Originally Posted by
Simon Barden
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.