JMI Vox AC-15 Twin
Thomas Vox "Kent" - 1965








2_Footer
© 1998 - 2017 The Vox Showroom, all rights reserved. No use on online auctions, eBay or Reverb.


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 1965 AC-15 Twin shown on this page was manufactured by JMI Vox in the UK and marketed in America by Thomas Organ as the Vox "Kent."

The 1965 AC-15 Twin pictured on this page has a gray control panel and is covered in black basket weave pattern vinyl. This amp has plastic corner protectors, three black vinyl Vox logo handles, and a rotary voltage selector. The Vox logo is made up of individual V, O, and X letters.

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.





North Coast Music offers many factory licensed cosmetic replacement
and repair parts for the Vox AC15 Twin. Some are shown below.

Complete replacement cabinets for the AC-15 Twin
Tubular Steel, chrome plated "Beatle correct" amp stands with "L" shaped pins - made by and offered only at North Coast Music
Road Cases for the AC-15 Twin, plywood, foam lined, twist latches, swiivel casters, all plywood with carpeted exterior, swivel casters
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 amp grill
Replacement JMI Vox Speaker Stickers for the AC-15
Replacement Vents for the AC-15 Twin
Exact Replacement Handles for the AC-15 Twin
Vox Egg Foot Switch - cast aluminum case
AC-15 Twin Covers (NCM-039)



2_Footer

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



The VOX Showroom!


Photos and editorial content courtesy Gary Hahlbeck, North Coast Music


Any and all material presented herein is protected by Copyright.
© 1998 - 2017 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/ac15twin-1965.html