What exactly is "tube warmth"?

[Hambone70]

It’s right up there with tonewoods.

 

[agedhorse]

If no to second point, maybe. I don't know where I heard of that even harmonic thing, but I like point 7 in Agedhorse's post, that nonlinearities, not just raw clips, can make harmonics. I don't know what specific mechanism in a valve circuit would make even ones though.

Great summary.

Of these, aside from “warmth”, what accounts for the so called “3D effect” that some have used to describe the quality imparted by tubes in certain preamps (and some tube-based pedals)?

Is it a combination of subtle even-order harmonics, compression, and an element of psychoacoustics?

Good question(s). In addition to tone, there is texture that's associated with (bass and guitar) amplification. Some of this texture is related to the harmonics that are generated, and some of them are related to the non-linerity of how these harmonics relate to the fundamental. For example, as you drive a circuit harder, the harmonic ratio to fundamental also increases as does the distribution of the harmonic series.

valves are non-linear. What you get out is not an exact (but louder) copy of what you put in. Transistor amps are linear and are a "hi-fi" reproduction of the input signal. In practice, what the non-linearity means is that if you put a sine wave in, you get a deformed sine wave out. Think a pure sine wave that is -5V to +5V. Through an exaggerated valve amp, the first 1V would be the same, the next one would only be 0.9V, the next one 0.8V, then 0.7 then 0.6V. The top of the sine wave is a bit flatter than it should be. That, in a sense, emphasises odd numbered harmonics (so 3x the fundamental, 5x, 7x etc).

When you drive a transistor amp into distortion, you get a sine wave with a flat top and very sharp corners between the sloping sides and the flat top. That adds huge amounts of very high harmonics which can sound harsh unless you have a low pass filter. Valve amps continue with the non-linearity. You eventually get a flat top to the wave form, but with rounded corners between the top and the sides. Tube distortion is less bright and less harsh.

This is not true, both transistors and tubes are non-linear devices and when used in similar circuits will behave in similar, though not exactly the same ways.

The sharp corners and flat top that is often associated with "solid state clipping" is in fact due to high global feedback, and can occur in both tube and solid state circuits. It's MUCH more common in op-amp circuits (op-amps are made up from a large number of transistors) because op-amps have high open loop gain, and a large amount of global feedback is used to reduce the closed loop gain to what's needed for the design.

There are also discrete transistor and tube circuits that use 2 or 3 voltage gain stages for a lot of open loop gain (not as much as an op-amp though) and when global feedback is used in these circuits, the behavior is more similar to an op-amp than tube or transistor circuits with low amounts of local feedback.

This old wive's tale has been around for a long time, generally perpetuated by the marketing folks associated with tube amps, and because of the "advertising", have been believed as gospel by those dedicated to these topologies.

 

[Wisebass]

Nice thread!

So, does anyone know what exactly is the "warmth" that tubes have?

I know!

Perhaps ironically, many tube amps are voiced rather bright.

Tubes are not inherently shy on low or high end, in fact it's quite easy to design a tube amp that extends flat beyond the range of human hearing.

An exception to this is the Ampeg SVT. The tend to be fairly flat

Usually amps are not designed to have a sound! (but to amplify and reproduce a given sound 1:1)

But that won't work here

you can't make a Hiwatt sound like a Fender. You can't make either sound like an SVT, and vice versa. Which is best depends on what you want.

All of them have their sound (and you can hear the warmth )...

tube warmth is when the tube heats up your hands when your near it .

... you can feel the warmth

And of course you can place your beer on top

and taste the tube warmth!


wise(b)ass

 

[Mike Marshall]

This must have been asked before, but I couldn't find any specific threads.
I was checking some videos comparing class D and tube amps. And even the reviewers said that tube amps gave some type of "warmth" to the sound. I know what the word means, but I have trouble putting in context of sound.

So, does anyone know what exactly is the "warmth" that tubes have? is it a boost in some frequencies? is it volume or dynamics related?

I can hear some differences in the amps that are compared in videos, but is there a physics definition of what we describe as "warmth"?

I listen with my ears and I'm old, what tubes have for the most part is a depth to the tone that class D lacks. Not all tube bass amps are the same by any means. I use a 62 blond Fender Bassman it has depth yet the presence knob adds a high end sheen that can change the tone to a slightly grinding Marshall tone. Pure Tone amps made in Greece have a tone similar yet beyond the 62 Bassman check out the Pure Tone Offset. A good example of Depth for me in a current amp is the Ampeg PF20T tube head and a portaflex cabinet I can see studios getting into this amp it has a fat tone at low volumes and is capable of getting pretty loud. Studios used to go one direct line and a miked B15 or an SB12 for warmth.

 

[ToddB1963]

This is a really good question... one that I ask every time I design an amp. Not just tube based but also in some cases when designing an amp to be different than the PERCEPTION of what a tube amp is thought to be.

A few semi-organized thoughts, put together from both the design education and the experience of having done this many times.

1. Perception is just that, the perception of something that may or may not be real, but also based on what MAY have been a real thing with some but not all tube amps.

2. Tubes are not inherently shy on low or high end, in fact it's quite easy to design a tube amp that extends flat beyond the range of human hearing.

3. Both tubes and transistors share similar properties when used in SIMILAR APPLICATIONS, this means in similar circuit topologies. Preamp tubes are almost always used in circuits with no global feedback, transistors (and specifically operational amplifiers comprised of transistors) are almost always used in circuits with some (or a lot of) global feedback. In power amplifiers, solid state amps almost always have global feedback and tube power amps sometimes do and sometimes don't, but almost always when present is used at much lower levels.

4. In preamp circuits, transistors often are used with considerable local AC feedback, tubes are often used with no local AC feedback.

5. Perceived "tube tone" appears to have more to do with topology and amount of feedback than the device iteself, for example if you design a tube preamp circuit with no global feedback and minimal to no local AC feedback, it begins to behave more like a tube circuit designed similarly. Op Amp devices have generally MUCH higher global feedback factors because they VERY high open loop gain. If the open loop gain of an Op Amp is 10x the open look gain of say a tube or transistor circuit, then 10x the global feedback needs to be applied to end up with the same closed loop gain.

6. One characteristic of low feedback circuits is greatly increased distortion. Both tube and transistor circuits behave similarly in this regard. Both have similar proportions of even and odd orders for similar circuits, but accessory circuits can alter these ratios to some extent. It's not uncommon to see preamp circuits operating linearly yet generating 2-5% THD, which is perceived as enhancing the tone. This is why rating output power at 0.1% or even 1% is absurd within the context of the entire amp where the distortion is an essential part of the essence of the amp.

7. Distortion being added only during clipping is a myth... in most tube amps and some solid state amps, small to moderate amounts of distortion are what make these amps appealing to players looking for that "tone". It's the amount of gain and the distribution of the distortion products that help define the particular tonal characteristics (in combination of eq/voicing).

8. There can be a perceived compression effect from low feedback circuits, especially as they enter and exit clipping. Actually, one of the characteristics of a low feedback circuit is how it enters and exist clipping and what happens throughout that transition. This is not just the gain ratio of the transfer function, but also the symmetry of the waveform during this process.

9. Many of these aspects of tube and transistor circuits can be emulated using op-amps, but without these emulation sub-circuits, op-amps generally have very low distortion, and enter/exit clipping quite abruptly, symmetrically and not terribly gracefully. That said, op-amps are MUCH more stable, predictable devices, with high input impedance, low output impedance, precise gain numbers defined by the circuit and not the device itself. This makes them ideal for use in filter and eq circuits where these parameters improve the function (ie. slope, Q, bandwidth) of these circuits.

10. In power amps, one of the primary differences is that the output impedance of tube power amps is much higher than of solid state power amps, this is due to the devices themselves, the output transformer losses, and the lower levels of global feedback applied. The tubes themselves (aside from their higher plate impedance) don't behave all that differently than transistors, and in circuits where higher global feedback can be applied, they clip quite similarly to solid state power amps. This is the basis for some of the output emulation circuits used on the various Subway models (damping factor control, symmetry control, soft clip limiting).

11. Many players actually prefer what is commonly associated with solid state amplifiers, so when developing amps to appeal to a wide range of players, implementing "tube emulation" is not an all or nothing exercise. In fact, this is especially true since many tube amps use some techniques to bring them closer to the solid state perception. The SVT is one example of an amp that shares several design similarities with solid state amps, though also shares some similarities with tube amps... reinforcing the fact that it's NOT an all or nothing proposition.

Now here's a man who knows his stuff !

 

[Lostgallifreyan]

In addition to tone, there is texture that's associated with (bass and guitar) amplification. Some of this texture is related to the harmonics that are generated, and some of them are related to the non-linerity of how these harmonics relate to the fundamental. For example, as you drive a circuit harder, the harmonic ratio to fundamental also increases as does the distribution of the harmonic series.

This is the detail I like because to me it relates to synthesiser design, which I have done for some time. Unlike formants (cabs, acoustics in general), this direct extension of the instrument is critical for interest, If this didn't happen we'd just have amplitude, and that would suck, it's deeply uninteresting on its own. Part of the art of making a good phase modulation synthesiser is looking for new ways to get these entertaining nonlinearities into it. Anyone who knows the relatively glassy and undynamic nature of most DX7 sounds knows what that means... Unlike using natural sources of modification, when making a digital synth it's like 'painting by numbers', and any nonlinear method has to be deliberately put there.

One reason I love pounding away at a bass is that for much of the time I don't have to think about this stuff. I love the results of doing so, but I always felt the sting of those statements by people, decades ago, who told me synths couldn't be interesting like guitars. They were wrong, but I benefit hugely from exploring the bass, and all that makes it sound good.

 

[groovaholic]

"Tube warmth"?
If I'm being literal, it's because the transformer on a tube amp has an output that's designed for the exact purpose of heating the tubes.

Colloquially, it's exactly what people are commenting on here - a tone that feels like a flannel blanket that just came out of the dryer.

That feeling is most likely due to dynamic compression, subtle saturation, attenuation of both extremes of the audio spectrum, and the dynamic feedback of an output transformer coupling with the speaker cabinet.

A decent pedalboard can mimic most of those attributes - but IMO the interaction of the output transformer is the hardest thing to emulate.

 

[Nuage420]

For me it is a light but noticeable thickening of the notes that make runs sound more fluid and that thickening best translates into a warming of the note. The harshness that many modern amps bring tends to make the tube amp sound even more pleasant to the ear.

When you play tube amps these days, particularly old ones, younger (than me) bass players in particular, notice the different sound immediately and will come and ask questions. I never got that during the transistor or hybrid decades of my playing. When I started playing, everyone used tube amps so there was no discernible difference between us. These days, there definitely is.

Another point is that tube sounds don't demonstrate that accurately on video as no Utube reference demonstrated what standing next to the speaker cabinet will.

If you keep a permanent search on ebay for Marshall Bass amp as I have had for close to 2 decades, it will astound you that an amplifier company with the largest profile and huge amount of heads produced, hardly ever has a single bass amp for sale. Kinda says something.

Thank you for this! I've failed many times to explain exactly what you just said. Especially the part about standing next to the cab. It's this extra umph to every note that is felt as much as it is heard.

 

[agedhorse]

"Tube warmth"?
If I'm being literal, it's because the transformer on a tube amp has an output that's designed for the exact purpose of heating the tubes.

Colloquially, it's exactly what people are commenting on here - a tone that feels like a flannel blanket that just came out of the dryer.

That feeling is most likely due to dynamic compression, subtle saturation, attenuation of both extremes of the audio spectrum, and the dynamic feedback of an output transformer coupling with the speaker cabinet.

A decent pedalboard can mimic most of those attributes - but IMO the interaction of the output transformer is the hardest thing to emulate.

Actually, the transformer is relatively easy to emulate.

 

[MDBass]

This must have been asked before, but I couldn't find any specific threads.
I was checking some videos comparing class D and tube amps. And even the reviewers said that tube amps gave some type of "warmth" to the sound. I know what the word means, but I have trouble putting in context of sound.

So, does anyone know what exactly is the "warmth" that tubes have? is it a boost in some frequencies? is it volume or dynamics related?

I can hear some differences in the amps that are compared in videos, but is there a physics definition of what we describe as "warmth"?

It’s the even harmonics being added to the signal from driving the tubes.

Generally described as “warm”, “thick”, or “round”.

 

[Jaygetsreal]

A very interesting thread - thank you OP. Posts from sublime to ridiculous but on balance I'm learning.
It seems to me that this conversation would be greatly helped by something visual. Anyone got oscilloscope traces, graphs etc that show the frequency response/distortion etc? Preferably side by side tube Vs solid state.

 

[jdjk7]

This must have been asked before, but I couldn't find any specific threads.
I was checking some videos comparing class D and tube amps. And even the reviewers said that tube amps gave some type of "warmth" to the sound. I know what the word means, but I have trouble putting in context of sound.

So, does anyone know what exactly is the "warmth" that tubes have? is it a boost in some frequencies? is it volume or dynamics related?

I can hear some differences in the amps that are compared in videos, but is there a physics definition of what we describe as "warmth"?

I think "warmth" is a combination of particular a) EQ characteristics, b) clipping characteristics, and c) compression characteristics, at least where tubes are concerned. "Warmth" more generally can mean different things, to me. Tubes are hard to accurately emulate, and not many have truly done it with solid state electronics. All-tube amps really have their own sound, and even tube hybrid amps (tube preamp, solid state poweramp) don't quite hit the mark, even if it's otherwise a fine amp.

In my experience with tube amps, tube hybrid amps, and solid state tube emulations, the first two characteristics are generally reasonable to emulate in solid state electronics, but tube compression is a much harder effect to nail.

 

[Alessandro Cruz]

As technologies evolve, there will ALWAYS be "nostalgic" people trying to put a mythical aura...

Remember the days when carburators were the mainstream stuff and fuel injection came around?

Vinyl vs CD?

Tape vs MP3?

It goes like this...

 

[Wasnex]

Both tubes and transistors share similar properties when used in SIMILAR APPLICATIONS

One of the tube amps in my collection is the renowned Sadowsky SA200. This is largely considered an all-tube amp, but it actually has a few solid-state devices in the signal path. The tone control network is basically a copy of the Fender tone stack with some variation in the component values. It has two extra bands of tone controls that are separate from the "Fender" tone stack.


The SA200 sounds great with the tone controls all straight up (12:00). The other amp in my collection that gets closest to this sound is an Epifani UL902C, which is 100% solid state and also class-D. Let me be clear, the SA200 can be adjusted to create a range of tones that the UL902C cannot cover. Likewise, the UL902C can be adjusted to cover a range of tones that the SA200 cannot cover. But there is a little nexus in the settings where they sound and feel very, very similar to each other.

AFAIK, the UL902C does not really do distortion. The SA200 can be setup for distortion, but I wouldn't say the distortion it produces is exceptionally good.

 

[themickster]

This must have been asked before, but I couldn't find any specific threads.
I was checking some videos comparing class D and tube amps. And even the reviewers said that tube amps gave some type of "warmth" to the sound. I know what the word means, but I have trouble putting in context of sound.

So, does anyone know what exactly is the "warmth" that tubes have? is it a boost in some frequencies? is it volume or dynamics related?

I can hear some differences in the amps that are compared in videos, but is there a physics definition of what we describe as "warmth"?

Even worse is 'vintage warmth'. You can get solid state stomps that sound like valve heads. I believed the squit about valves being better than SS, until I listened to the difference between them.

 

[Old Garage-Bander]

This is a really good question... one that I ask every time I design an amp. Not just tube based but also in some cases when designing an amp to be different than the PERCEPTION of what a tube amp is thought to be.

A few semi-organized thoughts, put together from both the design education and the experience of having done this many times.

1. Perception is just that, the perception of something that may or may not be real, but also based on what MAY have been a real thing with some but not all tube amps.

2. Tubes are not inherently shy on low or high end, in fact it's quite easy to design a tube amp that extends flat beyond the range of human hearing.

3. Both tubes and transistors share similar properties when used in SIMILAR APPLICATIONS, this means in similar circuit topologies. Preamp tubes are almost always used in circuits with no global feedback, transistors (and specifically operational amplifiers comprised of transistors) are almost always used in circuits with some (or a lot of) global feedback. In power amplifiers, solid state amps almost always have global feedback and tube power amps sometimes do and sometimes don't, but almost always when present is used at much lower levels.

4. In preamp circuits, transistors often are used with considerable local AC feedback, tubes are often used with no local AC feedback.

5. Perceived "tube tone" appears to have more to do with topology and amount of feedback than the device iteself, for example if you design a tube preamp circuit with no global feedback and minimal to no local AC feedback, it begins to behave more like a tube circuit designed similarly. Op Amp devices have generally MUCH higher global feedback factors because they VERY high open loop gain. If the open loop gain of an Op Amp is 10x the open look gain of say a tube or transistor circuit, then 10x the global feedback needs to be applied to end up with the same closed loop gain.

6. One characteristic of low feedback circuits is greatly increased distortion. Both tube and transistor circuits behave similarly in this regard. Both have similar proportions of even and odd orders for similar circuits, but accessory circuits can alter these ratios to some extent. It's not uncommon to see preamp circuits operating linearly yet generating 2-5% THD, which is perceived as enhancing the tone. This is why rating output power at 0.1% or even 1% is absurd within the context of the entire amp where the distortion is an essential part of the essence of the amp.

7. Distortion being added only during clipping is a myth... in most tube amps and some solid state amps, small to moderate amounts of distortion are what make these amps appealing to players looking for that "tone". It's the amount of gain and the distribution of the distortion products that help define the particular tonal characteristics (in combination of eq/voicing).

8. There can be a perceived compression effect from low feedback circuits, especially as they enter and exit clipping. Actually, one of the characteristics of a low feedback circuit is how it enters and exist clipping and what happens throughout that transition. This is not just the gain ratio of the transfer function, but also the symmetry of the waveform during this process.

9. Many of these aspects of tube and transistor circuits can be emulated using op-amps, but without these emulation sub-circuits, op-amps generally have very low distortion, and enter/exit clipping quite abruptly, symmetrically and not terribly gracefully. That said, op-amps are MUCH more stable, predictable devices, with high input impedance, low output impedance, precise gain numbers defined by the circuit and not the device itself. This makes them ideal for use in filter and eq circuits where these parameters improve the function (ie. slope, Q, bandwidth) of these circuits.

10. In power amps, one of the primary differences is that the output impedance of tube power amps is much higher than of solid state power amps, this is due to the devices themselves, the output transformer losses, and the lower levels of global feedback applied. The tubes themselves (aside from their higher plate impedance) don't behave all that differently than transistors, and in circuits where higher global feedback can be applied, they clip quite similarly to solid state power amps. This is the basis for some of the output emulation circuits used on the various Subway models (damping factor control, symmetry control, soft clip limiting).

11. Many players actually prefer what is commonly associated with solid state amplifiers, so when developing amps to appeal to a wide range of players, implementing "tube emulation" is not an all or nothing exercise. In fact, this is especially true since many tube amps use some techniques to bring them closer to the solid state perception. The SVT is one example of an amp that shares several design similarities with solid state amps, though also shares some similarities with tube amps... reinforcing the fact that it's NOT an all or nothing proposition.

And that's how @agedhorse goes to 11!
Thanks. I thought it very interesting, all the thought that goes into amp design, before solder ever sees heat.

 

[mjmeche]

Also the slight compression with even harmonics slight overdrive as opposed to odd harmonics of solid state.

 

[Slevage]

For me it is a light but noticeable thickening of the notes that make runs sound more fluid and that thickening best translates into a warming of the note. The harshness that many modern amps bring tends to make the tube amp sound even more pleasant to the ear.

When you play tube amps these days, particularly old ones, younger (than me) bass players in particular, notice the different sound immediately and will come and ask questions. I never got that during the transistor or hybrid decades of my playing. When I started playing, everyone used tube amps so there was no discernible difference between us. These days, there definitely is.

Another point is that tube sounds don't demonstrate that accurately on video as no Utube reference demonstrated what standing next to the speaker cabinet will.

If you keep a permanent search on ebay for Marshall Bass amp as I have had for close to 2 decades, it will astound you that an amplifier company with the largest profile and huge amount of heads produced, hardly ever has a single bass amp for sale. Kinda says something.

A Marshall guitar amp with a well designed bass enclosure is killer.

 

[ThisBass]

It’s the even harmonics being added to the signal from driving the tubes.

Generally described as “warm”, “thick”, or “round”.

Nope, there are even and odds added.
The 2nd may be strongest in context, that's all.

 

[mike buzzetti]

I have tube amps that are no where near WARM ! guitar amps , they SCREAM