If the transformers are the heart of your amp and the preamp is the brain, then the soul must be the tubes. But the thing is, there are so many tubes available, how can we discern the differences?
So what is a tube? (Some call it a valve, those terms are interchangeable). You can think of the tube as a very elaborate light bulb. As current flows through it via a series of screens, the current is being amplified. You could say that tubes are fragile, glass tube that allows a specific amount of current to run through it (hence the other name: valve). What is most interesting for us as guitar players are the different types of tones tubes allow us to get. Due to the inner constructions of amp and tube it is highly recommended not to change your tubes yourself.
Please go see a qualified tech in your area if you want to change your tubes! There are very high voltages running in your amp that may potentially kill you if you don’t know what you’re working with!
Having said that, let’s take a look at the different types of tubes, paying particular attention to power tubes. There are many types of preamp tubes, but those types all fall in the same ‘family,’ as it were. When it comes to power tubes there are more varieties.
The EL34 is a classic design that harks back all the way to 1953. It’s a pentode design, which means it has five elements (cathode, anode and three grids). The internal design of this tube is slightly different than of its main ‘competitor’ the 6L6 (more on that later). Since the EL34 is a European design, many European amplifier brands such as Marshall and Hiwatt used the EL34 in their amplifiers. This helped cement the ‘British’ tone, apposed to the ‘American’ tone created by 6L6 tubes. The EL34 is hailed for its strong midrange. EL34s generally sound a bit dirtier than the 6550 or 6L6. Not really more distortion, just a bit more nasty, gnarly. More aggressive, if you will.
The 6L6 is a completely different design than the EL34. Where the EL34 is a true pentode, the 6L6 is a beam tetrode. That means that is has ‘just’ four elements. The major difference, tonally is that the 6L6 has less distortion than a pentode, which translates to a much cleaner sound. The 6L6 was an American-designed tube to circumvent the patent Philips held for their pentode design. Due to better availability in the USA (albeit under various pseudonyms, for example 5881) many American amp builders used this tube for their amps. The amp that ‘started it all’, the Fender Bassman was loaded with a duet of of 5881s. The 6L6 has a cleaner sound than the EL34. Its mids are less powerful, less honky and the overall tone is less gritty and much more fluid.
The beam tetrode 6550, the standard tube in the Marshall AFD100, is similar to the 6L6 and came to life in 1955. Simply put, the 6550 can be used to create an amp that has a bit more power than with either the 6L6 or EL34. Tonally, it’s in the same vain as the 6L6: clean, little distortion and an almost ‘glassy’ feel. I love the look of this tube! I can’t explain, I just like the bulby look! I don’t find the tone is really up my alley, although you may beg to differ. There’s a grittyness to the midrange that I can’t seem to appreciate personally – it’s too choppy for cleans and too ‘sizzly’ for my dirty tones. Leads do get a fluid feel to them I do love, though.
The KT88 is based on the 6550 but even more refined and boosted. It has even less distortion and an even cleaner tone then the 6L6 or even the 6550. It isn’t used in guitar amps as often as it is in hi-fi audio amps, but that being said, I know a few guys who totally swear by the KT88. I love the KT88 in a super-clean amp for maximum clarity, definition and chime or in an amp that’s got so much gain and power on tap that I want to clear up the tone. In other words, I use the KT88 instead of the 6550 (which seems to be the to-go tube for those kind of applications!).
The KT66 is the predecessor of the 6L6. The tonal differences between the 6L6 and the KT66 are ambiguous. Some say the KT66 is very much like the 6L6 with minor differences, others say the difference is huge: smoother in the highs, smoother in the mids, more open sounding, better dynamic response… If you feel that your tone would benefit immensely from these tubes in your favorite amp, don’t attempt to just pop them in yourself – contact a local tech and have it done right! This warning goes for all power beam tetrodes like the KT88 and the 6550, by the way!
The sound of the KT77 is usually described as being somewhere in between an EL34 and a 6L6; they sound a little bit more compressed than 6L6s, but not as much as the EL34. The KT77 is often described as a pentode because it shares the same pin configuration and bias as the EL34, but that’s inaccurate. The KT77 is in fact a tetrode, but with the same base and biasing as an EL34 tube so they can be substituted in any EL34 amp.
The EL84 was developed to work in a circuit without a drive tube, which gives this power pentode a bit more gain than comparable designs. In other words, it could amplify a smaller signal more easily than other pentodes. The EL84 is notably more sensitive than the 6v6, another ‘small’ power tube. Tonally the EL84 has a midrange bark, and since it requires a lower voltage to work, if you put in a cleaner guitar tone you get a slightly dirtier tone than with most other tubes.
While the 6v6 and EL84 are both smaller tubes, the 6v6 gives a completely different tone than the EL84. Where the EL84 gives a grittier, gainier feel and structure to the tone, the 6v6 has a sparkle to the top end and a chewiness to the bottom end the EL84 only has in its dreams. The 6v6 was used in many Fender amps like the Deluxe Reverb and the Princeton. So, that amazing lead tone on Kid Charglemagne laid down by Larry Carlton was done through a duet of 6v6s and is to my ears the penultimate tone to describe the 6v6. If the EL84 barks, the 6v6 might be described as roaring when you crank up the overdrive.
The differences and nuances between power tubes can be very subtle. This clip shows the differences as well as possibilites. The difficulty is that, of course, the rest of the amp’s design is just as important.
This clip also showcases a few tubes. The differences are here also subtle, but now you hear the differences at higher gain settings too.
Also, check out Ola Englund trying some power tubes in his Randall Signature Amp, the Satan. This clip shows how even with a very high gain setting the characteristics of the tubes come through. I personally love the KT88 and the EL34s and would love to hear two duets of each in a 100 watt amp!
By the way, if you’d like to dig further here are two clips running for almost 45 minutes which go much deeper into the workings of tubes. They are very, very informative and take you step-by-step on how an amp works, with enough physics and math to sketch a complete picture but not so much that it overwhelmes or becomes too complicated.