Part one introduced the concept of tube amplifiers.
The two biggest reasons (opinions may differ) for choosing emulation over the real thing are: volume, and cost. The cost aspect is pretty straightforward: tube amplifiers were never cheap to make, and even less so now that tubes are a bit of a novelty item. The volume aspect is a bit more involved.
In the previous post, you may have noted that the amplifier adds desirable distortion when the signal is large, which means when things are loud. In a live gig setting, as well as a studio recording setting, the guitar amplifier is recorded with microphones. That recorded signal is then played back through the venue PA system, or your home stereo speakers, at the desired volume.
In those situations, in order to get full range of tube distortion, the amplifier needs to be turned up to a loud enough level where the sound engineer at the pub will be glaring at you, or where your landlord will be evicting you (for those non-home-owning millennial guitarists). I’ll leave it to Tim Pierce to explain how he converted his garage into an isolation booth so he can record his amp with that sought-after distortion.
Emulating tube distortion, on the other hand, can be done at any level. So the final sound can be played at a volume that doesn’t violate city by-laws.
Readers who’ve played with amplifiers might be thinking: wait, but you can turn up the gain, and then turn down the volume, and get the distortion without bursting your ear drums. Yes, and no. You get distortion from the pre-amp but not from the power amp. Wait what?
There are roughly three parts to a “combo amp” (what most people think of as an amplifier): the pre amplifier, the power amplifier and the cabinet / speaker. The pre amplifier (or pre-amp) takes the guitar signal, runs it through a few tubes and increases its voltage. The power amplifier takes that amplified signal and adds more current to it so that it can deliver enough power to drive a speaker. Finally the cabinet / speaker is just that: a speaker mounted in a cabinet. The amplified signal is run to the speaker which, in turn, creates the actual sound. Suffice to know that there are two stages to amplification, and then a speaker is needed to create the sound.
And it turns out that the distortion / compression caused by the power amp (the one controlled by the volume knob, which makes things loud no matter what) is the one that guitarists will often describe with creative vocabulary choices such as note bloom.
Part three explains how a computer can be used to emulate a tube amplifier.