"tube watts" versus "solid state watts"

I'm sure you, like me, have seen it written more than once that brand X tube amplifier, rated at 30WPC, sounded more powerful / more authoritative than brand Y solid state amplifier, also rated at 30WPC. Or that brand Z tube amp, only rated at 15W, was comfortably able to drive brand A speakers, because those 15 were tube watts and therefore up to the job. Heck, I think I've even heard the phenomenon with my own ears.

My question is: is there any basis in electrical engineering for this effect? Can we say scientifically what's going on here?
which is heavier a pound of nails or a pound of feathers? A watt is a watt no matter how you look at it. I would say what makes one amp sound more powerful has noting to do with wattage but frequency response.
My question is: is there any basis in electrical engineering for this effect? Can we say scientifically what's going on here?

Yes we can. There are probably several factors that are involved, but I believe that the most significant one is that solid state amplifiers clip more abruptly than tube amplifiers. So that if the power capability of the amplifier is momentarily exceeded by a small or perhaps moderate amount, the resulting distortion will be much more noticeable on a solid state amp.

-- Al
Actually...a watt is current times voltage. It is unlikely that an amplifier will "run out" of voltage capability at the same time that it runs out of current capability. You could have an amplifier with low "rail" voltages that could deliver huge amperage at a low output voltage. This would, of course, be a waste because the amperage actually drawn is determined by the voltage and the impedance of the speaker. Similarly an amp might swing a high voltage into a light load, or for a brief time, but be limited in steady state performance by current delivery capability. Still, there are enough technical grounds to support an audiophile argument that a "tube watt" is better than a "SS watt". It doesn't take much technical ground to support an audiophile argument :-)
2-left: this current thread (nopunintended) has some pretty good explanations and resources:http://forum.audiogon.com/cgi-bin/fr.pl?htech&1235758642&openmine&zzNsgarch&4&5#Nsgarch
A Watt is a Watt.
Beyond that my hyper technical response is . The measure of an amp of either tube or SS is how much it weighs. The question of where the watt is generated lies somewhere in the transformers . This is of great importance to the success of a tube Amp not the wattage. Indeed the better SS Amp has an enormous power trannie. In fact they weigh a lot if you aren't speaking of those wimpy on/off Chip amps. I will hold my opinions about the chip amp to myself.
So when looking for an amp first decide if you prefer a tube or SS amp by listening to both and ignore all this nonsense about the difficult nature of tube amps, it is pure B.S. Then buy the heaviest one you can afford, and please forget the watts.
Thought I'd expand a bit on my previous response. An example of how amplifiers are rated (which is done in accordance with requirements that are imposed, I believe, by the Federal Trade Commission), is as follows:

100 watts/channel continuous rms power at 8 ohms, from 20Hz to 20kHz at less than 0.5% THD (total harmonic distortion).

There are many reasons why two amplifiers that are identically rated in this manner at "100 watts" may differ significantly in real world power delivery.

First, for brief musical peaks an amplifier will be able to deliver significantly more power than its "continuous power" rating, the difference being referred to as dynamic headroom. The amount of headroom depends on many factors in the design, such as the amount of energy storage in its power supply.

An amplifier will have a clipping point, at which its output voltage cannot swing any further without a large increase in distortion. As I alluded to in my previous post, it is pretty well recognized that the onset of clipping for a tube amplifier is more gradual (less abrupt) than in the case of a solid state amplifier. That means, everything else being equal, that a tube amplifier will be able to exceed its supposed power rating by a greater amount than a solid state amplifier, FOR EQUAL LEVELS OF DISTORTION. A watt is a watt, but a "100W" tube amplifier will be able, at least on short term peaks, to be able to put out more clean, listenable watts than a solid state amp, everything else being equal. Power needs to be specified in the context of distortion to be meaningful.

There are many other factors than can make one "100W" amplifier more or less powerful (both subjectively and objectively) than another "100W" amplifier, some of which have been alluded to above, but which do not directly relate to the tube vs. solid state question. Different designers will provide differing amounts of margin in their power ratings, to allow for component tolerances, variations with line voltage, temperature, etc. As has been noted, amplifiers with similar power ratings into 8 ohms will differ in how much current they can deliver into low impedance speaker loads, or loads which dip down to low impedance levels at certain frequencies, and how well they can handle reactive loads, back emf, damping factor, etc.

-- Al
I agree 1000% with Almarg; I'm no engineer but I'm quite sure that the main difference is that tube amps clip more softly and gradually than solid state. It's not so much a question of what different amps do when they're playing within their power range, but rather how they sound when pushed beyond it.
Beyond the fact that tube amps clip softer than solid state: There's the fact that their distortion when driven hard is composed more of even order harmonics, than SS's predominiately odd order(much more disturbing to our senses). Then there's the fact that(generally speaking) the filter caps found in tubed amps store more Joules than SS amps, and can supply higher instantaneous energy when transients require it. Part of that relates to the much higher voltages at which tubed circuits function. Then(in some amps) there's capacitor construction/speed to be considered. Some info: (http://www.wyetechlabs.com/links/articles/energystorage.html)
Arahl...Some SS amps have circuitry which detects when clipping is imminent, and limit gain so as to behave very much like a tube amp when overdriven. Also, consider that SS amps are, for any price point, much more powerful than tube amps, so that when the tube amp is clipping the SS amp is not.
A Watt is a Watt is a Watt.
An inch is an inch is an inch.
Despite logic and reason, measurements seem to be gathered by perception.
Tubes are men.
Transistors are women..
Unsound...How about a Hybrid?
Wouldn't that be a hermaphrodite?
Would that be AC/DC ?
>>Tubes are men. Transistors are women.<<

Everybody chant with me:

So if its not clear- the difference in tube and transistor watts has to do with distortion- generally speaking tubes have more distortion, but most of that is lower order which the human ear/brain system does not care about. Transistors tend to have more higher ordered odd harmonics, and while in only trace amounts (100ths of a percent) these are the harmonics that the ear/brain system used to detect loudness, so the human ear is very sensitive to them.

The result is that a tube amp may have a higher percentage of usable power, even though it has less overall power.

In the case of clipping, a tube amp will seem to compress dynamics slightly before audible clipping occurs. With transistors the clipping characteristic is very audible and instantaneous. This is why guitar players, who overdrive their amps on a routine basis, prefer tube amps. In fact, it is the guitar amplifier business that drives tube availability, not high end audio!
2 amps with identical RMS ratings will probably have similar drive characteristics into a resistor.
As soon as you start driving complex loads with current ahead or behind voltage, that's where you'll start hearing significant differences.
Amps which can tolerate such complex loads will have more apparent power.
As a test you could find 2 different speakers of similar sensitivity..make them 91 to 93db rating. Also, 1 speaker should be a fairly benign load and the other as wretched as you can find. Huge phase angles and large impedence swings.
Try a couple or more amps. amp#1 should be a very high quality unit. Nearly any of the 'biggies' should work. Find another amp of similar power. This one should be of much lower quality, but similar RMS power rating. They should both play an easy load to similar loudness levels. However, when driving a difficult load, the better amp will play louder, longer without heating, and provide a much superior listening session.
Distortion usually increases with demand. More Watts = less clipping. Having more Watts, lessens the chances for distortion. On a cost per Watt basis, ss usually offers more Watts for yer dollar than tubes. One could argue that avoiding distortion is preferable to accepting more pleasing distortion. While it's an old fashion notion, and certainly not the end all towards choosing an amplifier, perhaps we should re-consider $ss to $tube Watts as part of this discussion.
To me,most tube amps seem to sound like they have more power than a SS with equal power.Also if I remember right,tube amps have even order distortion vs. odd order distortion put out by SS amps.This may already have been mentioned.
I think it has to do with the tubes warming up the hair in your ear. The vibrations of the ossicular chain displace the basilar fluid in the cochlear, causing the hairs within it, called Stereocilia, to vibrate. Hairs line the cochlear from base to apex, and the part stimulated and the intensity of stimulation gives an indication of the nature of the sound. Information gathered from the hair cells is sent via the auditory nerve for processing in the brain. That's why men love tubes.
It's not a tube vs transistor issue. I've had numerous amps all rated for 100W both tube and SS. Some sounded like kilowatts, some sounded like milliwatts. Some were just not a good match for certain speakers. Just trade-offs in the design/price. I could be more specific with the individual models but that would serve little purpose and be misconstrued as criticism.