Tube amp power watts equivalent to Solid State?

03-16-14: Dopogue
My 12wpc SET monoblocks (845 output tubes, EL-34 drivers) designed and built by a friend sounded much more authoritative and powerful than the CJ Premier 12 monoblocks (140 wpc) I used to have.

So, for me, this contention -- tube watts delivering something more than SS watts -- must have SOME validity. It may not make sense, but I heard it, at length, playing all kinds of music, in my own system. Ten years later I'm still astonished at the twelve-watters. And my speakers are not of the ultra-sensitive variety.

Fortunately I'm not an engineer or I'd know this couldn't happen :-)

Not necessarily :-)

In addition to ZD's point, what stands a very good chance of having been a major factor, as alluded to by Lewinski and Unsound, is the amplifier's interaction with the impedance vs. frequency characteristics of the speaker.

Many speakers have impedances which rise to extremely high values (e.g., 40 or 50 ohms) at some frequencies in the deep bass region, and also to values of perhaps 20 or 30 ohms in the vicinity of mid-range crossover points. For a given input voltage, the power delivery of a solid-state amp relative to its power delivery into 8 ohms will decrease in direct proportion to that impedance rise, when the amplifier is operated within its maximum power capabilities. Its maximum power capabilities into those high impedances will also decrease dramatically, relative to its 8 ohm rating. Both of those effects will occur to a much smaller degree in the case of a SET amp, and many other tube amps.

It should be clarified, btw, that the CJ Premier 12 is a tube amp. However, it has an output impedance (measured by John Atkinson as 0.87 ohms at mid and low frequencies) that is much closer to being in the solid state camp than in the camp of nearly all SET's and the majority of other tube amps. Everything else being equal, that will cause the Premier 12's interaction with speaker impedance vs. frequency variations to be more solid state-like than that of most other tube amps.

Also, nobody has yet mentioned that the characteristics of the distortion rise that occurs with tube amps as their power output approaches and somewhat exceeds their rated maximum capabilities tend to be less offensive than in the case of solid state amps, i.e., they clip more softly. That can contribute to a tube amp being seemingly more powerful than its rating would suggest.

Also, see this thread, which includes extensive discussion of related issues by some very knowledgeable folks.

Regards,
-- Al

A point I would add to my previous post: SETs will typically have distortion levels that are vanishingly low when small amounts of power are being delivered, and that increase considerably as their maximum power ratings are approached or exceeded. That kind of variation of distortion as a function of output power can be perceived as an increase in dynamic contrast, which can in turn contribute to an increased perception of "authoritativeness."

Regards,
-- Al

I understand ... the output impedance interaction with the load impedance. I don't understand why the ohms law computation of power in different frequency ranges due to different load impedance has any bearing. As long as the frequency response is unaffected, what difference does it make how much power is being dissipated?

Not sure I understand your question, Bob. The frequency response IS affected by the output impedance interaction with the load impedance.

Best regards,
-- Al

Hi Bob,

Thanks for the clarification. Bruce's (Bifwynne's) explanation of my comment that you questioned was a good one, and I think that your understandings that you've described are also correct. In the case of nearly all solid state amplifiers (excluding special cases such as some of the First Watt amps that have tube-like or higher output impedances), the interaction of amplifier output impedance and variations of speaker impedance as a function of frequency will not result in significant frequency response differences as a function of what amplifier is being used.

And I was not implying anything to the contrary in my previous posts. Note the two sentences in my post which followed the ones you quoted just above, especially the second sentence:

Its maximum power capabilities into those high impedances will also decrease dramatically, relative to its 8 ohm rating. Both of those effects will occur to a much smaller degree in the case of a SET amp, and many other tube amps.

My posts were addressing reasons why a tube amp, especially a SET amp, may seem more powerful and "authoritative" than its power rating would suggest. As you'll realize, one of those reasons is that its power delivery into the very large low frequency impedance rise I referred to will not decline as sharply (relative to its delivery into lower impedances at other frequencies) as it would in the case of nearly all solid state amplifiers.

So although the interaction of amplifier output impedance and variation of speaker impedance as a function of frequency will not cause significant differences in frequency response among nearly all solid state amplifiers, with many and probably most speakers it will cause significant frequency response differences between solid state and tube amplifiers, and often between different tube amplifiers as well. As Bruce indicated, if one such combination (solid state or tube) results in flat frequency response, the other one probably won't, unless the speaker has an uncommonly flat impedance curve (although some speakers do have an impedance curve that is flat enough to make this a non-issue).

Best regards,
-- Al

Hi Unsound,

I essentially agree, except that I would substitute the words "the majority of speakers" for "typical speakers." I would put it that the majority of speakers are designed to provide flattest frequency response when driven by solid state amplification.

Best regards,
-- Al