Most tube amps exhibit relatively high second-order distortion (and other even order products) that may sound "richer" or "fuller" compared to SS amps that have lower overall distortion. A minor quibble with Ralph: in addition to distortion, output impedance is IME a big factor in the difference between SS and tube amplifiers. The high output impedance of most tube amplifiers means they are more sensitive to the load and thus tend to exhibit greater differences in frequency response than SS amps (very general statements, quite sure to be untrue for specific amps of either flavor). Again IME amp/speaker matching is more critical with tube amps than SS amps (again, a line of exceptions is sure to form).
One thing I have long suspected is that intermodulation distortion is part of the reason folk prefer tubes. SS usually has lower distortion, but it tends to be odd-order by design, whereas tube circuit tend toward single-ended designs that are dominated by even order harmonics. Even-order intermodulation distortion (IMD), created when two or more signals are present, falls at frequencies far removed from the two original tones (roughly DC and twice their frequency assuming the two tones are closely spaced). In contrast, odd-order IMD appears very near the original tones, at frequencies related to the difference between the tones, so adds non-harmonic distortion close to the original tones. Close enough, and it is masked as Ralph said, but often enough is not close enough and creates a dissonance to the sound. Feedback solves that problem, but again as Ralph said it takes careful design to make that work.
@Gregadd : Not trolling at all! But my answer isn't likely to help, still wishy-washy... Richer/fuller compared to SS is what I think I meant (been a while since that exchange). Having played with bands and in an orchestra for many years, I feel it is almost impossible to know how closely a recording matches the original. A tympani can be tuned loosely or tight, a piano is usually "stretched" in tuning and how the string groups are tuned varies with the tuner and desires of the artist, different reeds sound different, brass players can create a huge range of sounds depending upon their desires (plus instrument, mouthpiece, mutes, etc.), and so forth. Then there is the impact of the venue (room), mic technique, etc. So I tend to listen for what I like rather than thinking how how true to the source it is, unless it is grossly "off".
The low damping factor (high output impedance) of many tube amps tends to allow the bass drivers to "ring" a little and that causes a little "bloom" to the sound, a little boost in the mid and lower bass. It actually sounds good to most folk, myself included, but is not accurate to the recording. Again in my experience, and what I have measured, which does not include an Atma-Sphere amplifier nor really any modern tube amplifier on a system and in a room I know. And again IME "accurate" is not always what audiophiles seek; there is room for preference. There are things I miss about my old tube amps, knowing they are not as accurate as my SS amps, but I can enjoy them all.
FWIWFM/HTH/IME/IMO/my 0.000001 cent (microcent) - Don
One thing I have long suspected is that intermodulation distortion is part of the reason folk prefer tubes. SS usually has lower distortion, but it tends to be odd-order by design, whereas tube circuit tend toward single-ended designs that are dominated by even order harmonics. Even-order intermodulation distortion (IMD), created when two or more signals are present, falls at frequencies far removed from the two original tones (roughly DC and twice their frequency assuming the two tones are closely spaced). In contrast, odd-order IMD appears very near the original tones, at frequencies related to the difference between the tones, so adds non-harmonic distortion close to the original tones. Close enough, and it is masked as Ralph said, but often enough is not close enough and creates a dissonance to the sound. Feedback solves that problem, but again as Ralph said it takes careful design to make that work.
@Gregadd : Not trolling at all! But my answer isn't likely to help, still wishy-washy... Richer/fuller compared to SS is what I think I meant (been a while since that exchange). Having played with bands and in an orchestra for many years, I feel it is almost impossible to know how closely a recording matches the original. A tympani can be tuned loosely or tight, a piano is usually "stretched" in tuning and how the string groups are tuned varies with the tuner and desires of the artist, different reeds sound different, brass players can create a huge range of sounds depending upon their desires (plus instrument, mouthpiece, mutes, etc.), and so forth. Then there is the impact of the venue (room), mic technique, etc. So I tend to listen for what I like rather than thinking how how true to the source it is, unless it is grossly "off".
The low damping factor (high output impedance) of many tube amps tends to allow the bass drivers to "ring" a little and that causes a little "bloom" to the sound, a little boost in the mid and lower bass. It actually sounds good to most folk, myself included, but is not accurate to the recording. Again in my experience, and what I have measured, which does not include an Atma-Sphere amplifier nor really any modern tube amplifier on a system and in a room I know. And again IME "accurate" is not always what audiophiles seek; there is room for preference. There are things I miss about my old tube amps, knowing they are not as accurate as my SS amps, but I can enjoy them all.
FWIWFM/HTH/IME/IMO/my 0.000001 cent (microcent) - Don