A lot of audiophiles (myself included) are in love with
due to their superior sonic properties. What makes tubes so special,
anyway? People have been trying to figure that out for years.
Personally, I don't think it's any one specific thing. Although tube
amps figuratively fall of the test bench when it comes to hard
measurements, people hardly listen to white noise or square waves in
real life (well, unless they're autistic or something). The attributes
of tube sound we find most pleasant are clarity or resolution of fine
detail, and freedom from "harshness" or "grain".
In terms of resolution I think that there are two
factors, one being high voltage, the other being space charge. Tubes
operate at much higher voltages, up to 600V for a power amp, compared
to transistors, which don't usually go beyond 48V in that application.
I believe higher voltages in an amp reduce the resistance relative to
the potential in things like sockets and hookup wires, and allow
capacitors to charge and discharge more quickly. Speaking of
capacitors, those rated for higher voltage are generally more well
constructed than their low voltage counterparts. The other contribution
is from the "space charge". The hot cathode of a tube is surrounded by
a cloud of free electrons. This space charge provides a pool of
instantly available electrons, smoothing out the supply and providing
extra power for rapid transients in excess of what the power supply by
itself would be able to deliver.
There are a few likely candidates for the reduction in
provided by tubes. One of the most popular ideas is that tubes don't
clip the same way as transistors. Clipping is where a signal is
overdriven past the point that the power supply would be able to
handle. Tubes are usually operated in a way that minimizes clipping,
this is mainly to protect the cathode from "stripping", but has the
fortunate side effect of minimizing their ability to clip large signal
transients. When a tube does clip, it doesn't clip abruptly. When a
transistor amp clips a signal it tends to literally cut off the top of
the wave, while a tube amp would round it off, which more closely
approximates an undistorted signal.
Tube amps typically produce more total harmonic
(THD). While I've seen transistor amps with THD as low as 0.001%, there
are some highly regarded tube amps (single-ended triode, mostly) that
have up to 5% THD. This is counter-intuitive, since low THD is widely
considered to be a good thing. Some people think that the (over)
application of negative feedback, which is used to reduce THD can lead
to a very dry, artificial sounding amplifier.
But what about tube rectifiers? Some maintain that tube
rectification makes a difference in the sound of an amplifier.
Personally, I think this is only evident in systems with very simple
power supplies. Tube rectifiers do have less switching noise than a
solid state diode, and may help to filter out some external powerline
noise. I haven't really made up my mind about this yet, but I do use
tube rectifiers in my projects, mainly because it's cheaper and easier
than wiring up timers and relays for a soft start.