Need help understanding tube wpc

Theoretically, I don't believe there is any electrical difference between a ss watt and a tube watt--a watt's a watt. But lots of people think that, since, among other things, tubes overload more gracefully than transistors, you don't need as many 'tube watts' as you do 'ss watts'. I've never seen anyone state a 'factor' by which you could multipy (or divide) ss watts to get 'equivalent' tube watts,,,but maybe someone here knows.

My rule of thumb is 50% -- that is, for a given speaker, you probably need 50% of the ss watts if you run tubes. It's only a worst-case rule though: if the ideal ss power is 120 wpc, you may be able to get by with 60 tubed watts. (Better to have the same powered amp in either case, it's just that high-powered tube amps tend to cost a lot to buy and run.)

When choosing a tube amp over a solid state amp, though, it is more important to consider the speaker's impedance curve (over frequency) than the amp's output. A decent solid-state amp can usually cope with a wide impedance variation. A smoother impedance curve, with a minimum point not much less than 4 ohms, will generally be easier for a tube amp to drive.

I have heard the same thing that Tobias states from a reputable manufacturer of both types of power amps. There is a difference between tube and ss wattage as I have experienced it first hand; same manufacturer of ss and tube amp.