There are a lot of mysterious components within any guitar amp, many of which remain puzzling even to hobbyists who have built a DIY project or two. In the third part of Mojotone’s new series What Does This Thing Do? we’re taking a look at preamp tubes, the smaller tubes whose essential function enables the rest of the amp to crank up the volume.
We’re here because we love the sound of tubes in guitar amplifiers, right? But even if you feel that’s something you know inherently, which tubes are you actually listening to when you’re enjoying that “great tube tone?” Several different types of tubes perform different duties within any all-tube amp—with more types and a greater variety of duties existing within the more complex and feature-lade amps available—so it’s worth isolating the job of each somewhat in order to better understand how they’re all working together to make the amp function.
First off, it’s worth emphasizing that none of the tubes within any guitar amp should be considered to have “a tone” all on their own, irrespective of the circuit in which they are being used. Other components such as capacitors and resistors help to voice an amp by determining the general frequency range in which it operates, and the range and function of its tone controls and so forth. That aside, different tubes do also present different sonic characteristics, and this means that even different types used to perform the same function can sometimes slightly alter the overall feel and sound of an amp.
As a rule, the preamp tubes are the smaller tubes in most types of guitar amplifiers. They’re generally found positioned nearer the input, usually arrayed between the input and the larger output tubes. So-called preamp tubes might be used in different stages of the amp, however, which can include the preamp proper (the first gain stages of the amp, or extra gain stages which follow other stages); EQ stages; any built-in tube-driven effects such as reverb and tremolo; and the phase inverter which feeds directly into the output tubes.
The vast majority of preamp tubes are of the nine-pin variety (sometimes call noval), which became popular in the mid 1950s as a so-called “miniaturized” tube type to replace the somewhat larger eight-pin (octal) preamp tubes that are still found in some vintage amps and retro designs. The 12AX7 is far and away the most common nine-pin type, which you’ll often see listed as its British designation ECC83, though some equivalents and similar-looking, though differently-functioning, nine-pin preamp tubes are also in use. 12AX7s and their equivalents are dual-triode tubes, which means they contain two small amplification units in one bottle, each made up of one element, plus a shared filament, or “heater”. Some other nine-pin preamp tubes still in use today such as the EF86 or the 5879 are pentode tubes that contain only one amplification unit, but made up of five elements. (For that reason, don’t try to swap one for the other, and always check equivalency and compatibility before swapping any tube types.)
In short, the job of the preamp tube—when used in an actual preamp stage rather than an effects or phase-inverter stage—is to take a low-voltage audio signal that comes into it and ramp that up to a higher-voltage signal that is sent along to the next stage of the amplifier. In most common guitar amplifiers, the first significant thing the very low-voltage signal from your guitar pickups hits after it goes through the amp’s input is a preamp tube that comprises that amp’s first gain stage (usually via just a single resistor and a short length of wire). The preamp tube or tubes that do this job don’t transform the signal into one that can drive a speaker, but they quite literally amplify it to a point where the amp’s output tubes can receive it, amplify it even further, and send it on to an output transformer, which can indeed drive a speaker.
A preamp tube doesn’t physically “shape” or “voice” the guitar signal in and of itself, but each time the signal hits a gain stage all along the signal chain from input to output stage the entirety of it is amplified by that tube, so the way the tube performs this amplification task has a significant impact upon the resultant sound and feel of the signal. And different makes and types of preamp tubes do their thing a little differently—less or more efficiently, cleanly vs grittily, tightly vs more compressed, and so on. Some preamp tubes distort earlier or later, react relatively quickly or slowly, are predisposed to enhancing the highs or lows or midrange, or whatever. However slight these differences might be, they can add up to noticeable variations in tone from different tubes plugged into the exact same circuit.
When driven hard, preamp tubes can also distort, and will particularly distort when one preamp tube is driven hard into another, and perhaps yet another, along several chained-up gain stages in some high-gain amps in particular. As much as some players like to rave about “output-tube distortion” (something we’ll cover more in a future installment) the majority of distortion you hear from most tube amps is either largely or at least partly the result of preamp-tube distortion.
All of this adds up to tell you that, as far as altering the overall tone of any guitar amp via tube swaps goes, a change of preamp tube often has the biggest effect. And of those swaps, a change of tube in the first gain stage—what’s often referred to as “V1” (for “valve one,” using the British term for “tube”)—makes the most significant impact on tube-induced tonal tweaks. Fortunately, you can almost always change preamp tubes within your guitar amps without making any internal adjustments to the amplifier. Let them cool down before swapping, wiggle them gently on the way out, line up the pins accurately for replacement (you’ll see there’s a gap in the array to help you fit it into the socket), and push gently but firmly into place.
Hopefully understanding a little bit more about what preamp tubes do enables you to better fathom how they may—or may not—help fine-tune your amp’s tone, and get you a little closer to sonic bliss as a result.