tube Watts vs solid state Watts

It isn't. Contrary to audiophile myth WATTS ARE WATTS. It makes no difference whether they are tube or SS watts. They all measure the same.

A watt is a watt, an amp is an amp, they all just amplify a signal, and probably sound alike. Or not........

The limiting factor isn't just wattage - the audibility of clipping comes into play as well.

Tube amplifiers "soft-clip" - that is, they do not generate as much harsh-sounding high-order harmonic content when they clip as solid state amps do.

Perceived loudness is more a function of the average SPL than of the peak SPL, and a tube amp's relatively benign behavior when clipped means that it can be driven farther into clipping before the distortion becomes audible and/or objectionable. So the average SPL with a 20 watt tube amp might be 2 or 3 dB higher than with a 20 watt solid state amp by the time clipping becomes audibly objectionable.

Duke
dealer/manufacturer

Duke, excellent answer. Clipping behavior is the reason that people perceive tube amps to be more powerful even when the wattage is the same.

This is also the reason that electric guitar players love tube amps. Many of the most memorable "sounds" we hear from the great guitar players are due to the way they drive various gain stages of their tube amps into intentional distortion. Do the same thing with a transistor amp and it typically just sounds nasty.

Just for full disclosure, I current use one of the solid state class T amps. If you stay out of clipping the difference between solid state and tubes is much more subtle.

The limiting factor IS wattage. Tube amps have a high output impedance and low current which means their power is limited to the output tube plate dissipation. A transformer's needed to reflect the low speaker impedance and this limits the amplifier's power output. When impedance is high, the current is lower and within the amp's power rating (but will put out less power), as the impedance drops the amp clips. Softly, it's agreed.

A solid state amp has a low output impedance and higher current which in turn requires higher capacitance and, more important, a power transformer rated higher than the amp's output power. So SS amps usually have more in reserve and put out more juice as speaker impedance drops. Maybe a 20 wpc amp at 8 ohms can put out 30 or 40 wpc at 4 ohms. Any way you slice it, this is twice the power at that given load. And the load is the defining part. You cannot say they're the same or that a tube amp is more powerful unless you consider the same range of impedances - an area where tubes generally cannot compete.

BTW 20 watt Class A amps consume more power than 20 watt Class B amps if both are rated the same - they are not more powerful watt for watt. They're just power pigs; a 20 watt Class A amp requires a 100 watt minimum power supply whereas a Class B may require a 70 watt supply.