Vox AD100VT/VTH Amp Head
A Look "Under the Hood"

© 1996 - 2021 The Vox Showroom, all rights reserved. No use on online auctions, eBay or Reverb.
The AD100VTH was the only head 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 tiny audio output transformer combined to allow these models to simulate the tone of an all tube power amp.

A simplified 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 a reasonable price.

The circuitry for the AD100VTH was mounted on two circuit boards. The upper circuit board 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 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 100 watt RMS solid state portion of the Valve Reactor circuit was housed on a single, integrated chip that is mounted to a fan cooled finned aluminum heat sink. The 12AX7 tube is the only consumer serviceable part in the power supply and output amp circuitry.

Changing the 12AX7 Tube in the AD100VTH
Unplug the amp from the wall. Make sure that the tube has cooled before proceeding. The 12AX7 may be accessed through a hole in the bottom of the head cabinet.



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Photos and editorial content courtesy Gary Hahlbeck, North Coast Music

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