You might think this is weird. I sure do. Vacuum tubes used to be used in everything: radio transmitters and receivers, audio amplifiers, televisions – anything at all that required a signal to be amplified, or AC to be rectified. Beginning in the 1950’s semiconductor transistors began replacing the vacuum tube, and now we have integrated circuits that can include billions of transistors on a single tiny chip! Yet we still persist in using vacuum tubes in guitar amplifiers.
Some of this, for sure, can be chalked up to tradition, or voodoo, mojo, or whatever mumbo-jumbo tends to make electric guitarists some of the most irrational folks on the planet. There’s also the worship of an amazing time in rock history when guitar tones were first being invented by folks such as Eric Clapton, Jimi Hendrix, Pete Townshend, and Jimmy Page. This is also seen in our continued devotion to Vox, Marshall, and Fender amps.
However, the science behind what the tubes in our amps do also bears evidence that there might be something behind the woo-woo. Now, whether any of that stuff matters when you’re playing with a zealous drummer in a really loud room is matter for another discussion…
The ways which tubes and transistors respond differently to audio signals are many, but there are primarily two characteristics of vacuum tubes that make them preferable to some for audio amplification. Interestingly, both of these things relate to distortion.
All audio circuits will introduce a measure of distortion to a signal, meaning that a truly transparent passage of signal through any circuitry is theoretically impossible. Every component a signal runs through will introduce some measure of distortion. Vacuum tubes distort a signal more than transistors do, but we tend to prefer the sound of this distortion over the cleaner sound of transistor amplification. This is because the type of distortion tubes create sounds good with the original signal (even-order harmonics, for you physics geeks). The result is something we enjoy, and would describe as sounding richer, warmer, and more lively.
The second characteristic that we love about vacuum tubes is what happens when they saturate, also known as overdrive or clipping. There is a limit beyond which a tube will not amplify, and if the signal coming into the tube pushes it past that limit, clipping occurs. It’s pretty easy to overdrive preamp tubes. When we use a boost pedal and it causes distortion, this is usually what is happening. It can sound pretty good, but it’s nothing like the rich, creamy loveliness of what happens when power tubes are pushed into saturation. On the People’s Amp, this happens at about 2 o’clock on the volume knob. From there until it’s wide open, the amp doesn’t really get any louder, but it sure does get fatter!
Again, the harmonics produced by tubes saturating are ones that we seem to like, whereas transistor saturation tends to sound brittle or harsh to us. Some folks also say that tubes exhibit a quality known as “soft clipping.” This means that as the tubes start to move into overdrive, it happens gradually in response to touch, rather than all at once, which is what happens with transistors. To my taste, this makes tube amps seem more responsive to the small nuances in my playing, similar to how an acoustic instrument responds.
Then there is this issue of “sag.” The theory says it’s real, and I must confess that I think I’ve experienced it, but I wonder how much of this is psychosomatic. With an amp that uses a tube rectifier, the voltage being supplied to the power supply can decrease when a louder attack causes the power tubes to draw more current. This causes a momentary dip, or “sag” as the voltage recovers. Solid-state rectifiers do this more quickly. Sag affects more of low frequencies, which take more power to amplify. This can make an amp feel very responsive, like it’s “breathing.”
Are you infatuated with tubes? Do you love the orange glow? Do you think it’s quaint that they can misbehave in unpredictable ways, like a spirited child? Like me, do you get tickled using ancient technology?
Long live the thermionic valve!