I just checked out Sweetwater's new demo of that new pedal that uses one of Korg's NuTube's. It was a very informative demo if you haven't checked it out yet. After digesting that I downloaded the NuTube datasheet from the Korg website so I could take a look at its electrical qualities. I'll break my thoughts into two categories... sound of that pedal from the video and then the side of NuTube technology.
To be as nice as possible, it sounded okay. My very first thought was that sounds like butt, but then he bypassed the pedal and played clean and it still sounded like butt. Okay, upon more listening I thought to myself, "where's the headroom?" That marvelous headroom that you can only get from real vacuum tubes running at very high voltage... not 9, 12, or even 18. So basically, they're selling a starved plate pedal for $250. But is it a real vacuum tube?
I took a very thorough look over the data sheet with my vacuum tube scientist hat on. It appears as though the NuTube is a tube with a directly heated cathode. That means no cathode biasing, grid only. That also means no cathode follower circuits, which are great for driving low impedance loads. The data sheet specifies the plate resistance to be 330k Ohms, which is significantly higher than any vacuum tube I can think of. That means any circuit utilizing these "tubes" will have a much higher output impedance than a typical vacuum tube (which is high compared to a transistor). That's probably why that new pedal doesn't have a bass knob. A treble roll off pot (like on a guitar) doesn't need a driving force to make it work. Having the ability to boost and cut, as in a bass & treble configuration, generally requires a low impedance stage to drive the EQ circuit properly, thus ruling out the use of a NuTube. Another point to make is it will be next to impossible to design a circuit with these in which the NuTube drives a line output transformer. I'm thinking anything studio related - mic preamp, compressor, microphone, etc. But even if it were possible to incorporate transformers, why would you? The NuTube's greatest flaw when comparing to a traditional vacuum tube is its lack of headroom. It surely has more headroom than a transistor, but with a maximum plate voltage rating of 80v, its no where near tube town. Headroom comes from the voltage. More voltage = more headroom. If you ever compared a starved plate pedal next to a high voltage pedal, you'll know what I'm talking about. Big difference in feel and tone. When it comes to a studio application where high fidelity sound is paramount, tons of headroom is most desirable. One of my favorite things about tubes is the insane amount of headroom you can get out of them. Its the reason why tube gear tends to sound "big." Lastly, the NuTube is not as small as Korg likes to tout. Dimensionally speaking, it is thinner than a 12AX7, but there were many vacuum tubes developed that are far smaller than the NuTube. Take the NuVistor for example. Where do you think Korg got the name NuTube from? That's right, the NuVistor was invented in the 1950's to be the vacuum tube's answer to the coming transistor. Its literally smaller than the size of a thimble. Freaking tiny. Korg states their NuTube to last 30,000hrs, which is longer than a miniature vacuum tube's (ie. 12AX7) rated 10,000hrs. But, and I mean a very big BUT, the NuVistor had a rated 100,000hrs!!!
In summary, the NuTube is an interesting development and well suited for starved plate distortion circuits. It does exhibit excellent power efficiency (2% of a 12AX7), even better than a NuVistor, (~40% of a 12AX7... bearing in mind that was way back in the 1950's!!!). But where the NuTube fails is its diversity due to high plate resistance, directly heated cathode, and low headroom. With a current price of $47.79 USD per NuTube, I can guarantee you I won't be designing these into any new products. You can get a decent current production 12AX7 for under $15 or a great NOS tube for under $45!
12AX7 Amplification Factor = 100
6CW4 NuVistor Amplification Factor = 65
12AU7 Amplification Factor = 20
NuTube Amplificaiton Factor = a measly 14.5
About the Author
Mike Congilosi II, Owner/Designer/Electronics Engineer at Lightning Boy Audio and Owner/Audio Engineer/Music Producer at LBA Studios.