MIM: Tube Basics

Written by
Dave Hunter
Published on
January 21, 2021 11:20:47 AM PST January 21, 2021 11:20:47 AM PSTst, January 21, 2021 11:20:47 AM PST
Article by Dave Hunter

There are few things as uncomfortable in the guitar world as having to ask an experienced player or repairman that shamefaced giveaway question: “Uh, so what is this tube… and what does it do?” The fact is, though, that everyone has to start somewhere, and the so-called experts weren’t born knowing this stuff either. If you’re a noob or a novice who’s eager to dig into the wonderful world of tube amps—and maybe BUILD, MODIFY and/or REPAIR your own—what better than a quick guide to answer exactly those pressing questions? To that end, this piece on Tube Basics will explore which tubes are which within most traditional guitar amps, and what they do.

The Three Main Tube Categories

Any genuine “all-tube” guitar amp will contain at least two different categories of tubes, and many will contain three. These include preamp tubes, output tubes, and rectifier tubes. Within each of those broad groups, however, there are many different types of tubes that are all designed to perform slightly differently. In most amps, these different categories of tubes will be grouped somewhat closely together. This way, the signal flows through them from one type to the other, beginning with the preamp tubes and ending with the output tubes. (Note that some people will also refer to output tubes as “power tubes,” which isn’t necessarily wrong, although I prefer the term output tubes since they exist in the output stage of the amp, while the rectifier tube functions in the power stage of the amp.)

Rectifier tubes perform a different task that is not at all in the signal chain (as discussed below in more detail), and which is sometimes performed by solid-state diodes contained within the amp’s chassis. Whether an amp has a tube rectifier or not, it can still be considered and all-tube amp if both preamp and output tubes perform the majority of its signal-amplification duties.

Gain Stages

You’ll see the term “gain stage” used occasionally throughout this article, in reference to jobs performed by preamp tubes in particular. A gain stage is simply any stage within the amp at which the guitar signal is increased; this, of course, is the tubes’ main and most common job in the first place. Some amps have just one or two gain stages--achieved with the use of preamp tubes--and some have several. Note that preamp tubes can also be used for other functions—reverb, tremolo, buffered FX loops, phase inverters—so a quick headcount of the number of preamp tubes in any amp doesn’t always equate to its number of gain stages.

Preamp Tubes

In the vast majority of tube amps made since the mid or late ’50s, preamp tubes are identifiable as the smaller of the three potential categories of tubes in any amp. They’ll be about 1 ¾" long excluding their pins, and ¾" to 7/8" in diameter, with nine metal pins extending from the base and a pointed tip on top. In most cases, you can also identify preamp tubes as those nearest the amp’s input, because that’s where their job begins.

Preamp tubes take the guitar signal, which comes into the input as a very low-voltage signal from your guitar’s pickups, and amplify it—that is, they enlarge it, essentially. Guitar amps might have only one or two of these stages in the preamp section of the circuit, or they might have several depending on how “hot” the designer wants the signal to get before it hits the output tubes. Along with increasing the signal, preamp tubes also play a big part in shaping the overall tone of the amplifier (along with any other EQ controls and stages between them).

The 12AX7 is by far the most common type of preamp tube used in guitar amplifiers, and has been since at least the mid ’50s. It belongs to a category of preamp tubes called “twin triodes,” because they contain two gain stages within one tube—“twin” for the fact that there’s two of them, “triode” for the three elements required for any tube to do its amplification job. These two triodes can be used independently, making any 12AX7, or other twin-triode, essentially two tubes in one. In a tweed Deluxe, for example, one half of the first preamp tube is used for the first gain stage for Ch1, while the other half is used for Ch2.

You’ll also sometimes see a 12AX7 preamp tube listed as an ECC83, which is the British designation (hence its use in amps from Vox, Marshall, and others); or as a 7025, which was an extra-rugged version often used by Fender and other U.S. makers in the ’60s and ’70s. Each of these can be swapped one for the other since they do the same thing, but there are also several other twin-triode preamp tubes that have different amplification characteristics, some of which can also be swapped in place of a 12AX7 and the ilk.

A 5751 fits into the same size socket and provides two triodes with slightly less gain than a 12AX7, while a 12AY7—as used in many tweed amps of the ’50s—provides even less gain still. Somewhere between a 12AY7 and 5751 gain-wise, a 12AT7 can also be substituted for a 12AX7 in many amps, although some of its electrical characteristics are different. You shouldn’t make such substitutions without referring to your amp’s documentation or contacting the manufacturer directly, but this at least gives you some idea of what’s available in preamp tubeland.

All amps other than single-ended types (more of which below under Output Tubes) also have a stage called a “phase inverter,” which splits the signal departing the preamp into two strands while also reversing the phase of one strand, and then feeds them to the output tubes. This job is also most often achieved by a 12AX7 or sometimes a 12AT7. The phase inverter is usually the last preamp tube in the lineup before the output tubes take over.

Another preamp tube that we sometimes see in guitar amps is the “pentode,” which has just one tube stage within each bottle. The most common pentode preamp tubes in use are the EF86 (U.S. equivalent: 6276) used in classic British amps like the early ’60s Vox AC15 or more recently in amps like the Matchless DC30 and Dr Z Z-28 and others; or the 5879 used in Gibson’s GA-40 Les Paul Amp of the ’50s and several current models from Divided by 13. These pentodes have an even higher gain factor than the 12AX7 and other twin-triodes, meaning they can amplify your preamp signal even further; but they also distort less in and of themselves, so in many amp designs they excel at being hit hard with overdrive pedals in front of the amp without collapsing into fizzy mush. Note that you can never substitute a pentode preamp tube for a twin-triode, nor can you substitute an EF86/6276 for a 5879 and vice-versa.

Before these compact nine-pin preamp tubes came into use around the early ’50s, preamp tubes mostly used eight-pin (aka octal) bases that made them look a little like output tubes. These are seen far less often today, although some boutique amp makers in particular still like to use them for their alternative sonic flavors. To find these octal preamp tubes, look for designations such as 6SC7, 6SL7, 6SN7, 6SJ7, and 6SQ7; all of which might easily be mistaken for a slightly stumpy octal-based 6V6GT output tube, as discussed below.

Output Tubes

Output tubes are the larger of the tube types within your amp, or the longer at least, and are generally found toward the opposite end of the chassis from the amp’s input and preamp tubes. These tubes receive the guitar signal that the preamp tubes have already amplified slightly and amplify it much more, into a signal that can be pumped through a speaker (via an output transformer), to make some noise. The output tubes also play an important part in shaping the overall voice of any guitar amplifier, but the proportion to which they do this relative to the preamp tubes can vary a lot, and is dependent on the amp in question and what the designer was seeking to achieve with it.

The most common output tubes traditionally used in American-made guitar amps are the 6L6GC and 6V6GT. Each uses a similar base, which includes a dark brown or black plastic section that encases the lower ¾" or so of the glass tube, and has eight pins protruding from it, with a “key post” in the center to help correctly guide the tube into the socket. The 6L6GC can provide an output of around 50 watts max when used in pairs, or 85 to 100 watts max when used in quads (think Fender Twin Reverb). The 6V6GT will yield around 20 watts in pairs (think Deluxe Reverb) and 35 to 40 watts in quads. These tubes are also sometimes found in “single-ended” amps, which use just one output tube, in which case they put out about 8 to 10 watts and 4 to 6 watts respectively.

The smaller (i.e., lower-powered, but also physically smaller) of the traditionally “British” output tubes is the EL84, which looks somewhat like a tall 12AX7 with nine pins in the bottom, and fits into the same type of socket as those preamp tubes… but should never be put into one. A pair of EL84s will put out from 14 to 20 watts, while a quad will put out around 30 to 36 watts (as in the fabled Vox AC30). Note that the approximate output potential of all of these tube types can vary dramatically according to different design parameters; amp manufacturers will use them in different ways to achieve different ends, but these figures give you the general range and some indication of the maximum achievable output from each type.

The larger of the classic British output tubes is the EL34, as commonly seen in everything from the Marshall Plexi to the big Hiwatt models. These tubes use an eight-pin base like the 6L6GC and 6V6GT, and will put out from around 50 to 60 watts as a pair or 100 to 120 watts as a quad. These days, American and British makers alike are using many different varieties of output tubes, since those tubes are mostly made in Russia, Eastern Europe, and China, rather than actually in the U.S. and U.K. and Europe; because of this you’ll see EL84s and EL34s in loads of American-made amps, and 6L6GCs and 6V6GTs in British-made amps—although we still tend to think of each as representing the “flavor” of the countries in which they were commonly used in the ’50s, ’60s and ’70s.

Rectifier Tubes

Put briefly, every amp has a rectifier, which converts the AC voltage from its power transformer into the DC voltage that powers the tubes. Some amps use a solid-state rectifier made up of a string of diodes mounted within the chassis, while others use a tube rectifier, which you can see standing in a socket mounted outside the chassis alongside the other tubes. For what it’s worth, neither is superior, it’s just a matter of horses for courses. Tube and solid-state rectifiers have slightly different performance characteristics which might be preferable in different circumstances, but neither inherently “sounds” better or worse than the other ( in fact they have no sound at all).

Most tube rectifiers—common types like the 5AR4, GZ34, 5UG4 and 5Y3—have a plastic base much like that on 6L6GC, 6V6GT, and EL34, although it has fewer pins, and as a result these rectifiers can sometimes be mistaken for output tubes. Many vintage British amps of around 20 watts or less also used a nine-pin rectifier tube known as the EZ81 (U.S. name: 6CA4), which can easily be mistaken for an EL84 output tube. The EZ81 has found favor again with many makers seeking to re-create that classic British lower-wattage tone, so you’ll also find it in TopHat’s Club Royale and Supreme 16, as well as 65 amps’ London.

In the vast majority of guitar amps, whatever their size, you’ll only find only one rectifier tube (the Matchless DC30 and larger amps in Mesa/Boogie’s Rectifier Series are notable exceptions). The rectifier tube will also usually be found positioned fairly close to the amp’s AC power inlet and the power transformer, which is generally the larger of the transformers mounted to the outside of the chassis. Tube rectifiers are also uncommon in amps of more than 50 or 60 watts, with the big Mesa/Boogie Rectifier Series models once again being an exception to the rule.

So those are the tubes you’ll encounter in just about any guitar amp, in their various sizes and guises. Now you can at least walk the walk and talk the talk regarding what goes where and why. In future articles, I’ll explore the different sounds of several different types of tubes within each of the major categories.

Dave Hunter is a writer and musician who has authored several gear books including The Guitar Amp Handbook and The British Amp Invasion and is a regular contributor to Guitar Player, Vintage Guitar, and Premier Guitar magazines. He lives in Portsmouth, NH, with his wife and their two kids, where he fronts the indie-rock band A Different Engine.