Photo by Andy Ellis

Most guitarists who habitually play tube amps understand that, to keep their rigs in optimal shape, it’s vital to replace those glowing bottles on a routine basis. Fewer players, however, explore the very real sonic variations that can result from careful and considered tube swapping. Different brands of the same kind of tube might each sound a little different, but you can achieve more dramatic results by auditioning the several different types of tubes that are compatible in the same preamp-tube sockets and circuit positions. The effects of such thoughtful tweaks reach beyond the realm of tone into the world of gain and headroom. Because those qualities are the central defining characteristics of most guitar amps, even swapping just one crucial preamp tube for a different type can entirely alter the feel and response of your favorite amplifier.

Let’s investigate how several different tube types can change your amp’s sound and overdrive content when swapped into preamp and phase-inverter positions. Maybe you’ll discover a tweak to fine-tune your sound.

Everything to Gain
You may already be familiar with the term “gain staging,” a process that controls how the different gain stages within your guitar amp (or pedalboard, in some instances) flow from one into the next. In brief, the amount of gain added to your guitar signal at any stage within the amp’s circuit—and the degree to which that is further ramped up or tamped down as it is passed along from stage to stage—is arguably the most significant factor in shaping the character of any classic or modern guitar amp. (For an in-depth look at the subject, read “All the World’s a Gain Stage.”)

The tube types we’ll discuss next can alter your amp’s sound and feel because they are your amp’s gain stages (albeit ones that are regulated by the circuitry surrounding them). This is where the magic happens. The gain induced by each preamp tube throughout the signal-carrying part of the circuit—along with the resistors and capacitors that enable correct tube operation, and the coupling capacitors that pass the signal from one stage to the next—determines just about every aspect of how an amp “sounds.” Which is to say, how the amp translates your guitar pickups’ meager electrical signal into that mighty jolt that belts out of the speaker at the other end of all that wire.

The amount of gain by which different tube types inherently increase a given guitar signal helps to lay the foundation for what we call tone: It shapes the ratio of clean to crunch, determines how saturated the signal is (or isn’t) with harmonic overtones, and partly governs how hard you can play before your clear and articulate chords and lead lines fold over into delectable, singing overdrive … or possibly dirt and mush. And yes, you can reduce the gain level of any channel in your guitar amp by just turning down the first gain knob (variously labeled volume, gain, drive, etc.), but changing it up via the correct tube swap can alter the nature of that gain stage and your amp’s overall dynamic response. The results of this kind of change are often heard as a more all-encompassing alteration of the amp’s sonic characteristics, including the degree and onset of distortion.

Here’s the cool thing: Most amps that are leaning a little too far one way or the other for your personal liking can be significantly altered by simply changing one of the original preamp tube types in the design for another compatible type. It takes no more effort than pulling out one tube and pushing in the other—plus the four-lattes-worth, about $12 to $15, of cash it costs you to acquire the replacement tube in the first place.

If you’re looking to tame a first gain stage that’s a little raw and hairy and needs some tightening up, the right 12AT7 might do the trick.

Change Where It Matters
With amps that have just a couple of preamp tubes, it’s pretty clear that changing one or both to different types will likely make an impact on your tone. In others—amps with lots of extra tube-driven features, or even classics from the Fender blackface/silverface template with six preamp tubes—you need to know which tubes to replace to most effectively alter the amp’s performance.

Since we’re talking preamp tubes here (the smaller tubes toward the front of the circuit, rather than the big output tubes near the end of the signal chain), the first tube your guitar signal hits after entering the input jack generally has the biggest impact on the overall character of any amp’s sound and playing feel. In amp circles, that’s generally referred to as “V1” (for “valve number one,” using the British term for “tube”), and it will usually be found hanging from the chassis in a position that is roughly in line with the location of the input jack itself.

For example, consider the typical Fender Deluxe Reverb. If you spin the amp around so you’re facing the back panel with the tubes hanging beneath the chassis, V1 is the tube furthest to the right side of the back of the amp. On the other hand, if it’s one of many amps based on the Marshall plexi platform—many of which have their chassis mounted tubes-up in the bottom of a head cabinet—V1 will be furthest to your left as you face the amp from behind.

To get the most out of your tube-swapping adventures, it’s best to familiarize yourself with your amp’s schematic or tube diagram, with reference to the job each tube performs within the circuit. In Marshall-style or Fender tweed-style amps, for example, V1 will form the first gain stage for each of the amp’s two channels, so changing it will affect both simultaneously. (A nine-pin tube in the dual-triode family—the common 12AX7, for example—has two individual gain stages within it, each of which can act as an independent “tube” for pre-amplification purposes.) In amps designed on the Fender blackface/silverface template, on the other hand, V1 provides gain stages before and after the tone stage on the same channel, so V1 covers the Normal channel and V2 covers the Vibrato channel.

If you want to more dramatically alter the gain characteristics of modern high-gain amps, you might need to go further and also swap out the next tube in line, if the channel involved uses more than V1 in its gain structure. (To determine this, refer to your amp maker’s tube chart or schematic, or contact the company directly.) Also, many amps that use an independent tube to drive the tone stage, or to make up lost gain after the tone stage, can be further tweaked with some attention to that tube, but the results are usually less immediately obvious than those achieved by swapping the first gain stage in the channel.

We’ll mostly concentrate on the effects of changing V1 for a different tube type, because that’s where most amps reveal their foundations for sound and playing feel. If you’d like to go further, you can use the same guidelines discussed below to swap other tubes, but it gets into a game of revolving variables pretty quickly if you don’t go one tube at a time and take good notes along the way.

It’s also worth putting some thought into the phase-inverter tube. The phase inverter is the last stage in the amp before the output tubes, and in all amps other than single-ended types with just one output tube (Fender Champ and Vox AC4, for example), this stage splits the signal into two reverse-phase strands to send on to those big bottles for final amplification. The tube in this position is still a preamp tube of the type used in the front end of most amps, but it doesn’t affect the tone of the signal passing through it so much as it does the gain. That, however, can still have a significant impact on the way your amp behaves. Why? Different types of compatible phase-inverter tubes can drive the output stage harder or less hard, accordingly. We’ll check out the effects of some swaps in this position after looking at the V1 position.