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VOX VALVETRONIX™ AMPLIFIERS — A TECHNICAL OVERVIEW
THE FRONT END, WHERE IT ALL STARTS
The preamp section of the Valvetronix amplifiers employs Korg’s proprietary REMS™ (Resonant structure and Electronic circuit Modeling System) technology. The critically acclaimed REMS has the ability to accurately reproduce the complex circuitry within the modeled amplifier’s signal path and re-create the tonality originally created in that amplifier’s circuitry. For example, REMS replicates the unique tone-stack network of each of the Valvetronix’ 16 amplifier models and, as a result, the Valvetronix Tone Controls operate and interact as they do in the amplifiers being modeled.
VALVE REACTOR TECHNOLOGY, THE BIG DIFFERENCE
The power amplifier section—and the relationship and interaction between the output tubes, output transformer and speakers—are a critical part of any great tube amp. In fact, each and every classic amplifier’s power amp has its own set of unique characteristics. VOX realized that to accurately model any amplifier they would need to precisely model its power amp section, too. And, to effectively model a variety of great amplifiers, VOX would need a power amp that could change itself to model each and every power amp.
How was this achieved? By inventing a new type of modeling, tube, power amplifier. VOX’s patent-pending Valve Reactor™ power amplifier consists of a tube power amp with tubes (dual triode in a single envelope) and an output transformer that is electronically coupled to a totally transparent solid-state power amp in such a way that the relationship between the output tubes, output transformer and speakers is unaltered.
In the Valve Reactor power amplifier, a 12AX7 dual-triode tube (12AX7s are typically used as preamp tubes) is connected directly to an output transformer and produces 0.4 Watts. The amp’s designers obviously had to develop a way to raise that level to one which could actually drive speakers to normal volume levels while maintaining the integrity of the tube-amp sound and feel. They accomplished this by creating a circuit of proprietary design dubbed the VOX VariAmp Power Circuit. The output transformer is connected to this new VariAmp Power Circuit which uses Constant Current design and Reactive Feedback technology. The VariAmp Power Circuit cannot be overdriven, is totally transparent and can be configured to be 1, 15, 30 or 60 Watts.
There are a number of benefits. Unlike power attenuators that change the sound of the amplifier being attenuated (especially when one lowers the output level to “bedroom” volumes), the tonality of the VariAmp Power Circuit remains constant at any level. This means that the player can crank the power amp to “11” and listen back at any level without tonal coloration. Another advantage is that Valvetronix amplifiers are better able to capture the sound and feel of the amps being modeled because output power can be configured to be exactly the same as the original amp. A 60 watt AC15 may have a cool and interesting sound but the original is 15 watts.
As mentioned earlier, the VariAmp Power Circuit cannot be overdriven and is completely transparent. It does not color or change the signal in any way and the resulting output tone is pure. It also ensures that the output signal maintains the high dynamic range associated with traditional tube amps— something most solid-state amplifiers simply cannot deliver, and one of the primary reasons why tube guitar amps normally sound and feel so much louder and more alive than solid-state amps boasting the same output power rating.
The Reactive Feedback technology used in the VariAmp Circuit “reads” the impedance curve of the speaker and then reports this reactive information back to the secondary side of the output transformer. This information is fed back to the primary side of the output transformer and therefore changes the loading on the tubes, another important part of the vital role an all-tube power amp plays in the creation of traditional tube tone.
Unlike most solid-state amplifiers
used in guitar amps which are Constant Voltage designs, the Valvetronix Constant
Current design used in the VariAmp allows the output voltage of Valvetronix
amplifiers to vary along with the speaker’s changing impedance, just like in
a traditional tube amplifier.
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As the diagram above shows, the output of the amplifier remains constant. Typical solid state power amps use Constant Voltage designs where the output impedance is relatively low. The amplifier’s low output impedance resists the varying impedance of the loudspeaker and therefore acts as a “brake” to its movement. The amplifier is exerting ultimate control, and, in guitar amp terms, removes the special X factor “life” that we expect from guitar amps and speakers, but, is perfect for reproducing sound from keyboards, PA, and HiFi.
On the other hand, the high output impedance of a “CURRENT” type amp does not restrict the loudspeaker’s movement but “follows” the loudspeaker’s impedance/frequency curve, and, when correctly set-up, the amp works in harmony with the loudspeaker.
Another unique aspect of VOX’s Valve Reactor is its ability to reproduce various circuit characteristics of the all-tube power stages of the amplifiers being modeled. The Valve Reactor has the ability to automatically switch between Class “A” and Class “AB” depending on the amplifier it is modeling. For example, all of the VOX models in Valvetronix amps employ Class “A” circuitry. When the player selects a VOX model, the Valve Reactor automatically switches to Class “A.” If the player selects an amp model that has a Class “AB” power amp, the Valvetronix amp instantly switches to that class.
In addition, the Valve Reactor can select whether or not a model will have a negative feedback loop, and how much and what kind of feedback there will be. If, for example, the player selects a VOX model, the feedback loop is not connected. When a model of an amp is selected that does feature a negative feedback loop, the Valve Reactor exactly replicates the original’s feedback circuit. This adds to the overall accuracy of the model’s sound and feel because the feedback circuit determines how much presence and, or resonance will be present in the model.
Being able to match such vital characteristics (except power output, which is user defined) helps ensure that each and every one of the amps modeled is authentic in every way possible. Only VOX Valvetronix employs this PAF (Patent Applied For) power amp technology.
Using this unique technology, combined with REMS, the VOX Valvetronix amplifiers offer 16 different types of popular amplifiers, as follows:
1. Black 2 x 12
2. Tweed 1 x 12
3. Tweed 4 x 10
4. AC15
5. AC15TB
6. AC30
7. AC30TB
8. UK Blues
9. UK ‘70’s
10. UK ‘80’s
11. UK ‘90’s
12. UK Modern
13.Recto
14. US High Gain
15. Boutique OD
16. Boutique Clean
There are 32 Programs consisting of 8 Banks, each with 4 channels. A Manual Mode overrides the Programs and the sound of the amp is determined by the control panel settings.
THE SIGNAL PATH
Signal path is another critical factor in the design of a modeling amp if it is going to be capable of accurately modeling the sound, tone and feel of desirable classic and modern amplifiers and effects. When laying out the Top and Rear panels of the Valvetronix AD60VT and AD120VT amplifiers, VOX looked at how guitarists typically set up their rig and laid out the Valvetronix’ signal path exactly like the real deal.
Guitar —> one of 10 Stompboxes —> one of 16 Preamp models à one of five Modulation effectsà one of three Delay effects à one of three Reverb effects à Virtual Effects Loop à Analog Master Volume à Power amp model —> Celestion guitar speaker(s).
The effects in Valvetronix amps are not an afterthought. Each is a carefully designed model of one of a variety of classic and popular effects. The 10 different “gotta have” pedals include Compressor, Acoustic Simulation, VOX Wah, Auto Wah, U-Vibe, Octave Divider, Treble Boost, Tube Overdrive, Fat Overdrive and Fuzz. The continuous controller pedal on the optional VC-4 Foot Controller can be used to control both the U-Vibe and VOX Wah models as they would be on the originals. As pedals, they appear before the preamp models so they can drive the preamp in the same way they do in a traditional rig. For example you can choose the Treble Booster to overdrive the input of a VOX AC30 for the sweet, massively compressed tone made famous by one of VOX’s most illustrious users.
BUILT-IN DIGITAL EFFECTS/FX LOOP
VOX has taken great care to ensure that the highest level of signal integrity is maintained at all times. The built-in ambient effects are configured in three sections (Modulation, Delay and Reverb), all of which can be used simultaneously. The ambient effects sit between the preamp and power amp sections, just like in a professional guitar amp/rack. In addition, they contain a virtual, direct, signal path that insures that the integrity of the guitar/amp signal is maintained at all times and the player’s sound remains full, rich and tight. The effects themselves are parallel to the direct signal.
The Modulation Section includes accurate models of popular Chorus, Flanger, Phaser, Tremolo and Rotary effects. The Delay Section delivers three distinctly different types: Delay, Tape Echo and Multi-Head delay. A Tap button is provided for setting the delay time. A Reverb Section with three reverb types, Spring, Plate and Room, is also provided. Each section can be controlled independently from the top panel, or, in the case of Modulation from the optional VC-4 Foot Controller.
Valvetronix amps also feature a series FX loop between the preamp and power amp for use of outboard effects.
Plus, VOX has even included a built-in Korg auto chromatic tuner to eliminate the need for an external device.
