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May 2014
more... Amp DIYDIY65506L66V6EL34EL84KT66February 2009

Everything You Ever Wanted to Know About Output Tubes

Everything You Ever Wanted to Know About Output Tubes
However many fancy gizmos the techs devise to give us “warm, tube-like” guitar sounds, real tonehounds know it’s only happening in one place: real tubes! If you want to get it happening for you, though, you’ve got to understand what’s going on with different tube types, and learn the various ways the wide range of tubes available will affect your sound.

 Click here to read part one: Everything You Ever Wanted to Know About Preamp Tubes
In the last issue I discussed preamp tubes, along with a basic primer on how and why tubes do what they do. This trip under the hood we’ll explore output tubes—the big bottles at the back of your amp that pump out the serious wattage. Although some people refer to them as “power” tubes, I feel “output” is far more fitting: for one thing, these tubes create your amp’s output; for another, the term “power” might cause confusion with the third type of tube in an amp—a rectifier tube—which lives in what is correctly described as the power stage of the amplifier. In any case, you mainly just need to be aware that the output tubes are the “amplifier within your amp”: while the preamp tubes ramp up the signal from the guitar, the output tubes are the babies that really make it loud.

Output tubes can be recognized as the biggest, or at least tallest, tubes in the back of your amp, although a tube rectifier (if your amp has one) can also be mistaken for one of several output tube types. Your clue here will be that there’s usually only one rectifier, but at least two matching or similar output tubes in any amp, other than small single-ended “practice” amps such as a Fender Champ or a Gibson GA-5. Many, many types of output tubes were used in the glory days of thermionic devices, when they appeared not only in guitar amplifiers, but in radios, stereos, TVs, and many other applications. Today, only about half a dozen varieties of output tubes are regularly used by contemporary amp manufacturers, and just four of these are seen in any great numbers. The four most common output tube types are the 6L6GC, 6V6GT, EL34, and EL84. A handful of contemporary makers still offer amps with KT66 and 6550 tubes, and a few even manufacture unusual designs using more esoteric tube types, but you’ll see one of those first four in a good ninety-nine percent of amps you encounter today.

Other than EL84s, which are the same diameter as preamp tubes (although taller) and use the same 9-pin socket, all of the most common output tube types use large 8-pin (octal) sockets. While they might appear interchangeable in terms of socket size, however, most have different circuit, voltage, and bias requirements, so they cannot simply be substituted one for the other in most amps. There are a few makers today producing amps that are specifically designed to let you swap between output tube varieties for “tube tasting,” and models such as THD’s UniValve and BiValve, and Victoria’s Regal II and Two Stroke can take any of the common 8-pin types without needing rebiasing or other adjustment. For the most part, though, a maker will design an amp with a very specific tube type in mind, and will work very specifically to the performance and sonic characteristics of that tube. Let’s take a little look at the signature tones and capabilities of some of these more common output tube types.
Note: photos courtesy of thetubestore.com

6L6GC. Think “big Fender amp tone” and you’re thinking 6L6 (also sometimes substituted for the interchangeable 5881, essentially a ruggedized 6L6). 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 forty to fifty watts in an efficient Class AB amp; a quartet (with two pairs working in teams on each side of the phase-inverted signal) can put out up to one hundred watts. In less efficient, but juicily toneful, cathode-biased designs (socalled “Class A” amps) like TopHat’s Super Deluxe or Carr’s Rambler, or a mid-fifties tweed Fender 5E5 Pro, a pair of 6L6s will put out around twenty-five to thirty 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.

6V6GT. Think small-tweed amp and you’re hearing the 6V6GT. Smaller American-made amps of the nineteen-fifties, sixties and seventies 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 many Fender designs—the Champ, Princeton, and Deluxe lines among them— some great vintage Gibson amps like the GA-40 Les Paul Amp of the nineteen-fifties and early sixties, and countless others. From the late eighties to late nineties no reliable current-manufacture 6V6s were available, so few manufactures designed new amps around this tube. This is the course of events that led to the virtually unthinkable release of smaller Fender amps that used EL84s, such as the Blues Junior (the early-sixties Tremolux, which briefly carried EL84s, being something of an anomaly). The release of a rugged and reliable 6V6 first from Electro-Harmonix, then from other contemporary makers, has led to a renewed popularity for this tube, and it proliferates again in the twenty-watt-and-under range.

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