I always get funny looks around here because I tend to conventrate on a device's behaviour many times, as usually the others hae covered the freq response and such, or sometimes behaviour is a different angle that bears looking at. Both tuebs and transistors are capable of perfectly good tones, depending on the application. Almost all of the legendary mic pre's and compressors are transistor, with
the LA-2A being a common tube exception. While almost any decent distorted guitar tone starts with a tube preamp.
"solid state =shit tone
tube =killer tone
sheppard "
pretty general, and generally crap
An important bit to look at is how these devices ACT. To simplify, to WAY oversimplify, and this is not exactly 100 % accurate always but a decent way to look at for a start:
tubes are controlled by voltage. Your guitar sends out varying amounts of voltage depending on how hard you whack your strings. This is why even with quite a bit of distortion, hitting your strings harder on a tube preamp, will make a louder sound, and even with insane amounts of distortion hitting harder will make a DIFFERENT sound than hitting soft. Transistor guitar PREamps tend not to do this. This is because transistors are controlled by current(usually)
Tubes are many times configured to act in a NON linear way. what goes in is intentionally not what comes out. As levels get higher going into a tube stage, it can easily exhibit its own form of compression, and many times this is what causes " warm " and " fat ". It can also be distorted intentionally. A widely held notion is that tubes will create even order harmonics, while transistors create odd order harmonics. When distorted, tubes many times make a somewhat rounded wave, transistors often make nasty ass square waves.
But its 6 of one half a dozen of another. One reason transistors make square waves is that they can switch or cycle way faster than tubes. This means that transistors on a given similar circuit design can pass more, or more accurate or higher hi frequencies than a tube can. Of course many will say tubes kill that " harshness", while others will say " tubes make it muddy"
its a matter of degrees and taste.
If a device is meant to be accurate and respond in a LINEAR way, often transistors are better. Say you have an instrument that you LOVE the way it sounds as is, and also you want the highest dynamic range possible, no interest in compression or gain control besides what your fingers do. Good candidate for a transistor. Or you want to capture hi's into the dog ear world, another good candidate for transistors.
TRUE, well designed tube circuits can ADD to what you are doing, controlling gain a little and fattening up with nice smooth distortion, and rich harmonics
the trouble today is so many manufacturers caught on to the tube trend and just stick glowing bottles onto gear that they made long ago, using a full on solid state circuit path. In these devices a tube is usually nothing more than a heat an light source and may actually harm the circuit and the sound much more than if it had been ommitted
just cuz something has a "toob " in it dont mean its gonna sound good!