JMI Vox AC-15 Twin - 1963

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If a Vox AC-30 was not in the budget for a young guitarist living in the 1960s, the AC-15 Twin was a good option.

The AC-15 Twin and AC-30 cabinets had the same external dimensions and both had two 12" speakers. Unlike the AC-30, which featured two Celestion Alnico Blues, the AC-15 Twin utilized two Goodmans Audiom 50 speakers.

The 1963 AC-15 Twin pictured on this page has a copper control panel and is covered in black basket weave pattern vinyl. This amp predates the use of plastic corner protectors and has three black vinyl Vox logo handles.

The AC-15 Twin used the original AC-15 circuit designed by Dick Denney. 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.

Most guitar amps utilize a bank of 12AX7 (or ECC83) tubes in the preamp area. Denney's design for the preamp section of the AC-15 followed a different appproach. The AC-15 preamp not only included three 12AX7 and one 12AU7 tubes, it also included a high gain EF86 pentode tube. The inclusion of the EF86 tube in the AC-15 preamp design further enhanced the harmonically rich tone of the AC-15.

One of channels in the AC-15 Twin included Vib/Trem, an effect that combined variable tremolo (pulsed volume) with variable vibrato (wavering pitch). A cast aluminum remote foot switch canceled the effect.

Features: AC-15 Twin
Output Power 17 watts
Tube Compliment 3 x ECC83, 1 x EF86, 1 x EZ81, 2 x EL84
Channel One two inputs,
one volume,
one tone.
Channel Two two inputs,
one volume,
one tone,
one Vib/Trem
one speed
Speaker(s) Two Goodmans 12" Audiom 50
Size 27.15" H x 20.5" W x 10.4" D
Accessories Cover, Foot pedal.


The VOX Showroom!

Photos and editorial content courtesy Gary Hahlbeck, North Coast Music

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