Line 6 Helix Amp Models

• Drive—This is the first volume control on the amp. It’s called “drive” on the model to fit the pattern of all the Helix models. On the real amp is says “Volume.”
• Bass, Mid, Treble—The normal tone controls on the amp (called a tone stack by amp nerds), located between the first and second gain stages in the preamp. These have different ranges than traditional guitar amp tone controls. Also, the whole voicing of the tone stack can be changed with the “Rock/Jazz” switch, which will be explained later.
• Presence—This is like the presence controls on other guitar amps. It changes the amount of high frequency in the power amp by modifying the EQ filtering in the power amp’s negative feedback loop.
• Ch Vol—This controls the output level of the amp model. It has no effect on the tone or distortion of the amp model
• Master—This is the master volume on the front panel of the amp. It is located between the preamp and power amp and can be used to get more or less power amp distortion. This amp is VERY loud, and most players would probably run the master volume on the lower side. If the master volume is cranked, the power amp distortion can be pushed into unpleasant territory. Most players would never crank the master in real life as the actual output would be way too loud for most musical settings.
• Mid Switch—This switch changes the value of the treble capacitor in the tone stack. When it is off, the amp has more of a scooped sound. When it is on, there is a noticeable upper-mid boost.
• Jazz/Rock—This switch changes the wiring of the tone stack circuit. It allows for two totally separate tonal voices. Jazz is quieter with a lower center frequency for the mids. Rock is louder with a more traditional mid frequency center. Tone controls rarely translate well between the Jazz and Rock settings. If a good sound is achieved in one mode, it is not guaranteed that the same settings in the opposite mode will still sound pleasing.
• OD Switch—This turns the two-gain-stage tube overdrive circuit on and off. This circuit is located AFTER the tone controls and Drive knob. When the overdrive is turned on it’s as if a third and fourth gain stage is added to the preamp. Generally, it’s best to set up the base tone of the amp with the Drive and tone controls first, and then adjust the overdrive circuit to work with the desired base tone.
• OD Drive—This controls the amount of drive or saturation in the overdrive circuit. Since the whole overdrive circuit is after the amp’s regular drive and tone controls, the range of OD Drive knob will change based on those earlier knob settings.
• OD Level—This controls the output level of the overdrive circuit.
• Bright—This is a three position switch. The settings are “off” and two different values of bright capacitor. This bright capacitor works with the Drive (volume) knob earlier in the circuit, and is similar to other amps that have bright switches. When the Bright switch is engaged, the effect is more pronounced with lower Drive settings. The bright becomes less effective at higher Drive settings. When the Drive is at 10, the switch is effectively removed from the amp circuit, and changing the switch settings has no audible effect.
• FET Boost—The GSG100 has a solid state, FET (Field Effect Transistor) boost circuit at the very beginning of the amp circuit. It is akin to placing a FET Boost pedal before the amp. On the physical unit there are two input jacks, but on the model it is placed on a switch and can even be made foot-switchable. The FET Boost has a fixed boost amount of about 7 to 9 dB and also gives a slight EQ change.
• PAB—This stands for “Preamp Boost.” The PAB works by removing the tone controls from the circuit. Tone controls work by removing frequencies and signal level. Engaging the PAB circuit returns all of this lost signal level, but the side effect is that tone controls no longer work. It truth, the treble knob does work slightly, and the mid switch will change how much lows and mids comes through the circuit. In general, though, the PAB trades tone control functionality for a full blast level between tube gain stages 1 and 2.
• Sag—This is a control that is added to all the Helix models. Every tube amp has some amount of power supply sag, which feels like compression, squish, and sustain to the player. This control makes the sag amount user-adjustable.
• Hum—This is a control that is added to most of the Helix models. Preamp tube heaters in tube amps will leak a little bit of 60 cycle hum into the audio signal. When this hum mixes with the distorted audio signal, a non-musical distortion is created at low levels. To some players, this low-level, non-harmonic distortion adds a bit of realism to the amp model. The best way to put it is that sometimes the model sounds more “wrong” without the hum. However, if you don’t like it, you can just turn it down.
• Ripple—This is a control that is added to most of the Helix models. Power amp circuits will sometimes let a little bit of rectified 120Hz hum (that the power supply filter caps can’t quite fully remove) into the audio signal. When the power supply is being pushed hard, more of this ripple can get through the audio path. Much like the hum, this provides a bit of non-musical distortion to the power amp at distorted settings. To some people, this sounds like harmonic complexity that is enjoyable and realistic. Other players just don’t like it and turn it off.
• Bias—This control is in most Helix models. It adjusts the bias of the tubes in the power amp, causing a change in tonality and the distortion characteristic.
• Bias X—This is the most difficult parameter to describe in Helix models, so hang on. All tube amps need to bias the power amp tubes. This is usually achieved by applying a negative voltage to the input audio signal. (Cathode bias works differently, but that’s a story for a different time.) However, when the power tubes are distorting, free electrons can form around the input grid and cause a shift in the bias voltage. This shift only happens during the moments when distortion is occurring. This shift causes a tonal and texture change much like adjusting the bias control. However, once the tubes leave the distorted state, the free electrons dissipate and the bias returns to normal. Another way to put it (less accurately) is that this is a level/envelope controlled bias shift. This behavior is modeled in all Helix amps, and the Bias X control allows users to control the amount of bias shift that is happening. It is a very subtle change, so please don’t expect high drama from this knob.

“The other new key feature of The Elmsley’s power amp section is the Negative Feedback (NFB) parameter, where the amount of feedback within the power section’s response can be dialed anywhere from wild and unhinged, tight and punchy, and anything in between. As with many amplifier designs, the presence and depth controls are part of this negative feedback circuit and functions to control the brilliance and resonance of the power amp, respectively. As the NFB param is reduced, the presence and depth controls’ influences are also influenced, and they become effectively deactivated when the NFB knob is at zero. These interactions allow the player to create countless tonal variations to suit their needs.” —Sam Hwang, Sound Designer