What’s the Big Deal About Output-Tube Distortion?

March 12, 2013

TUBE-AMP FANATICS TALK PLENTY about output-tube distortion, and how it’s preferable to preamp-tube-generated distortion if you’re looking for fat, juicy, classic-rock lead tones. It sounds good in theory—I mean, you would think those bigger bottles have just got to sound fatter—but what’s the whole story?

When we say “output tubes,” also known as “power tubes,” we’re talking the big bottles that are last in the signal chain, the ones that ramp up the wattage to drive speakers—most commonly 6L6s, 6V6s, EL84s, EL34s, 6550s, and a few other types. The theory—and there’s some truth to it—is that the type of overdrive sound achieved when an amp’s output tubes begin to clip is thicker, richer, and more dynamic. That it simply has more guts to it, so there’s more body to the distortion tone, and a broader sonic range overall.

Preamp-tube distortion, on the other hand, is clipping that is generated in the smaller preamp tubes toward the front of the amp’s circuit, most commonly 12AX7s and their equivalents. While detractors often say the lead tones produced by these smaller bottles are somewhat fizzy, soft, and constricted, the reality is that plenty of amps use preamp-tube distortion to generate highly acclaimed overdrive sounds. Check out Dumble, Soldano, Bogner, Mesa/Boogie, EVH, Two-Rock, many contemporary Marshalls, and so on, and the scorching lead tones you hear from them will be largely preamp generated. Used in the right circuit, and not just cranked to the max with one gain stage rammed into the next, preamp-tube distortion can be plenty juicy and dynamic.

The other major truth behind this argument is that very few amps actually produce a lot of pure output-tube distortion. Take the plexi Marshalls, Vox AC30s, and tweed Fender Bassmans and Twins that come to mind when we think of classic-rock overdrive. Such amps really aren’t going to kick their output tubes into clipping until they are wound up to the top of the dial. Even then, the preamp tubes—as well as the often-overlooked phase-inverter tube—will still contribute their own distortion to the brew, and therefore play a big part in what we might claim to be output-tube distortion.

Having said all that, if you’re still burning for the juicy goodness of an overdrive tone that’s more output- than preamp-tube generated, how do you get it? Conversely to what you might imagine, it is found most easily in amps with simple, single-gain-stage preamp circuits and lower-wattage output stages. The 12AY7 in a tweed Deluxe, for example, which is a lower-gain tube than a 12AX7, lets you push a cleaner signal on through to the back end of the amp and clip it there when you play it loud. Stevie Ray Vaughan achieved the same trick by putting a lower-gain 5751 preamp tube in his blackface Fenders.

At the end of the day, though, finding an overdrive tone that sounds and feels right for your own playing is a lot more important than worrying about where within the amp it was created.

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