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 tenth part of Mojotone’s new series What Does This Thing Do? we’re examining output tubes, the big bottles that do the heavy lifting near the end of the signal chain.

Despite all the technological advances made in the eight decades since guitar amplifiers first brought players out from the shadows of the rhythm section and into the spotlight, that archaic component the tube continues to rule the roost for optimal sonic splendor. Even ultra-modern digital modeling amps achieve their binary magic by emulating or replicating the sound and function of tubes in one way or another, so they remain at the heart of our sound even when we’re not actually using them in our rig. For those interested in probing the workings of the real deal, however, and possibly building such tone tools for themselves, a little knowledge of these miraculous glowing bottles is essential.

Any genuine all-tube amp carries two breeds of signal tubes:

Preamp tubes, which we covered in “What Does This Thing Do? Pt3,” and output tubes, which we’ll discuss here. (In Pt2 we covered the third type of tube that is found in many amps, the rectifier tube.) The vacuum tube—tube for short, or “thermionic valve” (valve for short) in the UK—is itself technically known as an “amplifying device,” and indeed the output tubes are the part of the guitar amp that really performs the duty of making your guitar signal louder. Everything else in there—the resistors, capacitors, transformers, and the wire that connects them all—is responsible for shaping the tone of the signal and providing the correct voltages to the tubes. In short, the guitar signal that has been amplified to a slightly higher voltage by the amp’s preamp stages enters the input of the output tubes, where it is amplified much further and sent along to the output transformer. The output transformer (discussed in Pt7) translates the high-impedance amplified signal from the output tubes to a low-impedance one that will drive a speaker. 

The reason we love tubes so much, however, is that they don’t just make a guitar louder, they make it louder with style.

Tubes handle the signal peaks (the surges) in a way that is musical to the ear, with a natural roundness and degrees of compression and frequency attenuation that flatter the amplified electric guitar. Where solid-state amplification devices, or at least ones without a lot of added circuitry designed to make them sound “tube-like”, rise smoothly up toward the peaks but clip hard and harshly when pushed into distortion, tubes—used in an amp that is designed and built well—smooth out the transition into distortion, and offer a distortion that is relatively more musical and harmonically appealing as a result. 

That’s the basic 101 on the functional characteristics that all output tubes share, but there is an enormous variation in the sound that different tubes make in the course of doing their job, so let’s also explore some of the archetypal tones associated with specific types of output tubes. To be clear, no output tube will result in “a tone” all on its own, and the sound it helps to put out is just part of a bigger overall system. But classic and common varieties are generally associated with certain known styles and sounds, and these do help us to form a base level of what to expect from any specific type of output tube

6L6GC

The larger of the output tubes traditionally seen in American-made amplifiers, it has a bold, solid voice with firm lows and prominent highs, which can be strident in loud, clean amps, or more silky and rounded in softer, “tweed” style amps. A pair of these will generate around 40 to 50 watts in an efficient class AB amp, and a quartet (with two pairs working in teams on each side of the phase-inverted signal) can put out up to 100 watts. This is the tube of anything from the Fender tweed Bassman and blackface Twin and Super Reverbs, to early Marshall JTM45 heads and “Bluesbreaker” combos, to the Mesa/Boogie Mark Series and beyond. Amps designed for 6L6GCs can usually also use 5881 output tubes and the European KT66 is also swappable for either type, and is a little bolder, fatter and louder.

EL34

This is the classic big-amp tube from the other side of the pond, commonly found in larger British amplification from the mid 1960s onward (and occasionally before). The EL34 can be driven to produce a little more output than the 6L6GC, and it sounds somewhat different, too, characterized by a fat and juicy but softer low end, sizzling highs, and a midrange that exhibits a classic crispy-crunchy tone when pushed into distortion. This is the tube of post-1966 Marshalls like the JMP50 “plexi” and “metal” panel amps, the JCM800, and the majority of modern models; they also appear in the classic Hiwatt models, and plenty of modern amps seeking a big Brit-rock sound. (Note that some Marshalls distributed in the USA years ago carried 6550 output tubes instead of EL34s. The 6550 is probably best described, in brief, as a “bolder, louder 6L6”).

6V6GT

Smaller and mid-sized American-made amps of the 1950s, ’60s and ’70s most often carried 6V6 tubes, which are known for their juicy, well-rounded tone and smooth, rich distortion, which occasionally exhibits an element of grittiness that is not necessarily unappealing. They produce about half the output of their big brother, the 6L6, and are therefore more easily driven into distortion. The 6V6 was used in all versions of the Fender Deluxe, Princeton and Champ, the Gibson GA-40 Les Paul amp of the 1950s and early ’60s and others, and countless great American-made amps besides.

EL84

Best known for its appearance in classic Vox amps such as the AC15 and AC30, this tall, narrow, 9-pin output tube is most often used in nominally “Class-A” circuits. More correctly termed “cathode-biased,” these seek to achieve a sweeter, more harmonically saturated sound at the expense of a little output efficiency. The EL84 can still exhibit a pretty firm, chunky low end in the right amp, but is most known for its chimy, sparkling highs and a midrange that is crunchy and aggressive when pushed. A pair in a cathode-biased output stage (a la Vox) will put out around 15 to 18 watts, and a quartet double that. These tubes also appear in many modern amps that emulate the “Class-A tone”, including models from Matchless, TopHat, Dr Z and others. Gibson’s unusual, wedge-shaped GA-79T stereo amp of the early 1960s also utilized these output tubes, as did Fender’s later Blues Junior and Pro Junior.

In addition to having their own sonic characteristics according to type, different makes of the same types of output tubes will also sound slightly different. Once you have pinned down the right genre of tube amp for your style, it pays to experiment with a few different sets of quality output tubes to see which will work best for you. You’ll be amazed to hear how simply swapping output tubes can take an amp, in some cases, from soft, fuzzy and bluesy to bold, punchy and twangy. As with all things tonal, there isn’t necessarily any better or best here—whatever suits your sound is best for you.