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Vox AD15VT Combo Amplifier
A Look "Under the Hood"

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The AD15VT was the entry level model in the "AD Chrome"  
Valvetronix amp series offered by Vox from 2004 to 2008. The AD Chrome series used the second generation version of the "VR" or Valve Reactor power amp circuit.

The original VR circuit designed for the Valvetronix "blue grill" amplifiers coupled a 12AX7 with a small audio output transformer to make a one watt power tube amplifier circuit. The output from this circuit was used to drive a tonally transparent solid state power amp to boost the RMS output to performance levels. In 2002 literature promoting the original "blue grill" Valvetronix amps, Vox explained that the interaction between the 12AX7 power tube and the tiny audio output transformer combined to allow these models to simulate the tone of an all tube power amp.

A revised version of the VR power amp circuit was used in the AD Chrome amps. The new VR power amp circuit in the chrome grill AD amps eliminated the output transformer, replacing it with a coupling capacitor. This modification simplified the power amplifier circuit and contributed to the cost savings that allowed Vox to introduce the "AD Chrome Series" amps at such an affordable price.
The circuitry for the AD15VT was mounted on two circuit boards. The upper circuit board was suspended below the control panel and contained all of the REMS  (Resonant Structure and Electronic Circuit Modeling System) preamp circuitry. This included the volume, treble, and bass controls along with the digital processors for effects such as reverb, echo, chorus, flange, tremolo and amp modeling. The upper circuit was held in place by the control nuts. There are no consumer serviceable parts in the preamp circuitry.

The lower board contained the power supply and Valve Reactor power amp circuitry. The 12AX7 tube for the Valve Reactor power amp was located toward the rear of the lower circuit board in the area under the rotary "Amp" model switch. The 15 watt RMS solid state portion of the Valve Reactor circuit was housed on a single, integrated chip that was mounted to the finned aluminum heat sink.

The AD15VT cabinet was constructed of particle board covered in traditional Vox basket weave vinyl. It included a 8" speaker of Oriental origin. The ferrite magnet on the back of the speaker was stamped with the model number "AD-15" and a manufacturing date code. The date code stamped on the speaker shown above was "C0507," suggesting an May 2007 production date.

Changing the 12AX7 Tube in the AD15VT
Neither the Vox Showroom or North Coast Music accept responsibility for personal injury or damage to your amp while performing these steps. Proceed at your own risk.

Unplug the amp from the wall. Loosen, but do not remove the eight large phillips head screws that secure the chassis to the cabinet. Rermove all the screws in the back and take off the back panel. Carefully lift the 12AX7 tube out of the socket. If there is a foam retaining block glued to the chassis above the tube, gently compress the top end of the tube into the foam until the pins on the tube clear the socket. Be careful to not shatter the tube. Reverse the steps above to complete the process.



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Photos and editorial content courtesy Gary Hahlbeck

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