All said, there is much more difference between individual
designs, whether tube or transistor, than there is between tube
and transistor designs generically. You can make a fine amp
from either, and you can also make a lousy amp from either.
Although tubes and transistors clip differently, clipping
will be rare to nonexistant with a good amp, so this
difference should be moot.
Some people claim that tubes require less or no feedback
while transistor amps require significant feedback. In
practice, all amps require some feedback, be it overall,
local, or just "degeneration". Feedback is essential in
amps because it makes the amp stable with temperature
variations and manufacturable despite component variations.
Feedback has a bad reputation because a badly designed
feedback system can dramatically overshoot or oscillate.
Some older designs used excessive feedback to compensate
for the nonlinearities of lousy circuits. Well designed
feedback amps are stable and have minimal overshoot.
When transistor amps were first produced, they were inferior to
the better tube amps of the day. Designers made lots of mistakes
with the new technologies as they learned. Today, designers
are far more sophisticated and experienced than those of 1960.
Because of low internal capacitances, tube amps have very
linear input characteristics. This makes tube amps easy to
drive and tolerant of higher output-impedance sources, such
as other tube circuits and high-impedance volume controls.
Transistor amps may have higher coupling from input to output
and may have lower input impedance. However, some circuit
techniques reduce these effects. Also, some transistor
amps avoid these problems completely by using good JFET
There is lots of hype out on the subject as well as folklore
and misconceptions. In fact, a good FET designer can make a
great FET amp. A good tube designer can make a great tube amp,
and a good transistor designer can make a great transistor amp.
Many designers mix components to use them as they are best.
As with any other engineering discipline, good amp design
requires a deep understanding of the characteristics of
components, the pitfalls of amp design, the characteristics
of the signal source, the characteristics of the loads, and
the characteristics of the signal itself.
As a side issue, we lack a perfect set of measurements to
grade the quality of an amp. Frequency response, distortion,
and signal-to-noise ratio give hints, but by themselves are
insufficient to rate sound.
Many swear that tubes sound more "tube like" and transistors
sound more "transistor like". Some people add a tube circuit
to their transistor circuits to give some "tube" sound.
Some claim that they have measured a distinct difference between
the distortion characteristics of tube amps and transistor amps.
This may be caused by the output transformer, the transfer
function of the tubes, or the choice of amp topology. Tube amps
rarely have frequency response as flat as the flattest
transistor amps, due to the output transformer. However, the
frequency response of good tube amps is amazingly good.
For more information on tubes, get one of the following old
reference books, or check out audioXpress Magazine (see the
magazine section of the FAQ for more info on audioXpress).
The Receiving Tube Manual (annual up to 1970)
The Radiotron Designers Handbook
Fundamentals of Vacuum Tubes" by Eastman 1937, McGraw-Hill