Quote Originally Posted by Simon Barden View Post
A tube/valve will only start to ‘compress’ the sound once it reaches its clean gain limit and starts to distort slightly. From that point on, you’ll be getting an increase in compression at the same rate as harmonic distortion increases. Unlike a compressor which kicks in on the whole signal once it crosses the threshold and only returns to non-compression mode after the signal level drops below the threshold for a period related to the release time of the compressor (almost always a preset value on non-studio compressors and any compressor with three knobs or less), a tube will only reduce the peak signal value above the clean limit. Doing this will alter the signal wave shape, so introducing distortion and adding harmonics. So it’s a very different type of compression to a normal compressor, and hand-in-hand with extra harmonics and an initial ‘warmer’ sound, before audible distortion kicks in and you know the tube is definitely overdriving and adding grit.

Apparently, when the tube is being lightly overdriven, and you're getting that warm sound out of it, the distortion consists of mainly even harmonics due to asymmetric soft-clipping, I remember doing some experimenting with some 12AX7 tubes, a variable HT supply, a signal-generator and an oscilloscope, I built a simple triode amplifier stage on a breadboard and fed a sine wave signal into it while monitoring the output on the oscilloscope, I noticed that below a certain signal-level the output signal was pretty much a clean sine-wave, as I increased the input signal level, the bottom half of the output signal became more rounded and compressed, whereas the top half became elongated and peaky, then, when I increased the input signal further, the top half of the waveform started to clip sharply while the bottom half remained gently rounded, according to what I can remember, I might see if I can do a thread in the amplifier section one day about tube/valve distortion.