Designed in 1957 and first produced in 1958, the AC-15 is the Vox amp that "started it all." The amp at left has the second generation cabinet and was manufactured in 1960. Many thanks to Brian Machacek for allowing me to take photos of his 1960 AC-15 amp for the Vox Showroom.
Designed by Dick Denney, the tone of the AC-15 became the sonic blueprint for all JMI Vox tube amps to follow. The heart of Vox tone comes from the power amp section, and three key design concepts were combined in the AC-15 power amp design to create the characteristic Vox sound.
Denney's design used two small bottle EL-84 power tubes to make the first component of the Vox signature tone. The EL-84 is a highly efficient tube. It was capable of producing 15 watts per push/pull pair at a relatively low circuit plate voltage of only about 350 volts. By comparison, EL-34 and 6L6GC output power tubes required plate voltages that approached 450 to 500 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 imput of the power amp. This not only cleans up the distortion, but removes some of the harmonics in the signal. After listening tests, Dick Denney decided he preferred the harmonically rich tone of the AC-15 amp without negative feedback. He also liked the way the amp distorted when overdriven. The second ingredient in the creation of the Vox sound was to eliminate the negative feedback circuit in the power amp.
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. Denney felt that this non traditional approach to biasing the output tubes yielded a superior sounding amplifier.
The 1960 AC-15 had a Fender style "TV" cabinet and was covered in a light tan vinyl with a small diamond pattern. The speaker baffle was covered in traditional brown Vox diamond grill cloth grill cloth and had a gold -V-O-X- logo in the upper right corner. A small stamped "Jennings" logo was fastened to the upper front face of the cabiinet, similar to Fender logos installed on Bassman and Twin amps of the same period. The amp had a single hinged handle and no corner protectors. The black control panel had four inputs and six pointer knobs. The amp had two channels, one with the "Vibravox" effect. This was an fetaure that combined tremolo with vibrato. No top boost circuit was included, tone was adjusted using the "Cut" control.
The 12" Celestion/Vox alnico speaker was introduced to Vox amps in the 1960 AC-15.