Need help understanding tube wpc

My equipment has always been solid state so bear with me (i'm sure this has been asked before but having trouble finding the threads) . I don't follow the wpc differences between SS and tubes and how to match tube power with speaker efficiency to ensure that they'd be driven okay.

Theoretically, I don't believe there is any electrical difference between a ss watt and a tube watt--a watt's a watt. But lots of people think that, since, among other things, tubes overload more gracefully than transistors, you don't need as many 'tube watts' as you do 'ss watts'. I've never seen anyone state a 'factor' by which you could multipy (or divide) ss watts to get 'equivalent' tube watts,,,but maybe someone here knows.
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.

Tubes typically have FAR higher rail voltages than SS amps, which therefore gives them more dynamic headroom. Tubed amps also clip in a "harmonious" manner, making them less objectionable when overdriven. This means that a smaller tubed amp can play "louder" than one would think, as it has quite a bit of dynamic headroom and doesn't rip your ears / eyes out when it does clip.

The biggest problem with tubed gear is that it typically lacks current and is bandwidth limited, both on top and bottom. The lack of current is what gives most tubed gear that "round, tubby" bass that many folks dislike. At the same time, this "added warmth" tends to "fill out" many of the leaner digital recordings that we hear. The limited bandwidth up top tends to soften the treble response, making hard, bright and edgy digital sound smoother and more listenable.

Like any other amplifier, you want to look for a design that utilizes very large core transformers. With tubed gear, the output transformers are as important ( maybe moreso ) as the power supply transformer. As such, look at the build quality of the amp more than the actual power rating, as most tubed power ratings are "bunk" anyhow. If you applied the same standards for measuring distortion in tubed gear as one does to SS gear, most of these tubed amps would be rated at about 1/4 to 1/3 of what the manufacturer advertises.

There are obviously designs / products that fall outside of these basic recommendations i.e. output transformerless ( OTL ) designs, digital power supplies, etc.. These are all non-standard designs, so the "rules" are apt to be different for these products. In such cases, proceed with caution and just make sure that you're dealing with a reputable manufacturer. Sean

PS... Buy more than what you think you need, as running tubes quite hard on a regular basis also means replacing tubes on a regular basis.
I like Sean,Rl,Viris comments and would ad that regardless of tube/ss when an amp is capable of X watts/ch RMS it is capable of a different number of watts for an instantanious peak. A better amp can do a peak perhaps 50-100% above the rated RMS power. This is based on a better power supply and design, etc. Music is peaks. Not steady. Then there are Class design differences too. There are a lot of factors involved. Good luck!

watts is watts, which means if if you get more usable watts because of soft clipping then you got more watts!

Someone mentioned distortion, distortion can be percieved as being 10 times louder than it really is. Accepted practices of one percent distortion setting the standards for the amplifiers quoted power rating as Sean mentioned makes most 60 watt tube amplifiers like 20 watts.

But unfortunatley everyone missed the point and continues to miss the real reason tubes sound louder than solid state.

See figure 9 on the link above.

Halcro DM88 Monoblock is playing at 100 watts and look at the complete lack of harmonic content with a 50hz input

Now compare.

Yamamoto tube amplifier look at the harmonic by products which are ADDED to a simple 50hz signal, now these byproducts do not effect the voltage or WATTS, so in this case the tube 100 watts offer a great deal more sound than the SS watts.

The factor I just demonstrated with the comparison and the soft clipping has everything to do with tube watt sound like more than a solid state watt.

But depending on your speakers getting tubes may be a bad idea as speakers are not always compatible with two very different amplifiers. Many speakers are not designed well enough to have solid state run on them and the ones that are do not sound very good with tubes. So your speakers are going to determine which amplifier type is best for you.
Thanks for all of your input - getting a real education.
My speakers are Tylet Linbrook System 2 - 8ohm 89db are there certain tube design amps that would not work well with these. Have seen that Ty uses Jolida at times during shows?
Here is an article from Stereophile that delves a bit deeper into the subject and suggests that tube wattage and ss wattage are not the same; but we already knew that.
Czbbcl: ???
All that article seems to say is, what Sean mentioned above + that tubes are current "power" devices whereas SS are voltage devices. They act differently under load.

However the basic spec of "power" remains the same.

You, yourself as well as scores of others have known this for a long time now.
D_Edwards offered:
>>Yamamoto tube amplifier look at the harmonic by products which are ADDED to a simple 50hz signal, now these byproducts do not effect the voltage or WATTS, so in this case the tube 100 watts offer a great deal more sound than the SS watts. <<

Ummm, D, you may want to take ANOTHER look at those graphs, the Yamamoto is being measured at 100 mW, that's 1/10 of a watt versus the Halcro being measured at 100 W. The Halcro is pumping out 1000x the wattage. And, why in the world would you want your amp ADDING distortion to the signal?

Frankly, the Yamamoto looks like a WAY under-powered POS to me.

Watts are watts, whether ss or tube. How loud an amp can play without objectionable distortion (to the ear) is related to how gracefully it overloads. Tube amps are better in this in that they use less negative feedback. If the same excessive feedback was applied to a tube amp as typically used in ss designs, the overload characteristics would be just as bad if the amp was dc-coupled and far worse if ac coupled as most tube amps are. Fortunately, engineers have not been able to apply large amounts of negative feedback in tube amps because phase anomalies in the output transformer won't allow it.

Of course, lower amounts of negative feedback results in higher output impedance of the amp, resulting in measurement specs gurus freaking out.

David Berning
What the specific test results that D. Edwards linked to tries to point out is that many ( NOT all !!! ) tubed designs typically sound "richer, fuller & airier" because they are generating TONS of harmonic distortion.

These harmonics tend to make "sparse" sounding digital recordings sound more natural, hence their ability to not only "warm things up" due to the aforementioned lack of bass control / current output, but also to "breath life into" these recordings. They do this through emphasizing all of the harmonic overtones that may / may not have been present originally.

If you look at the Halcro vs the Yamamoto's spectral content, the Halco produces the primary signal and is -100 dB's down by the second harmonic. The Yamamoto on the other hand, is only about -40 dB down. This means that the Yamamoto is producing 60 dB's more output than the Halco at this point, let alone WAY more output across the entire spectrum. It does this at any given time or amplitude. Bare in mind that the Halco is doing this at 100 watts of output whereas the Yamamoto is at a watt or less !!!

Quite honestly, this is NOT a very fair comparison, but it does make a point. Using a poorly designed and possibly misadjusted ( read the text ) SET tube amp to compare to a high negative feedback SS amp just isn't fair or right. Maybe as an example of what is possible on both extremes, but not as a general example. Most designs are going to be somewhere between these two extreme examples.

On top of that, i really don't think that anyone shopping for something along the lines of the tubed Yamamoto ( rated at 2 wpc ) is going to be comparing that to the Halcro. They are completely different design approaches producing very diffferent sounds and system compatability issues. Whereas the Yamamoto might sound "larger than life" due to all of the spurious harmonic distortions added, the Halcro tends to sound thin, sterile and lifeless. Most of that is probably due to use of too much negative feedback.

Whereas the Yamamoto is a prime example of why some people refer to tubed audio gear as "distortion generators", some would say that the Halcro is a prime example of why some categorize SS gear as being "unmusical, lifeless and sterile". While one could be said to be "more technically correct than the other, it all boils down to system synergy and personal preference. After all, the bottom line is building a system that you can listen to and enjoy the music without being fatigued, annoyed or distracted by the gear itself. Some do that with tubes, some do it with SS. Some use a combo of the two, looking for the "added harmonic richness" of tubes with the speed, focus and authority of SS. Whatever you choose to go with, just make sure of one thing. That is, it makes YOU happy, as YOU are the one that has to use and listen to YOUR audio investment : ) Sean

As usual, Sean nails it!
There is NO reason that an SS amp couldn't make use of very high rail voltages like that of a tubed amp. I've mentioned before that the amp with the highest rail voltages and greatest current capacity would be the most universal in application, so long as the rest of the circuit was fast and "sounded good". Sean

PS... Whereas tubes are primarily a "voltage device", transistors are a "power device", hence their ability to double down when properly designed. Having the extra current capacity doesn't necessarily make up for the extra voltage capacity that most tubed designs bring with them.
"The biggest problem with tubed gear is that it typically lacks current and is bandwidth limited, both on top and bottom. The lack of current is what gives most tubed gear that "round, tubby" bass that many folks dislike. At the same time, this "added warmth" tends to "fill out" many of the leaner digital recordings that we hear. The limited bandwidth up top tends to soften the treble response, making hard, bright and edgy digital sound smoother and more listenable." [sic]

This statement is patently untrue, but is a very common misconception. There are tube amps with bandwidth to 100KHz and there are tube amps with LF cutoffs as low as 1Hz at full power. Some of these are the same tube amps. So bandwidth is clearly *not* the issue.

Similarly, lack of current has nothing to do with this either. Ohm's Law (which is inviable, BTW) reveals that a shocking (no pun intended) low amount of current is needed to drive low impedance speakers to quite high powers! The 'high current' mindset is an outcome of the introduction of large amounts of feedback in transistor amps, which is one of the major reasons that SS amps have more odd-ordered harmonic content than tubes.

In fact the issue of tube vs SS power does have to do with the rules of human hearing- which audiokinesis outlined earlier. Humans are sensitive to odd-ordered harmonics and transistors make more of those than tubes. To get around the problem you have to have a very big transistor amp so you don't come anywhere *near* clipping.

The idea of voltage rails having something to do with this is incorrect also. The voltage 'rails' merely determine how much power the amp will make- tube *or* solid state- it does not describe headroom at all. As a specification, headroom is more a function of the class of operation (class B amplifiers having the *most* headroom), but it turns out that class A amplifiers carry more authority, and for their size tend to behave as if they have 'more power'. What they *really* have is more *usable* power, and that is what this thread is all about- how much *usable* power the two technologies have.

My experience has been that in general, a tube amp will have the same amount of musical *usable* power when it is between 1/10 and 1/4 the power of a transistor amp. Variables that throw this generalization off are class of operation ( for example, a class A transistor amp will have more *usable* power), elegance of construction (don't expect a $500 SS amp to do what a $5000 SS amp of the same power will do) and the like.

Definitely muddy waters!
Forgive me for generalizing, but most tubed amps SUCK in terms of their power bandwidth. This also means that their transient response sucks too, hence the amount of low frequency tilt and rounded square waves that are seen in most test reviews. Yes, there are units that don't follow this trend, but they are a small portion of what is available out there.

The Atmasphere amps, as far as i know, are the fastest mass produced tube amps that i'm aware of. As such, one can see why Ralph would take umbrage at these statements. He doesn't want his products "lumped in" with those that i am criticizing.

The higher the rail voltage, the less likely the amp is to clip. So long as the amp can deliver the current needed, the voltage doesn't sag and larger peaks can be delivered without hesitation or distortion. This maintains a higher level of OPERATIONAL headroom than an amp that can deliver the same amount of current, but with a lower rail voltage. After all, musical peaks are voltage driven, not current driven.

The only time current comes into play is when speaker impedances require it. Selecting a speaker that maintains a higher than average impedance without any radical phase angles makes for an easy speaker to drive. In such a case, a VERY low current amp with a reasonably high rail can easily get the job done. This is how / why some SET amps, with their miniscule current capacities, can drive some speakers phenomenally well.

When one starts using low impedance loads, long excursion drivers that generate a lot of reflected EMF and / or highly reactive loads, the amp MUST have high current capacity. If the amp doesn't have the current it needs, the voltage sags and linearity is lost.

In extreme cases like this, the amp temporarily "loses control" of the driver diaphragm. The end result sounds horrible, especially with larger, higher mass woofers. In some cases, you can literally hear the voice coil "bottom out". The lack of control from the amp in such a situation coupled with the high velocity movement of the driver mass results in enough kinetic energy to "slam" the driver against its' mechanical limits. Take my word for it, JBL's sound especially bad / scary when this happens.

As far as Class of operation goes, the lower the bias, the more operational headroom the amp is likely to display. That is, all things being equal ( HA HA HA ). Thermal stress lowers maximum voltage and current capacities, so lower operating temperatures are typically a desirable thing. At the same time, the higher levels of bias that generate "bad" levels of heat also display the highest levels of linearity. As such, design decisions and production trade-off's have to be made in order to produce a reasonably priced marketable product. This is why we have more AB amps on the market than just Class A or Class B. They strive to achieve the linearity of Class A at lower levels with the lack of thermal stress / cost cutting of Class B at higher signal levels. Switching amps take this even further, and depending on their design, can offer some very real benefits in several different areas.

We could go on and on and on and on here, but if one does as i suggested i.e. buy more than you think you need, you'll pretty much be covered. Just make sure that it sounds good in YOUR system to YOUR ears. If it sounds good AND measures good, you've probably got one helluva good sounding component and / or system. Sean

Yes you are correct the equation for how you calculate power is the same. How that power reacts to real world loads is another story. In the end all that matters is how it sounds to the individual and not how some set of equations says it should sound.
11-22-06: Sean “If it sounds good AND measures good, you've probably got one helluva good sounding component and / or system”

As opposed to it sounds good but measures bad then you have a bad system? Sorry, I cannot agree with that. Some of my amps have THD numbers as high as 3%, but can reproduce piano recitals with more realism than anything I have ever heard, and believe me, I am intimately familiar with piano.

Measuring is probably very useful on a production/assembly line, but I have never found any correlation between measured numbers like THD and what the amp sounds like to my ears. In fact have heard amp with excellent THD number that are useful only as doorstops. I know, I own a few amps like those.

There is only one measure that determines how a component sounds like - your ears. In my experience the folks let their ears be led by measurements have the least realistic sounding systems (read, total crap sounding systems) out there. And yeah, I have heard many of them too.

As i've mentioned before, you have to look at ALL of the pertinent specifications on the whole and understand how the results were achieved i.e. specific test methodology used. Singling out one spec on ANY product in the world won't tell you ANYTHING about how well it works in ANY given situation, let alone on the whole or universally. Audio components aren't any different. Sean
Pauly, No, no:
As opposed to it sounds good but measures bad then you have a bad system?
Not at all. You surely know the quote: "if it measures well and sounds good -- it's good. If it measures bad and sounds good -- you're measuring the WRONG things" ;)
"After all, musical peaks are voltage driven, not current driven."

In the world I live in, musical peaks being reproduced by a power amp produce power, not merely voltage. Voltage in a speaker cannot exist without current and this is inviably defined by Ohm's Law and the Power formula (Power=Voltage X Current).

So- a musical peak delivered by a power amp has a voltage component- *and* a current component. Since the peak represents a peak in power as well as voltage, current must therefore be peaked also.

The idea that the peak is voltage driven comes from the Voltage paradigm I've mentioned in a few threads here. What I've not mentioned is that paradigm is actually that- a paradigm, and not one based on reality. It is in fact an artifact of the 50s and 60s when transistors were making their way into audio. A central precept of the Voltage paradigm is the use of negative feedback (ostensibly to reduce THD); such use is in violation of the rules of human hearing which we all subscribe to by default.

The violation, for those curious, is the addition of odd-ordered harmonic content that negative feedback brings.

A curious artifact of the Voltage paradigm is that nothing but voltage matters in the response of an amplifier. Another is that 'voltage source' amplifiers are also defined as 'high current'. Yet another is the idea that speakers are 'voltage driven'. English speaking people will note some contradictions.

The Power paradigm aims to correct these oddities. First, *all* speakers are power driven. All power amplifiers produce power. 'High current' does not exist for musical reproduction without the generation of power, and the same is true of voltage. In this way, the power formula and Ohm's Law are satisfied within the conversation of power amplifiers and speakers and at the same time the meanings of English words are also satisfied.

Within the conversation of this thread, the issues relating to why tubes are somehow able to produce more *usable* power when their total power is less than that of transistors is easily revealed by the Power paradigm, which has it roots based on the rules of human hearing, rather than a thought model conceived to sell transistor amplifiers in particular.

The answer is that tubes generate power in a way that satisfies more of the rules of human hearing than transistors do. For example, SETs get their dynamic punch out of their harmonic generation: odd ordered harmonics are masked by even orders, so while the amp sounds lush, the odd orders are triggering the human ear to hear dynamics on peaks. It is an illusion.

Other, lower distortion tube amplifiers still manifest a greater percentage of *usable* (musical) power by the simple use of components that are inherently more linear than transistors, and usually with less stages (meaning less places for things to get messed up), objectionable distortions are minimal, resulting in little or no feedback being required for the amp to do its job. Thus the human ear is not *as* offended, and the bottom line is more of the amplifier's power generated can be used for meaningful musical reproduction.

In the Power Paradigm the rules of human hearing are not ignored so an amplifier can measure well and sound good too because the pertinent specifications that are important are the ones that get measured.

In the Voltage paradigm, as Pauly points out:

"I have never found any correlation between measured numbers like THD and what the amp sounds like to my ears. In fact have heard amp with excellent THD number that are useful only as doorstops."

-that there is no correlation between specs and sound. There is a huge disconnect here! The Voltage Paradigm seems to equate to the Emperer's new Clothes. After nearly 50 years- *that* would seem to be a little old :)
This is simple Ohm's Law. One can try to make it as complicated as they like, but it is actually just as simple as it sounds.

The amount of current needed in the circuit is dictated by the impedance of the load at the frequency that is being reproduced. After that, it's all a matter of whether or not the amplifier can swing enough voltage in time to keep up with the amplitude of the signal as it changes on a dynamic basis.

Since we've already acknowledged that high current capacity is not as necessary as most would think with reasonable speaker loads, the only reason that an amp would clip has to do with running up against a limited voltage capacity and / or the inability of the circuitry to slew fast enough to deliver the voltage required. Both result in clipping, whether it is due to limited headroom and / or a lack of speed.

With high rail voltages, high current capacity, high speed / wide bandwidth and good circuit stability, you can drive any load that you want with reasonably low levels of distortion. If you limit even just one variable in the aforementioned list though, the versatility and performance potential of the circuit is drastically reduced. This dictates more careful matching of the associated load ( speaker and speaker cables ) to that of the performance limited amplifier.

Since all amplifiers are limited in one way or the other to some degree, and no speaker is purely resistive with a higher nominal impedance, some matching is always required. Having said that, the more competently designed the amp, the more consistent the performance that it will deliver, regardless of the load. Obviously, one can go to extremes coming up with radical speaker loads that might embarrass all but the most advanced amplifier circuitry, but as a general rule, these are just that i.e. extremes and not the norm.

If speaking of extremes, anything is possible and specifics must be mentioned if we are to have any type of meaningful conversation. I have strictly been speaking in general terms, as i've not seen any mention of specifics in this thread. Sean

PS... The fact that Ralph and i are "debating" should tell you folks quite a bit. That is, even though we both find many of same attributes very desirable i.e. wide bandwidth, high speed, reasonably low distortion, low negative feedback, high stability, high bias circuits, etc... there is still plenty of other things for us to disagree about. This is one of the reasons there are SOOOOO many different products out there with different designs.

As can be seen by our responses here, most of this boils down to what the best way is to achieve all of those goals simultaneously without having to cut a lot of corners to get there.
Not to change the subject, but i wanted to point something out. Ralph aka Atmasphere said: "Since the peak represents a peak in power as well as voltage, current must therefore be peaked also".

This is not always true. When a speaker resonates, it produces a peak in output, even though voltage and current input from the amp are at a minimum due to the high impedance encountered. How intense this peak is has to do with the alignment of the speaker system and how well the amp can control the reflected EMF.

Some speaker designs seek to tame this form of "self oscillation" ( low Q sealed designs, transmission lines, etc... ) whereas others try to exploit it ( many vented designs ) in hopes of getting "free output". That "free output" doesn't really come "free" though, as this type of system typically lacks damping while producing rather inarticulate bass at or near the point of resonance(s).

Whereas "uncontrolled oscillation" is typically frowned upon anywhere near the audible bandwidth in every type of component that i can think of, this seems to be the accepted norm with many vented loudspeaker designs. This has always baffled me ever since i understood the sonic and technical differences in various bass alignments. Sean
In AC circuits there is often an inductive nature that can result in current lagging behind voltage. So it is true that current may not be peaked in all cases of loudspeaker; the more 'resistive' the speaker (i.e. magnetic planars, Avalon) the more the current will be peaked with the voltage.

Nelson Pass wrote a very interesting article in Audio Express about 2 years ago that offers some interesting insight into the issue of 'high current'. I agree with his conclusions, although it would be easier to understand if his language were not part of the Voltage Paradigm conversation (although for those who only understand the Voltage paradigm, his text is a nice introduction to some very real issues!). His more recent designs are clearly based on Power paradigm concepts.

I agree that Sean and I probably do have similar concerns regarding amplification! -despite the fact that we (apparently) have very different ways of going about it.

I prefer an amplifier with both wide bandwidth (I play bass so I need good 20Hz square wave response to be convinced) and low distortion if I can get it without putting a sheen or hardness up top. Having done a lot of live recordings over the years I have to say I've gotten picky and have yet to hear an amp with negative feedback that sounds right (although there was a time when I thought all that stuff was just fine- my personal victory over the past is to have found the breakthrough results that left that behind).

I do recommend to any audiophile who is trying to create a reference-quality system to start with a set of condenser microphones, an Ampex tube recorder (I use Neumann U-67s and an updated Ampex 351-2) and some living musicians. Create a library of recordings in different venues; very qucikly you find out what a large variance exists between tube and transistor playback!! Its great to spend time at live concerts but you are powerfully in touch with what is happening when you also have decent recordings of them!

I hope y'all had a great Thanksgiving!!