Vox VT100 Combo Amplifier
A Look "Under the Hood"

The VT100 was one of the models in the VT Valvetronix Series
offered by Vox from 2008 to 2010. The VT Series used the same second generation version of the "VR" or Valve Reactor power amp circuit as used in the prior "AD Chrome" series.

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 the tube and transformer drove 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 helped to simulate the tone of a tube power amp.

A modified version of the VR power amp circuit was used in both the VT and the AD Chrome amps. The power amp circuit in the chrome grill AD and VT amps retained the 12AX7 tube but eliminated the output transformer, replacing it with a decoupling capacitor. This revised power amplifier circuit contributed in part to the cost savings that allowed Vox to introduce the "VT" amps at such an affordable price.

The circuitry for the VT100 was mounted on two circuit boards. The upper circuit board was mounted under 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 processing chips 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 VT100 introduced a number of feature and control enhancements over the AD100VT it replaced. The number of amp models was increased from 11 in the AD100VT to 22 in the VT100. A separate reverb control was added along with sixty-six preset programs, twenty-two "song preset" programs and the capacity to save eight end user programs.

The lower circuit board was mounted to the floor of the chassis and contained the power supply and Valve Reactor power amp circuitry. The 12AX7 tube for the Valve Reactor power amp was located near the edge of the circuit board away from the control panel and held in place with a spring loaded clip. The 100 watt RMS solid state portion of the Valve Reactor circuit was housed on a single, integrated chip that was mounted to a fan cooled aluminum heat sink.

The close up picture of the power amp board (lower right) reveals some unrealized plans for the VT power amplifier circuit. The circuit board appears to be designed to incorporate a second tube socket and supporting circuitry. I spoke to several sources at Vox regarding this subject, but no one seemed to know what was planned. Perhaps Vox was considering a stereo VT amp using a fully populated version of this Valve Reactor power amp board.

The VT100 cabinet was constructed of particle board covered in a smooth black vinyl. It included two 12" speakers of Oriental origin.

Changing the 12AX7 Tube in the VT100
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 upper back and the wires from the speaker. Remove the four phillips head screws that secure the chassis to the top of the cabinet and slide the chassis out of the enclosure. Carefully lift the tube retention spring over the top of the tube and swing it clear, being careful to not shatter the tube.




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Photos and editorial content courtesy Gary Hahlbeck, North Coast Music


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