Amp Module: Preamp
3-Band Pre Filters
These are custom-designed filters positioned before the distortion tube stages. They can boost or cut the bass, mid and high frequencies before the guitar signal goes into the tube-simulation stages.
For a clean and glassy tone, you can slightly cut the LOW SHELF control and boost the HIGH SHELF control. For a metal tone, cut the LOW SHELF control quite a lot to prevent too much bass signal from going into overdrive (which might cause too much saturation or a boomy bass response), and cut the HIGH SHELF control to prevent distortion from sounding too fuzzy.
Ah, tubes. We love ’em, don’t we? A quick and easy tube change can make a more interesting and dramatic tonal change—achievable from a single component swap within our amplifiers—than any other besides perhaps a speaker change. It plays the most important role in shaping the distortion characteristics, it influences the interaction between the tube and the circuit built around it, and it generates so many interesting dynamic circuit interactions that are pleasing to our ears.
Based on our MESH MODELING technology, we carefully considered the many aspects of the vacuum tube, including its transconductance curve, bias circuit topology, input and output impedance, internal effect, and much more. You can experiment with using a different combination of tube types to create your ultimate, warm distortion.
Many amps contain a simple bright cap circuit with a switch that kicks up the upper frequencies. The effect may be subtle or pronounced depending on the overall amp design.
The gain sets the amount of preamp input gain and thus controls the overall preamp distortion. You can use the gain knob with the master knob (see below) to tweak the sound to be clean, slightly broken up, moderately overdriven, or completely distorted.
We also implemented the widely used gain-knob shelving circuit, which gradually increases the bass response when the gain is turned up.
The distortion knob is an extra control that adjusts the amount of distortion in the selected tube stages. It provides greater resolution to fine-tune the distortion—the break-up point—to match various amp designs.
Some modern high-gain amps are designed specifically to create extreme yet controllable preamp tube distortion by cascading multiple gain stages, with gain and master volume controls between each stage to control their individual drive levels. Used in this way, preamp tubes can produce a scorching, harmonically saturated lead tone that sustains all day—what we usually hear as a classic shred or contemporary rock tone—in an amp that relies on its output tubes to amplify the sound rather than add further distortion to it. When driven into distortion in a simpler, more basic amp with fewer gain stages (a category that might nevertheless include some very high-end, “boutique” tube amps), preamp tube distortion becomes just a part of the amp’s overall distortion character and is blended with clipping at the phase inverter and output stages (and often at the speaker, too).
The number of tube stages provides a straightforward yet effective way to instantly manage the overall gain amount. The more tube stages, the more cascading gain stages and the more gain.
High Cut Frequency (Miller Effect)
In electronics, the Miller effect accounts for the increase in the equivalent input capacitance of an inverting voltage amplifier due to amplification of the effect of capacitance between the input and output terminals.
As most amplifiers are inverting, the effective capacitance at their inputs is increased due to the Miller effect. This can reduce the bandwidth of the amplifier, restricting its range of operation to lower frequencies. The tiny junction and stray capacitances between the base and collector terminals of a Darlington transistor, for example, may be drastically increased by the Miller effect due to its high gain, lowering the high-frequency response of the device.
The High Cut Freq knob captures the essential Miller effect and uses one simple knob to adjust the amp’s bandwidth. The higher the High Cut Freq knob setting, the less extended the high-frequency response will be in the preamp’s tube stages.
Low Cut Freq (Cathode Cap)
The cathode capacitance adjusts the bandwidth of lower frequencies. The higher you set the Low Cut Freq knob, the less extended the bass response will be in the preamp’s tube stages.
In order for a vacuum tube to operate in a safe and fairly linear region of its characteristic curve for low signal, the grid element must be maintained at a certain bias voltage. In the real world, properly setting up the bias voltage will prolong the tube’s life cycle, and changing the bias point can adjust the characteristics of the tube distortion.
BIAS Amp provides the most accurate simulation of the actual bias circuit. It faithfully adjusts the operation point as in the real world’s tube stage, but in BIAS you can experiment with extreme settings without worrying about damaging the tube. Lowering the BIAS Adjust control will provide a cleaner tone, while higher settings will provide a warmer tone.
3-Band Post Filter
These are custom-designed filters positioned after the distortion tube stages. They can boost or cut the bass, mid and high frequencies after the tube-simulation stages. For a cleaner tone, you can slightly cut the Low Shelf control and boost the High Shelf control to create a glassy tone. For metal tones, boost the Low Shelf control to increase rich bass response, and cut the High Shelf control to prevent the distortion from sounding fuzzy.