Guitar 101: What Is a Tube Amplifier? Learn the Difference Between Tube Amplifiers and Solid State Amplifiers

A tube amplifier is an amplifier that uses vacuum tubes or valves to amplify the electric signals produced by a musical instrument. For electric guitars and basses, those signals are produced by pickups—typically of the electromagnetic variety—and they pass through the preamp tubes and power tubes of the amplifier before exiting the amp through a speaker.

Tube amps for guitar fall into two categories: Class A and Class AB. The archetypal Class A amplifier is a Vox AC30, while the most famous Fender, Marshall, and Mesa/Boogie amps are Class AB machines.

• Class A amps tend to distort more quickly as you increase the volume. This is considered positive by many players. When players or audiophiles speak of a “warm” sound, they’re frequently referring to small levels of aurally pleasing distortion.
• When cranked to the max, Class A amplifiers provide cascades of overdrive. For instance, Queen’s Brian May achieves his signature roar by cranking a Vox AC30 and further boosting his signal with a treble booster pedal. That’s it: apart from a brief period of ‘80s experimentation, he uses no other effects.

Class AB amps are preferred by players who value “headroom,” which refers to the ability to turn up an amplifier without it distorting.

• Class AB amps are also able to project bass more easily than the typical Class A amp, and because of how they manage power.
• However, they don’t burn through tubes as quickly as Class A amps.

Many professional players own both Class A and Class AB amps, which they utilize according to the nature of a current project. And some high-end amps can run on both Class A and Class AB circuitry. The Mesa/Boogie Mark V, for instance, uses Simul-Class switching. This lets it be a Class A amp when it runs at 10 watts, but it switches to Class AB when running at higher wattages.

A tube amplifier is largely characterized by the types of power amp tubes that it uses. Vacuum tubes were not originally built for musical amplifiers—they can be found in all sorts of twentieth-century electronic devices—but certain models seem to produce more “musical” sounds than others. Here are some of the most popular models:

• 6L6. These are famously “high headroom” tubes, which means they can get quite loud before they start distorting. 6L6 tubes are found in Class A amplifiers, but they’re even more commonly utilized in Class AB amps. 6L6 tubes are most closely associated with large Fender amplifiers like the Deluxe, the Hot Rod, and the Tremolux. When players talk about an “American” tube sound, they usually are referring to Fender amps with a 6L6 (Fender is an American brand based in Southern California). Mesa/Boogie amps use a variation on Fender circuitry and are also strongly associated with the 6L6. (And like Fender, Mesa/Boogie is also based in California.)
• 6V6. A close cousin to the 6L6 and another “American” sounding tube. A 6V6 will distort or “break up” at lower volumes than a 6L6, which make them popular among lead guitarists playing single notes high on the neck. On the flipside, when compared to the 6L6 they tend to produce less precise bass tones at high volumes. So while they’re good for blues, classic rock, and country, they wouldn’t be the best choice for heavy metal riffing.
• EL34. If the 6L6 is the classic “American” power tube, the EL34 is the classic “British” tube. This is due to its strong association with British amplifier companies Marshall and Orange. But it’s not just British players like Jimmy Page who get their sound from EL34s. American players like Slash and Australians like Angus Young are also synonymous with their Marshall amps and the EL34s that drive them.
• EL84. Another “British” tube that’s actually equally popular in amps from America, Japan, and even China. The EL84 drives the Vox AC30 (think Brian May, The Edge, Peter Buck) and is generally a popular choice for Class A amplifiers. They distort readily when their volume is pushed and can quickly create creamy saturation—albeit one that can be a bit treble-focused.

Many amps, particularly those made by brands like Mesa/Boogie and Bogner, generate a lot of distortion from their preamp circuitry, but there’s actually far less variation in preamp tubes across different models of amplifiers. The 12AX7 is the standard preamp tube (the 12AT7 is also used with some frequency), but their function varies greatly depending on an amp’s circuitry.

Solid state amplifiers forego vacuum tubes in favor of electronic transistors. These transistors operate differently from tubes in the sense that they don’t pleasantly distort when pushed to their limit. By contrast, most players will tell you a tube amp sounds its very best when pushed to the max.

• Solid state amps are great for players who want maximum headroom (a.k.a a loud, clean, undistorted signal). But without a bit of natural distortion, an electric guitar can sound a bit brittle. As such, solid state amps are more popular with bassist and keyboard players than they are with guitarists.
• This isn’t to say that guitarists completely eschew solid state amps. Jazz players, many of whom play with almost no overdrive at all, tend to favor solid state amps. This is partly for tonal reasons, but it’s also because solid state amps are almost always lighter than tube amps, and many gigging musicians value the convenience of a light amp. Perhaps no place is this truer than in New York City, often considered the jazz capital of the world.
• Rock musicians also use solid state amps. Andy Summers of The Police is famous for his use of the Roland JC-120 Jazz Chorus amp, which is unapologetically solid state (and incredibly loud). John Fogerty’s guitar leads on Creedence Clearwater Revival records were achieved with a Kustom solid state amp. Meanwhile, Radiohead’s Jonny Greenwood tends to use tube amplifiers for clean tones and a solid state Fender Eighty-Five for distorted tones—literally the opposite of what most players would do.

Digital audio technology has taken massive leaps forward in recent years, and today’s digital software can emulate the great sound of tube amplifiers with impressive accuracy. Companies like Zoom, Line 6, Headrush, Tech 21, and Kemper have created digital modelers that let players dial in the sound of a Fender, a Marshall, a Vox, a Mesa/Boogie, and Orange, a Bogner, and more, all on a single device.

• Many of these digital modelers are housed inside stompbox pedals. This allows touring guitarists to travel without amplifiers: they simply plug these stompbox digital amps into a venue’s PA system and they get amp-like tones without an actual guitar amp.
• Computer software has also embraced digital amp modeling. Digital Audio Workstations like ProTools, Logic, Digital Performer, and Cubase contain guitar amp effects that allow composers to plug a guitar directly into the computer and produce amp-like tones. Even free programs like GarageBand now offer these features, which produce surprisingly good sound.
• Tablet and smartphone software also contains a selection of digital amps. A few brave guitarists have played concerts by plugging their guitars directly into an iPhone or iPad and letting specialized apps sculpt their tone.
• Do digital amps sound as good as genuine tube amplifiers? No. But they come a lot closer than they once did, and when you factor in considerations like price and portability, they make sense for certain types of guitarists.

Learn more about amplifiers and guitar playing with Carlos Santana here.