[roymail]

I seem to frequently read that there's a significant difference between tube watts/ power and that of solid state amps.  Can you tube guys help me understand this keeping in mind that I don't really have much experience with tubes.  Thanks guys!

[JoshK]

objectively watts are watts.  subjectively, there is a perceived difference.  there are a number of plausible reasons for this, you may or may not agree with them, but they are plausible

1) tube amps have much higher distortion with rising level, unlike many SS amps which rise more gradually.   This is not true in general, but it is probably a decent rule of thumb.  We know higher distortion results in a louder perceived sound, all else being equal. 

2) tube amps have a tendency to clip more gracefully.  While distortion may rise appreciably, tube amps with moderate levels of feedback tend to have a "soft clip" whereas SS amp's clipping is usually quite hard and disconcerting. 
I've been at many raves (NYAR) where the tube amp was clipping, I felt noticeably, but as far as I could tell no one else seemed to notice or care.  I am no golden ears, this is just to say that it sometimes isn't very noticeable, unlike a SS amp clipping.

[Niteshade]

Good explanation! 

You will find allot of generalizations made about tube amps. 

If an amp is rated at 30 watts, it should not clip until that point regardless of whether it's tube or SS. Some tube amps will clip suddenly. It depends on how they're set up. Others will clip gradually (compress then clip).

I prefer a more sudden clip rather than allot of compression leading to clipping. The vacuum inside a tube (soft or hard) has something to do with clipping as well. Feedback and power supply spec's have a tremendous amount of clipping influence.

Great subject!

[richidoo]

SS amps have much lower output impedance which allows better current delivery to control drivers.

[Pez]

Also the type of distortion matters. Solid state has a lot of 3rd order harmonic distortion. Tubes have 2nd order distortion which is much less obvious or rather much less offensive to the human ear.

One other thing I would like to point out, people in general have absolutely no clue what watts really are. Is 100 watts twice as loud as 50? Ask 100 people on the street and 99 will say yes. Obviously to those of us who know 100 watts is 3 db louder than 50. And to that point most speakers can be driven to 90 db with about 4-8 watts of power. 

[Ericus Rex]

I find tubes to be more dynamic as well.  This might be why many perceive them as being louder watt for watt.

[Wind Chaser]

Most tube owners buy under powered amps and push them too hard.

[roymail]

Thanks, guys, for the replies and insights.

Wind Chaser said, "Most tube owners buy under powered amps and push them too hard."

That raises the question, what's the lure of the low power SET amps?

[Wind Chaser]

Within their limits, they can sound spectacular with the right speakers.

[Pez]

I'm not sure I follow what ol' windy is trying to say re: people driving their speakers. Yes SET is pretty much synonymous with lower power levels (at least when it comes to what mortals can afford), but for the most part SET users tend to be high efficiency speaker users as well. Most people hear "high efficiency" and think 103 db 1 watt 1 meter. But I for instance us a 5 watt SET amp for the midrange panels and tweeter for my RM40s and it drives them very effectively and they are approximately 90 db 1w 1m without any wimping out and certainly it isn't being under powered by any stretch of the imagination. I also have a 60 watts push pull Bella Extreme that obviously has a little more overall power capability, yet in reality the 5 watt amp performs amazingly in my system and is not outdone at even high volume levels by the Bella which is a phenomenally great amp.

Now the Bella might do better at insane SPLs like 95 or higher, but nobody in their right mind listens to music that loud very often.

[JackD201]

Also the type of distortion matters. Solid state has a lot of 3rd order harmonic distortion. Tubes have 2nd order distortion which is much less obvious or rather much less offensive to the human ear.

One other thing I would like to point out, people in general have absolutely no clue what watts really are. Is 100 watts twice as loud as 50? Ask 100 people on the street and 99 will say yes. Obviously to those of us who know 100 watts is 3 db louder than 50. And to that point most speakers can be driven to 90 db with about 4-8 watts of power.

+3db being on a logarithmic scale really is about twice as loud right? 3dB is the difference in SPL between 1 speaker and 2 speakers fed equal power carrying a mono signal.

[Pez]

I didn't say it was inconsequential, but twice as loud is 10 db if you want to get technical.

Now 3db is what you get if you double power for example sake 90 db with 10 watts would be 93 db at 20 watts. This all of course completely neglects dynamic compression which happens with every speaker ever made so really if you double down you might only get 2.7 db difference or something like that.

[mackortoyota]

If I'm correct, isn't a 10-fold increase in wattage needed for the volume to double?

[Pez]

That is incorrect. Because it is logarithmic a 10 db difference is always "twice as loud" so 70 db is "twice as loud" as 60 etc.

So wattage would look something like this:

70 db - 1 watt
73 db - 2 watts
76 db - 4 watts
79 db - 8 watts
which would put 10 db at about 8.5-9 watts which does not jive with a 10 fold increase in wattage.

EDIT... Err actually I mean that is correct. It is a 10 fold increase in wattage.

[JackD201]

The fact remains that 2 speakers playing the same signal with equal wattage yields only an additional 3dB to just one of them. It doesn't matter if they are tiny speakers on 1 watt or behemoths with a kilowatt. The 3dB gain is what it is regardless of the reference level.

I don't know where you got the little tidbit that twice as loud is 10dB Pez. Really. I would just looooove to know. A doubling of either power or intensity yields a 3dB not 10dB increase. Your own table shows this. 70,73,76,79...thats 3dB everytime you double your wattage output. A power factor increase of 10x not 2x will yield 10dB. Lest we forget, the dB scale was employed precisely to reflect the perceptional increase or decrease in relation to the measured output. 3dB is simply twice as loud.

[Niteshade]

A tube amp can have any impedance output desired. They can be made 1/2 ohm stable if required. It just depends on the output transformer's secondary windings. Damping factor is not necessarily a quality trait nor is impedance output.

Specifications, like IQ's don't mean a darn thing as to how well a device (or person) can do their job. They're supposed to provide an  indication of performance and that's all. This is why two amps (or people) with the same spec's can be worlds apart.

There are different guidelines to measuring power output. Amps can be measured conservatively, meaning power out before compression begins or they can be measured right at the clipping stage. Obviously the two numbers will be completely different. It all depends on where compression begins. Compression can begin at 20 watts with a 50 watt rated amp. It all depends on how the amp is designed and the tubes used. It is traditional to measure at the beginning of clipping. Compression is not always thought of as bad, but it will add a signature to the music.

[JoshK]

I don't think either one of you have it right.  Power follows the 10 log rule, amplitude (level) follows 20 log rule. 

http://en.wikipedia.org/wiki/Decibel

So 2 times the perceived level (SPL) is 20 log (2) = 6db.   2 times the power is 10 log (2) = 3db.  So twice the power equals 3db increase in SPL but you need 6db to get a 2 fold increase in perceived sound.  edit: that part isn't right.

[srb]

The problem is the terminology between "louder" and "twice as loud".
 
A 3dB increase is the generally accepted increase in sound pressure level that most people can definitely discern as being "louder".  (Obviously some people can also detect 2dB and 1dB increases as being louder.)
 
That 3dB increase requires 2X the power to produce, as all of us seem to agree on.  The difference is that while a 3dB increase is perceptually louder, 10dB is considered to be the level that most people percieve as twice as loud and requires 10X the power (and a 20dB increase requires 100X the power, etc.)
 
It is further complicated by the fact that apparent loudness with an increase in dB level varies with frequency, and that everyone's eardrum to brain translation is different.
 
Steve

[JoshK]

Yeah, now I think I botched it.  6db is 2 times the voltage in electronics, but SPL is 10db to "twice as loud". 

This does a decent job of explaining the confusion.  http://trace.wisc.edu/docs/2004-About-dB/

Where I went wrong above is that perception of loudness does not follow the SPL scaling.

[Pez]

lol this topic really highlights the problem with scientifically analyzing something tht is based on human perception.

http://wiki.answers.com/Q/Is_a_three_dB_difference_in_sound_twice_as_loud