Need help understanding tube wpc

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.

A watt is a watt. Watts = Volts x Amps.

The equation has no idea whether you used a tube or a transistor. SS amplifiers usually have more current (amps) and tube amplifiers usually have more voltage, and their interaction with the speakers impedance at various frequencies account for the differences you hear.

Agree with Rlwainwright, except that there is one more difference, and that is that tubes clip very gracefully, rounding off the top of the waveform, essentially compressing it. SS amps tend to clip more abruptly, flat topping the waveform with a spray of odd-order distortion products. So the theory is that, if a tube amp clips now and again, it will barely be noticed, whereas if a SS amp clips, it is very unpleasant. The theory is very flawed, in that one should have the requisite amp power to drive the loudspeaker to the desired peak levels without either compression or flat-topping of the waveform, and that will require the same amount of wattage, tube or transistor.

While it's true that watts is watts whether tube or solid state, the ear's sensitivity to different types of distortion is what's coming into play in a "tube watts vs solid state watts" discussion.

Tubes soft-clip; that is, they produce less high-order harmonic distortion when driven into clipping. High-order harmonic distortion is quite audible and objectionable even in fairly small amounts. Generally speaking, tube amps can play louder than equivalant-power solid state amps before the ear detects the distortion. This matters because usually the problem that's most noticeable in clipping is the distortion rather than the dynamic compression.

Given that recorded music can have an average-to-crest ratio of 20 dB or more (which would call for a 100-fold peak in amplifier power), clipping can and does happen more often than we'd probably like to admit. In theory enough reserve power to avoid clipping would be great, but I'm not sure that's always practical.

Duke