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.
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