Vox AD50VT Combo Amplifier
A Look "Under the Hood"

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The AD50VT was one of the models 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 served to drive a tonally transparent solid state power amp to boost the 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 audio output transformer allowed 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 circuit eliminated the output transformer, replacing it with a decoupling capacitor. This simplified power amp circuit contributed to the cost savings that allowed Vox to introduce the "AD Chrome Series" amps at an affordable price.

The circuitry for the AD50VT was mounted on two circuit boards. The upper circuit board was suspended below the control panel and contained all 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 were 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 50 watt RMS solid state portion of the Valve Reactor circuit was housed on a single, integrated chip that is mounted to the finned aluminum heat sink. The 12AX7 tube is the only consumer serviceable part in the power supply and output amp circuitry.

The AD50VT cabinet was constructed of particle board covered in traditional Vox basket weave vinyl. It included a Celestion 12" speaker of Oriental origin. Earlier versions used the 12" Celestion Model 7080, but later versions, such as the one pictured above, used the 12" Celestion VX12.

Changing the 12AX7 Tube in the AD50VT
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. Remove the back. Make sure that the tube has cooled before proceeding. The 12AX7 protrudes downward from the outside bottom of the chassis. Pull down to remove the tube. Be careful to not shatter the tube.



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Photos and editorial content courtesy Gary Hahlbeck, North Coast Music

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