For the Love of Tubes

Tubes rule; we all know that. Even those of us firmly in the digital modeling camp still enjoy a good tube amp. There’s something about the warm glow, the smell, and the sense of history that appeals to us.

Many pedals require some sort of amplification circuit, so putting a preamp tube or two into a pedal would seem like an obvious choice; and indeed, there have been some great tube- powered pedals over the years. The Mesa V-Twin and Bottle Rocket both utilized a pair of 12AX7’s for preamp and overdrive capabilities.

As a manufacturer of tubes themselves, EHX has a range of five or six tube pedals including EQ and modulation pedals. Effectrode out of the UK is one of the few companies manufacturing exclusively tube-powered pedals including buffers, delays, and modulations, as well as gain pedals.

Compared to solid state though, the choice of tube powered pedals is small. Our expectations for effects pedals require that they are small, low cost, low powered, and bullet proof. Tubes don’t fit well within these constraints. Tubes are large and fragile, and they require protection against damage yet need ventilation to keep from overheating. They deteriorate over time, sometimes needing replacement, and require high voltages and special power supplies to operate.

It would be nice if we had a small, low power, low temperature, robust and long-lived miniature tube. Wouldn’t that be great for effects pedals? Enter the Korg Nutube 6P1 directly heated dual triode.

Those of us old enough to have used calculators and Hi-Fi systems in the seventies and eighties will recall the now retro-cool blue glowing displays that were often used on these devices. Wikipedia describes these vacuum fluorescent displays as hot cathode, anodes, and grids encased in a glass envelope under a high vacuum condition. Doesn’t that sound familiar? Some folks at Korg thought it did. They have partnered with display specialists, and VFD pioneers Noritake Itron (who are still in the business fifty years on) to produce an audio tube module based on a VFD.

The Nutube 6P1 has some impressive advantages compared to a 12AX7:

  • 30% of the size
  • 2% of the power
  • 30,000 hours continuous life expectancy

At just 12mW per channel from a 5V input, the Nutube should be appropriate for a typical Hammond-style effects pedal enclosure. Small linear regulators can be used so the pedal could be run from a battery or conventional 9VDC pedal power supply. The design generates almost no heat compared to a conventional vacuum tube. The module also looks cool too, with its distinctive blue glow from the phosphor on the VFD.
Let’s take a quick look at some of the figures compared to a GE 12AX7.

12AX7

  • Filament Voltage: 6.3V
  • Filament Current: 300mA
  • Anode Voltage: 250V
  • Anode Current: 1.2mA

Nutube

  • Filament Voltage: 0.8V
  • Filament Current: 17mA
  • Anode Voltage: 80V
  • Anode Current: 20uA

The low voltages compared to a traditional tube make Nutube an easier proposition for boutique pedal builders that can create pedals that run on a 9V battery. They also do not have to worry about high voltages and the associated safety concerns and regulatory requirements.

Korg doesn’t divulge exactly how they have optimized the VFD into an audio tube, but we can determine the basic function just by observing the part. The 6P1 is a twin triode like a 12AX7. The two white squares are the anodes (plates). The centers of the anodes have a phosphor coated layer that glows in operation in a similar way to the original displays. I’m not sure if this serves any functional purpose. It may just be there for visual appeal. The square grid overlays the anodes. You can see the grid pattern if you look closely. The filaments are the two thin wires running across the center of the glass. The circle at rear center is probably the getter that’s used to help maintain the vacuum. I’d recommend checking out the Applied Science Youtube channel, and search for the CRT disassembly video. This includes a great explanation of how a getter works in a vacuum tube.

The Nutube is potentially suitable for any pedal that can benefit from a tube amplifier; including buffers, boosts, overdrives, distortions, modulations, and delays. Because it’s a directly heated device, it may be susceptible to generating microphonic noise, where vibrations are picked up by the filament and reproduced as noise in the signal. This could be an issue for pedals that are typically floor mounted. Acoustically isolating the component inside the enclosure in some form such as plastic assembly, rubber mounts or damping material in the chassis may be necessary.

Nutube was announced a couple of years ago. Korg has produced a number of proof of concept devices, mainly small amplifiers. The output from the tubes is not sufficient for a power amp, but pairing with a Class D power stage is possible. The first full commercial Nutube product looks to have done just that. The Vox MV50 range of small form factor guitar amps (Vox is a subsidiary of Korg) utilizes a Nutube preamp stage with a solid state power amp. I’ve seen a few one-off effects pedals but nothing on a commercial scale yet. I imagine that’s about to change.

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