I do personally avoid tubes in the amplification part [preference] but I do love tubes in other parts of the system. I.E. a tubed vinyl pre, a tubed CDP and tubes in line with other digital sources. How might this type of combination relate with what you mentioned?
I harness and avoid.
Tom
Tubes in the phono preamp is amplification. The main thing we are interested in is linearity. Tubes in signal applications can be quite low distortion so I advocate their use, especially since they often lend themselves to simpler circuits.
I've seen the math for this and its only for a limited range of amounts of feedback that the higher harmonics increase. I think there's a paper by Bruno Putzeys where he covers this, but he didn't originate the math. In my understanding most SS amps use more than 20dB of feedback and if I remember correctly that's about the amount where the higher harmonics start going down again.
I haven't found this to be so in my experience. I start out with SS amps which sound bright and harsh, modify them for appropriate noise control and they then sound smooth and non-fatiguing. I doubt very much that their harmonic distortion spectrum has changed much, although I haven't measured. The harshness I associated with transistors (particularly low-noise bipolar opamps, the LM4562 is one of the worst) is I believe related to the input stage's sensitivity to low-level RF signals. Deft use of ferrite beads, common-mode chokes, attention to PSU cleanliness and star grounding removes it to my satisfaction. The LM4562 has practically immeasurable distortion according to its datasheet yet still sounds harsh and fatiguing without these kinds of countermeasures being applied.
<edit> I realized I'd like to add that in my estimation the reason that SS amps are harsh and fatiguing is that's the sound that customers want. We discussed this on the recent RMAF thread - people who want smooth sound buy valve amps, in general. I'm something of an exception, preferring the low maintenance, weight and better energy efficiency of transistors so I design my own stuff.
I have seen people that don't like smooth sound- but in every case it was someone who had no intention of spending more than a few hundred dollars on the entire system, and also seemed to lack a lot of listening discernment. I think we often deal with this at the entry level- a public so used to seeing cheap equipment described as state of the art that they believe that rather than their own ears.
However it is a fact that our ears use odd ordered harmonics to determine the sound pressure of a sound and in the 21st century this should not be a matter of debate (its been known since the 1960s at the very least). It is also easily proven if one has access to a sine/square generator, an amplifier, a speaker and a VU meter. I can give the entire procedure if interested.
It is also true that at about 20 db on many amps, the odd-ordered harmonic distortion does indeed start to 'go back down'. However this is a very different comment from 'suddenly is totally gone' if you get my point. Nelson Pass has a wonderful article on the subject on his website- what you see is that you keep adding feedback and the odd orders do decrease from some peak value, but never to a point less than the original signal. Long before that you run out of gain, and so have to add another gain stage, which will require more feedback- ultimately you wind up always behind the goal.
It is true that there are other things that will cause an amplifier to sound smoother as you mention. Keeping an amplifier immune to RF difficulties, whether tube or transistor, is always good practice. But compared side by side the smoothest of the best transistor amps I have heard still have an unnatural hardness that the better tubes always seem to lack. I should point out that while I work with tubes, I am not of the opinion that is *has* to be done with tubes, only that its a hellava lot easier to do a competent design with low odd-ordered harmonic generation using them.
Its not rocket science to put the two concepts together; if you follow the data points a picture emerges of what has to happen (build an amplifier with low odd-ordered harmonic generation = more musical, more relaxed). I often hear about how tubes are all colored because they have a 2nd harmonic distortion, which is often true. But that can be eliminated by the use of fully differential operation, the same way it often is in transistor designs. While I have yet to hear a transistor amp that lacks odd orders in the way that tubes amps often do, that is not the same as saying that I don't think it can happen.