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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

EL34. Take your aural imagination across the pond, conjure up that big, British crunch tone, and your mind’s ear is hearing the EL34. The classic Marshall tube, the EL34 was the big boy of British amplification from the late nineteen-sixties onward. It can be driven at higher voltages 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. It also appears in the classic Hiwatt models, and plenty of modern amps seeking a big Brit-rock sound. Many contemporary American makers, such as Rivera and VHT, have also used EL34s for high-gain amp designs, and plenty of boutique makers also employ this output tube.

EL84. Sometimes described as “a baby EL34” because it is another classic British output tube, the EL84 really has a tone all its own. 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 chimey, 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 fifteen to eighteen 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.

KT66.
Rarely seen for many years other than in vintage amps that carried them (notably early-sixties Marshall JTM45s, following their brief use of 5881/6L6s originally), the KT66 (pictured at right) is a direct substitute for the 6L6, but really has a character all its own. This tube of European origin is a little bolder, firmer, and fatter than its American cousin, and can put out a little more volume. A few good recent reissues of this tube type have led some amp makers to design around it again, and Dr Z’s Route 66 is one example of a popular boutique amp that takes advantage of the KT66’s potential.

6550. Marshall amps exported to the USA from around the mid-seventies to the mideighties were modified to use 6550 output tubes instead of the EL34s they were originally designed for, apparently for reasons of availability and reliability. The change altered their character somewhat, as the 6550 doesn’t sound especially like an EL34, but more like a bigger, louder 6L6 (in approximate terms). That’s not to say it’s a bad thing, just different. Many other makers have designed amps around this lesser-seen output tube, such as Alessandro and ENGL. The 6550 is perhaps more commonly seen as an output tube in big bass amps, and was used for a time in Ampeg’s SVT, and currently appears in models by Traynor and others.

While each of these output tube types has its own characteristic tone, different makes of tubes of the same type can sound quite different, too. Take six different pairs of 6L6GCs from different manufacturers, for example, some old and some new, and each will sound just a little different in your amp (sometimes a lot different). Tube connoisseurs rave about NOS tubes (new old stock), meaning tubes that were manufactured in the USA or Europe many years ago, but have never been used—and certainly these can represent the pinnacle of output tube quality, provided you can find a good, tested pair that is genuinely NOS and not just used, pulled from an old hi-fi, and polished up a little. You’ll hear tubeheads go all gooey over black-plate RCA 6L6s, Mullard EL84s, Brimar 6V6GTs, GEC KT66s and many other types that carry the great brand names of old, and certainly there’s a lot to be said for them. Any pair you can find in genuinely good condition, tested, and guaranteed will also be very expensive these days. Grab some if you can (and certainly if you find some going cheap from an old supplier who is selling out; but also be aware of fakes and forgeries being sold online these days as NOS—there are plenty of them around). But there are also many, many excellent current-manufacture tubes today that are very good—with better quality and better selection than was available even ten years ago—and these exhibit different sonic characteristics too. Read about their respective pros and cons online (there isn’t room to go into full detail here), or try a few different pairs of the types that seem like they’ll suit you, and see how they change the sound of your amp. When you locate a pair that’s just right, you can always keep the others for back up.

As discussed briefly in Part 1, distortion occurs in all stages of a tube amp, but the resultant overdrive tones sound a little different depending on which type of distortion is generated where. Preamp tube distortion, as much fun as it can be, will sound a little more fizzy and gritty, while output tube distortion will sound comparatively thick, rich, and dynamic (in broad terms). Old-school tone freaks tend to enjoy the distortion tones generated at the output stage, which is why you see many such players going for vintage—or vintage-styled—amps with simple circuits, no master volume (or one that’s bypassable), and a minimum of bells and whistles such as channel switching and added gain stages. Such amps aim to drive the output tubes more than the preamp tubes, and to generate that creamy, harmonically saturated overdrive tone when cranked up. This love of output-tube distortion is also what’s leading a lot of players, touring pros included, to use smaller amps on stage. Few players really need a big double-stack to be heard on stage these days, and it’s harder to push such amps into overdrive without incurring the wrath of the soundman and blowing your band mates off the stage. Use a smaller fifteen to thirty watt combo or mini-stack, however, and you can hit the sweet spot and still (hopefully) dodge the tinnitus until well into late-middle age.

Be aware that many types of amps also need to be rebiased when output tubes are changed. This is something you can do yourself with the help of a kit (several types are available), or have done for you by a qualified tech for a nominal charge. An amp’s bias is like a car’s idle speed: it needs to be set correctly for the amp to operate efficiently, and an incorrect bias setting will also seriously impede your tone. Confusingly enough, “fixed bias” amps are the ones that generally have adjustable bias levels that need to be checked and reset when you change tubes. Cathode-biased amps, on the other hand, which are often billed as “Class A amps,” have a bias level that is set at the factory with a fixed resistor. With these, you just pop in a good, matched pair of new tubes and away you go.

It’s also worth knowing that any new set of output tubes, whether NOS or new manufacture, will need some playing-in time. They won’t sound their best until you have put a few hours on them, and maybe as many as forty or eighty hours of playing time to get them into the tone zone. Not unlike a vintage bottle of wine, output tubes need to “breathe” a little before they will be at their peak. Similarly, once you uncork that prized NOS pair that has rested on the shelf for three decades and start playing them, they won’t last forever. Hopefully the tonal payoff will live up to the anticipation. Test, taste, sample, enjoy—there’s gold in them thar tubes!

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