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What's the Big Deal About Tube Rectifiers

January 30, 2014
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If we generally agree that good old all-tube guitar amps are still the tone tool of choice for the majority of pros, does it follow that putting a “solid-state” rectifier in that circuit is necessarily going to compromise something? To answer that, let’s first look at what any type of rectifier really does in an amp, and how tube and solid-state versions perform differently … if indeed they do.

Every tube amp needs a rectifier to convert the AC current coming from its power transformer into the DC current that makes its tubes do their thing. In that sense, the job of the rectifier is pretty simple: Two streams of alternating current (AC) go in one side, and are married and converted to one stream of direct current (DC) that comes out the other side. Several varieties of tubes can perform this job, as can a simple pair of silicon diode chains (what we call solidstate technology). The real differences between the types are in how much DC they make out of the AC given them, and how quick they are at doing the job when hit hard—say, when you’re hammering power chords or squeezing out blues solos with the amp cranked.

Tube rectifiers exhibit what we call “sag” when hit with a request for a lot of current when the amp is working hard. This is because they can’t do their job instantly, and need a few milliseconds to get back up to speed. The result is a type of compression in the sound and playing feel. There’s a slight sponginess, some give at the front of the note that softens the attack, and a gentle blooming as the tubes roar back to full voltage. The thing is, different tube rectifiers do this to different extents. Of the most common types, a 5Y3, found in many smaller amps, exhibits the most sag. On the other hand, a GZ34 (aka 5AR4), used in slightly larger amps or amps that need a tighter response, might not sag at all until hit really hard. There are also other gradations between these two.

Roughly parallel to this, the smaller, “softer” tube rectifiers generally produce less DC with the AC given them; that is, your amp’s other tubes will see higher voltage coming out of a GZ34 than out of a 5Y3, even if your power transformer is putting 350 VAC into both. (Some amps allow you to swap rectifier tube types to achieve the desired effect, but many don’t. Never try it without consulting your manual or the manufacturer.)

There are also different configurations of solid-state or diode rectifiers—some designs used in smaller amps, some in larger—but they are more universal in the way they work. All commonly used types convert more DC out of the AC at their front end than tube rectifiers do, and all of them exhibit less sag, too (how much less depends on what tube you compare them to).

This means you often get somewhat less of that spongy, compressed playing feel from an amp with solid-state rectification, as well as a faster response. At high volumes, the attack is likely to be more immediate. But other factors can blur this apparent dichotomy: Preamp and output tubes will also compress—especially in smaller amps—creating some of that same sag, even in amps that don’t have tube rectifiers. All else being equal, though, the tube-rectified amp will compress more at the same given output level.

This doesn’t by any means equate to “diodes bad, tubes good,” and for some players it might even be the other way around. What it does mean is that you can select your amp’s style and configuration according to your playing needs, rather than just following the current hype. Plenty of great amps, old and new, have been made both ways: 100-watt Marshall plexis, Fender blackface Twin Reverbs, Carr Ramblers, and Soldano SLOs all have solidstate rectifiers; the Fender tweed Deluxe and Bassman, Vox AC15 and AC30, and Matchless Clubman and DC30 all have tube rectifiers. If you still can’t decide, think about this: As the name suggests, the Mesa/Boogie Dual Rectifier series of amps allows you to switch between tube and solid-state rectification with the flip of a switch. The best thing you can do is try a bunch of amps with various rectifier types and see what you think. Whichever works for you is the only “right” choice.

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