tube Watts vs solid state Watts

Hi folks, can anyone explain to me why 20W tube amp is more powerful than a 20W solid state amp? Further: a 20W pure class A amp is more powerful than a 20W class B amp. Why is that? I've always thought Watt = Watt.

It isn't. Contrary to audiophile myth WATTS ARE WATTS. It makes no difference whether they are tube or SS watts. They all measure the same.
A watt is a watt, an amp is an amp, they all just amplify a signal, and probably sound alike. Or not........
The limiting factor isn't just wattage - the audibility of clipping comes into play as well.

Tube amplifiers "soft-clip" - that is, they do not generate as much harsh-sounding high-order harmonic content when they clip as solid state amps do.

Perceived loudness is more a function of the average SPL than of the peak SPL, and a tube amp's relatively benign behavior when clipped means that it can be driven farther into clipping before the distortion becomes audible and/or objectionable. So the average SPL with a 20 watt tube amp might be 2 or 3 dB higher than with a 20 watt solid state amp by the time clipping becomes audibly objectionable.

Duke, excellent answer. Clipping behavior is the reason that people perceive tube amps to be more powerful even when the wattage is the same.

This is also the reason that electric guitar players love tube amps. Many of the most memorable "sounds" we hear from the great guitar players are due to the way they drive various gain stages of their tube amps into intentional distortion. Do the same thing with a transistor amp and it typically just sounds nasty.

Just for full disclosure, I current use one of the solid state class T amps. If you stay out of clipping the difference between solid state and tubes is much more subtle.
The limiting factor IS wattage. Tube amps have a high output impedance and low current which means their power is limited to the output tube plate dissipation. A transformer's needed to reflect the low speaker impedance and this limits the amplifier's power output. When impedance is high, the current is lower and within the amp's power rating (but will put out less power), as the impedance drops the amp clips. Softly, it's agreed.

A solid state amp has a low output impedance and higher current which in turn requires higher capacitance and, more important, a power transformer rated higher than the amp's output power. So SS amps usually have more in reserve and put out more juice as speaker impedance drops. Maybe a 20 wpc amp at 8 ohms can put out 30 or 40 wpc at 4 ohms. Any way you slice it, this is twice the power at that given load. And the load is the defining part. You cannot say they're the same or that a tube amp is more powerful unless you consider the same range of impedances - an area where tubes generally cannot compete.

BTW 20 watt Class A amps consume more power than 20 watt Class B amps if both are rated the same - they are not more powerful watt for watt. They're just power pigs; a 20 watt Class A amp requires a 100 watt minimum power supply whereas a Class B may require a 70 watt supply.
Perceived loudness and amplifier wattage is not the same thing. Agreed a watt is a watt, no matter what (excuse the pun).

On a thread some time ago (I am to lazy to search now) a couple of posters stated that even if the fundamental remains the same, humans will perceive an increase in volume when the even ordered harmonics increase in db. So when a tube amp clips in a benign fashion, additional gain will allow the harmonics to increase in db, allowing the listener to hear an increase in volume/loudness.

I use a 12wpc SE tube amp and it most certainly plays louder than my 25wpc chip or 35wpc transistor amp. So while it is technically a watt is measured the same no matter what amplifier topology we talk about, for a specific rating a tube amp will deliver substantially more perceived loudness than a transistor amplifier.

And what about pure class A solid state Watts and class B Watts? Many audiophiles (who are often non techies) say that a 50W pure class A solid state amp is comparable to a let's say 200W class B amp. Is this bullshit? Why do people give that kind of deceiving comments?

When comparing two amps, I think the power rating is the power rating - no matter whether Class A or B as long as you are comparing at the same THD. Perhaps the discrepancy is due to type of distortion as mentioned above because Class A and Class B definitely have different distortion characterisitics. But I also feel (and this applies to tube amps too) that tonality can play a role. I often hear that such-and-such low-power tube amp has better bass than some big SS amps does and that gives the impression of more power.

Another thing I have heard a few times now is that tubes are able to produce a huge amount of power for a very short amount of time - much moreso than SS amps can. I am not sure about the validity of this claim but from a signal level conduction resistance point of view, it makes sense since you are comparing vacuum with "sand."

But when you are dealing with human perception, everything gets cloudy. Everyone is entitled to their opinion so take what you hear and read with a grain of salt but don't get upset by it. :)

In a tube amp there is (usually) an output transformer, and it, as much as the power supply or tubes, will limit the distortion-free power output of the amp. The real heart of a tube amp is its transformer. Good ones are few which is why OTL amps exist.
A Watt is a Watt.. No doubt.. This might be slightly off topic,
However I have 20 watt tube monos that Shocked the H3LL out of me, that put out more clean power and musical response than 200 watt SS monos that by the way costed in the 4 k range. Tube power is very different regardless how many here want to say its the same.. it is on paper but seems to be very different in actual use..

Another example is why would a 50 watt Marshall Guitar head amp just need 50 watts to go into concert levels? Yes a big reason is the far more efficient drivers, and the limited frequency range being not too deep of bass taxing the amp, but fact is I would still take a 50 watt marshall over a 300 watt peavy SS and walk all over it!

So don't be afraid of low power Class A tubes, they still pull way less power from the outlet than a 200 watt boat anchor like a adcom or mcintosh that dims the lights in the house when turned on, just make sure you have efficient speakers and it will not matter. Don't get me wrong I have had great amps from both camps, but low powered tubes are probably much more impressive on first listen due to very little power and big sound, vs. many solid state amps seeming to be fairly closed in and restricted regardless of power ratings. Just another partial audio mystery to most, and even urban legends that small tubes can't produce acoustical power, mostly they do more so until you get into some really serious solid state stuff.
Undertow: what is your explanation for these a bit counter intuitive findings? So those people who say that tube Watts and solid state Watts are not alike should be taken seriously?

Dazzdax, simply put I think it has to do with a
200 watt SS will put out about 10 watts of good power at lower levels, and it is not continously putting out 200 watts at all gain levels. It switches into this high wattage in peaks..

Class A tubes are running 100% at output at all times, same temperature, same stress, same outlet power draw etc...

When a SS hits it then taxes your power system in the house very quickly and can sound less impactful for the instantaneous peaks they actually perform on and off.. Tubes seem to be much more "Consistant" watts..

But measurably I am sure both would say the same on a meteer, but two totally different devices deriving these watts.
By the way interestingly I have shut down thermally exhausted and or overheated several ample powered SS devices in my day :-)

Never have I ran out of Tube power.. but again Tubes burn out due to they normally run 100% of their output at all times including Idle, Basically what I find is the only way a TUbe device fails is when the tube reaches the end of its life, which yes can be prematurly by other bad components in the unit giving out, or just the tube being not so great in the first place... However whether they would blow up under Hi volume or idle, I think it would normally just be by coincidence if they fail under hi power output to the speakers, it just could happen.

I have run several SS devices out of steam or shut them down with no better volume and actually more compressed volume than you get with much lower Tube power. Probably due to most Tube designs transformers and power supplies are Way overbuilt, and SS are built to handle their output levels in steps and part time peaks.

I am not an engineer, and actually am just going on my observations, but don't be shocked to see many tubes putting down some nasty power and seeming to be even more effortless with 10 watts opposed to 100 or even 1000 watts from solid state.. Again I am sure a 10 k, or 30 k SS device it would be no problem showing the true colors of full on SS power no doubt, and reliability is I am sure very excellent..

I am sure many more on this forum knows much more, just from some experience I have had this seem to be interesting and sometimes misunderstood.. Hey My uncle had a 30 watt Harman Kardon Receiver when I was a kid from the mid 80's and it had better sound and power output up to top output over a brand new very well known Receiver we tried 2 years ago at 250 watts and that I could not believe ~! So go figure.
While on one level a watt is just a watt, we also have to take into account how humans hear. As Duke noted early in the thread, we perceive "loudness" far more on the average level than peaks.

Take the following example with a 20 watt tube amp and a 20 watt transistor. Both are plugging along into a 87 dB efficient speaker. We're using roughly 10 watts to crank out a loud 96 - 97 dB average volume. We encounter a 3 dB peak in the music which requires doubling the needed power. Both amps deliver their maximum unclipped volume without clipping.

However, if that is a 6 dB peak, we now need 40 watts so both amps clip. The tube amp will give a better behaved clip that is less likely to draw attention to itself so we leave the volume control alone. The transistor amp clips harshly. Many people will respond to that harshness by turning the volume down until the harsh sound goes away.

This is the mechanism that causes many to say tube amps sound more powerful, watt for watt, than a transistor amp. Nothing real complicated going on.

Keep in mind that powerful transistor amps are widely available so if well matched with the speakers, this isn't a big problem these days.

As for transistors delivering more current & power as impedance drops, this only happens at certain frequencies, generally in a reasonably narrow band. You only get the extra power if your peak just happens to be composed primarily of energy at the correct frequencies. Music isn't generally composed and played with much consideration that the loud parts conform to a particular frequency profile. As such, it is a roll of the dice as to whether a transistor's extra power at lower impedances will be a benefit when playing any given piece of music.

All of that said, one is still better off buying an amp of appropriate power for your speakers and listening habits. The better solution is to avoid clipping and the need for impedance-related power all together.
No doubt the tube amps I have owned play louder and have more "power" then higher rated (watts) SS amps. Very much my experience. My current 45 watts monblock tube amps have more speaker driving power then all 50 - 100 watt SS amps I have owned. I have owned many good ones to be sure.

My experience confirms this on many, but not all of the tube amps I have owned. It depends on the builder. Some tube amps are spec'd at 50 watts and only put out 32 when measured. Other tube amps put out more then the stated power.

The same is true for SS mps. I owned an Aloia 60 watt amp that was measured to put out 100 watts. Again, really depends on the builder and what they decide to list in the spec's.

However, in general, tube amps sound more powerful than like wattage SS amps. This is in general.

Not all ss amps clip harshly, if at all. My CarverPro ZR1600 have clip detection and automatic gain reduction if clipping occurs. I have never seen the CLIP LED illuminate. However, in home audio application this is probably the usual case as the darned thing is rated at 600 watts into 4 ohms.
Other than the tubes vs. solid-state argument and the class A vs. class B argument, an amp with a larger power supply will often sound louder and more dynamic due to its ability to drive more current, i.e. dynamic headroom. When I was in school I learned this and was amazed at a demo at my local Circuit City. The salesman there was comparing a 26 wpc Harmon Kardon receiver versus a 90 watt Pioneer integrated amp. I and others there expected the more powerful Pioneed to blow away the HK, right? So we were all amazed to hear not only that the HK had much better sonics, it played much louder. The sales guy then asked someone to stand on a scale he brought in and the HK was twice as heavy, hence a much bigger power supply.
I suspect that part of the perception that tube watts sound louder than solid state has to do with people being deceived into thinking they need WAY more power than they actually do need.

Solid state can deliver a lot of power cheaply. So, makers of a lot of solid state gear push the benefits/need for a lot of power. Power is much harder to supply from tubes, and personally, I find most high powered tube gear sounds pretty bad.

So, when one hears a demonstration where a tube amp sounds really good while being rated at 20 watts and a 200 watt solid state amp sounds less impressive, one tends to conclude that the 20 watts of tube power sounds stronger. Actually, it is more likely to be the case that the speakers really only need a few watts, when played at normal levels, so the extra power of the solid state amp was wasted. Oversized solid state amps are particularly cheated in such demonstrations because a lot of solid state gear does not sound its best when idling along while tube gear is at its best at low output.

I see so many people argue about how much power is needed to achieve "realistic" levels, based on measurement of SPL at concerts and factoring how much power is needed to achieve peak levels. But, the simple fact is that NO commercial recordings ever deliver that full dynamic range so one never needs what "theory" supposedly dictates. I'll take great performance over the first watt of power any day over ability to deliver a lot of power for peaks that may never actually be in a recording. One watt on most speakers, even inefficient ones, is surprisingly loud.
Ok folks, so the point is here not that the tube Watts are more "powerful" than solid state Watts in absolute terms (when measured) but they are more powerful in terms of psycho acoustics. It's the subjective finding that not all Watts are the same. So when someone is saying that a 20W tube amp is more powerful than a 100W solid state amplifier you have to correct that somewhat by using abovementioned explanation.

I took the trouble to actually measure the voltage being applied to my 4 ohm MG 1.6 speakers by my 600 watt amps during peak volume intervals of VERY loudly played music. I wanted to know if a lower powered amp would be OK. Most of the time the maximum rms voltage over a brief (1 second) interval was 10 volts or less, which correspionds to 25 watts. (At times when the music was quiet the voltage was only a couple of volts, which is one watt). However, there were a few moments when I saw about 40 volts and this would correspond to 400 watts. A 40 volt rms sine wave would have instantaneous voltage peaks of 56 volts. Since the CLIP LEDs did not illuminate I guess that my amp was able to follow a 56 volt peak waveform,

So, I concluded that 600 watts is not as much of an overkill as you might suppose. Nevertheless, because this test was done with music much louder than I would actually use I bought some new amps which are only(!) 350 watts into 4 ohms, and they are OK. (I did not buy some 60 watt tube amps which I had been considering). In general, my experience has been that every time my amp power has been increased the speakers sounded better. I think that high powered amps have characteristics other than their maximum power capability which are helpful.
This discussion is similar to the high torque versus low torque engines producing the same horsepower discussion.

HP is HP and IT is what determines the max speed that a car can go.

There is the term, however, of 'usefull' power (HP), which could be used as 'usefull watts' for amplifiers.
Interesting concept. Can you explain that a bit please? What is in this case the high torque engine: the tube amp or the pure class A solid state amp?

Neither. The point is that power (watts or HP)is equal to volts (potential or torque) X amps (current or rpm). The high torque engine produces the HP at lower rpm, thus, and this is debatable, at a more useable range.
Amplifiers (tube or SS)can be designed to be either 'high torque' or 'low torque' for the same power. This analogy can only go so far, so lets not push it.
Salut, Bob p.