Valve saturation in plain English
A valve is a glass tube with most of the air pumped out. Inside, a heated metal part called the cathode gives off electrons, and a plate called the anode collects them. Between the two sits a wire mesh called the grid. A small voltage on the grid controls a much bigger flow of current to the anode, so a small signal in becomes a large signal out. That's amplification, and before transistors took over, nearly every microphone preamp, mixing desk, compressor and guitar amp did it with valves.
The simplest amplifying valve, the triode, has just those three parts. Its gain isn't perfectly even across its range, so it amplifies one half of the wave slightly more than the other, even at normal levels. Push it with a bigger signal and the difference grows until the peaks on each side run into a limit. The wave comes out lopsided and rounded off, and a changed shape always means new frequencies, called harmonics, at whole-number multiples of the note.
Americans call valves tubes, and plug-in makers mostly follow, so tube saturation, valve saturation and tube warmth all describe the same thing. The British name comes from the earliest valves, which let current flow one way only, like a valve in a pipe.
This page goes further into one kind of saturation. If you haven't met drive, transfer curves or harmonics before, start with what is saturation?, which covers every mode in the lab.
Bias and the lopsided curve
A valve needs a steady voltage on its grid to sit at a sensible resting point, and that voltage is called the bias. The music then swings the grid up and down around it, along a response with a limit at each end. At one end the valve stops conducting altogether, which is called cutoff; at the other, the grid starts drawing current of its own and the valve can't be pushed much further.
Those two limits have different shapes, and the bias rarely sits exactly halfway between them. So when the signal gets big, one side of the wave runs into its limit sooner, and harder, than the other. A lopsided curve like that is what makes even harmonics: the 2nd, 4th, 6th and so on. A curve that treats both halves the same way can only add odd ones.
In the lab, the Character knob is the bias. Turn it up and the valve curve leans further, which widens the gap between how much the top and the bottom of the wave get bent. Unlike the lab's soft and hard clippers, the valve curve leans a little even at 0%, because a single valve stage is never symmetrical.
With no extra drive and the character at 0%, the harmonic screen shows a 2nd harmonic 30 dB below the note and a 3rd 43 dB below it. Move the character to 60% and the 2nd comes up to −21 dB, while the 3rd falls to −77 dB. Only one new frequency clears −60 dB on the harmonic screen, and it's exactly an octave above the note.
A leaning curve also means the wave's average level is no longer zero. The output picks up a small DC offset, a constant push to one side. Real valve circuits pass the signal on through a capacitor, which blocks it, and the lab runs a filter that does the same job, so the offset never reaches your speakers.
What does valve saturation sound like?
At low drive, mostly like the original with more body. The 2nd harmonic is an octave above the note, so it lines up with the pitch and doesn't add a new note of its own. You tend to hear it as the sound getting fuller and closer, which is where the word warm comes from, and it's why a valve preamp run a little hot can make a vocal sound bigger and still pass as clean.
Push the drive and the sound changes in a way you can follow on the harmonic screen. The 2nd harmonic keeps growing, but the 3rd grows much faster. With the character at 60%, the table shows what the lab measures on a 110 Hz sine as the drive goes up.
| Drive | 2nd (220 Hz) | 3rd (330 Hz) | 4th (440 Hz) | Louder than −60 dB |
|---|---|---|---|---|
| 0 dB | −21 | −77 | −63 | 1 |
| 8 dB | −14 | −44 | −40 | 4 |
| 12 dB | −12 | −30 | −30 | 6 |
| 16 dB | −12 | −20 | −23 | 8 |
| 20 dB | −14 | −14 | −19 | 11 |
| 24 dB | −18 | −11 | −20 | 11 |
Between 0 and 12 dB of drive the 2nd harmonic rises by 9 dB and the 3rd by 47 dB. Around 20 dB they meet, and past that the 3rd leads. A hard-driven valve runs into its limits on both sides of the wave, and once both halves are being flattened the curve is close to symmetrical again, which brings the odd harmonics in. They also reach higher up the spectrum, so the sound gains edge and buzz on top of the body, and you're into valve distortion, the sound of a cranked guitar amp.
Reading the lab
The lab above is the saturation lab from the flagship lesson, set up with the valve curve first and three others for comparison: a soft clipper, a hard clipper and tape. While the lab has focus, keys 1 to 4 switch between them. The Bits and Rate knobs belong to the digital modes and are switched off here.
The screen to watch is the harmonic one, under the transfer curve and the close-up. The lab runs a pure 110 Hz sine through your exact settings and measures what comes out. The first bar is the note, and every bar after it is something the curve added, in dB below the note. Even harmonics are amber and odd ones pink, so a valve set up for warmth shows one tall amber bar at 220 Hz and very little else.
The transfer curve shows the lean directly. A symmetrical curve looks the same if you spin it half a turn around the centre; the valve curve doesn't, because its top bends over sooner than its bottom. The close-up next to it shows one peak of the loop sample by sample, where you can see the two halves of the wave come out different shapes.
Auto gain is on, so the lab keeps the output as loud as the original whatever the drive, and A/B switches to the original at matched loudness. The written panel in the lab explains the current settings using those same measurements and updates when you move a knob.
Valve saturation vs a symmetrical soft clipper
The lab's soft clipper and valve modes are built on the same rounded curve. The valve version is set to start bending later and always leans to one side, while the soft clipper's curve sits dead centre, so with the character at 0% it bends the top and bottom of the wave by exactly the same amount. At 12 dB of drive it adds a 3rd harmonic at −12 dB, a 5th at −20 dB and a 7th at −27 dB, and every even bar is empty.
Flip to the valve at the same drive and 60% character, and the 2nd harmonic leads at −12 dB while the 3rd is down at −30 dB. On the bass loop the soft clipper sounds hollow and edgy next to it and the valve sounds rounder. Some of that is drive, because the soft clipper starts bending sooner, but no amount of turning it down will give it the even harmonics.
Single-ended and push-pull
Not every valve circuit sounds like a single triode. Many power amplifiers, including most guitar amp output stages, are built push-pull: two valves are fed opposite copies of the signal, and an output transformer combines what they put out. In a balanced push-pull stage the even harmonics from the two halves cancel, so what's left is mostly odd. A single-ended stage has one valve doing all the work and keeps its even harmonics. The Fender Champ, a small guitar amp with a single 6V6 output valve, is a well-known single-ended design.
So valve covers more than one sound. A valve preamp on a microphone tends to add a clean, octave-led colour, while a push-pull guitar amp turned up loud adds a lot of odd harmonics and sounds much more aggressive. Plug-ins that offer several tube types are usually modelling differences like these.
The 2nd harmonic is also one of the main differences between valve and tape saturation. Tape's curve is close to symmetrical, so it adds mostly the 3rd and softens the top end as it's pushed. Press 4 in the lab to hear tape on the same loop, and see tape vs valve saturation for the full comparison.
Where valves turn up in a studio
In a studio, valves mostly sit in gear that was designed to stay clean. Valve circuits run at a few hundred volts, so they have a lot of room before they clip, and they add a small amount of 2nd harmonic well before that point. Turn the input up and you get more of it.
- The original Neumann U47 microphone has a valve inside it, and valve condenser microphones are still made for what they add to vocals.
- Many preamps and channel strips use a 12AX7, also sold as the ECC83, a small twin triode with an amplification factor of about 100. Turning the input gain up and the output down drives it harder at the same level.
- The Teletronix LA-2A compressor does its gain reduction with a light and a photocell, but its amplifier stages are valves. The Fairchild 670 and the Manley Variable Mu go further and use the valves themselves to turn the sound down.
- The Pultec EQP-1A is a passive equaliser followed by a valve amplifier that makes up the level it loses.
- Guitar and bass amps are the valve circuits most often pushed into real distortion, in both the preamp and the power stage.
Most valve gear has transformers too, and they saturate in their own way, most of all on loud low notes. When people describe the sound of a piece of valve hardware, they're often hearing the transformers as much as the valves.
In software, most saturation plug-ins include a valve curve. Ableton Live's Dynamic Tube has three tube models and a Bias control that does the same job as the lab's Character knob, plus an Envelope control that moves the bias with the input level. If your plug-in has a bias, asymmetry or even/odd control, it's adjusting the lean.
How to add valve saturation, step by step
- Set the bias for even harmonicsTurn the bias or character control up until the 2nd harmonic leads. In the lab that's anywhere from 40 to 80%; on a plug-in without a meter, start in the middle.
- Push the drive past what you wantGo until the sound is clearly distorted, so you know what the valve is adding. Watch for the point where the 3rd harmonic catches up with the 2nd.
- Back off until it's warmLower the drive until the edge goes and only the extra fullness is left. On most sources that's well before the 2nd and 3rd meet.
- Trim the top if it fizzesIf the highest harmonics sound harsh, pull the tone control down to between 8 and 12 kHz, or put an EQ after the saturator.
- Blend it if you need the attackOn drums and plucked sounds, drive harder and bring the mix down to 20 to 40%.
- Match the level and compareSet the output so bypassed and saturated are equally loud, then A/B. Keep it only if you still prefer it.
Where to use valve saturation
Vocals
Valve saturation is a common choice on vocals because the 2nd harmonic adds body and the voice still sounds natural. In the lab, 8 dB of drive at 60% character puts the 2nd at −14 dB and the 3rd at −44 dB, which is enough to bring the voice forward. Listen to the S and T sounds as you go, since any saturation exaggerates them; a de-esser before the saturator keeps them under control.
Bass
A 55 Hz bass note gets a 2nd harmonic at 110 Hz, which a laptop speaker reproduces far better than the note itself, so you can still follow the line on speakers that can't play 55 Hz. Valve suits this job because that octave sounds like part of the note. Keep the drive moderate: once the 3rd takes over, a bass loses its roundness.
Guitars and keys
Organs and electric guitars spent decades going through valve amps, so a valve curve sounds natural on them and on electric pianos too. They can take more drive than vocals, and pushing into the range where odd harmonics join in gives a gentle crunch that suits them.
Drums
At 100% mix, valve saturation rounds the front of each hit. Parallel saturation keeps the snap: drive hard, then mix the result in under the clean drums. With 24 dB of drive at 30% mix, the lab shows the strongest harmonic falling from −11 dB to −23 dB, because most of what you hear is still the clean loop. The parallel saturation lesson covers setting it up on a send.
The mix bus
A little valve saturation across a whole mix can make the parts feel more connected. Keep the drive low and the bias moderate, and if you can clearly hear it on an A/B at matched loudness, back it off.
Starting settings in the lab
Drive amounts depend on the plug-in and on how hot the sound is going in, so these are starting points for this lab. Use them to learn what each range sounds like, then let the A/B decide.
| Source | Drive | Character | Mix | Listen for |
|---|---|---|---|---|
| Lead vocal | 4–10 dB | 40–60% | 100% | More body with clean S sounds |
| Bass | 8–16 dB | 60–80% | 100% | The line still audible on small speakers |
| Keys or clean guitar | 12–20 dB | 50–70% | 100% | A soft crunch on the loudest notes |
| Drum bus | 20–28 dB | 60% | 20–40% | More weight with the snap still on top |
| Mix bus | 0–4 dB | 30–50% | 100% | A difference you only just notice on A/B |
What a plug-in curve leaves out
The lab models a valve as a fixed curve, which captures the lean and the harmonics. Real valve gear does a few things a fixed curve can't. Its transformers saturate more on low notes than on high ones. In a guitar amp, the power supply's voltage can drop under a loud chord and squash the sound for a moment, which players call sag. And when a valve is driven hard enough for its grid to draw current, its bias can shift for a short time after each loud hit, so the colour changes as the music plays. Better valve plug-ins model some of this, but on a vocal or a mix bus most of the sound is in the fixed curve.
Common valve saturation mistakes
Hearing louder as warmer
Saturation nearly always raises the level, and a sound that's a decibel louder tends to seem fuller even with no saturation at all. Switch Auto gain off in the lab and the panel tells you how much louder the result is. In your DAW, match the output to the bypassed level before you judge it.
Driving past the warm part
The 2nd-harmonic sound most people want sits in the low to middle drive range. Turning the drive up in search of more warmth gets you more odd harmonics instead, which is distortion. If a valve plug-in has started to sound edgy, the fix is less drive.
Leaving the bias centred
On a plug-in with a bias or asymmetry control set to zero, the curve may be symmetrical, and then it can only add odd harmonics, whatever the name on the front says. If the result sounds hollow, turn the bias up.
Saturating every track
A valve plug-in on each channel stacks 2nd harmonics in the same low-mid range, and the mix gets thick and cloudy. Use it on the parts that need help to come forward and leave the rest clean.
Train your ears
The quiz under the lab plays the current sound through the valve, the soft clipper or the hard clipper, at the same loudness as the original with the screens covered, and asks which you heard. The valve and the soft clipper are the hard pair to tell apart. Listen for whether the sound gets fuller, which is the valve's 2nd harmonic, or picks up a hollow edge, which is the soft clipper's odd ones. The bass and the chords make it easiest, and your best score is saved in your browser.
Questions people ask
Is a tube the same as a valve?
Yes. Vacuum tube is the American name and thermionic valve, or just valve, is the British one. Plug-ins and hardware made in the US nearly always say tube, so you'll see both names on the same kind of gear.
Why does valve saturation sound warm?
A single valve stage bends one half of the wave more than the other, and that lopsided shape adds a strong 2nd harmonic. The 2nd is exactly an octave above the note, so it thickens the sound without adding much edge, which is what most people mean by warm.
Is valve saturation better than tape saturation?
Neither is better. Valve saturation leads with the 2nd harmonic and keeps the top end intact. Tape adds mostly the 3rd harmonic and softens the highs as you push it, so it sounds a little darker. Reach for valve to make a part fuller and for tape when you want it smoother.
Do I need real valve gear to get the sound?
No. A plug-in that models a valve curve gives you the lopsided shape and the 2nd harmonic, which is most of what you hear. Hardware adds things that are harder to model, like transformers and power supplies that change under load, and how much of that survives into a finished mix depends on how hard the gear was pushed.
What does the bias control on a tube plug-in do?
It moves where the signal sits on the valve curve. Off-centre bias makes one half of the wave hit the bend sooner than the other, which adds even harmonics. In the lab it's the Character knob, and in Ableton Live's Dynamic Tube it's called Bias.
Should valve saturation go before or after compression?
Before the compressor if you want it to react to the extra harmonics and sound denser. After it if you want the amount of colour to stay the same from quiet notes to loud ones, because the compressor has already evened the level out. On vocals, start with it after, so the colour stays the same from line to line.