The Vox AC-15 - 1961 Model













Features
Output Power 20 watts
Tube Compliment ECC83 (x2), ECC81, EL84 Duet + GZ34
Channel One Two inputs,
one volume,
one tone,
Channel Two Two inputs,
one volume,
one tone,
one Vibravox control.
Speaker(s) One Vox Celestion 12" speaker
Size 20" W x 21" H x 10" D
Accessories Cover, Foot pedal.





2_Footer
© 1998 - 2014 The Vox Showroom, rights reserved. No use in online auctions.
Originally introduced in 1958 with a Fender influenced "TV" style cabinet, the AC-15 is the Vox amp that "started it all." The amp at left is the third generation of the AC-15, from 1961.

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.





.................





The VOX Showroom!


Photos and editorial content courtesy Gary Hahlbeck, North Coast Music


Any and all material presented herein is protected by Copyright.
© 1998 - 2014 The Vox Showroom and North Coast Music, all rights reserved

The images and editorial content in this web site may not be copied or reproduced in online auction sites such as eBay
and Craig's List. Sellers may provide a link to the Vox Showroom web site if they wish to refer to this copyrighted material.


URL: http://www.voxshowroom.com/uk/amp/ac15.html