What, measurement wise, is "tube sound?"

[Jim Shaw]

I'm looking for factual sources of information that attempt to objectively characterize "tube sound."

There is no end to internet and print pundits that try to subjectively characterize it. I'm not interested in what you or they think you heard. At least, not until I learn more about what tests and measurements data and findings show.

Anyone out there know some papers or written pieces that cover this? I'm no longer a member of the AES, so I can't just go digging around in their shorts to study the topic. I'd prefer peer reviewed papers, but generally trusted ones will help too. If members here can perhaps forward one PDF copy (or such) of such a tome, I would be pleased. I will be discrete about republishing or distributing it.

I'm not afraid of doing some reading. I can get through reading at the level of Toole, et al. My 2nd order differential equations and transforms faculties have been somewhat transformed by the ravages of time.

TIA and regards,

 

[Jim Matthews]

I've heard OTL designs that sounded different from any Williamson derived amp or my favorite Direct Heated Triode amps. I suspect what we refer to as "tube sound" is the result of output transformers in the circuit path.

Good tube amps have "big iron" that doesn't readily saturate during voltage swings and responds quickly without excess hysteresis distortion.

In my opinion the best Class D amplifiers employ similar filtration to audibly pleasant effect.

https://purifi-audio.com/2020/04/28/dist/

 

[DonH56]

The usual suspects are output impedance, discussed in the link below, the generally even-order harmonic structure (inaudible in a decent design until near clipping), and softer saturation (again near clipping). Sometimes higher noise and microphonics contribute. IME output impedance is the biggest player for a competent amp. Partly driven by the intentional design choice to use limited or "no" feedback in tube amplifiers, probably to generate that "tube sound".

Since you mentioned dif eq, I'll note the Maclaurin expansion for a single-ended tube's harmonic series is factorial, whilst for a BJT it is exponential, so at the device level a tube should have lower distortion than a transistor. But feedback cancels all... And transistor designs are more likely to be differential, biased differently, exhibit lower output impedance, and of course don't have output transformer saturation to worry about (but plenty of other things).

But aside from the output impedance interacting with the speakers (which is of course load dependent) much of the "tube sound" is a myth IME/IMO. Or the result of intentional design decisions to "voice" the amplifier.

FWIWFM - Don

Amplifier Output Impedance (Damping Factor) and Speakers
https://www.audiosciencereview.com/...amping-factor-and-speakers.23968/#post-807327

 

[dc655321]

I'm only half joking when I say that a fMRI may be the best tool to answer this question

 

[Wes]

as for sources, try to look thru the Bell labs archives, or mebe Western Electric

 

[Newman]

I'm looking for factual sources of information that attempt to objectively characterize "tube sound."

Don’t we first have to subjectively determine that “tube sound” even exists? And that means DBT, not “every time I hear a tube amp it has this characteristic, um, sound.”

I say this because IIRC back in the day (see, I do have something in common with you, Jim), amp designers saying it is trivially easy to build a tube amp indistinguishable from good SS amps, simply by using sufficient feedback intelligently and using it with a speaker that presents a friendly load, and keeping both amps away from clipping.

I see I am agreeing with @DonH56 … it’s probably a myth that wouldn’t survive controlled listening.

My 2nd order differential equations and transforms faculties have been somewhat transformed by the ravages of time.

Hey, I resemble those faculties…. and what happened to them!

 

[SIY]

There is no such thing. When asking people to define it specifically, one hears “it’s bright, forward, and alive” and “it’s mellow, laid back, and soft.” Ahhh, ok.

There’s frequency response differences having nothing to do with active device choice. In edge cases, there’s gross distortion which again is achievable with any choice of active device. The only thing unique to tubes, and only certain ones, is acoustic feedback from rattling filamentary cathodes.

 

[MakeMineVinyl]

I have a feeling the members of this forum are generally not the target market for tube amplifiers, so whether or not they have a particular 'sound' is a rather pointless academic argument. People who want tube amplifiers I would wager don't give one whit about academic discussions and will simply buy them for reasons that mean something to them, or as in my case, solves a particular technical problem and/or they like that tubes simply look more interesting.

 

[SIY]

For a different take on the subject, look up the Tube Cad Journal. Lots of information there.
Jim

John is an old buddy of mine and does fine work.

 

[Ron Texas]

Extreme high and low frequencies in the audible range roll off. Lots of 2nd order harmonic distortion, high noise floor.

 

[DSJR]

I don't mean to step on SIY's toes here but I think the popular valve/tube amps are anything but state of the art. E.A.R amps were designed by a master, but I think he deliberatelky tailored in a soft warm tone in most of his company's products (his work for Musical Fidelity years ago could be anything they wanted). I think other popular valve/tube amp makers design loadsof distortion in because that's what the punters want.

Just my take, but people here in the UK seem to like (used usually) low powered soft clippers with high output impedance (the latter alone will screw with the speaker response) and plenty of second and third order harmonic distortion that may well add a kind of pleasant creamy 'fog' to the tone. Add to this low bass transformer saturation and there you have it.

 

[SIY]

I don't mean to step on SIY's toes here but I think the popular valve/tube amps are anything but state of the art. E.A.R amps were designed by a master, but I think he deliberatelky tailored in a soft warm tone in most of his company's products (his work for Musical Fidelity years ago could be anything they wanted). I think other popular valve/tube amp makers design loadsof distortion in because that's what the punters want.

Just my take, but people here in the UK seem to like (used usually) low powered soft clippers with high output impedance (the latter alone will screw with the speaker response) and plenty of second and third order harmonic distortion that may well add a kind of pleasant creamy 'fog' to the tone. Add to this low bass transformer saturation and there you have it.

Funny you should mention EAR. I knew Tim, well enough that he critiqued my designs, and his engineering was creative and very much directed at neutrality.

I’d like to see the claim of brand-based “warm sound” demonstrated instead of rumored or asserted.

 

[Blumlein 88]

Funny you should mention EAR. I knew Tim, well enough that he critiqued my designs, and his engineering was creative and very much directed at neutrality.

I’d like to see the claim of brand-based “warm sound” demonstrated instead of rumored or asserted.

I seem to recall an interview where Tim said he could get the sound he wanted with SS or tube.

 

[SIY]

I seem to recall an interview where Tim said he could get the sound he wanted with SS or tube.

Yes, my point exactly.

 

[Mountain Goat]

I love tube amps. I've got one, hooked to 4 10-inch speakers. I remember every stain on it:

 

[Wes]

There is no such thing. When asking people to define it specifically, one hears “it’s bright, forward, and alive” and “it’s mellow, laid back, and soft.” Ahhh, ok.

There’s frequency response differences having nothing to do with active device choice. In edge cases, there’s gross distortion which again is achievable with any choice of active device. The only thing unique to tubes, and only certain ones, is acoustic feedback from rattling filamentary cathodes.

you forgot "a sense of liquidity in the muzak"


However, the fact that a designer can achieve the same sound qualities via SS or tubes does not mean that there is no separation of most SS designs from most tube designs. It simply means that a principal components analysis would show some overlap of the two clouds. An example is "men are taller than women."

 

[Helicopter]

I call it the 'tube amp sound.' Tubes don't actually have to do anything audibly different, but they are often used in circuits that have the colorations Texas Ron mentioned above. This could be due to lack of feedback, output transformers, etc.

 

[MrPeabody]

The problem with the notion of "tube sound" is that the notion implies that essentially ALL tube amplifiers sound similar to one another but different from solid state amps. If this were the case, the cause should have been identified decades ago, and everyone would know what it is and why it is something that doesn't occur with solid state amps and why it cannot be avoided with tube amps.

There possibly exists a plurality of tube sounds, each associated with one of the potential causes that have been identified, singularly or in combination. If there is such a plurality, there is no "tube sound" per se.

 

[MrPeabody]

you forgot "a sense of liquidity in the muzak"


However, the fact that a designer can achieve the same sound qualities via SS or tubes does not mean that there is no separation of most SS designs from most tube designs. It simply means that a principal components analysis would show some overlap of the two clouds. An example is "men are taller than women."

I'm not sure whether I've ever heard anyone say exactly that. Perhaps, "On average, men are taller than women." More to the point, your example is concerned with a particular and exact attribute, height. Perhaps it would make sense to speak of tube sound if there were a fuzzy blending with overlap, with respect to some very specific sonic attribute, but only if the sonic attribute is very specific and very well defined. And only if it has been properly established that this very particular sonic attribute is in fact the attribute by which people distinguish the sound of tube amplifiers from the sound of solid state amplifiers.

 

[Blumlein 88]

Just anecdotally, I've thought the sound of push pull tube amps is in the transformers mostly. I've had a few tube pre's back when people had the idea you could get much of tube sound by using a tube pre into a ss power amp. I didn't find it to be the case. Good tube preamps didn't really have a sound. Most put out more voltage than necessary, and sometimes adjusting gain was a good step, but they were not noticeably different than good clean SS or FET preamps.

Now I've done some careful comparisons of tube power amps, but all sighted. Discount that as you see fit, but I found Ultralinear tube amps to have a similar and non-transparent sound, triode push-pulls had a sound (which was my favorite) and SET's have a sound (though these varied more than the the others). It was most apparent when I did series tests where I put a power amp with loaded output between the source and the power amps driving the speakers. I could then switch the power amp under test in and out of the circuit. I found there were FR differences, but also they seemed to create a 3d space, good ones seemed smoother, and usually seemed to punch up subjective dynamics. The FR differences were real and easily measured. The creation of space, smoothness and dynamics not so easily measured. Other gear with transformers often seems to have a smoothness, and ability to add some spaciousness to sound. Not always does it add the dynamics, so maybe more than just the transformers did that or maybe at power amp levels where saturation can be an issue it creates false dynamics. I've thought a simple push-pull triode preamp using 12ax7s or 12at7's which fed a small output transformer might be a good thing to do. Basically a mini push pull triode amp to pre-color the signal fed onto your power amps. Tim's E.A.R. preamps used transformer coupling at the output.

What I found is that good SS amps can fully and faithfully re-create all that spacious, dynamic and smooth sound of a triode power amp. Because if you put a triode amp in front of the SS amp you get the full triode sound.

But none of this answers definitively the OP's question.