There are all sorts of tube circuits, some of which actually deliver The Sound people associate with Vacuum Tubes.
Did you know, for example, that asymmetrical distortion generates even-order harmonics while symmetrical distortion produces odd-order harmonics? Neither vacuum tubes nor transistors are exclusively responsible for either type of distortion, it’s really about circuit design. Figure 1 shows distortion variations in red and violet. Notice how the violet wave follows the green wave perfectly during its positive excursion but falls short of the mark during the negative swing. The red wave is more symmetrical, but does not “clip” like an overdriven IC op-amp. This is what I like to call “The Sonic Airbag,” or Electric Crayon effect. Op-amps are linear-clean right up until the point of clipping, some discrete transistor and vacuum tube designs have a non-linear window between clean and clipped.
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