One of the few useful and credible measurements published on the web is Stereophile. Here is a random example of a tube amplifier and a solid state amplifier.
tube amp
Take a look at figure 10. It's a graph of response generated by the amp when the input signal is 50Hz pure sine wave. The vertical axis is normalized to the amplitude of fundamental signal at 50Hz. The 2nd harmonics of 100Hz is seen at -40dB of fundamental, and the 3rd harmonics of 150Hz at -60dB of fundamental.
solid state amp
The response plot of the same signal is shown for a random solid state amp in figure 7. There is almost no 2nd harmonics, and 3rd harmonics is at -90dB of the fundamental.
The relative levels of those harmonics may be low enough, but they are not present in the input signal of pure sine wave, and they are the by product. I've picked random samples, but in general tube amps tend to generate more harmonics, and solid state amps tend to generate less. There are exceptions of course.
tube amp
Take a look at figure 10. It's a graph of response generated by the amp when the input signal is 50Hz pure sine wave. The vertical axis is normalized to the amplitude of fundamental signal at 50Hz. The 2nd harmonics of 100Hz is seen at -40dB of fundamental, and the 3rd harmonics of 150Hz at -60dB of fundamental.
solid state amp
The response plot of the same signal is shown for a random solid state amp in figure 7. There is almost no 2nd harmonics, and 3rd harmonics is at -90dB of the fundamental.
The relative levels of those harmonics may be low enough, but they are not present in the input signal of pure sine wave, and they are the by product. I've picked random samples, but in general tube amps tend to generate more harmonics, and solid state amps tend to generate less. There are exceptions of course.