15 tone comes from the power amp circuit. Three key design concepts were combined to create the characteristic Vox tone. The ingredients were: EL84 output tubes, no negative feedback in the power amp and a Class A "self biased" output stage.

Denney's design used two small bottle EL-84 power tubes as the first component of the signature Vox tone. The EL-84 is a highly efficient tube capable of producing 15 watts per push/pull pair at a relatively low circuit plate voltage of only about 340 volts.

The efficiency of the EL-84 also had a downside. EL-84 tubes were a bit more prone to distort due to their reduced "headroom." Simply stated, when pushed hard, the distortion level could creep up into the 7 percent area. This distortion was normally controlled by the incorporation of a circuit design called "negative feedback." Negative feedback sends a bit of the signal coming out of the amplifier back to the input of the power amp. Negative feedback not only cleans up undesirable distortion, it also removes some of the pleasing even order harmonics from the amplifier output.

After listening tests, Dick Denney decided he preferred the harmonically rich tone and the natural sounding overdrive of the AC-15 amp when negative feedback was not employed. Although unconventional in design for the period, eliminating the traditional negative feedback circuit in the power amp made a major contribution to Vox tone.









The final ingredient involves the method of biasing the output tubes. Bias is a controlling voltage sent to the control grid to keep the current passing through the tube within safe prescribed limits. Most tube power amps have a manual bias adjustment for the output tubes, typically adjusted from time to time by a trained technician.

Denney discovered that his AC-15 design sounded better when the traditional manual bias adjustment was abandoned in favor of a self biasing or "Class A" output circuit.

Vibravox
Many amplifiers include a simple effect called tremolo. The AC-15 featured vibrato. While tremolo pulses the volume of the amp, vibrato varies the pitch of the signal. The "Vibravox" vibrato effect on the AC-15 was engaged either by a control panel mounted toggle switch labeled "Tremulant" or by a remote foot pedal. A three position rotary switch adjusted the speed.

In an interview with Guitar Player magazine, Dick Denney admitted that he "reverse engineered" the complex Vox Vibravox circuit from a home console organ before incorporating it into his prototype AC-15 design. Ironically, Vox was so protective of the Vibravox circuit design that they initially encapsulated a number of its key electronic components into a set of sealed modules.


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.

Cabinet
E
arly AC-15 amplifiers featured a TV front cabinet with rounded corners in the front and square corners in the rear. A removable cardboard panel with die cut ventilation holes enclosed the back. The cabinet was covered in a diamond patterned tan rexine vinyl. Traditional brown Vox grill cloth was installed on the front panel. Individual horizontal letters spelled out "VOX" in the upper right corner of the grill. An embossed "Jennings" logo was mounted to the top front edge of the cabinet.



Control Panel
The 1958-59 version of the Vox AC-15 featured a black control panel. From left to right, the control panel included four guitar inputs, a "Tremulant" toggle switch, a three position vibrato speed switch, a foot switch jack, two volume controls, a tone or "cut" control, a pill style voltage selector, power switch and a pilot lamp. AC-15 amps produced in 1958 and 1959 were typically equipped with white knurled control knobs. Occasionally Vox installed a "chicken head" pointer knob on the vibrato speed switch, as evidenced in this 1959 Vox AC-15 product flyer.


The control panel was mounted beneath a rectangular slot that was cut into the top of the cabinet. This design made the control panel appear as if it were an "island" on the top of the amplifier. The control panel cut out on later AC series Vox amps would continue to the rear of the amp.

This first version of the AC-15 predated the use of slider boards to mount the chassis to the cabinet. The first generation AC-15 chassis was bolted to the cabinet.

Removable Power Cord
A Bulgin power recepticle was mounted to the bottom rear of the left side of the amplifier cabinet. A removable power cord with a matching Bulgin plug connected the amp to the AC wall outlet.

Speaker
The amp was fitted with a 12" 16 ohm Goodmans Audiom 50 speaker.


The A.C.1 Becomes the AC-15
When first introduced in 1958, Vox called this amplifier the "A.C.1." The "1" in A.C.1 pointed to this amp being the first commercially successful guitar amplifier offered by Vox.

As Vox increased the their amp line with new models rated at 6, 10 and 30 watts, the RMS power rating of the amp was combined into the model name. The A.C.1 then became the AC-15. It was joined by new Vox amplifiers named the AC-6, AC-10 and AC-30.

Photo Credit
Many thanks to Mike Handley who generously offered photos of his 1959 Vox AC-15 to the Vox Showroom. Mike's son, Danny, is the guitarist in the Animals.




1958 - 1959 Vox AC-15 Amplifier
Specifications and Dimensions

Size: 20¾" x 20¾" x 10¼"
Weight: About 40 pounds
Channels:

Normal Channel
Vibravox Channel
Power Output: 17 watts RMS
Speaker:

12" 16Ω Goodmans
Audiom 50, 10 watts
Tube Complement: 4 - 12AX7
1 - 12AU7
2 - EL-84
1 - 5Z4
Total Production
Estimate:
500 - 900




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Photos and editorial content courtesy Gary Hahlbeck, North Coast Music


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