Tape vs valve (tube) saturation

At a glance

What's the difference between tape and valve saturation?

Both are kinds of saturation: something that handles quiet sounds cleanly and bends loud ones, so the shape of the wave changes. A changed shape means new frequencies, called harmonics, at whole-number multiples of each note. A 110 Hz note can pick up harmonics at 220, 330, 440 Hz and on up. What sets tape and valve apart is which of those harmonics they add, and how the amount changes with level.

Tape saturates magnetically. The record head magnetises tiny particles on the tape, and as the level goes up, the particles get closer to holding all the magnetism they can, so each extra decibel in gives a little less out. The positive and negative halves of the wave meet the same limit, which makes the curve close to symmetrical, and a symmetrical curve can only add odd harmonics, at 3, 5 and 7 times the note.

A valve amplifies by letting a small voltage on a wire grid control a much bigger current. A triode, the simplest amplifying valve, doesn't amplify both halves of the wave by quite the same amount, and the steady voltage that sets its resting point, called the bias, rarely sits exactly halfway between its two limits. So one side of the wave bends sooner than the other. A lopsided curve adds even harmonics, and the 2nd, an octave above the note, is the loudest of them.

Each one has a lesson of its own: tape saturation goes into head bump and tape speed, and valve saturation explains triodes and push-pull stages. This page puts the two side by side.

The same amount of distortion, two different recipes

A fair comparison needs both curves adding the same total amount of harmonics. If one is adding twice as much, the difference you hear is mostly the amount. Engineers measure that total as total harmonic distortion, or THD: the level of every new harmonic added together, as a percentage of the note.

The first two steps in the lab are set up that way. Tape gets 12 dB of drive with the character at 0%, and the valve gets 8 dB at 50% character, because the lab's valve curve starts bending later. On the 110 Hz test sine, both add harmonics worth 17% of the note, made up like this.

Tape spreads its distortion along the odd harmonics, with the 5th and 7th following the 3rd. The valve puts nearly all of it into one harmonic, the 2nd, and the next one down is more than 20 dB quieter. On the bass loop you can hear it. The tape version has a firmer, slightly hollow edge on each note, while the valve version sounds like the same bass with more body, because an octave lines up with the note and doesn't add a pitch of its own.

What each one does to peaks and transients

Look at the last row of that table. The crest factor is the gap between a sound's peak level and its average level. Tape brought the bass loop's crest factor down from 17 dB to 11.4 dB, so its peaks were squashed. The valve, adding the same 17% on the sine, left it at 17.2 dB.

The reason is the shape of the curves. Tape bends the top and the bottom of the wave the same way, so every peak gets flattened a little, whichever way it points. The valve curve squashes one half of the wave and stretches the other: in the lab at 8 dB of drive, a sine peak of 0.9 comes out at 0.23 on one side and 0.47 on the other. One set of peaks gets shorter while the other gets taller, so the gap between peak and average hardly changes.

The two also grow differently. At low levels a lopsided curve's 2nd harmonic rises by about 1 dB for every decibel the input goes up, while a symmetrical curve's 3rd rises by about 2 dB per decibel. So tape saves most of its work for the loudest moments, which are the peaks, and the valve spreads its colour across the whole sound.

Drums show it best, because a drum hit is a tall transient followed by a quieter body. With 12 dB of tape the drum loop's crest factor falls from 19 dB to about 14 dB, and the kit sounds denser and less spiky. With the valve settings from step 2 it rises a little, to about 20 dB. That rounding of the hits is what engineers mean by tape compression, and a valve only starts doing it once it's driven hard: at 24 dB of drive the bass loop's crest factor is down to 10.8 dB.

Neither is a compressor, which measures the level and turns it down over time. Both are curves that change the shape of each wave as it passes. Saturation vs compression covers where the two overlap.

Does tape or valve sound warmer?

Both get called warm, and they get there by different routes. A valve's warmth is the 2nd harmonic: the sound gains weight an octave up and seems closer to you, while its edges stay where they were. Tape's warmth is mostly about what it takes away. The peaks lose their sharpest edge and the top end softens a little, which makes a sound smoother.

That softer top end comes from the way a tape machine records. It boosts the high frequencies on the way onto tape to keep hiss down and cuts them again on playback, so the highs are the first to run out of room, and a real machine pushed hard gives you back duller cymbals and S sounds. The lab's tape mode models this with a gentle roll-off that starts at 16 kHz and moves down as you add drive. In the lab the change is small: the bass loop comes out 1 dB darker above 3 kHz through 12 dB of tape, and about the same as the original through the valve. Tape plug-ins set to a slow tape speed lose more.

If someone asks you to make a part warmer, work out which of those they mean: a part that sounds thin and distant wants the valve, and one that sounds harsh and spiky wants tape.

What happens when you push them harder

Turn tape up and its recipe stays the same. The 3rd harmonic keeps leading with the 5th and 7th climbing behind it, and the sound gets thicker and more squashed until it turns fuzzy. Turn a valve up and the recipe changes, because a hard-driven valve runs into limits on both sides of the wave. Once both halves are flattening, the curve behaves like a symmetrical one, and odd harmonics take over.

Here are both modes in the lab as the drive goes up, tape at 0% character and the valve at 50%.

The valve's 2nd harmonic tops out between 12 and 16 dB of drive and then falls, while its 3rd climbs all the way from −58 dB to −11 dB. Around 20 dB they meet. At 24 dB the valve's 3rd matches tape's at −11 dB, and the two sound much more alike than they did at low drive. The valve still adds a little more in total up there, 35% against tape's 33%, because its lean puts even harmonics on top of the odd ones.

So the valve sound most people are after lives at low to moderate drive. Turn a valve plug-in up looking for more warmth and you'll get more odd harmonics, and less of the thing you wanted. Tape has no such turning point; it gives you more of the same until it's too much.

Reading the lab

The lab has two curves loaded, tape and valve, and while it has focus you can press 1 and 2 to switch between them as the loop plays. Auto gain is on, so whichever you pick comes out as loud as the original, and A/B switches to the original at the same loudness. The Bits and Rate knobs belong to other modes and are switched off here.

The screen that answers this comparison is the harmonic graph. The lab runs a pure 110 Hz sine through your exact settings and measures every harmonic that comes out, with odd ones in pink and even ones in amber. Tape at 0% character shows a row of pink bars and no amber at all. A valve set for warmth shows one tall amber bar at 220 Hz and very little after it.

The Character knob means something slightly different for each. On tape it adds a small lean, the way a real machine's electronics add a little 2nd harmonic. Keep it low: at 100% and 6 dB of drive the 2nd comes up level with the 3rd, which is no longer the usual tape recipe. On the valve it's the bias, and the valve curve leans a little even at 0%, because a single valve stage is never symmetrical. The Right now panel reports the crest factor and the change in top end on the loop that's playing, which are the two numbers to watch when you switch modes on drums.

The lab's curves are simplified models. Its tape leaves out head bump, wow and flutter and hiss, and its valve leaves out transformers and power supply sag, so the drive numbers won't line up with the meters on a hardware unit or a particular plug-in.

When to use tape and when to use valve

Vocals

Start with a valve. With 6 dB of drive at 50% character, the lab puts the 2nd harmonic at −17 dB and the 3rd at −43 dB, so the voice gets fuller and still sounds clean. Tape on a vocal rounds off the loudest words, which can help a jumpy performance, but its odd harmonics and the way it treats high frequencies make the S and T sounds saturate first. If the sibilance gets worse with either, put a de-esser before the saturation.

Drums

Start with tape. It does part of a bus compressor's job by rounding off the tallest part of each hit, and the kit sounds more like one instrument. If the snare loses too much crack, blend the tape in parallel so the clean hit stays on top. A valve on drums is the better pick when you like the hits as they are and want more tone in them.

Bass

Both work, because both add harmonics that small speakers can play. A 55 Hz note gets a valve's 2nd at 110 Hz, or tape's 3rd and 5th at 165 and 275 Hz, so the note still comes through on a speaker that can't play 55 Hz. The valve version sounds rounder. Tape sounds firmer and evens out the level of the low notes a little.

The mix bus

Tape is the usual choice, a few decibels of it before the limiter. It brings the loudest drum hits down a touch and leaves everything else nearly alone. A valve across a whole mix adds 2nd harmonics to every part at once, which can make the low mids thick, so keep the drive and bias low if you use one there.

How to use tape and valve saturation together

Plenty of analogue recordings went through both. The microphone fed a preamp, which could be a valve one, the result was recorded onto a multitrack tape machine, and the finished mix was often recorded onto a stereo tape machine as well. You can follow the same order in a DAW, with valve on the individual parts that need body and tape on the groups where you want peaks rounded off.

1. Put the valve on the sourceAdd a valve plug-in to the vocal, bass or keys. Set the bias so the 2nd harmonic leads, raise the drive until the part sounds fuller on A/B, and stop before it gets an edge.
2. Put tape on the busAdd a tape plug-in to the drum bus or mix bus, with hiss and wow and flutter switched off. Raise the drive until the loudest hits just start to soften.
3. Match the levelsSet each plug-in's output so bypassing it doesn't change the loudness. Saturation nearly always makes things louder.
4. Check the top endSeveral stages of tape add up, so if the cymbals have gone dull, take one out before you reach for an EQ.
5. Compare the whole chainBypass everything at once with the mix playing. If you can't say what got better, lower the drive on both.

If you want both colours on a single track, put the valve first. That's the order the sound took through an analogue studio, and it means the tape rounds off the peaks of a signal that already has its 2nd harmonic.

Tape and valve plug-ins

Tape plug-ins range from close models of specific machines, such as Universal Audio's Studer A800 and Ampex ATR-102, to more general ones such as u-he Satin and Softube Tape. CHOW Tape Model is free and open source. Valve curves turn up in Ableton Live's Dynamic Tube, which has three tube models and a Bias control, and in multi-mode saturators such as FabFilter Saturn 2, which has both tape and tube styles. Whatever the name on the front, the harmonics decide what you get, and a tube mode with a centred bias is symmetrical and behaves more like tape.

Common mistakes

Comparing them at different amounts of drive

Put tape and a valve plug-in on the same part at the same drive setting, and one is almost certainly adding more distortion than the other, because nothing about the two drive knobs matches. In the lab, 12 dB of tape and 8 dB of valve add the same total. Match the amount by ear, or with a harmonic analyser, before you decide which recipe you prefer.

Choosing the louder one

Saturation raises the level, and a louder version nearly always sounds better for a few seconds. Turn Auto gain off in the lab and the Right now panel says how much louder each mode is. In a DAW, match each plug-in's output to the bypassed level.

Driving a valve to get more warmth

Past about 16 dB of drive in the lab the valve's 2nd harmonic starts to fall, and by 20 dB the 3rd has caught up with it. If a valve plug-in has started to sound gritty, you've gone past the warm part, and the fix is less drive.

Tape on every track

Each tape stage takes a little top end off and rounds a few more peaks. On one bus that's what you wanted, but across twenty tracks the mix goes dull and loses its punch.

Train your ears

The quiz under the lab plays the current sound through tape or the valve with the screens covered, at the same loudness as the original, and asks which it was. Its settings are heavy enough to hear easily. The bass and the chords make it easiest: listen for whether the low notes gain body, which is the valve's 2nd harmonic, or gain a firmer, slightly hollow edge with the peaks rounded off, which is tape. The tape round also rolls the top end off from 5 kHz, so it comes out duller than the valve round. Your best score is saved in your browser.

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