Guitar Amplifiers » The Valve Reactor

What is Valve Reactor technology?
The Valve Reactor circuit used in the power amp of the Valvetronix+ combo amps are based on new technology. Most of the sound creation and tonal shaping is performed in the digital domain, but the Valve Reactor power amp is 100% analog. Passing the guitar signal through the analog-domain power amp stage is an important factor in capturing the feel and tone of the original amp for that model.

The Signal Path

Valve Reactor’s power stage is a miniature version of a true tube push/pull power amp. It uses a 12AX7 tube (dual triode; in other words, two vacuum tubes in one), and is similar to a true tube amp with an output transformer. Rather than connecting the vacuum tube directly to a speaker via an output transformer, the signal is converted by a virtual output transformer in which solid stage components emulate the output transformer, and the final output stage circuit is connected to a specially designed VariAmp power circuit. This allows the output of the power stage to be continuously varied from minimum output to maximum output.

This VariAmp power circuit is completely transparent – i.e., it does not modify the signal – so the output tone will be the pure sound of a tube amp. The wide dynamic range seen in a conventional tube amp is also maintained. This dynamic range is a characteristic that is difficult to obtain from a solid-state amp; this is why a tube amp sounds more powerful than a solid-stage amp of the same output specifications. The output of the Valve Reactor power amp “reads” the constantly changing impedance curve of the connected speaker system, and feeds this information back to the vacuum tube. In response to this information, the operation of the amp’s tube stage will vary according to the speaker load (impedance). This too is an important element in the sound of a true tube amp. In addition to producing lively tube sound, “circuitry characteristics” distinctive of the full-tube power stage in the amp being modeled can also be simulated. These characteristics include class-A, class-AB, and power output. By adjusting these characteristics (except for the power output, which the user specifies), every sound of that amp model can be faithfully reproduced. This power amp technology, for which a U.S. patent has been obtained, is unique to the VOX Valvetronix amps.

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