Designed by Dick Denney, the tone of the AC-15 became the sonic blueprint for all JMI Vox tube amps to follow. While the AC-15 preamp section is unique in its own right, the heart of Vox tone comes from the power amp. 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 two tone color scheme. I understand the "blue over light grey" color combinaton shown in the photos at left was quite common. 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 at left has a replaced handle, the original handle was hinged. As this was the bass version of the AC-15, there were only four control knobs and no "Vib/Trem" circuit. "Top Boost" circuitry had not yet been invented, the tone was adjusted using the "Cut" control.

A 12" 20 watt Goodmans Audiom 70 speaker was standard.

North Coast Music offers these repair parts and accessories for the AC-15 Current reissue brown diamond Vox grill cloth Reproduction Serial Plates Vox Logo for the AC-30 (NCM-025) Model "Flags" as located in the lower right corner of the Replacement JMI Vox Speaker Stickers for the AC-15 Vox Egg Foot Switch - cast aluminu