Solid State History
So what's to stop someone designing a solid state amplifier to work like a valve amplifier? Unfortunately, early attempts are responsible for the appalling reputation solid state amplifiers have in this application. The main objection to these early amplifiers was their harshness and limited dynamic range, caused almost entirely by the super clean sound and wide frequency response up to their maximum output, after which the onset of clipping occurred quite abruptly and harshly.
When the amplifier is played near its maximum output, the attack of notes (immediately after the note is struck) pushes the amplifier briefly into the clipping region, producing a "squashy, spitting" sound at the start of each note. If the amplifier is overdriven well into the clipping region, the type of overdrive was typically very dirty and un-musical.
There was much debate at the time that valve amplifiers produced overdrive with primarily even harmonics (musically related to the fundamental and therefore desirable), while transistor amplifiers produced odd harmonics which do not have a musical relationship to the note(s) played. I don't believe this is entirely valid, because a type of distortion circuit has become popular with heavy metal players in the 80?s (and a variation for grunge players in the 1990's) which replicates the harsh clipping of solid state amplifiers. This type of distortion has been made palatable by following it with some very carefully tuned equalisation to minimise the harsh components, without sacrificing too much high frequency response.
Nevertheless, valve designs produce predominantly low order harmonic overdrive (which is musically related), while solid state designs generally produce a full range of harmonic distortion, including the objectionable high order harmonics. This is due to a number of factors, including the transformer output of valve designs, and the large amounts of negative (electronic) feedback required in solid state designs. Valve designs need very little negative feedback, and some good sounding amps don't use any at all!
Another popular theory of the time was that the difference was due to valves being voltage driven, while transistors are current driven. Hopes were therefore high with the introduction of power FETs, because these are also voltage driven. Some companies produced power FET amplifiers with the usual claims of their likeness to valve sound, and again, failed. These early designs still exhibited the transistor cleanliness with abrupt and harsh clipping.
To my ear, the main undesirable elements of a solid state power amplifier are that there is no high frequency roll-off to remove the harshness of the brutal clipping produced, but more importantly, the onset of distortion is just too sudden. Distortion itself is the addition of higher harmonics not present in the original signal, and both the smoother onset of distortion, and their design means they add different harmonics. Valve amplifiers use an output transformer which results in the lower order harmonics being predominant in the early phases of overdrive, giving the characteristic smooth overdrive. More recent solid state power amplifiers have begun to address these design goals with some success by using volume limiting, and flexible preamplifier overdrive circuits. Marshall Valvestate and Peavey Transtube offer some very valve-like dynamics with their solid state power amplifier designs.
One option is to use a very powerful solid state amplifier and speaker system, and produce the required sounds entirely within the preamplifier stages. Indeed, this approach is favoured by some heavy metal players, and for related styles.
Typical Designs
Most designs now (valve and solid state) allow the preamplifier to be overdriven, with a master volume level to control the level produced through the power amplifier. This allows the player to use a variety of clean to overdriven sounds at various volume levels. Generally, the player will establish a fixed master volume level to suit the performing situation, and footswitch additional preamplifier overdrive as required. It is common to provide gain in the hundreds, just within the pre-amp stages!
