Part 1: Preamp Tubes
Ah, tubes. We love ’em, don’t we? But to help them really love us back—and I mean with big, juicy, bear-hug love—we need to know a little bit about what makes them tick, how they vary, and what they do for our precious guitar tone. A quick and easy tube change can make the most dramatic tonal alteration achievable from a single component swap within our amplifiers, other than perhaps a speaker change, but unless you know a little about what different tubes can achieve sonically, that change will be entirely random. Welcome to a two-part feature in Premier Guitar that will give the uninitiated all the basics needed to help them launch their quest for tubehead status. We’ll also provide plenty of under-the-hood details to further bolster the knowledge of players who are already in the know about these glorious audio devices. I’ll discuss preamp tubes this issue and output (power) tubes next issue, but before diving in, let’s take a brief look at how tubes perform their sonic magic in the first place.
Once upon a time, vacuum tubes were used all over the place. They glowed their little hearts out in our television sets, car and home radios, hi-fi systems, and guitar amplifiers, and were crucial components in myriad military applications, from radar technology to missile guidance systems and more. Bit by bit they have been replaced in all of these functions by other forms of more compact and more stable technology… except in guitar amps, where they maintain their preeminence over all kinds of far more advanced electronics. Is this just nostalgia, or mere perversity on the part of guitarists? Not in the least: when used to amplify electric guitars, tubes still simply sound better than anything else out there. Sure, there are some respectable sounding solid-state amps, and digital modeling amps have also made inroads into the market, but ninety-nine out of a hundred serious pros (if not more) continue to use tube amps for both recording and touring, and these little glowing bottles still define the cornerstone tones of rock, blues, and country guitar.
To get quickly to the heart of tube magic, stop thinking of them as amplification devices and start thinking of them as tone-generating devices. A tube-based amp makes your guitar louder, sure, but tubes amplify your electric guitar so beautifully mainly because of the way they distort. To put it as briefly and concisely as possible: push a simple transistor circuit hard, and it clips (distorts) in a sudden, harsh, “square wave” way; push a tube into clipping and it distorts more gradually and more smoothly—it “rounds off” into distortion—and slathers on a gorgeous gravy of harmonics along the way. There are a lot of other factors involved, of course, but that gets us to the nut of it.
This is why any decent sounding solid-state amp requires a lot of extra circuitry to do what a very simple tube amp circuit can do naturally. And be aware, too, that when I’m talking about distortion, I’m also referring to sonic elements that influence your so-called “clean tone.” Most tube amps, even when set to clean levels (unless you’ve got the volume of a powerful amp set extremely low) are still distorting a little, and that distortion creates layers of harmonic depth that sweetens and fattens up that thing that we call our tone, even when we’re playing “clean.”
All amplification tubes carry at least four elements within their vacuum-sealed glass bottles: a cathode, a grid, a plate (also called “anode”), and a filament (or “heater”). The most basic tubes are called “triodes,” named for the first three of these elements (a filament is always present, so it’s ignored in the naming process). Pentode tubes, which account for most output tubes and a few preamp tubes, carry two further grids—a screen grid and a suppressor grid—that help to overcome capacitance between the control grid and the plate.
In simple terms, a tube’s job is to make a small voltage (guitar signal) into a bigger one. How do they do this? Pluck a string on your guitar and the pickup sends a small voltage to the input of your amplifier, where it is passed along to the grid of the first preamp tube (think of it as the “input” of this tube). The increase in voltage at the grid causes electrons to boil off of the cathode and onto the plate at a correspondingly increased rate and, voila, the sound gets bigger. This slightly bigger signal from the preamp is passed along to the output stage, where the output tubes make it even bigger, to carry it on to the speaker via the output transformer.
(Note: some people refer to the latter as “power tubes”, but I prefer “output” tubes because that better defines their function, whereas “power” might be confused with the power stage within the amp, AC/DC voltage conversion, and the work done by rectifier tubes, which is a different function altogether.)
Preamp tubes and output tubes do essentially the same thing, just with varying degrees of bigness, if you will. Tubes are literally the amplifiers at the heart of your amplifier: they do the real amplification work, and everything else inside the box is there to help them run efficiently and to help pass along the signal. Of course, in addition to early amplification duties, preamp tubes are also used for other functions within the amp: to drive reverb or tremolo stages, for example, or to split the signal and reverse the phases of the two legs that are fed to the output tubes.
Preamp tubes are easily identified, in most cases, as the smaller bottles in your amp, and are usually positioned to correspond to your amp’s inputs and early gain and tone stages. Sometimes they are covered with metal shields, which are easily removed. Since the mid-fifites, preamp tubes have mostly been of the smaller ninepin variety, although some older amps will still have bigger eight-pin (or “octal”) tubes that fit the same sockets used by many types of output tubes. The most common type by far is the 12AX7 (also known by the designation ECC83 in Europe, or the high-grade US alternative 7025).
Some other types you will occasionally see look much the same, other than the numbers printed on them. These are: the 12AT7, often used in reverb-driver and phase-inverter stages; the 12AY7, original equipment in the first gain stages of many legendary Fender tweed amps of the mid and late fifties; and the 5751, a lower-gain replacement for the 12AX7. All of these are what we call “dual triode” types, because they contain two independent tubes within the same bottle. They are mostly differentiated by their gain factor— the degree with which they increase the signal they are given. The 12AX7 has the most gain of the bunch, and the 12AY7 and 5751 are direct substitutes with less gain, which in many cases means they’ll distort the early stages of the amp less. The 12AT7 also has less gain than the “AX,” but requires a slightly different bias voltage for optimal operation (it can be directly substituted in a pinch).