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Tone Tip: Output Tubes

Dave Hunter | 09.10.2008

Players continue to rave about a much-loved piece of archaic technology known as the tube amp. If you’re already into these potentially toneful beauties, or are considering buying your first tube amp, it’s worth knowing a little about the variety of key components that make them tick: namely, the tubes. Any genuine all-tube amp carries two breeds of signal tubes: preamp tubes and output tubes. I covered preamp tubes in a previous installment of Tone Tips, so let’s take a brief exploration of output tubes this time around. Note that some tube amps will also carry a third type of tube, known as a rectifier tube. This tube isn’t in the signal path, but is instead responsible for converting AC voltages from the power transformer into the DC voltages that the tubes use to amplify your guitar signal. Perfectly good amps are made without rectifier tubes, instead using solid-state diodes for AC/DC conversion, and because this circuit isn’t in the signal path we still legitimately call them “all-tube amps.” Tube rectifiers will affect other aspects of an amp’s playing feel and performance, but that’s an investigation for another Tone Tips. 

The vacuum tube—tube for short, or “thermionic valve” (valve for short) in the U.K.—is itself technically known as an “amplifying device,” and indeed this is 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, or, in the case of the output transformer, translating the high-impedance amplified signal 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 and “round” to the ear. 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 quick lesson in “why we like tubes;” there’s a whole lot more to it, of course, but I offer that as a segue to the subject at hand. Sure, these are characteristics that most tubes share, but there is enormous variation in the sound that different tubes make in the course of doing their job, and knowing a little bit about the sonic characteristics of different output tubes can help you to better pin down the amp that will achieve the tone in your head.

Output tubes, sometimes also called power tubes, perform the final amplification duties after the preamp tubes boost the signal to a level they can work with. (The signal is passed from preamp to output stage via another preamp-sized tube known as a “phase inverter,” because it splits the signal into two opposite-phase paths that feed each side of the output stage). The output tubes are the big boys in the amp, and are almost always larger or at least taller than the preamp tubes. Hundreds of varieties of output tubes have existed through the years, but the ones that have survived for guitar amplification duties have been narrowed down to four main varieties (and their siblings):

6L6GC This is the big-amp output tube traditionally seen in American-made amplifiers, and 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. Gibson’s own GA-20RVT and GA-40RVT carry these tubes. Amps designed for 6L6GCs can also use 5881 output tubes (originally a ruggedized version of the same tube), and the European KT66 is also swappable for either type, and is a little bolder, fatter, and louder.

EL34 The classic Marshall tube, the EL34 was the big boy of British amplification from the late 1960s onward. It 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 driven into distortion. This is the tube of post-1967 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 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 the Gibson GA-40 Les Paul amp of the 1950s and early ’60s (and appears in the Gibson GA-20RVT today), as well as the tweed and blackface era Fender Deluxe, Princeton, and Champ.

EL84 This tall, narrow, 9-pin output tube is best known for its appearance in classic Vox amps such as the AC15 and AC30, and is most often used in “class A” circuits, which 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.

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 nailed 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.