Solid state vs Tube

This debate is very, very old. In the beginning there were vacuum tubes. These electronic devices were used to build audio amplifiers. When the electric guitar was invented, naturally , tubes were used to build amplifiers for the guitar signal. In the 1950's, the transistor was invented. It took many years for transistors to percolate up to guitar amplification, but by the 70's ,  transistor based amplifiers were common. However, there was a complaint coming from the musicians. They claimed that the transistor amps "had no tone", "lacked  warmth", or just plain "sounded shitty". Audio engineers would look at the specifications and scratch their heads. "What are these long hairs talking about.". The THD is low, the amplification head room excellent, the frequency response stellar  etc.  However, the musicians were adamant that tubes sounded better.  So much so that they continued to buy tube amps even as they climbed in price as tubes were becoming obsolete for every other electronic application. Was this just a case of being nostalgic for old technology? That might account for some of it, but when musicians are willing to pay 3 times as much for the same audio power if it is delivered by tubes, then there has to be more to it. And it turns out, there is. Engineers tend to focus on the linear models used for amplifier design. Gain and Bandwidth are the primary linear modeling parameters for an  audio amplifier. Distortion is characterized by how much is produced via the 'Total Harmonic Distortion' spec, but the spectral composition of the distortion  is not considered with THD, only the  total spectral power generated by non-linear amplification. The reason is simple, distortion is bad, and when designing an audio amplifier you want to minimize it. However, guitar amplifiers are not used as linear amplifiers. They are often driven into deep non-linearity by the guitar player to generate the characteristic "fuzz" of rock guitar. So the gap was that standard amplifier design methodology did not consider in detail the spectral composition of harmonic distortion. When they did, they found out one key difference between the transistor and tube amplifiers. Tubes tend to produce even order harmonic distortion and transistors produce more odd order harmonics. This means that if the string frequency is f, a tube amp will produce overtones at 2f,4f... Where as a transistor amp will produce 3f,5f. Even order harmonics are musical  and pleasing to the ear. Odd orders sound dissonant. This was the first breakthrough at understanding something that was only qualitative before.  The even order harmonic generation by tubes occurred because tubes are what are called a "square law" devices. The tube transconductance is expressed approximately as a I=K*V^2.  Transistors on the other hand were exponential.  It turns out that a certain class of transistors had square law behavior. These were MOSFETS. Guitar amp manufacturers started designing transistor based amps with MOSFETS and achieved some pretty good results. The day of the "buzzy" transistor amp was gone. Still musicians weren't satisfied and pros continued to buy tube amps. Amateurs now had a lower cost alternative that produced fairly good overdriven tone. In the end, it would not be possible to completely replicate the dynamic characteristics of vacuum tubes. In addition, some of the effects produced by tube amps  resulted from the high output impedance tube amps connected to a low impedance speaker via a transformer. The dynamic characteristics of the transformer also helped shape the sound due to a phenomenon called core saturation. Sharp transients into the magnetic transformer would be softened by the magnetic saturation of the transformer core. There are many more differences such as open loop design vs negative feedback. But the point is that trying to make a transistor based circuit sound exactly like a tube amp just isn't possible. So in the end, replication of tube amp sound by solid state electronics would not happen until DSP based amp modeling became available. Here a digital model of the tube amp and speaker produce very accurate sound.   So to conclude, if you are a gigging musician, what type of amp should you use? If you are playing solo or at relatively low volume, use a solid state amp with an amp modeler. Using a solid state amp has many advantages : they weigh less, are more reliable and also more durable. The amp modelers are so good now that you can get any guitar sound you want. If you are a rock band operating at high volume you still need tube amps. Amp modeling won't work  because if you are already simulating an overdriven stack you can't run that through an overdriven stack!  In the studio, there is no competition. Amp modeling all the way.