What Are Rectifier Valves?

If you’ve got a valve guitar amp, you’re no doubt well aware that the specific valves inside it largely determine your sound. You might also know that there are roughly 25 different valves used for guitar amplifiers, four of which you’ll bump into all the time. What you might not know, however, is that there’s a very specific valve that kick-started what we now call ‘vintage’ sound way back when: the rectifier valve. And guess what? It’s regaining popularity again!

12AX7, 6L6, EL-84, EL-34…if you put the most commonly used guitar amp valves in a row, it almost looks like a mathematical equation. Found in amplifiers made by all big-name brands, including Marshall, VOX, Fender and Mesa Boogie, these specific valves should ring a bell for any veteran guitarist.

What Are Rectifier Valves?

The 12AX7 valves are classic preamp valves while 6L6s, EL-34s and EL-84s typically come in pairs to drive the power amp section of valve guitar amps. In specific configurations, they produce an instantly-recognisable sound that’s often associated with one of the amplifier brands I just mentioned.

Classic Valve Combinations

If you’ve been playing the electric guitar for a good while, then like hordes of your fellow guitarists, you probably link 6L6 valves to Mesa Boogie and especially Fender, who’ve built their Californian tweed-sound on these valves. Across the pond, Marshall mainly loaded its amps with EL-34 valves back in the day, while VOX’s go-to valve for their well-known British rock-grind sound is the EL-84.

What Are Rectifier Valves?

Nowadays, those classic valve load-outs are mostly outdated. Fender will happily opt for 6V6 valves these days, and Marshall has not only been using old-school KT66-type valves again, but has been ‘borrowing’ EL-84s from VOX. Mesa Boogie, meanwhile, has been outfitting its valve amps with classic American as well as classic British valve pairs — sometimes even both. All of this has everything to do with the fact that us spoiled guitarists want it all, from that famed tweed sound to vintage blues tone, and preferably crammed into a hefty cab loaded with 1950s-style electronics. I mean, ask any old-school guitarist how they feel about modern modelling amps, no matter how good the simulated valve sound actually is, chances are their answer will start with “Oh please…”

Boutique Amplifiers

The rise of modelling amplifiers coincides with the rise of boutique amplifiers, which is the collective name for eye-wateringly expensive, valve-driven amplifiers that can do it all. Boutique amps treat you to cleans supported by masses of headroom, shape full-fat overdrives and crunch, and even deliver metal-ready distortion topped with a ton of harmonics. No matter what kind of drive and attack you need, most boutique amps can hook you up, courtesy of a special type of valve. A valve that, until recently, was collecting dust in the warehouse of so many amp builders: the rectifier valve.

Now I know what you’re thinking: Wait, you rambled on about 6L6 this and EL-34 that, and now this? A rectifier thingamabob? Are you deliberately trying to confuse me?

I promise you I’m not. The ‘forgotten’ rectifier valve is what started it all in the first place and is now on its way back. In fact, Mesa Boogie has already launched a special Rectifier Series, and they’re not the only manufacturer of high-end guitar amps that has rediscovered rectifiers.

What Are Rectifier Valves?

And don’t worry, I’m not turning the rest of this article into a deep-dive on the technical specifications of rectifier valves. I’m simply going to tell you how they might enhance your sound and inject the same magic that, if you listen closely, you’ll hear on old blues records by bands like John Mayall & The Bluesbreakers.

The Feel

Rectifier valves mainly have an effect on the ‘feel’ of the amplifier and essentially make things a little spongier compared to amplifiers that are equipped with diodes to turn the alternating current coming from your wall socket into direct current — a process known as rectification, hence rectifier valve. Put simply, what sets rectifier valves apart from power amp and preamp valves is the fact that the actual audio signal doesn’t run through them at all. Nevertheless, rectifier valves do have an effect on the sound, which is more commonly known as sag: a sudden, short-lived dip in the sound that happens shortly after the plectrum hits the string, reducing the attack. In the case of blues notes, this gets you a silky-soft response followed by long, lilting sustain, which is the result of the short power dip caused by the rectifier valve, followed by the literal resurgence of the amplifier valves. Most boutique amplifiers feature a sag switch so you can toggle this effect on and off at will. Some models will even automatically turn off the sag effect when the lead channel is fired up, which is particularly useful for fast metal riffs that wouldn’t sound quite right with a more silky ‘feel’.


If you’ve already got an amp kitted out with a rectifier valve, then it’s worth knowing that you can experiment with the sag effect by plugging in a different type of rectifier valve. Quick warning: never swap a power amp valve for a rectifier valve! Mesa Boogie are known for using three different types of rectifiers, ranging from mild to intense sag. Found in the Marshall Bluesbreaker, Marshall’s Astoria Series amps, the VOX AC30 Handwired and the Orange AD30, the 5AR4 (aka GZ34) is one of the least saggy rectifier valves you can get.

What Are Rectifier Valves?

The 5U4GB offers a juicier sag without immediately sacrificing sonic flexibility. It can be found in the Mesa Boogie Dual and Triple Rectifiers, the Road King, the Roadster, the Stiletto Ace, the Deuce and the Trident models. Boasting the most liquid response, the 5Y3 rectifier valve comes packed into the Fender EC Tremolux Eric Clapton as well as Mesa Boogie’s Lone Star Special. If you demand maximally lush attack, this rectifier is your best bet.

What Are Rectifier Valves?

Now that you’ve made your way through a dense jungle of guitar amplifier valve types, all that remains is to give the old forgotten rectifier valve some love. Pick up a 5Y3, 5AR4 or 5U4 and stick it in your amp. They’re surprisingly cheap so you really can’t go wrong with any of them. Just remember not to swap one out for one of your power amp or preamp valves because the result would be catastrophic!

2 responses
  1. Steve says:

    I find your articles very interesting and light. Not too much tech-speak but enough to get the point across.
    I do think you could use a proof reader though. Typos tend to make me think that the writer is in a hurry and doesn’t really care about the work he/she has produced. E.g. VOC AC3, Eric Claptop plus a paragraph in what I presume is Dutch, which I think shouldn’t really be there.

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