All About Rectifiers

October 29, 2008

RECTIFICATION 101
In basic terms, a rectifier is the diode component within a tube amplifier that converts AC voltage from the power transformer into a DC voltage required by preamp and output tubes to do their amplification business. In doing so, a rectifier also increases the resultant DC voltage level slightly. In fact, the voltage levels are increased twice on their way to the output tubes: the AC that comes into the amp from your wall socket at around 120 volts is first ramped up by the power transformer to around 330 volts (as in a Fender Deluxe Reverb), and then converted by the rectifier to around 400 volts DC to feed the Deluxe’s two 6V6 output tubes.

The rectifier is not in the signal chain portion of the amp, and your guitar signal does not pass through the rectifier at any time. But this doesn’t mean that the rectifier has no impact on the sound of your amp. Preamp and output tubes perform differently at various voltage levels, so a rectifier that converts your power transformer’s 330 volts to 400 volts DC will result in your amp sounding a little different than it would with a rectifier that converts it to 350 volts. Rectifiers also have frequency sensitive internal impedances that cause voltage loss or “sag” when power demands are high. This affects the attack, compression, dynamics, and touch-sensitivity that any tube amp gives you, and is something players tend to notice most about how their rectifiers are performing—whether they know it or not. Tube rectifiers are four-pin devices that typically fit into octal (eight-pin) sockets. However, only four pins are connected internally— two that take in five volts AC from the power transformer to heat the filaments so the tube can function, and two that take in the high-voltage AC current for conversion. So where does our DC come out? Through one of the pins that is taking in the filament supply. Sounds impossible, I know, but one pole of the AC (which is bipolar, alternating current) can enter while the lonesome DC exits. Some nine-pin tube rectifiers also exist, such as the EZ81 that was used in the early Vox AC15 and Marshall 18-watter of the mid ’60s. Recently back in fashion, the EZ81 looks a little like an EL84 output tube, and fits into the same type of socket, although only four of its pins are connected internally. Some small early guitar amps also used sevenpin- socket rectifiers such as the 6X4 or 6X5 (again, with only four pins “live”), but these tubes aren’t very common today.

SOLID-STATE RECTIFIERS
Up until the late 1950s, almost all guitar amps used tube rectifiers. Even high-powered amps that required a lot more current—such as Fender’s high-powered tweed Twin—still used tube rectifiers. But the demands put upon them by four 5881or 6L6GC output tubes running at high voltages verged on being more than they could handle. Toward the end of the ’50s, the arrival of reliable and affordable silicon diodes provided manufacturers of high-powered amps with an excellent alternative to tubes. A solid-state rectifier is typically comprised of two, four, or six diodes in what’s called a “bridge” configuration (see diagram). Simply by switching to silicon rectifiers, amp designers could deliver higher voltages to the output stage and squeeze more power from the tubes. And with less sag too, giving their big amps a more immediate and consistent response.

When Fender changed to solid-state rectification in many of its Professional Series amplifiers in 1960—including the blonde Twin, Showman, Vibrosonic and a few others— it bragged about the fact in its advertising literature, pointing out that silicon rectifiers reduce both chassis heating and servicing problems. Silicon diodes also cost a fraction of the price of a tube rectifier, and since they require no filament supply, the transformers used in amps that employ diodes for rectification can be made without this extra winding, and therefore cost less to manufacture. Eliminate the socket that a tube rectifier requires, and you’ve got a threefold savings. Sonically speaking, solid-state rectifiers truly are beneficial to certain amp designs regardless of cost, so many manufacturers used them for the right reasons, and still do. In addition to these great early Fenders (as well as some of the larger blackface models that followed them) legendary amps employing solid-state rectification include Marshall’s plexi- and metal-panel JMP50 and JMP100, Hiwatt’s thunderous 50- and 100-watt heads, Mesa/Boogie Mark Series amps of the early ’70s and beyond, and modern classics such as the Soldano SLO and Peavey 5150.

Still, the slightly compressed feel that a tube rectifier produces when hit hard can be a big part of the magic in some tube amps. It softens the front edge of the player’s pick attack while producing an enticing, explosive swell of notes just behind that attack, and can often make an amp feel more dynamic and touch-sensitive. Players and philosophical amp designers alike will also tell you that they sometimes hear more air and dimension in tube-rectified amps; a sort of bloom and spaciousness in the notes that gives them depth and texture.

In addition to the beloved Deluxe, other amps from the golden-age of tone that employed tube rectifiers include Fender’s tweed, brown, and blackface Champ, Princeton, Deluxe (and Deluxe Reverb) and Super (and Super Reverb) and others; Vox’s AC15 and AC30; Gibson’s GA-30 and GA-40 Les Paul; Marshall’s JTM45; and almost any Valco-made amp. Reissues of these models from Fender, Vox, and Marshall follow the originals in the rectification stakes, and almost all boutique amp makers—including 65 Amps, Carr, TopHat, Victoria, Matchless, and plenty of others—lean heavily toward tube rectification. Some other small amp designs, however, exhibit plenty squash from the output tubes themselves, and adding rectifier sag to that would only make them too soft for a satisfactory performance. Cornford’s highly regarded Harlequin, for example, has solid-state rectification behind its juicy single-ended six-watt output, as does Bad Cat’s five-watt Mini II.

TUBE RECTIFIER TYPES
Different types of rectifier tubes provide different levels of AC-to-DC conversion. The most common small-amp rectifier tube is the 5Y3, as found in old Champs, tweed Deluxes, and many vintage Gibson amps. This tube works fine in small combos that put out anything up to about 15 watts, but when pushed hard it can start to sag noticeably— cool, in the right place, but potentially frustrating when you’re looking for punch and clarity. At the other end of the scale, the GZ34 (equivalent to the 5AR4 in the USA) has long been considered the king of the rectifier tubes. This powerful, sturdy rectifier tube makes more DC out of the AC it’s fed, and does so very elegantly and with great efficiency, while still inducing many of the most-loved characteristics of tube rectification into an amplifier’s performance. Show a tube amp freak a European-made Mullard GZ34 from the ’60s and watch ’em go weak in the knees and start scrounging around for the $150 or more to pay for it.

In between the 5Y3 and GZ34 are a number of rectifier tubes—such as the 5R4, 5U4GB, 5U4, and 5V4—which yield AC/DC conversion with ascending efficiency, and are generally found in mid-sized amps. Rectifier tubes can sometimes be swapped for a different type in cases when you find it preferable to run your tubes at slightly higher or lower voltages. Such a change should only be made, however, after consulting with your amp’s manufacturer or a qualified tech.

As with preamp and output tubes, different makes of the same type of rectifier tube—and even different examples that came off the line from the same factory on the same day—can perform their duties a little differently. By this I mean that if you obtained three NOS Mullard GZ34s or three Sovtek 5Y3GTs from the same year of manufacture, popped them into an appropriate amp one after another, and measured the DC voltages that each put out, you would find slight variances. Tube amp fanatics understand the need to obtain a range of good rectifier tubes and test them all in an amp to see which one makes it tick most sweetly. If you go to these lengths yourself and find a particular rectifier that your amplifier seems to groove on, it might even be worth “blueprinting” that stage of your amp by measuring the DC voltage coming out of the rectifier (often referred to as B+) and note it down for posterity. Just remember that a tube amp’s power supply voltages are potentially lethal, so have a qualified tech do this for you if you’re unfamiliar with the procedure and related safety issues!

Next time you need to change rectifiers, try a few until you find one that gets your amp sounding its best. Be aware, however, that even our more reliable domestic power today will vary from town to town, or even from one side of town to the other, or according to time of day, demands of the season, and other factors. Plugging the same amp, with the same rectifier, into a socket giving off 118 volts in Cleveland then another providing 124 volts in Cincinnati will result in different voltage levels within the amp. Finally, if you do swap your rectifier for one that produces a different DC voltage level within the amp, be aware that many fixedbias amps will need to have their output tubes rebiased, another job for a professional. Digest, tweak, and enjoy—the tonal rewards are worth it.

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