The first picture shows the unit distorting with signals at three different input levels. The Blue trace is basically clean. Part of the valve ‘sound’ is that Valves clip the ‘top’ and ‘bottom’ of the waveform in different ways. Positive excursions are clipped at the input by grid current limiting, while negative excursions are clipped by the output cutoff against the rails, potentially leading to an uneven waveform. This can be seen in the green trace, in which one part of the waveform is rounded off (‘soft clipped’. The Art preamp clipping LED glows orange when valves are clipping. In the pink trace the unit heavily slipped on both top and bottom of the waveform. The clipping is slightly more symmetrical, although it can be seen that one side is’softer’ than the other. The clip light goes red in this condition.
The second graphic shows the changes in the harmonics created by the clipping. The uneven clipping tends to emphasise even order harmonics, with 2nd harmonic at 2K being larger than 3K and so on. Confusingly, waveforms that are clipped evenly create more odd harmonics. YOu can see that 3K harmonic is now greater than the 2K, the 5K greater than the 4K etc.
So there you go. If you have a valve preamp repair job that you think I could help with them please do contact me.