What determines good distortion?

I have a friend using an Audio Research CA 50 integrated amp with 45 watts/channel into Vandersteen 2ce sig II. I use a 50 watt YBA integrated into the same speakers. We both listen at sane levels in small rooms 8 x 12. He thinks that it's better to use a 50 watt tube amp rather than a 50 watt SS amp because tubes when they distort sound more pleasant. I'm thinking that if you drive the amp into clipping it's bad with either a SS or tube amp because clipping distortion is bad whether or not you can tolerate it. Am I wrong?
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This is a really old question.
SS amps tend to clip 'hard'. They really have a flat top to the clip waveform, and thus create really strong high frequency output when clipping. (what that is, and means to a layman is too complicated for me to enlighten you, get a book on amplifier disign)
A tube amp clipping creates rather 'soft' rounded clips,(way less high frequency junk being created) and thus are gentler on speakers.
So, in general, the tube clipping is way more survivable, and rather 'easy' on both speakers and ears.
The SS amp will clip and sound grating and harsh, besides killing your tweeters.

So neither of you are 'wrong' You just have different experiences.
Yes clipping is bad. Yes a tube amp will be able to clip and be less problematic for both hearing, and tweeters.
So you are both right.
And he may just like the sound of tubes... which is his right in general.. Then you have those who say 50 tube watts are 'bigger' than 50 SS watts...

But I bet neither of you usually run the amp so hard they produce clipping anyway. So why worry?
Some solid state amps have clip protection. Such amps produce a waveform smoothly limited in the same way as a tube amp. Also, mine is good for 600 watts, so I don't think I need to worry.
What I've always heard is that SS produces odd order harmonic distortions (3rd,5th, etc) while tubes produce even order distortions (2nd, 4th, etc). I've also heard that musical instruments produce even order harmonic distortions, which is why most folks find tube equipment more "musical" or "natural", because it's distortions are more similar to that heard from a musical instrument.
J, I think you are mistaking tube for single ended and ss for ultralinear. An old Pass Aleph does not distroy even order harmonics like an old ARC push pull.
You will hear a lot about ss sound and distortions vs tube sound and distortions. First, eliminate the stupid. A well designed and built amp will be wonderful, regardless of whether it is tube or ss. As Jmcgrogan2 stated, ss designs "typically" produce odd order harmonic distortions, where tubes produce even order distortions. This is only true so far as how the amps were designed. circuit designs can correct for distortion effects. it totally depends on how much effort and money the designers placed into their designs. So, please do not buy into the ss sound one way and tubes sound another way. you can have a tube design that is basically crappy circuit design, same as with ss. So, it is a combination of the actual circuit designs (regardless of whether it is tube or ss) and then you add in whether they "corrected" for particular distortion effects in the design. I have heard (within specific price points) ss amps that were much better than tube amps and visa versa (within the price points). At the upper end of the price range, i.e. rediculously priced amps, I would take either one. The ss amps sounded wonderful and so did the tube amps. One is definitely not better than the other. It depends on price, circuit designs and other factors. Please don't buy into the tube is better than solid state or visa versa arguments. Pick a price point, then go demo in your home, using your equipment the various amps and see/hear for yourself. depending on the amps, you will probably pick a ss amp and in other cases, the tube amp will make you smile.

stay away from the hype.

Jmcgrogan2 is correct about odd and even order harmonic distortions. The best explanation I've read to explain the difference is this:
imagine you are listenning a sollo singer - this is your base sound. Now imagine you are listenning to a chorus - thouse are all your harmonic distortions. If every singer in the chorus sings in cynch (even order harmonic distortions) the sound, even though is different from a solo singer, is pleasant. However, if anybody in the chorus is singing out of cynch (odd order harmonic distortions) the sound becomes unpleasant.
Lewhite is correct, I was confused. It's single-ended amps that produce the even order harmonic distortion versus push-pull amps which produce odd order harmonic distortion. I think my confusion is because MOST single ended amps are of the tube variety. However, yes, the Pass Aleph series are single ended solid state amps.
For me, distortion is distortion. It is never good. Some are just less offensive than others. SOme might even sound pleasant. But look up the definition of distortion.

If your goal is to reproduce the original as accurately as possible, it is NEVER good.

If your goal is to make the original better in some way that you desire (although concurrently less accurate), then some distortions might accomplish that. Distortions resulting from soft clipping versus hard clipping is a common and valid audio example.

Note that soft clipping is more common but not unique to tube amps and hard clipping more with SS amps, but some SS amps may also soft clip though I am not sure if any tube amps hard clip?

Better to be safe and avoid clipping altogether by not skimping on the power needs of your speakers in your room. Effects of clipping can be subtle and not easily recognized way before the point comes where you can clearly hear it! THis is a very common scenario that affects many unknowing listeners out there who cannot understand why their system does not sound as good as it should.

I'm sure there are some things that could be done to even the Mona Lisa to make it more appealing to some, but that would result in a reproduction that is distorted in some way but for good reason perhaps. Would that be wrong?
I'm sure there are some things that could be done to even the Mona Lisa to make it more appealing to some, but that would result in a reproduction that is distorted in some way but for good reason perhaps. Would that be wrong?
given the contents of your post, it would be wrong for you (& probably me as well 'cuz I agree with you that distortion is never even if it sounds good to one's ears) but would be right for many others.
Audio is so subjective & it depends on what the listener's objective is that there is no straight answer to your question. (of course, you already knew that! ;-) ).

I'm with you.

Distortion, whether good or bad, is determined in the head of the listener! So it's the listener who makes the only meaningful determination in the end.

Avoid clipping distortions though always! Oveer the long term at least, I doubt anyone with good hearing would prefer even subtly clipped sound using any amp technology to the alternative.
Distortion is distortion. When you hear or/and feel it, it's bad. Whether it is tube or ss doing it, I have low tolerance.
The odd-ordered harmonics (above the 3rd) are used by the human ear/brain system to ascertain the volume of the sound being heard. This is pretty important to know if you want your system to sound like real music rather than a hifi.

If an amplifier has troubles with this, there will be two results- it will sound louder than it really is, and it will sound brighter than the music really is. All human ears are very sensitive to this!

The ear hears harmonic distortion as tonality. Electronics can have the fault of being overly 'warm' in sound, which is caused by the 2nd, 3rd and 4th harmonics. Quite often tubes get taken to task over this, but that really has more to do with the topology rather than the circuit being tube or transistor. If for example the tube circuit is fully differential in design, there will be no even ordered harmonics and so the circuit will have a more neutral presentation.

Conversely you can give transistor circuits a richer sound by building them single-ended- this will result in more of the lower-ordered harmonics.

Linearity of the devices themselves has a big effect on the sound of the circuit as well. The simple fact of the matter is triode vacuum tubes are the most linear form of amplification known. Additionally, it is easily demonstrated that even the most pedestrian tube amplifier will make less odd ordered harmonics than any transistor amplifier; a sine wave generator and an oscilloscope are all that's needed to demonstrate this.

This is why tubes are still around half a century after being declared obsolete.
"This is why tubes are still around half a century after being declared obsolete. "

Atmasphere, I often wonder what the SOTA is in tubes themselves these days?

Are NOS tubes still the SOTA or has tube production technology advanced at all since the heyday of tubes in general 40+ years ago?

I've noticed prices of new tubes seem to be going up. Is that negatively impacting the value proposition?

"Additionally, it is easily demonstrated that even the most pedestrian tube amplifier will make less odd ordered harmonics than any transistor amplifier; a sine wave generator and an oscilloscope are all that's needed to demonstrate this."

OK, if it is so easy, can you demonstrate that please? WHich amps specifically would you select to do that?

ALso remember demonstrating something does not prove it. Other demonstrations with different amps could have different results.

Even if true, does that mean any tube amp is better than any SS amp? I doubt that!

I like tubes in general as much as the next guy, but just trying to keep things in perspective and avoid people jumping to conclusions based on partial facts.
Mapman, the test is simple- put the amp on a speaker load (nearly any load will do) and run the amp up to clipping with the sine wave, and observe the results on a 'scope.

Of course others have already done that. Goto Google, click 'Images' then enter:

tube clipping characteristic

The first two images will show you the difference- the transistor characteristic being the first image. Note the squared-off waveform of the transistor test- as if someone cut off the tops and bottoms of the waveform with a knife. These sharp corners are evidence of odd ordered harmonics. This test is common to all transistor amps.

The tube amp has rounded corners- less odd-ordered and lower orders as well. This characteristic is common to all tube amps.

Now this test is at clipping but this difference between tubes and transistors has been quite well documented in the last 40 years and really isn't a topic of debate as it is so readily measured and heard. It is why nearly all guitar players use tube amp, BTW.

However, almost any audiophile will correctly argue that we don't listen to amplifiers at clipping. And I agree with you in your surmise in the second to last paragraph of your post. A cheap tube amp with lousy transformers and the like is still a crappy amp.

However, the fact of the odd-ordered harmonic distortion issue will not go away despite all this. All it says though that a transistor amplifier will sound harsh compared to a tube amp **generally speaking** (and with rare exception...).

A lot of people are OK with that, thinking that they can use a synergistic approach to deal with the harshness. But look back at my original post here- I specified the difference between sounding like *music* or sounding like a *good hifi*. Its that nuance, that nth degree, that I am talking about- not the ability to weld with the amp or the like.
YEs, I will chose to avoid clipping all together, thanks.

Easier to do with modern speakers these days with smaller and more efficient switching amp technology. These are game changers in the SS versus tube amp debate IMHO.

If my choice were limited to massive monster SS amp or tubes, I might chose the tubes as the lesser of two evils. Most likely I would go to high eff.speakers to avoid all the amp pain incurred with any large hot running, and heavy amp.

OF course some do not like the sound of switching amps they have heard either for whatever reason. Class A SS or bust! OR maybe tubes?

If you are going to clip, better off with a tube amp in general I would agree, though some SS amps claim soft clipping characteristics similar to tube amps though no the same.

I'm satisfied with the softening of the tube amp claim to accept that not all tube amps are inherently superior to any SS.

Tubes in the phono and line level devices is as far as I will go. What's involved to keep a tube power amp especially a large one running properly does not appeal to me personally, but may be the best solution for some.