What amp sag is: A temporary voltage drop in your amp's power supply when the output tubes demand more current than the power transformer can instantly deliver, producing a brief reduction in output that rounds the attack and makes the amp feel like it's yielding to your pick rather than pushing back against it.
The word "sag" appears in almost every conversation about tube amp feel. It explains why a cranked blackface Fender feels different from a tight solid-state amp, why an attenuated amp can lose something the cranked version has, and why modeler Sag parameters exist at all. The concept is real and measurable. The common description ("the amp breathes with the music") is accurate but unhelpful if you're trying to understand what to set on your Helix.
This is what's actually happening, and what to do with it.
What Causes Sag: The Power Supply Physics
A tube amp's output stage runs on high-voltage DC supplied by the power supply section. That supply has a limit on how much current it can deliver at any instant, set by the power transformer's rating, the rectifier type, and the size of the filter capacitors.
When you hit a hard transient (a picked attack, a palm mute, a chord strum), the output tubes suddenly demand more current. If the supply can't keep up instantly, its voltage drops — by a few volts in a stiff, solid-state-rectified supply, and by considerably more in a tube-rectified one. For a fraction of a second, the output tubes have less voltage to work with.
That reduction produces:
- A slightly softer initial attack: the transient's leading edge is rounded rather than sharp
- A brief reduction in output level on the note's peak
- A bloom effect: as the supply voltage recovers, the note can open up slightly, producing the "sustain that swells" characteristic of vintage clean tones
- A perceived arc to the dynamics rather than a flat, linear decay
The power supply recovers in milliseconds. But during that window, the amp's character shifts, and that shift is what players mean when they say a tube amp has "feel," "give," or "response."
How Sag Differs From Compression
Compression (as a pedal or plugin effect) reduces signal level above a threshold by a set ratio. Sag is not threshold-based or adjustable in the same way. It's reactive, proportional to how hard and suddenly you hit the amp.
| Characteristic | Compressor | Amp Sag |
|---|---|---|
| Trigger | Signal level exceeds threshold | Current demand exceeds supply capacity |
| Response | Adjustable attack parameter | Determined by transformer and cap design |
| Recovery | Adjustable release parameter | Determined by cap size and charge rate |
| Control | Direct: threshold, ratio, attack, release | Indirect: amp design or modeler Sag parameter |
| Signal position | Before the amp (as a pedal) | In the output stage, after the preamp |
| Effect on feel | Levels dynamics, may reduce pick sensitivity | Softens attacks, creates bloom, changes dynamic arc |
The practical difference is where the effect lives in the signal chain. A compressor pedal operates on the signal before the amp processes it. Sag operates after the preamp stage has done its work. A compressed signal through a non-sagging amp feels different from an uncompressed signal through a sagging one, even if the measured output curves are similar. Players who use both compressor pedals and tube amps are layering two different kinds of dynamic shaping, which is part of why the Nashville session clean tone sounds the way it does.
Why Sag Changes Feel More Than Sound
Sag comes up in feel discussions more than sound discussions because its effect is most pronounced on dynamic response: how the amp behaves differently depending on how hard you play.
Hard attacks get softened. When you dig in, the voltage drop rounds the leading edge of the transient. Aggressive playing feels more manageable. The pick attack registers, but the note doesn't spit back at you with full force.
Light passages feel more present. Because the power supply doesn't sag when you play softly, quiet playing retains full voltage. The subjective dynamic range between soft and loud is wider than the actual volume difference would suggest.
Palm mutes develop a characteristic shape. A brief sag on a mute's attack followed by quick recovery produces a bump that stiffer amps don't generate — useful on a crunchy rhythm part, a liability on tight high-gain rhythm. That's also why the Tube Screamer in front of a high-gain amp tightens mutes: the pedal cuts low end ahead of the amp, so each mute arrives with less of the low-frequency energy that makes a loose amp flub. It's the pedal's EQ doing that work, not anything it does to the amp's power supply.
Held notes sustain differently. A note played into a sagging amp has an arc: the initial attack is slightly rounded, the note blooms as the supply recovers, and then it trails off normally. This is the "violin-like sustain" quality associated with cranked vintage Fenders and Marshall Plexis. It's not just gain; it's the voltage arc.
Which Amp Designs Have More or Less Sag
Sag is not uniform across amp designs. Several variables determine how much the power supply droops.
Higher sag designs:
- Vintage Fender blackface/brownface amps: Deluxe Reverb, Twin Reverb, Super Reverb. Loose power supply filtering, tube rectifiers in some models.
- Early Marshall JTM45 and Bluesbreaker. Transformer limitations produce significant sag, part of what makes the JTM45 feel so different from the later JCM800.
- Vox AC30. Cathode-biased EL84 output tubes produce a different sag character than fixed-bias designs. The Top Boost channel's feel is substantially sag-driven.
- Any amp with a tube rectifier (5AR4, GZ34, 5Y3). These rectifiers have internal resistance that adds to the supply droop. Mesa/Boogie's Dual Rectifier "Vintage" mode engages a tube rectifier to deliberately add sag.
Lower sag designs:
- High-power Marshall heads: 100-watt Super Lead, JCM800 2203/2204. Larger transformers with more current headroom. Less sag than the smaller designs, but not zero.
- Mesa/Boogie Dual Rectifier on Raw/Bold (solid-state rectifier mode). Designed for tight, consistent output, which is why many metal players prefer the solid-state rectifier position.
- High-headroom solid-state amps: Roland JC-120, Fender Tone Master series. Virtually no supply sag by design.
- A modeler amp block with Sag turned all the way down. Defaults vary by platform and model, so check the value rather than assuming it's off.
Setting the Sag Parameter on a Modeler
Most modern modelers include a Sag parameter in the amp model's advanced settings. The implementation varies by platform.
Helix and HX Stomp
The Sag parameter appears in the amp block's Advanced panel. Range is 0.0 to 10.0.
| Setting | Character | When to Use |
|---|---|---|
| 0.0 – 2.0 | Tight, modern, high-headroom. No compression on attacks. | Metal, djent, high-gain where pick articulation must punch through |
| 2.0 – 4.0 | Subtle give on transients. Mostly tight. | Hard rock, classic rock, situations where you want some response without losing definition |
| 4.0 – 6.0 | Noticeable sag. Attacks round, held notes bloom. Feels close to a vintage blackface Fender. | Blues, clean country, edge-of-breakup tones |
| 6.0 – 8.0 | Heavy sag. Significant give on attacks, pronounced bloom on sustained notes. | Low-gain vintage clean tones, Hendrix-adjacent styles, atmospheric leads |
| 8.0 – 10.0 | Maximum sag. The attack nearly disappears. | Ambient swell playing, specific vintage tape-fed tones |
Starting point test: Load any clean amp model and set Master to 8. Set Sag to 0.0, play a hard clean chord and let it sustain for 3–4 seconds. Note the attack profile. Then set Sag to 7.0 and play the identical chord at the same strength. The difference in how the chord opens up in the first half-second is amp sag in direct comparison.
Sag Needs Master
That table only means what it says if the modeled power amp is doing some work, and on a Helix that's decided by Master. Master in the amp block isn't an output level — it sets how hard the preamp drives the virtual power tubes. Helix Help's parameter reference is explicit that Master interacts with every other power-amp parameter, and that the lower it sits, the less effect the rest of them have. Sag is one of them.
Which is why so many players conclude Sag does nothing. Most factory high-gain presets are built with Drive high and Master low, so all the distortion is in the preamp and the power stage is idling. A preset with Master at 3 and Sag at 7 is barely sagging. Set Master to 7 or higher before you judge Sag at all, and use the channel volume, not Master, to set output level. The same topology is behind the most common volume-knob complaint on modelers, covered in why your volume knob cleans up a tube amp and not a modeler.
Quad Cortex (Neural DSP)
Neural DSP's manual doesn't list per-model amp parameters, and we haven't been able to confirm a Sag control on the Quad Cortex's amp models — an earlier version of this page said there was one, and we can't stand behind it. What holds regardless is the Capture side: a Neural Capture is a snapshot of an amp at one setting, so its sag behavior is baked in at the volume and settings used during the capture session. If you want a low-sag capture of your amp, capture it at a conservative volume, where the power supply is running at stable voltage.
Fractal Audio (Axe-Fx III, FM9, FM3)
Fractal's implementation is more granular. Its Supply Sag parameter sets the virtual power supply's impedance: higher values mean more B+ droop and a more compressed, spongy feel, and Fractal's own guidance puts values around 2 near a solid-state rectifier and 4 to 6 near a tube rectifier. The models' defaults are already tuned to each reference amp, so adjusting it moves you away from that amp's behavior rather than starting from zero.
Kemper
The Kemper's equivalent is Power Sagging, in the Amplifier section. Kemper describes it much as this page does — a droop on hard transients that changes the feel far more than the sound.
Sag vs. Bias X
Sag gets confused with a second Helix parameter, Bias X, because both respond to how hard you play. They model different things. Sag is the power supply voltage drooping, and it mostly shapes the attack. Bias X is the output tubes' operating point shifting under a hard signal, and it mostly changes the texture of what follows the attack — a grainier, slightly compressed tail after a hard strum. Both are gated by Master in the same way. The operating-point side, including why raising Bias can make a clean preset cleaner, is covered in Bias and Bias X: the two modeler parameters nobody sets.
The Practical Decision
If you're building modeler presets:
- Set Master first. None of the numbers below mean much with Master low.
- For any clean or edge-of-breakup tone modeled on a vintage Fender, start Sag at 4.0–6.0 and adjust by feel.
- For a Vox or AC30-style tone, start around 5.0. These amps have pronounced sag as part of their character.
- For a Marshall Plexi or JTM45 style, 4.0–6.0. The JCM800 models can go lower, around 2.0–3.5.
- For any high-gain or metal preset, default to 0.0–1.5. Sag on high-gain tones blurs palm mute articulation and softens the pick attack in ways that work against tight rhythmic definition.
- For acoustic-style clean tones where you want pristine transient response, stay at 0.0–2.0.
Sag is not a "more is better" dial. It's a character dial. More sag doesn't mean better feel; it means different feel. The vintage Fender feel that sag produces isn't appropriate for every style. Knowing what it is lets you use it intentionally.
