OK EEs (Kevin!) and layman, Y do tube watts sound louder than transistor watts?

Guitarists and guitar hacks alike tend to know this.

"Total Harmonic Distortion" (THD) is used as a criteria for stating a given amp's output power. Typically one will see numbers like 100 Watts RMS at 0.1% THD.

The answer lies in the human ear. Tubes produce a 2nd order (even harmonics) where as transistors produce a odd order harmonic (3d order), if both are of the "push-pull" type. Transistor distortion sounds HARSH to the human ear, where as even harmonics are more easily tolerated. Thus a tube amp can be "pushed" further without our ears objecting.

100 watts at 0.1% is that though, and a tube amp will output the same as a transistor amp using the 0.1% criteria, and thus won't sound any louder.

[Kevin Bresnahan]

When you start talking about amps at the point of xx% distortion i.e. beyond their designed maximum gain and at the onset of compression, the biggest and maybe only difference is that tube amps clip soft and solid state amps clip hard. Overdriven tubes don't immediately sounding like crap. Overdriven solid state amps generally start sounding crappy as they enter the non-linear region, which is about 10 dBm down from the saturated gain.

However, this is science and all that really matters is your ears. It doesn't matter if use a mono tube amp or a single-ended transistor amp or anything else. Whatever you think sounds better to you, sounds better and nothing I, or anyone else says, should mean anything.

Later,
Kevin