Vox AD120VTX & AD120VTH Amplifiers - A Look "Under the Hood"

Let's take a look "under the hood" at what are possibly the
most complex amplifiers ever produced by Vox, the "blue diamond grill" Valvetronix AD120VTX combo and AD120VTH head.

The REMS Preamplifier Circuit
The preamplifier was based on Korg's proprietary REMS (Resonant Structure and Electronic Circuit Modeling System) technology. REMS had previously formed the basis for a number of stand alone Korg effects devices, but incorporating this technology into a guitar preamp was a first.

REMS bestowed a plethora of digitally based effects on the AD120VTX. In addition to reverb and tremolo, these amps offered effects such as delay, chorus, flanger, and a rotary speaker effect. It also offered tone models of sixteen of the world's most popular amps and simulations of a number of popular "stomp box" pedals.

The REMS circuitry was located on a PC board mounted beneath the control panel (shown above) and on the "digital" board (at left).

The Valve Reactor Power Amp Circuit
The heart of the Valvetronix Valve Reactor power amp circuit is the 12AX7 "twin triode" tube. 12AX7 tubes are ordinarily found in the preamp area of a tube guitar amplifier. One would normally

not expect to find a 12AX7 as part of a power amplifier. However, this is exactly how the 12AX7 was used in the original Valvetronix Valve Reactor power amp circuit.

The unique Valvetronix Valve Reactor or "VR" circuit adapted what would normally

be a preamp tube for use as a one watt tube power amp. The one watt RMS output from the 12AX7 tube was then directed

to a small output transformer, as done in traditional tube power amp designs. The interaction that existed between power tube and output transformer contributed to the tube-like tone of the VR amp circuit.

However, the one watt output power of this circuit would not be enough volume to get you through a gig. The output from the 12AX7 and output transformer was then sent to a tonally transparent, high powered solid state amplifier which in turn boosted the level of the amplifier to performance levels.

The three components of the VR power amp circuit are pictured above and at left. The photo at upper left shows the 12AX7 tube protruding from the bottom of the AD120VTX chassis. The VR output transformer and the back side of the tube socket are visible in the photo above. The photo at left shows the integrated solid state power amp chips mounted to an aluminum heat sink.

The complexity of the software incorporated into these early Vox "Blue Grill" amps becomes apparent as we now focus on further details of the Valve Reactor circuit.

A Vox AC-15 or an AC-30 amplifier has "Class A," or self biasing circuitry in the power amp section. Such amps tend to have more compression and less headroom. Fender and Marshall amplifiers typically have "Class AB," or fixed biasing circuitry in the output amp section. FIxed bias amps tend to have less compression and more headroom. The method used to bias the output tubes plays a major role in the tone of the amp.

The "Blue Grill" Valvetronix amp circuitry had the ability to configure the Valve Reactor output circuitry in either "Class A" mode or "Class AB" mode. This selection was determined by the REMS amp model selected on the control panel. Selecting the AC-30 model on the control panel would configure the VR circuit to "Class A" mode while choosing the "Blackface" model would engage the "Class AB" mode. This bias switching ability enhanced the tonal accuracy of the REMS amp models offered in the AD120VTX and VTH.

Tube output amplifiers often have a "negative feedback" loop. A negative feedback loop takes a small portion of the signal from the output transformer and introduces it back to the preamp. This practice reduces distortion in the circuit and is common in Fender amplifiers, among others.

Many Vox amps did not include a negative feedback loop. Lead Vox engineer Dick Denney discovered that eliminating the feedback loop on the AC-15 and AC-30 improved the harmonic content and tone of these amps.

The Valve Reactor circuitry also had the ability to choose whether or not an amp model would have a negative feedback loop, again determined by the REMS amp model selection.

Some of the complexities of the original Valve Reactor circuit, such as the inclusion of an output transformer and bias switching, were eliminated in subsequent versions of Valvetronix amplifiers.

My thanks to Mike Truttschel of the Britins for allowing me to take photos of his AD120VTX chassis.



The VOX Showroom!

Photos and editorial content courtesy Gary Hahlbeck, North Coast Music

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