There is so much tech out there that give us tube sounding guitar tone that it can be hard for some to tell the difference. Purists though know the difference. If you desire great guitar tone (or even bass tone for that matter), you’ve have to understand how different tube types work and learn the wide range of tubes available and how they affect your tone. Today is about output tubes. You may or may not to be aware that the output tubes are the main output of your amp. Pre-amp tubes pump up the signal from the guitar, while the output tubes are what really make the tone loud or soft.
Most output tubes can be recognized as the biggest and/or tallest tubes in the back of your amp. The hint is that there will be only one rectifier, but at least two matching or similar output tubes in any amp, other than small practice amps such as a Fender Champ or a Gibson GA-5. Today there are about many types of amp manufacturers that use tubes. Four of these are seen more commonly than others. These four common output tubes are:
A few manufacturers still offer amps with KT66 and 6550 tubes. Some even manufacture unusual designs using more unusual tubes, but you’ll see the aforementioned in a most amps you see today.
Other than EL84s, which are the same diameter as pre-amp tubes and are taller, many use the same 9-pin socket. Most common output tubes use large 8-pin sockets. They might seem interchangeable in terms of socket size, but most have different circuit, voltage, and bias requirements, so they cannot simply be swapped 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 choices for getting different types of tone. Some models such as THD’s UniValve and BiValve can take any of the common 8-pin types without needing re-biasing or other adjustment. Usually a manufacturer will design an amp with a very specific tube type in mind and will work specifically to the performance and sonic characteristics of that tube.
Tube types and their individual characteristics
6L6GC: “The” Fender amp tone usually powered by 6L6s (sometimes swapped for the 5881, a more rugged 6L6). This is the big-amp output tube traditionally seen in American-made amplifiers. It has a bold, solid voice with firm lows and crisp highs, which can be harsh in loud, clean amps, or more smooth and rounded in softer, tweed style amps. A pair of these will generate around 40-50 watts in a Class AB amp. Less efficient Class A amps like a Mid-50s tweed Fender 5E5 Pro, have a pair of 6L6s that put output around 25-30 watts. This is the tube used from the Fender Tweed Bassman and Super Reverbs, to early Marshall JTM45 heads and Bluesbreaker combos, to the Mesa/Boogie Mark Series.
6V6GT: Listen to a small-tweed amp and you’re probably hearing the 6V6GT. Smaller American-made amps of the 1950’s,60’s and 70’s often carried 6V6 tubes, which are known for their thick, well-rounded tone and smooth, rich distortion which occasionally have an element of “dirtiness” that is not necessarily unappealing. They produce about 1/2 the output of their big brother, the 6L6, and are easier to push into distortion tones. The 6V6 was used in many Fender designs like the Champ, Princeton, and Deluxe. Some vintage Gibson amps like the GA-40 Les Paul Amp of the 1950’s early 60’s and others. In the late 80’s to late 90’s there were no reliable current-manufactured 6V6s available, so few manufactures designed new amps around this tube. This is led to the creation of smaller Fender amps that used EL84s such as the Blues Junior. The release of a rugged and reliable 6V6 have led to a renewed interest for this tube in the 20 watt and-under range.
EL34: Classic British crunch tone produced by the very popular the EL34. The classic Marshall tube, the EL34 was the big boy of British amplification from the late 1960’s to present. It can be pushed at higher voltages to produce a little more output than the 6L6GC and it sounds somewhat different. This tube is characterized by a fat but softer low end, sizzling highs, and a mid-range that exhibits a classic crispy-crunchy tone when driven into distortion. This is the tube of post-1967 Marshall’s 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: Described as “the 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. These tend to have 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 mostly known for its chimey, sparkling highs and a mid-range that is crunchy and aggressive when pushed. A pair in a cathode-biased output stage like a Vox will put out around 15-18 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.
KT66: Rarely seen for many years other than in vintage amps that carried them (notably early-60’s Marshall JTM45s, following their short use of 5881/6L6s originally), the KT66 is a direct substitute for the 6L6. It really has a character all its own. This tube of European origin is a little bolder, firmer, and fatter than its American counter part, 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 were exported to the USA around the mid-70’s to the mid-80’s. They were modded to use 6550 output tubes instead of the EL34s they were originally designed for availability and reliability. The change altered their character somewhat, as the 6550 doesn’t sound like an EL34, but more like a bigger, louder 6L6 . Not necessarily 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 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 tone, different makes of tubes of the same type can sound quite different, too. Take 6 different pairs of 6L6GCs from different manufacturers, some old and some new, and each will sound just a little different in your amp. Sometimes drastically and horribly different. But not always. Tube nuts rave about “new old stock” (NOS) tubes, meaning tubes that were manufactured in the USA or Europe many years ago, but have never been used. 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 pulled from an old hi-fi, and polished up a little. Any pair you can find in genuinely good condition, tested, and guaranteed will also be very expensive these days. Grab some if you can especially if you find some going cheap from an old supplier who is selling out. Also beware of fakes and forgeries being sold online these days as NOS. There are many out there so do your research like a boss!
There are also many excellent current-manufactured tubes today that are very good with better quality and better selection than there was available even 10 years ago. These exhibit different sonic characteristics as well. You should get educated on their respective pros and cons online, 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 find a pair that’s just right, you can always keep the others for back up.
Output Tube Distortion
Distortion occurs in all stages of a tube amp, but the overdrive tones sound a little different depending on which type of distortion is generated where. Pre-amp 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. Tone purists enjoy the distortion tones generated at the output stage, which is why you see many such players going for vintage/vintage-styled amps with simple circuits. These have no master volume and a minimum of bells and whistles such as channel switching and added gain stages. Such amps drive the output tubes more than the pre-amp tubes 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 really need a big double-stack to be heard on stage these days. It’s harder to push such amps into overdrive without pissing off the sound guy and blowing your band mates off the stage. When using a smaller 15 to 30 watt combo or mini-stack you can hit the sweet spot and hope to avoid any amount of hearing loss.
Tube amp maintenance and re-biasing
Many types of amps need to be re-biased when output tubes are changed. This is something you can usually do yourself with the help of tube bias meters readily available for purchase, or 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, 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 use in a good and matched pair of new tubes and you are good to go.
Additionally its worth knowing that any new set of output tubes, whether NOS or new manufacture, will need some break-in time. They won’t sound their best until you have put a few hours on them, maybe as many as 40 or 80 hours of playing time to get them into the desired tone you are wanting. Similar to a vintage bottle of wine, output tubes need to “breathe” a little before they will be at their peak. Similarly as you uncork that prized NOS pair that has rested on the shelf for 30+ years and start playing them, they may not last forever.
We hope that you learned a little something today and hope you walk away with more than just an itch for tube tasting. Now get out there and play on, and play well!!!