There are many different desirable sounds produced by valve amplifiers, and they vary smoothly from completely clean to “full-on” overdrive. It’s not possible to describe every sound between these extremes, but there are three distinctive sounds that occur below, on and above the clipping point.

Clean

No overdrive is heard. Even when a valve amplifier operates well below its point of audible overdrive, it still exhibits an amout of sound compression, or sustain. Players usually desribe this effect as “warmth”, although this has nothing to do with warm tone settings (high bass or low treble).

The brown or thick, creamy sound

The amplifier is pushed into subtle overdrive at the start of notes and chords, but decays into a cleaner sound. This is considered the sweet spot of the amplifier, where sustain and variable clipping occurs over the duration of notes. This sound has so far eluded any solid state preamplifier or power amplifier design I’ve heard. I would even suggest that anyone who can truly capture this sound in a solid state design can probably make a lot of money from guitarists around the world, although I’m not sure how it might impact the valve manufacturing industry. Without wanting to deter anyone, you should be aware that many have tried and failed. The closest sound I can get uses a limiter followed by an overdrive circuit just on the verge of audible clipping, then some equalisation to reduce a little high frequency response. As mentioned above, Marshall Valvestate and Peavey Transtube offer some realistic sounds and dynamics.

Overdriven sounds, with long sustain and smooth overdrive

Modern preamplifier designs are capable of producing many of the popular sounds tonally, with some new sounds not achievable with any type of power amplifier. Many players agree that the sounds are good, but the “feel” (dynamic response) is not the same as a heavily overdriven valve power amplifier.

I believe the sound commonly sought by guitarists is produced in the power amplifier stages of a valve amplifier. For example, Mesa Boogie offers some of the most flexible valve preamplifier overdrives you can get, and the tones available are truly awesome, however, I don’t hear that creamy compression until you start to push the output stage (ie turn up the volume!).

Several recent designs are hybrid, with a valve in the preamplifier section capable of being overdriven, and a solid state power amplifier intended to be run within its power capability. An overdriven preamplifier valve does sound different to the popular solid state overdrive circuits, however, it does not sound the same as an overdriven valve power amplifier to me.