Valve Guitar Amplifier – Output Tubes Explained

EL34 (6CA7) Valves (Tubes) - JJ Matched Quad NEW TESTED

Here we take some time to review Valve Guitar Amplifier circuits and specifically the Output Tubes Explained. There are many solid state amplifiers on the market and modeling amps which claim to replicate the vintage valve guitar amplifier sound but any guitarist who has used any of these knows that the only way to get that great old style tube guitar sound is by using a designed valve guitar amplifier that uses quality amplifier valves.  I have written this article to give a basic understanding of  what is going on with different tube types, and learn the various ways the wide range of tubes available will affect your sound.

If you have not done it already, read my article on Pre-Amplifier Valves to find out about how these modify the incoming guitar signal. This article covers the output valves or power tubes that create the final volume and drive the speakers.

Output, Power and Volume

Output Valves 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. Since valves started to be manufactured there have been many different types of output valves used in many different applications, not only in guitar amplifiers, but in radios, stereos, TVs etc. Today only a limited number are used and of those only a handful show up in guitar amplifier designs. The most common output valve types used in about 90% of current amplifier designs are the 6L6GC valve, 6V6GT valve, EL34 valve, and EL84 valve. A handful of guitar amplifiers use KT66 and 6550 valve tubes.

Other than EL84’s, which are the same diameter as preamp valves (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.


Think of the big Fender amplifier 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 6L6GC valves will generate around twenty five to fifty watts depending on the amplifier design and a set of four (quad) will give out about 100 watts of power. The 6L6GC valve can be found in guitar amplifiers manufactured by Fender guitar amplifier, Marshalll guitar amplifier, Mesa Boogie guitar amplifier. And amplifiers like like TopHat’s Super Deluxe, Carr’s Rambler, Fender Tweed 5E5 Pro, Fender Tweed Bassman, Fender Blackface Twin and Super Reverbs, Marshall JTM45 heads and “Bluesbreaker” combos, to the Mesa/Boogie Mark Series and beyond.


Think small-tweed amp and you’re hearing the 6V6. 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. The 6V6GT valve will 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 Fender Champ Amplifier, Fender Princeton Amplifier, and Fender Deluxe Reverb Amplifier 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 6V6’s were available, so new amplifiers being designed at that time used EL84 valves. Since then valve manufacturers have produced better quality and reliable versions of the valve and it has started to appear again in amplifier designs in the 20 watt and lower range.


When listening to the classic British sound generated by Marshall amplifiers and you will be hearing the EL34 power valve. The classic Marshall Guitar Amplifier sound is created by the EL34 and was the big boy of British amplification from the late nineteen-sixties onward. The EL34 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.

The EL34 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.


This tall, narrow, 9-pin output tube is best known for its appearance in classic Vox guitar amplifiers such as the AC15 and AC30, and is most often used in Valve Amplifier designs where a sweeter, more harmonically saturated sound is desired. Sometimes described as “a baby EL34” because it is another classic British output tube, the EL84 really has a unique tone and sound quality. 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 of EL84 valves when used in a cathode-biased output stage (as per VOX) will give out 15w to 18w of power.

These valves also appear in many modern amp designs including models from Matchless, TopHat, Dr Z and others. The EL34 is manufactured by factories in Russia, China and Europe.


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 on the left) 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.


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.

Replacement Amplifier Valves

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 amplifier. When buying replacement vales it is best to ensure they are tested and matched to your specific amplifier design. Valve (or Tube) has improved greatly and now there are a number of factories around the World manufacturing high quality valves for use in new designs and as replacements for old stock.

Many types of amps also need to be re-biased when output tubes are changed. This is something you can do yourself with the help of a kit or have done for you by a qualified tech for a nominal charge. An amp’s bias 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 need to add a good, matched pair of new tubes.

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 to perform at their best. Output valves need to burn in before they will be at their peak.


As discussed in the Pre-Amplifier article, distortion occurs in all stages of a tube amplifier, but the resultant overdrive tones sound a little different depending on which type of distortion is generated where. Preamp tube distortion has a fizzy and gritty sound, while output tube distortion will sound comparatively thick, rich, and dynamic. 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 amplifiers with simple circuits, no master volume and the minimum amount of bells and whistles. Such amps aim to drive the output tubes more than the preamp valves, 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. Which modern technology and sound systems at venues few players really need a large amplifier on stage these days and a great sound can be achieved by using a smaller fifteen to thirty watt combo or mini-stack.

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