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 1961 AC-15 was covered in fawn vinyl and had brown Vox grill cloth. A single leather handle, two brass vents, and chicken head knobs were used. The amp had two channels, and one of these channels had “Vibravox.” This was an effect that combined tremolo with vibrato. No top boost circuit was included, tone was adjusted using the “Cut” control.
A single 12″ Celestion/Vox speaker alnico speaker was standard.