Korg Nutube OD-K1 Review

By Andrew Gregory. Posted

Every guitar geek knows that the best sound comes from valves. Lovely, archaic, inefficient, glowing expensive valves. When transistors, and later op-amps, became available to pedal and amplifier builders, one of the advantages was cost.

That’s still a factor today: transistors cost pennies, while valves cost many times more, are fiddly, will eventually wear out, and need mains voltage to amplify a signal from your guitar cable. If only there were a way to combine the best bits of both? That’s the premise of the Korg Nutube (£126.31), packaged here as part of a distortion pedal kit, distributed in the UK by RS Components.

Korg claims that these things give all the benefits of valve tone, with the added bonus of being smaller, more reliable, and less dependent on mains electricity.

To understand how they work, we need to look at how valves work.

A tetrode valve has a cathode, an anode, and an electron grid. The cathode is a source of electrons that are accelerated toward the anode (also known as the plate). Where electrons move over the grid is where the magic happens: small changes in voltage at the grid, relative to the cathode, cause electrons to be either attracted or blocked at the grid, causing a proportionately higher or lower voltage at the cathode. The components are sealed in a vacuum in a glass tube, hence the American name, vacuum tubes.

Transistors work in a similar way: the base, collector, and emitter are analogous to the anode, grid, and cathode of a valve, but a transistor works by amplifying current, not voltage, so some of the characteristics of valve amps are lost.

That’s where the Nutube comes in. It uses very similar technology to cathode tubes: vacuum fluorescent display, which also uses the same anode, grid, cathode system – in this case, to make a filament glow as part of (most usually) a seven- segment display. They’re common in tech from the early 1990s.

The versions used in the Nutube glow as voltage is passed over them, and they also produce signal gain, which Korg has put to good use in a range of products for the electric guitar.

The Nutube (bottom- right in the pic) glows with blue light as the guitar signal goes though it


What you get for your money is two PCBs; one for the Nutube, and a larger one for the rest of the through-hole components. You also get the enclosure, pre-drilled, all the components, and the knobs and switches to make it work. It took us two afternoons to put together – if you’re better at identifying resistors’ bands than us, you’ll be able to get it done more quickly.


From left to right, the controls are: volume, tone, shape, and gain. In common with a lot of boutique pedals, with the gain turned all the way down, the pedal produces no sound at all, not even a clean tone. There’s no arbitrary cut-off where a designer has chosen what the appropriate level of clean is for you – it’s up to you to experiment.

Turned up to 1 or 2, with the volume at about 5 or 6, and you get a clean tone that goes from dark to bright to cutting, depending on what you do with the tone knob.

It brightened up the neck pickup on our Les Paul copy, and rolled off the ear-bleeding shrillness of a Strat played through the bridge pickup. Happy days.

Judicious application of the gain and tone together takes us from slightly gritty, to fuzzy, Orange-like, blunt force distortion and almost, but not quite, up to high-gain metal-type distortion.

We’re also happy to report that the level of distortion varies according to how hard you hit the strings, just like valve distortion.

Apart from the greater harmonics in valve distortion compared with solid-state circuits, valves also impart a compressed, squashy quality to the sound. You can hear this in loads of old recordings, but REM’s Monster album and Seether by Veruca Salt are good examples. This is what’s affected by the shape knob and the trim pot mounted on the PCB.

With the trim pot set to half-way, the shape control gives you a way to tighten the distortion on the bottom end. It’s great for choppy, percussive clean sounds, à la Nile Rodgers, and it genuinely does give you an authentic valve-like playing feel.

Tune the trim pot all the way up and the sound kicks in about a quarter of a second after you hit the strings, which is a bit useless, but fun. And, as with the volume and gain, we like being in control.


Purists will turn up their noses, but that’s an iron law that Korg must be aware of. This isn’t for purists: it’s for tinkerers who want to mess about with building their own valve-powered effects, amps, and synths and who want to be able to power them from a battery.

The pinout from the Nutube is the same as from a pair of triode valves such as 12AX7s (probably the most commonly used amp valve), so once you’ve got a working circuit for one, you can work with the other.

Compared with what goes into the average distortion pedal, this is an expensive kit. We see this more as a gateway for home tinkerers who want a valve sound without having to take the risks associated with mains electricity.

Home-brew enthusiasts have already made amps using Nutubes, and Vox has brought out a range of Nutube-powered amps starting at £219, which looks like cracking value compared with £126 for this pedal.

There’s a load of smoke and mirrors in guitar land, and we’ve seen tricks such as fake valves lit up with LEDs to simulate the glow of hot electrons. This is no trick, and it’s not a gimmick either: it works, it sounds great, and it’s incredibly versatile, especially when you get your head around the shape control. Super stuff.

Verdict: 8 out of 10

A great use for a new old idea

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