Stolen from my Mesa/Boogie Manual... I have edited and condensed the article. Sorry for the long post. Also, I love my transistor amps too.
TUBES VS. TRANSISTORS
Much has been written on this topic...
The basic characteristic of a transistor is that it operates much like a simple switch -- going either full-on or full-off. That's why they're so good in computers and other digital applications. A tube, on the other hand, is basically an analog device: It's "full-on and full-off" switching operations are a bit hazy and strained because it would rather work the in between area of "partially-on", tracking a "now-more, now-less" type signal. Think of a tube behaving like its British name: a valve, regulating a flow of current that is dynamic and pulsing. Then think of the physical action of live music: it's a series of pressure changes in the air which act on the ear drum in the manner of a pulsing and dynamic flow (analog behavior) as opposed to a digital sequence of "full-pressure-or-no-pressure". So the basic nature of a tube's operation makes it the more natural choice for musical amplification. Yet solid-state amplifiers usually outperform their tube counterparts when it comes to producing great specs in the laboratory. Here's an explanation of why this is and why it doesn't matter.
DISTORTION AND NEGATIVE FEEDBACK
Negative Feedback is a correcting process used in virtually all amplifiers to "erase" distortion. It works by taking a portion of the output signal (traditionally right from the speaker jack) and re-inserting it into the input -- but out-of-phase with the original signal. By definition, anything extra produced by the amplifier is distortion. So by inverting a portion of the output signal and running it back through the amp "upside down", the distortion products tend to cancel themselves out and disappear (the original signal is also reduced so extra "gain" is designed in to more or less compensate). How much NFB is the "right amount"? It depends on the amplifier. A "perfect" amplifier would have no need for negative feedback... and in-fact your Strategy 400 uses about one-fourth as much as the previous best tube designs.
Transistor amps by contrast, use hundreds of times more NFB in order to force themselves to behave well and iron out their natural tendency to work abruptly -- like a switch.
TIME LAG AND TEST SPECS
Large doses of negative feedback certainly work better in the lab than they do in "real time" because virtually every amplifier has some degree of time-lag in getting the signal through to the output jack. An amp that needs heavy negative feedback has trouble with music because the time delay causes the corrective remedy to always lag behind the music. Distortion must literally begin to appear before it can be fed back to perform its therapy. And by then the attack of the note has already begun! And since music is complex and constantly changing, a lot of what you hear can be the amp trying to "clean up its act". This is called "transient intermodulation distortion" - TIM - and wasn't even discovered until well into the transistor era... because it was never known to occur before with tubes and their much lower dependency on corrective negative feedback. But when TIM was finally discovered, it went a long way toward explaining why the "low distortion" transistor amplifiers sounded harsh and distorted when compared to tube units. TIM was truly egg-in-the-face for some heavy designers who proclaimed that the harshness of their designs was actually the absence of distortion (their numbers backed them up) and that people would just have to get used to hearing the new sound of "truly pure, undistorted music!" Solid-state has come a long way since then. Now there are some "audiophile quality" designs available - but at three to ten times the cost of your Strategy 400, and to our ears, still not as good sounding.
All this specification controversy occurs because music itself is far too complex and fleeting for measurement of distortion under actual "real time" conditions. So a simple, steady tone is used for laboratory testing and that gives negative feedback plenty of time to act before distortion test numbers can be generated.
The distortion of your Strategy 400 remains well below the audible threshold up to -- and
even beyond clip -- because some types of distortion are far less noticeable than others. And this brings us to the other major difference between solid-state and tube amplifiers. You've probably heard such things as "tube amps clip soft, transistor amps clip hard" or "even-order distortion vs. odd order"... So here is a simple explanation of what happens when an amp clips and what kinds of harmonic distortion result.
HARMONIC DISTORTION
Think of an amplifier as a big lever where you're pushing up and down on the short end, close to the fulcrum. A small amount of motion on your end produces a much larger swing on the other end. But there's always going to be some point where the large end runs out of swinging room - for example: you've hit the ceiling. This corresponds to the amplifier's running out of power -- or headroom as it’s aptly called. The result is called clipping distortion because the tops and bottoms of the swing are limited and "clipped" off because they should have gone further if responding faithfully to the action of the short end. So as long as the swinging is confined to the region between the floor and ceiling -- isn't hitting either one -- the lever (amplifier) is operating in its "linear region". This simply means that there is a constant ratio to the leverage: push the small end up and down two feet and the big end moves six feet; swing the short end three feet and the long end moves nine feet. So in this example the ratio (also called amplification factor or gain) is three. And if we're swinging this lever in a room that's ten feet high, we want to maximize our swinging room (headroom) in both directions by locating the fulcrum half way between the floor and the ceiling. Engineers would call this "operating around the mid-point of the linear region". But what if we want to push the short end up and down four feet and the room is still only ten feet high? Obviously the big end is going to slam into the ceiling because it can't move any farther. And it's going to stay there for a moment before it begins to swing down toward the floor. Then again, it's going to crash into the floor and stay there for the period of time it would have taken to go one foot further down and back before it starts rising again. This is what happens when an amplifier runs out of power. And clipping distortion occurs because the extra swing dictated the "overdriven" movement of the short end cannot be achieved by the big end and the "linear ratio" breaks down. In an amplifier, the onset of clip is the point where the distortion increases radically by generating various added harmonics -- or overtones-that weren't present in the original signal. Now let's carry this analogy a little further and we can contrast the way clipping occurs in tube and solid state amplifiers. Picture this same ten foot high room now with a padded rubber floor and ceiling. The action of the lever's long end slamming into the elastic rubber is that it gradually comes to rest before pulling away -- not suddenly as though hitting concrete.
True, the simple linear ratio of motion no longer holds (and so there is distortion) because a four inch motion applied to the short end won't result in the proper 12 inch motion once the long end has contacted the rubber ceiling but it will move at least a few inches against the increasing resilience of the spongy rubber. And this is akin to the soft, compression action of a good tube amp when it clips, while the example of the concrete ceiling and floor fairly well represents a typical solid state amp running out of headroom. The motion ratio remains perfectly constant right up to the onset of clip where its travel is abruptly stopped and restarted again. It's this softness -- or abruptness -- that determines which harmonics are generated when the amplifier clips. And you know from your own experience with plucking a string how much the harmonics are determined by the way you set the string in motion. If the string is accelerated abruptly -- like plucking it with a dime -- you get strong upper harmonics, even if you pluck softly. But if you use something soft like the flesh of your thumb to set the string vibrating, there are almost no upper harmonics -- no matter how hard you thumb it.
LOW ORDER DISTORTION
As you probably know, harmonics (also called overtones) are what make up the timbre of each instrument so that the A440 of a clarinet sounds different from the A440 of a saxophone. The sax tone is rich in harmonics while the clarinet sound is almost pure fundamental without overtones. Certain harmonics occur naturally in music -- both in the overtones of timbre and as component notes in the harmony of chords. These overtones are often called the "low order" or "even order" harmonics such as the 2nd, 3rd and 4th. These overtones relate to the fundamental note by being one octave above (double the frequency); an octave and a fifth above (called the "quint" in pipe organs, an octave and a half above)and two octaves above for the 4th. The ear has a great tolerance for these overtones - because they relate in a musically consonant way and are already present in the original signal, as either timbre overtones, upper harmony or both. For example, it is often possible to add up to 20% second harmonic to a single note before any change is audible. Then all that happens is that the ear hears the timbre change... it does not hear "distortion trash." If you're a guitar player or keyboardist try this simple experiment. Play two notes, play the low one loudly and the high one very soft at first, then increasingly loud, an octave apart at the same time. Notice how loudly you can play the upper octave note before you can even hear it... and that what you do eventually hear sounds mainly like a timbre change in the lower note -- not two separate notes. Now try playing the same original note but this time also play a note two octaves plus one step above. No matter how softly you play this note, it is distinctly audible and separate from the original.
What we've done with this experiment is demonstrate the effects of different harmonics in contrasting distortion types. You've heard how inoffensive the low order (sometimes called "even order") harmonics can be... especially compared to the prominence of the upper order (often called "odd order"). And as you might guess by now, tube amp distortion is primarily low order while that of most transistor units is odd-higher order... and hence very objectionable. Clearly then, a tube power amp with a higher percentage of Total Harmonic Distortion actually can sound cleaner and clearer and much less harsh than a solid state amp which carries a lower total distortion rating. It's not the total distortion that counts as much as the distribution of distortion components: It's all but impossible to hear even a few percent of 2nd harmonic distortion whereas a fraction of "high order" type becomes audible as annoying hash.
When playing very loudly, you may notice all the LED power indicators are lit up on your Strategy 400 - and yet we seriously doubt that you'll hear distortion - even though clipping begins when the 0 db LED lights up. Two things are happening here: First, special care has been devoted to avoiding high-order overtones and containing the Strategy's clipping distortion to primarily 2nd harmonic. And second: The Strategy has high dynamic headroom and can produce substantial extra undistorted power for short (but musically significant) periods of time. Musical peaks - such as the attack of certain notes - are so highly dynamic that they demand 20 to 50 times the average power produced by the amplifier to come through unclipped. So good sound requires more than just plenty of brute power. It demands good performance after ratings have been exceeded as well as before because some clipping is almost unavoidable.
CONCLUSION
This discussion has concentrated on the extreme end of power performance because that's where the differences between amplifier types are most noticeable. And, we expect, extreme power performance is one big reason for your interest in the Strategy 400. Rest assured that all those factors that enhance high performance are also benefits at other times. Thanks for reading this far, we hope this explanation has bettered your understanding for the science --and magic -- of great sound. Most of all enjoy your music and enjoy your Strategy 400 Stereo.
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