Tube amp sag and preamp vs power amp distortion explained


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Many of the most celebrated guitar sounds in history have been those where a tube amp is being pushed to its limits. It’s that addictive tube-driven crunch that fuels our rock star dreams and inspires us to seek out the best gear possible in that never-ending quest for tone. But what causes our amps to produce these holy tones? And where do the preamp and power amp fit into all this? As usual, Blog Of Tone has the answers.

The recipe for great electric guitar tone has many ingredients.

The guitar, pickups, strings, cables, effects, amplifier – all the links in this long chain contribute to the sound that ultimately emerges from your loudspeakers.

It’s not just the speaker type that shapes your tone, though; the housing also has a say in what comes out of it.

With so many variables, it would appear that conjuring emotive tone that brings tears to your eyes, or at least goose bumps to your flesh, is more of an accident than an achievement.

But make no mistake, there’s a science to dialing in great guitar tone.

Yes, different guitar cables can change your sound

Fortunately, not all the links in this chain are equal.

Take, for example, two different brands of broadly similar, standard-issue guitar cables in roughly the same price range. We can measure their tone-shaping action with a meter, but we would have to listen very carefully to hear the difference.

Alastair Greene, H&K devotee and lead guitarist for the legendary Alan Parsons, feeling the difference onstage. Alastair's solo blues rock project is certainly a beneficiary of the whole preamp vs power amp tone thing!

Alastair Greene, H&K devotee and lead guitarist for the legendary Alan Parsons, feeling the difference onstage. Alastair’s solo blues rock project is certainly a beneficiary of the whole preamp vs power amp tone thing!

The same can’t be said of the two main components of an amplifier, the preamp and power amplifier.

American or British preamp?

Most experienced guitarists can tell if a preamp section has more of an American or English flavor.

There’s a unique, fire-breathing dynamic to a tube-driven power amp, and the majority of players can hear that dragon when it’s unleashed.

And most seasoned six-stringers – those of the rack age – can detect the product of an all-in-one, convenience-driven sound generator in 19″ format.

Tubes: still the best way to go

If the signal chain culminates in an all-tube amp, we can safely assume that it, alongside the guitar, is responsible for the lion’s share of the sound.

After all, the vacuum tube is still the best medium for creating pure, unadulterated rock ‘n’ roll tone.

And I’d wager that even those who swear by smart digital solutions such as profilers, modelers and DAW plugins would agree that in life, as in art, it doesn’t get any better than the original.

Power amp + preamp = win

So allow to me to assume we have a consensus on the USP of an old-school tube amp: it can deliver exquisite tone.

The more modern-minded sound sculptor would weigh in with the argument that this all-but-antiquated technology’s sonic riches are offset by its poor flexibility.

With six channels, three independent preamps, three independent power amps and three pairs of totally interchangeable power tubes, we'd argue that the TriAmp Mark 3 is certainly not lacking in flexibility!

With six channels, three independent preamps, three independent power amps and three pairs of totally interchangeable power tubes, we’d argue that the TriAmp Mark 3 is certainly not lacking in flexibility!

I object, and respectfully submit to you my evidence: there’s a vast treasure-trove of tone to be had at the confluence of the preamp and power amp.

The beauty of saturated tube tone

There’s clean tone, there’s lead tone, and then there’s that wonderful place in between that goes by many names.

We’ll call it ‘crunch’ tone.

This is where we throw a pinch of dirt in the works, a smidgen of saturation, a little grit into the gears, to happily grind away for days in a tone zone where the two stages of amplification interact so well.

Now, I’m as partial as anyone else to a tasty preamp that can deliver sweet sonic snacks ranging from clean to cream.

That’s a happy meal alright, but sometimes you just want to get supersized with the big, bold sound of a tube power amp at the brink of breakup.

The tube amp that spawned a thousand successors

Most of us know what a cranked Marshall Plexi head sounds like.

This amp and its many derivatives provided a blueprint for rock ‘n’ roll guitar tone. A lot of great music was built on this foundation.

For many players, it’s still the Rosetta Stone of rock tone.

Of course, you had to find a good specimen to get the mojo workin’. Some were sublime, others sub-standard.

Tube amps - and the tubes themselves - are (and were!) very sensitive pieces of equipment. Back in the day, getting hold of one in good nick was an exercise in persistence, luck, and expense.

Tube amps – and the tubes themselves – are (and were!) very sensitive pieces of equipment. Back in the day, getting hold of one in good nick was an exercise in persistence, luck, and expense.

Good tone was like a Nigerian prince

These days, process engineers use statistical methods such as scatter charts to improve manufacturing quality.

Back then, though, the variance in these amps’ quality was off the charts, so there was indeed just a scattering of good specimens to be had.

And they were as hard to find as that Nigerian prince that keeps sending you emails promising you money.

But if you did manage to bag that elusive beast, then you too could get the sound that sold a gazillion records.

The problem with tube amps and volume

That is, you could if you played venues large enough to crank the amp.

It’s not just the speakers that whimper for mercy when you dime a 180-watt powerhouse in an enclosed space.

It sounded divine, as if the signal had been sent directly from the deities of the rock ‘n’ roll pantheon. But it was so unrewardingly loud that this hammer of the gods could be heard and felt even up there in Valhalla.

Earplugs and master volume knobs

For a long time, the only way around this issue was earplugs.

Then came the master volume knob, which solved the volume problem, but the pain relief came at the cost of worse tone.

Uh-oh. The Master Volume is found on many amps, but it didn't have the best reputation back in the day! It's different now, though, so Master away at your toneful leisure...

Uh-oh. The Master Volume is found on many amps, but it didn’t have the best reputation back in the day! It’s different now, though, so Master away at your toneful leisure…

You could now dial in distorted tone at almost any volume, but the softer the level, the less impact the output stage had on your sound. The outcome was but a pale shadow of the power amp’s sonic glory.

“Turn it down!”

A power amp has to be pushed hard to hit that sweet spot where saturation kicks in.

If it’s idling in low gear, that cat may purr but it just ain’t gonna roar.

So, despite the master volume knob, many guitarists still had to play loud enough on stage to bring the power amp to the edge of breakup.

This is why the most frequently heard phrase at sound checks was the guy at the desk: “Say there, old chap, do be so kind as to turn it down a touch.”

Occasionally, the same sentiment was expressed in somewhat less polite terms.

The infamous tube amp ‘sag’

Getting back on topic, many tube amps seem to shine at around the midway point of their power spectrum.

A master knob set to the 12 o’clock position generally puts you in the ballpark.

As you ease the knob towards the right, the output stage starts distorting. Guitarists generally perceive this as very musical compression, with notes blooming rather than just sounding.

Many guitarists report that this inspires them to play better.

This effect is compounded by the sag – that is, a drop in power supply voltage – induced when the power transformer and/or rectifier is hit hard.

This sag enhances the amp’s dynamic, musical feel and makes it very responsive to your picking attack.

One decent tube amp and a solid axe is all you really need to unleash those stadium-filling rock tones on the unsuspecting audience of your choice. The rest is in your fingers...

One decent tube amp and a solid axe is all you really need to unleash those stadium-filling rock tones on the unsuspecting audience of your choice. The rest is in your fingers…

Sag your way to the sweet spot

People have come up with all kinds of colorful names for this sound and the subjective sensation of this wonderfully musical experience.

But whatever you wish to call it, it’s at this sweet spot where I feel most connected with the audience because the amp is at its most expressive.

And isn’t that what this is all about – expressing emotions that we can’t put into words?

At any rate, this is where I feel at home and it’s in this zone that I want to be when I’m playing.

A bit more preamp gain can work wonders

The preamp can enhance this effect, and that’s what makes the interaction between the tube preamp and the power stage such a smorgasbord of sonic delights.

If you nudge up the preamp gain to coax a little crunch from the circuit, you will eventually arrive at a point where the power amp clips when you dig in hard.

That’s when this sagging effect becomes as addictive as other far less wholesome pursuits.

The joys of this playing experience extend beyond the crunch territory and up into the higher gain ranges. However, at extremely high gain settings, the preamp’s natural compression flattens out the dynamics that are so typical for this tone zone.

If your GrandMeister Deluxe 40 is not sagging, you're not doing it right!

If your GrandMeister Deluxe 40 is not sagging, you’re not doing it right!

Even so, the spectrum of this type of sound is broad enough to cover vast musical ground.

If it sags, play it

So, whatever biological imperatives conditioned us to find sagging things unattractive, they don’t apply to tube amps.

Au contraire, sag is the very essence of rock guitar tone.

You can’t replicate true sag

But an overdriven preamp alone does not give you the tools to add true 3D depth to a relatively one-dimensional guitar sound.

Yes, preamps can deliver creamy distortion that works in many contexts, but they can’t offer the living, breathing vitality of a power amp that’s running out of headroom.

There are ways to model or compute this effect, and they are put to good use in many of today’s popular solutions.

But those are bloodless imitations of the sag of analog components interacting in a way that feels almost biological to me.

Step forward the lunchbox amp

If this natural sagging effect is to be produced in a natural way and enjoyed at a volume level that won’t make the player the neighborhood pariah, then the only solution is to go with a low-powered amplifier.

As we've said many, many times on the Blog Of Tone, a small amp can fill the biggest stage in the world if you do it right.

As we’ve said many, many times on the Blog Of Tone, a small amp can fill the biggest stage in the world if you do it right.

That’s what those lunchbox heads launched a few years ago are here to do.

Few things in life are more pleasurable than letting your inner troubadour emote at full tilt, but at a manageable volume.

The beauty of the power soak

The power soak on amps such as the TubeMeister and GrandMeister series models can intensify this refreshing experience.

If you handle it with care, you will be able to conjure arena-rock-approved tone.

And that’s exactly what the secret rock star in most of us is dying to do.

You’ll have to watch the compression that power soaks add to the sonic equation. Too much of that, and the arena dream is done for.

Just enough, though, and your bedroom could be Madison Square Garden, Wembley Arena and the Budokan rolled into one.

 

First published: December 15 2016. Most recent update: December 15 2016.

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Leave a comment

Nik Henville on December 17, 2016 Reply

This is why I, from time to time, trawl old tape-decks and gramophones for Mullard valves and the like, and then put them in my TM36. The “character” they bring is a real “ear-opener”… And here’s the thing. How many modellers and profiling amps emulate Valve amps, and how many do they emulate. And then ask yourself how many solid state amps they emulate. Then ask yourself why the former is a huge number, and the latter a smaller number. It ain’t rocket science (heck – it ain’t even advanced audio engineering…)

    Hughes & Kettner Hughes & Kettner on December 22, 2016 Reply

    Never a truer word spoken on here, Nik! And if you’ve got any pics or clips of those old valves being put to use in your TubeMeister, we’d love to see/hear them… 🙂

Huey T. on December 23, 2016 Reply

Fine words, my dear German tube nerds! Yes, the only time a (male!) guitar player allows something to sag is when it’s involved with the performance curve of a vacuum tube power amp section 😉 This always makes me smile. In all other cases, though, you should always avoid sagging from the beginning on! Regardless of whether you’re a man or a woman 😉

Sincerely, your upright-standing Blog Of Tone Fanboy Huey 🙂

    Hughes & Kettner Hughes & Kettner on December 23, 2016 Reply

    We believe you Huey, and we certainly agree – like we said in the blog: “sag is the very essence of rock guitar tone”… The best advice is this: keep your guitar tone sagging, and your body not sagging, and you’ll be enjoying music for many years to come 🙂

Michael Locher on January 20, 2017 Reply

I play a Grandmeister36 with the small 12″ speaker and absolutely love the sound and the features of this amp. I play it with different guitars but only with a Wah-Wah and a volume pedal as additional “effects” and it always sounds great: Live in the band, but also for recording directly to the recording desk !

    Hughes & Kettner Hughes & Kettner on January 27, 2017 Reply

    Thanks for the kind words about GrandMeister Michael! That’s exactly how we conceived it: as a kickass amp you can use for all the situations you mention 🙂 Enjoy!

Steve Winkler on March 20, 2017 Reply

Hmmm…well, one way to get sag in your tone at lower volumes is Brian Wampler’s Pinnacle Distortion Pedal. This is the one pedal / circuit he designed to have a sag characteristic as well as to be interactive with your playing in terms of picking attack but especially your guitar volume control. I use this pedal w a 50 watt hand wired plexi clone. The guitars that match well to it will have high output alnicos in the bridge position. With a little tweeking I’m able to get a really nice Marshall tone at lower volumes that cleans up really nice when you roll the guitar volume back, especially when I use my Fender American Deluxe Mahogany strat with the Fender Atomic high output alnico bridge PU. We do a wide range of stuff from Heart’s “Love Alive” to Van Halen’s “Panama”. At a guitar volume of 7, you’re right in that great midrange classic crunchy rock Marshall tone. At a guitar volume of 10, the power chords get flubby but the single notes – especially the highs just sing, more so when you use a clean boost pedal. Between 3 to 5 you get a much cleaner tone, especially when you switch over to strat switch positions “2” and “4”. In a small room the overall tone won’t be as good because you have to lower the master and keep the gain channel at “4”. Turn the master up and enter into the power section of the amp and it just sounds incredible.

    Hughes & Kettner Hughes & Kettner on April 21, 2017 Reply

    Some really interesting points here Steve… and now we need to get ourselves one of those Wamplers to see what all the fuss is about 😉 Thanks for taking the time to write!