Five Things About Tube Distortion

February 25, 2011

1 TUBES ARE WAY MORE THAN JUST “AMPLIFIERS.” Sure, tubes are the amplifiers within our amplifiers because they make the signal louder. But they do way more in the process. Players rave incessantly about the beauty of tube distortion (and we will do some more of that here), but even when you achieve what you perceive as a gorgeous clean tone from a tube amp, an element of tube distortion is still responsible for shaping that tone. Even when present at lower levels, distortion adds harmonics to the fundamental note the guitar delivers, helping your tone sound thick and lush and shimmery, rather than something like the pure note produced by a powerful audio amplifier with very low THD (total harmonic distortion).

2 TUBE DISTORTION VS. SOLID- STATE DISTORTION. The beauty of the way tubes distort— in a well-conceived circuit— is that they ease gradually and smoothly into breakup as a higher signal is applied, evoking a lot of depth and dimension and an ear-friendly quality as they do so. Transistors, on the other hand, clip off their output abruptly and harshly when finally pushed too far. Explanations of this phenomenon are usually accompanied by two diagrams: the smoothly rounded if slightly off-balance sine wave of a clipping signal through tubes, and the angular square wave of a signal clipping through transistors. Doesn’t even look as pretty, does it? Plenty of solid-state amps can sound great too, but the require a lot of added circuitry to help them sound “tube-like”.

3 THE “WARM” MYTH. One of the main adjectives applied to tubes, by marketing departments at least, is that they sound “warm”—but this can be misleading. The description is often used to plug gear that might include a lone preamp tube in an otherwise hybrid design that also uses transistors for other preamp and output functions, but the 12AX7 they are usually boasting about has an inherently crispy, bright tone when driven to distortion. Play an all-tube late-’60s Fender Deluxe Reverb alongside solid-state amps made by Kustom or Polytone at around the same time, turn each up to about half way, and you can bet the Deluxe Reverb will have more treble bite than the relatively warm tranny amps alongside it. Tubes sound great, but for several reasons other than their “warmth”.

4 PREAMP TUBE DISTORTION. Preamp tubes and output tubes produce rather different tones when distorting. Both can sound great for particular applications, but it’s worth knowing what you are after. In a good high-gain circuit, preamp-tube distortion can produce lots of juicy sizzle, endless sustain, and shedloads of jagged harmonics. Think lead channel on a Mesa/ Boogie Mark Series amps, Soldano SLO, Bogner Shiva, and the like. In a poorly conceived (or simply cheap-o) high-gain circuit, preamp tubes can sound buzzy, fizzy, and harsh, emitting that “can of wasps” tone that has given poorer such efforts a bad name.

5 OUTPUT TUBE GRIND. Output- tube distortion is produced when an amp’s preamp isn’t distorting significantly, but it is passing a full and high-level signal on to the output stage. This form of tube distortion is usually heard as thick, creamy, and dynamic, with perhaps a little more openness and body than preamp-tube distortion. Think cranked Marshall JTM45 or Vox AC30. In truth, we very rarely hear pure output-tube distortion because an amp’s preamp and phase inverter tubes usually distort at least a little by the time the output tubes break up, but we call this brew “output-tube distortion” regardless.

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