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Our hobby/profession of music is entirely subjective, as opposed to my real job as an engineer designing electronic products where a few nanoseconds separates a smash hit from a failure.
That subjectivity has really given me some chuckles the last few years with the resurgence of vacuum tube technology. In every magazine, I see the word "warm" to describe that tube sound. I have heard the difference between "analog" and "digital" myself. As a guitar player, I will never part with my old tube gear. But the hype is getting excessive. Let's look into it.
I learned electronics in the transition era between tubes and transistors, when there was still a decision to make with every purchase: tubes vs. solid state. The new solid state gear was touted as having vanishing distortion levels, and the 1970's saw an editorial war of one-upsmanship on solid state distortion specs -- just check out stereophile magazines from the period. At that time, the effort was to reduce the distortion of amplifiers, not increase it.
Even before that, high end tube audio gear (that is, for studios) was designed by engineers for the best performance. Techniques exist to reduce distortion in amplifier designs, and these were put to good use. Their goal was 0% distortion, nothing more.
Get into your vacuum tube time machine and travel back to 1960. Slap an electrical engineer on the back and compliment him for that "warm" tube sound. He looks puzzled and says, "Warm? What do you mean?" Go on to explain that at the end of the century, solid state devices have stripped sound of all its flavor, rendering it so squeaky clean that it is offensive. Tube amplifiers, on the other hand, add a certain distortion to the signal that makes it sound better.
The response: "Distortion? You are accusing my amplifier of producing distortion? I worked two years on this design! You better get your 'warm' little butt out of here before I kick it back into the future, without the benefit of your time machine."
You would get the same response from an audio engineer when accusing him of running his VU meters in the red just to get that "warm" analog tape saturation. That used to be called incompetence. But that was then and this is now, where we have music networks based entirely on the concept of lip syncing, and what formerly was stupid is now a big money maker.
Anything with the word "tube" on the label sells, have you noticed? As an engineer who has designed tube and solid state amplifiers, I know that the tube is only one component in the aural equation. Contributing to the tube sound is also a poorly regulated (or unregulated) power supply, transformers which are inherently nonlinear, and circuit configurations which can change the relative levels of even and odd harmonics. Solid state designs do not suffer these problems (or benefits). Just putting a tube in the box does not recreate the audio nirvana of the 1960s.
My point is, let's tone down the hype and not rewrite history, as if the oldtimers planned all that distortion. They were using the best equipment available at the time, running the VU needles at the right place (not in the red), and being very careful not to overdrive any component in the signal chain. And a good job of it they did.
Why Does Tube Gear Sound Different?
Tube gear is designed differently from transistorized gear because there is a fundamental difference between tubes and transistors. It's an impedance thing. Tubes are inherently high impedance devices (high voltage, low current), and transistors are low impedance devices (low voltage, high current). Those differences drive the design decisions.
Tube amplifiers have the following components and characteristics:
Transistor amplifiers have the following components and characteristics:
Let's look at the differences and how they affect the sound.
The power supply in an amplifier provides the raw direct current that is transformed into the audio power you hear. A tube amp has a power supply voltage of 400V to 600V (volts). A transistor amp has a voltage of perhaps 24V to 48V.
When you bang out a loud chord, the output stage of the amplifier demands a load of current from the power supply all at once. This causes the voltage to drop a bit.
An unregulated supply has no strict control of the output voltage, so large demands cause significant drops in the output voltage, and corresponding distortion of the output signal (that bite you love). Most tube amp power supplies are unregulated because the cost of the high voltage regulator tubes was considered prohibitive. Transistor amps use regulated supplies in many cases (or supplies with better filtering), because they are designed for lower distortion. Thus, the unregulated supply in a tube amp adds to the warmth or distortion of the sound.
The output transformer in a tube amp has much the same effect. A transformer is like a power conduit, moving power from the tubes to the speaker, changing the impedance in the process from thousands of ohms to 4 or 8 ohms. However, the conduit has a limited capacity. When too much power is demanded, the transformer cannot handle the signal peaks and softly rounds them off, causing distortion. This happens with interstage coupling transformers as well, present in some amps.
Transistorized amps typically have no transformers because they do not need the impedance transformation to interface with the speaker. That source of distortion is not present.
Higher power tube amps use a tube configuration called push-pull. This setup uses pairs of tubes to double the output power over one tube. It also has the effect of canceling even harmonics* across the output transformer, making the sound warmer. The tube amp, when producing distortion, produces the subjectively pleasing distortion that guitar players love.
Negative feedback is something that most of us hate, but amplifiers love it! That's because negative feedback reduces the distortion of an amplifier. However, this feedback reduces the amplifier gain and can only be used if there is sufficient gain to make up the loss.
With tube amps, adding more gain involves adding another tube stage, and the attending cost. With transistor amps, adding another 10 cent transistor is no big deal, so those amps usually have a larger raw gain and more feedback, lowering distortion considerably below the level of a tube amp.
I have a tube amp where someone added a switch to open the negative feedback loop. It adds about 3dB gain, but a lot of distortion, making the amp sound a lot hotter. (You can have a local amp technician make this modification to your amplfier.)
So we see that there are many reasons that tube amps sound different from transistorized amplifiers, not just the tubes.
The next time you are tempted to buy something just because it has a 12AX7 tube in it, remember what you learned here. If the tube circuit is running on a 9V battery, then the tube cannot be running on the voltage it was designed for (150V+). What you get may sure sound good, but it is not an authentic tube preamp. Ask about what's behind the front panel to tell the quality of the product. And please don't be suckered into buying tube-based cables, capos or cappuccinos!
What About Tube Matching?
When vintage tube gear was not vintage, but new, as in the 1960's or 1970's, it was often not possible to buy matched pairs of audio tubes off the shelf. If you wanted a matched pair, you had to order them or pick out a good pair using an instrument called a curve tracer, or perhaps a decent tube tester. The curve tracer plotted the characteristics of each tube, allowing detailed comparison. This was before computers, so there were no printouts or spreadsheets; matching tubes was an expensive manual operation. Not many guitar amp owners even knew about tube matching; it was the territory of audiophiles.
But do you think that Fender or Marshall actually matched tubes by hand? No way! If anything, they made the bias controls individually adjustable for each output stage tube. (I have never seen a bias control on any preamp tube.) In most cases, the bias controls were adjusted once at the factory (if any were present at all), and never again.
A good engineer designs amplifiers so that inexpensive commercial grade tubes and transistors can be used without the necessity for matching or special selection. As an engineering manager, one of the red flags I looked for in any design submitted for my approval was the presence of adjustment pots, matched parts, or parts marked SOT (Select On Test), because that drives up production and labor costs. Vintage tube amps were designed to tolerate the use of off the shelf components, for the most part without adjustment.
It really gives me a snicker to think of installing matched tubes in old amplifiers which are loaded with 5% and 10% tolerance components! So, even if you buy the matched tubes, you still have to adjust the bias controls to compensate for all the other sloppy components in the amplifier, many of which have drifted over the years. If you are going to adjust the bias anyway, then why buy the matched tubes in the first place?
I was working on a tube amplifier recently and found that the grid and screen resistors on the output tubes had drifted 50% to 100% higher in value since the 1960's when it was built. I also had to replace the power supply filter capacitors which had finally given out. If you have such an amplifier, you would do well to have it checked over by a qualified technician for drifting components, which will change the tone in unexpected ways that cannot be compensated for by matched tubes.
However, like other "premium" products for musicians, matched tubes won't hurt your amp, as long as they are installed properly. I expect that the tubes we buy today are actually much better quality than what we had in the 1960's due to modern statistical process control.
* Regarding harmonics in tube amplifiers, I read all sorts of explanations in the musician's press on this topic. I remembered from school (many moons ago) that push-pull amplifiers attenuate even harmonic distortion products because of their architecture, but several articles I have read indicated that it's the even harmonics that are pleasing to listen to, guitar-wise.
Reviewing my college textbooks once again, I find that a class B or AB push-pull amplifier in fact reduces the even harmonic content over a class A amplifier.
If you know a little about waveforms and their construction, this makes sense. For example, listen to a lead synth patch and you will hear a square wave. Think of Heart's synth lead in "Magic Man," or the synth lead in Emerson, Lake and Palmer's "Lucky Man." That fat sound is easily reproducible by a guitar player using an overdrive pedal. That waveform can be constructed using only odd harmonics of a fundamental frequency. That's what makes the fat, warm sound, the odd harmonics.
Even harmonics, if used to construct a waveform, make a triangular looking waveshape. The sound of that is quite harsh. The movement of a bow against a violin string makes a related sawtooth type of waveform. In the hands of an accomplished violinist, it can be beautiful, but in my hands it's only even harmonic dog bane. (Ask my dog.)
Class A amplifiers for guitar have a rich mixture of even and odd harmonics. This makes them sound a little more raw and untamed than their push-pull big brothers, which are lacking the even harmonic flavor.