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Tone Tip: Preamp Tube Tasting

Dave Hunter | 09.10.2008

I’d like to start this discussion of preamp tubes with a brief look at why so many guitarists still prefer tubes at all to solid state or digital modeling amps. Tubes are what make an amplifier amplify—which is to say, they are at the heart of what enables an amp to make your guitar louder—but it might help to think of tubes not simply as amplifiers (which is what they are, technically) but as tone generators. Sure, there are other ways of amplifying your guitar signal, but the beauty of tubes is that they don’t just make a sound bigger, they make it bigger with style.

Tubes amplify an electric guitar so beautifully mainly because of the way they distort. To put it as briefly and concisely as possible, push a simple transistor circuit hard, and it clips (distorts) in a sudden, harsh, “square wave” way. Push a tube into clipping and it distorts more gradually and more smoothly—it “rounds off” into distortion. If you view these two different types of distorted sound waves on a scope, a signal clipping in a solid-state circuit and one clipping in a tube circuit, you will see waveforms that are literally squared and rounded respectively. There are a lot of other factors involved, of course, but that gets us to the nut of it. This is why any decent sounding solid-state amp requires a lot of extra circuitry to warm up and smooth out the guitar signal, something a very simple tube amp circuit usually does naturally. And be aware, too, that when I’m talking about distortion, I mean the tone that influences even your so-called “clean tone.” Most tube amps, even when set to clean levels (unless you’ve got the volume of a powerful amp set extremely low) are still distorting a little, and that distortion creates layers of harmonics that sweeten and fatten up that thing that we call our tone, even when we’re playing “clean.” This time out, let’s address a few tips regarding preamp tubes.

A pair of Mullard EL34sThe fact that tubes distort so organically also means that no two tubes distort or even amplify exactly alike, even two tubes of the same type. Tubes are manufactured under fairly rigorous conditions, but even so they are imperfect devices. Every little fluctuation on the assemble line results in a slightly different sound and performance from each tube that comes along. That’s why good tube distributors need to routinely test tubes they sell: put even two high-quality preamp tubes from highly respected old American or British manufacturers* on a tube tester, say, a pair each of Mullard or RCA 12AX7 preamp tubes that came out of the factory on the same day in 1963, and they will most likely have slightly different readings for gain and other factors. Put enough of them up on a tester and some will even fail to meet standards. That’s the way it is. *What we call NOS tubes, for “new old stock,” tubes manufactured many years ago but never put into use.

What this all means for the guitarist is that it behooves you to get your hands on as many different tubes as you can reasonably afford, and different makes of tubes, and try swapping a few around to see which ones help you to best achieve the tone you are seeking. The first preamp tube position usually affects the tone of that part of the amp the most (read your amp’s tube chart or owner’s manual to make sure you know how to change tubes safely, and are changing the right tube, and please don’t touch any hot tubes! Let them cool down first). Try three different makes of 12AX7 (also called a 7025, or ECC83 in Europe) in that position—assuming that’s the tube type it uses, as the majority do today—and I’m willing to bet you’ll notice a slightly different tonality from each. Search the Internet and read up on what other players consider to be the best current-manufacture tubes coming out today (there’s too much detail on that subject to go into here), and also see if you can find any affordable NOS tubes, or can pull some used but functioning examples from old junker radio or hi-fi systems you find at garage sales and swap meets. Experiment a little, and see which ones work for you. 

In addition to trying different makes of the same type of tube, there are a few different tube types that are entirely compatible with the 12AX7—in an electrical sense—but which offer different gain and performance responses. To lower the gain of a preamp stage a little you can swap a 5751 into any socket that carries a 12AX7, or to lower it even more but retain the same performance characteristics (other than gain) you can use a 12AY7, like many of the classic “tweed” amps of the 1950s used. Many players think the last thing they want to do is lower the gain of a preamp stage, but in doing so you can often prevent your signal from dirtying up in the preamp, and thereby pass a beefy, full-frequencied signal along to the output stage when the amp is cranked, and thereby generate more output-tube distortion, which results in a fatter, fuller tone in many simpler tube amps. (Note that this tip doesn’t usually apply to high-gain type tube amps, whose whole raison d’etre is to generate preamp distortion). This 5751 swap is a trick that Stevie Ray Vaughan used, for one, to help generate his signature tone, and it has also been employed by plenty of other classic blues players. If you’re trying to achieve less of what you hear as preamp distortion and more output-tube distortion, you can also try using a 5751 in the phase inverter position, which is usually the last preamp tube before the output tubes. Again, check your owner’s manual. There’s a lot of fun to be had in swap land. Happy tube tasting.