Hi All,
In very basic terms:
Valves are high impedance voltage devices, Transistors are low impedance current devices, speakers are low impedance devices, valves therefore need some form of converting their high impedance to low, enough to drive a basic speaker impedance [ say 8 ohm] That’s where the output transformer comes in. Many would state that that is the main weak point of a basic valve amplifier, and to a point, I agree.
Now, if we look at Transistors, as stated, they are very low impedance compared with valves, and therefore an output transformer in most cases is not needed, [Mcintosh Amplifiers being amongst the few that still used output transformers in their solid state designs] Now it would seem from the examples given that transistors have the upper hand, however; this would only be true if the load they were driving remained constant.
Unfortunately that is far from the case where loudspeakers are concerned, i.e.; they may have a nominal impedance to start with, [4, 8, 16, etc] however; that can change quite dramatically once you put frequencies through them, and if we go back to the comparisons, when a speaker impedance falls [usually with low frequencies] the transistor output stage will put more power into the load, but as the speaker impedance rises, the power output will decrease, with a valve amp, with very minimal feedback, the opposite is the case.
Damping factor is another aspect of superiority grossly over played' by those promoting solid state designs over valve. Lets look at that for a moment, most well designed conventional valve amps manage a damping factor of around 12 to 20, now compare that to what a well designed conventional solid state amp is capable of i.e.; well into the hundreds, and again; those solid state fans amongst us here may be rubbing their hands with glee.
However, again’ all is not what it may seem, e.g., the full potential of that higher damping factor in solid state designs rarely gets put into play where loudspeakers are concerned, as most conventional speaker designs have a choke in line with the unit that would most benefit from it, the bass, or low frequency driver!
Anyway, I know there are many here knowledgeable enough to get where I am coming from on this, and can contribute their own thoughts, safe to say; there are good and bad aspects of both topologies, and because of this; there are very good, and very bad examples of both solid state, and valve designs.