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From the landing page: The real triode structure produces a warm, unique vacuum tube sound, delivering excellent linearity. "Tube sound" is nonlineari

From the landing page: The real triode structure produces a warm, unique vacuum tube sound, delivering excellent linearity. "Tube sound" is nonlinearity. WTAF. See also the NuTube-powered headphone amp here, which measures 1+% THD at all practical output levels: http://www.nutube.us http://www.nutube.us Actually-linear headphone amps regularly achieve under 0.01% THD... Beyond that, compared to a "classic" preamp tube like the 12AX7 (very popular in guitar amps), the NuTube looks like it has: * simplified structure: the 12AX7 has indirectly-heated cathodes (the filament heats the electrode). This permits the two triodes to be used more-or-less independently. OTOH, the NuTube has a shared directly-heated cathode (the filament IS the electrode). Not a huge problem, but it means some circuits are trickier. Not likely to be cloning a K2-W, ferinstance. * lower anode voltage: the 12AX7 is specified down to 100V on the anode, but the datasheet considers 250V "typical". The NuTube has a maximum anode voltage of 80V, presumably due to the reduced size of the physical elements. This has the expected consequences for performance: lower gain (10 vs 100), lower transconductance (10uA/V vs 1250-1600uA/V) and higher plate resistance (300k vs 80k) * considerably different grid characteristics: the 12AX7 specifies an absolute maximum grid voltage of 0V, because taking the grid positive causes it to conduct electrons from the cathode and heat up. The NuTube specs don't mention a maximum, but the anode curves are shown for up to +4V on the grid. AFAIK grid conduction is sometimes considered part of "tube sound" when it happens in power output stages, but it's definitely not "linear". Other than the line from Vox (owned by Korg) it doesn't seem like the NuTube has gotten much uptake in the industry until recently - 2018 reviews for the Ibanez NTS mention it's the "first pedal with NuTube".

i_am_proteus 8y ago
To a degree, "vacuum tubes" are like "machine learning" and "blockchain" in that their presence in a product has a magical money-attracting quality that's often divorced from their active role in the product's operation. That said, comparing %THD between all-tube circuits and solid-state circuits for hifi isn't entirely apples-to-apples, especially since "single-ended-triode" type tube amps' distortion is predominantly in the second-order harmonic, which listeners often perceive as loudness or "roundness" or "warmth." I've listened to 2% THD tube amps that sounded pretty good to my ears, whereas a 2% THD solid state amp will generally sound like ass. Although, honestly, I haven't spent much time listening to ass, so there's a chance I might just be talking out of mine.
- analog31 8y ago
  A 2% THD solid state amp would be pretty hard to design today, unless it's an effect you were trying to achieve. In that case, you can simulate the transfer function of one device with another.
- klodolph 8y ago
  The characteristics of a single-ended triode depends a huge amount on the circuit around it. A simple cathode resistor acts as local negative feedback. Bypassing it with a capacitor removes the local negative feedback for a range of frequencies. Then there’s the choice of load, resistor or transformer. All have different electrical characteristics, all fall under the “single-ended triode” umbrella. Personally, I’m quite skeptical about the idea that second-order harmonics are “warm” or “round” sounding. Whether or not distortion is “musical” depends on whether you are making music or adding noise to something that is already music; if you are playing a guitar then the distortion is music, if you are listening to a song, the distortion in your audio equipment is by definition noise. I understand that sometimes distortion is inevitable even when it’s not desirable. Your microphone preamplifier will sometimes get overloaded, or you might be listening to a song at high levels with transients beyond the power capabilities of your system. So for microphone preamplifiers, go ahead and stick a tube in there. But for listening, it is usually cheaper to fix your distortion problems by buying a more powerful solid-state amplifier with more headroom, and more expensive to fix your distortion problems by switching to a tube amplifier with better distortion characteristics.
  - madengr 8y ago
    I’d think since 2nd order distortion produces harmonics at octaves (which are naturally pleasing to the brain), vs 3rd order which produces IMD.
    - klodolph 8y ago
      That is simply incorrect. I think there’s a fundamental misconception about what distortion is, and how it works, at play here. Any distortion produces intermodulation distortion. A sine wave will produce exactly an octave harmonic if you distort it with only second-order distortion. However, nobody listens to pure sine waves. So if you feed anything more complicated than a pure sine wave, you will get something other than harmonics at octaves.
phasetransition 8y ago
Other questions I have: What is the change in input Z of the grid above 0V? Does it come crashing out of the megaohm range like a typical triode? (To the general reader: The grid conduction and the bias shift that it triggers is definitely a part of a "real" amp sound. If you've heard anything that sounds like a swirling sound out of an overdriven guitar amp, this is bias shift brought on by grid conduction) Do you need to continuously source current to the grid like most low voltage tube cases What are Cgk and Cga? What are the curves like near cutoff, and are the slopes different due to a different grid geometry relative to a traditional triode? --- PS, if tube amps for music are interesting to readers of HN, but you have no background, there is an exceptionally lucid book on the topic: "Designing Tube Amps for Guitar and Bass" by Merlin Blencowe.
wrycoder 8y ago
If the filament is the cathode, you’re going to have to feed it DC for heater voltage.
coldtea 8y ago
>"Tube sound" is nonlinearity. In balance. Too much nonlinearity is just noise. You want to have good balance, nonlinearity for warmth and linearity for fidelity.

i_am_proteus 8y ago

To a degree, "vacuum tubes" are like "machine learning" and "blockchain" in that their presence in a product has a magical money-attracting quality that's often divorced from their active role in the product's operation. That said, comparing %THD between all-tube circuits and solid-state circuits for hifi isn't entirely apples-to-apples, especially since "single-ended-triode" type tube amps' distortion is predominantly in the second-order harmonic, which listeners often perceive as loudness or "roundness" or "warmth." I've listened to 2% THD tube amps that sounded pretty good to my ears, whereas a 2% THD solid state amp will generally sound like ass. Although, honestly, I haven't spent much time listening to ass, so there's a chance I might just be talking out of mine.

analog31 8y ago
A 2% THD solid state amp would be pretty hard to design today, unless it's an effect you were trying to achieve. In that case, you can simulate the transfer function of one device with another.
klodolph 8y ago
The characteristics of a single-ended triode depends a huge amount on the circuit around it. A simple cathode resistor acts as local negative feedback. Bypassing it with a capacitor removes the local negative feedback for a range of frequencies. Then there’s the choice of load, resistor or transformer. All have different electrical characteristics, all fall under the “single-ended triode” umbrella. Personally, I’m quite skeptical about the idea that second-order harmonics are “warm” or “round” sounding. Whether or not distortion is “musical” depends on whether you are making music or adding noise to something that is already music; if you are playing a guitar then the distortion is music, if you are listening to a song, the distortion in your audio equipment is by definition noise. I understand that sometimes distortion is inevitable even when it’s not desirable. Your microphone preamplifier will sometimes get overloaded, or you might be listening to a song at high levels with transients beyond the power capabilities of your system. So for microphone preamplifiers, go ahead and stick a tube in there. But for listening, it is usually cheaper to fix your distortion problems by buying a more powerful solid-state amplifier with more headroom, and more expensive to fix your distortion problems by switching to a tube amplifier with better distortion characteristics.
- madengr 8y ago
  I’d think since 2nd order distortion produces harmonics at octaves (which are naturally pleasing to the brain), vs 3rd order which produces IMD.
  - klodolph 8y ago
    That is simply incorrect. I think there’s a fundamental misconception about what distortion is, and how it works, at play here. Any distortion produces intermodulation distortion. A sine wave will produce exactly an octave harmonic if you distort it with only second-order distortion. However, nobody listens to pure sine waves. So if you feed anything more complicated than a pure sine wave, you will get something other than harmonics at octaves.

phasetransition 8y ago

Other questions I have: What is the change in input Z of the grid above 0V? Does it come crashing out of the megaohm range like a typical triode? (To the general reader: The grid conduction and the bias shift that it triggers is definitely a part of a "real" amp sound. If you've heard anything that sounds like a swirling sound out of an overdriven guitar amp, this is bias shift brought on by grid conduction) Do you need to continuously source current to the grid like most low voltage tube cases What are Cgk and Cga? What are the curves like near cutoff, and are the slopes different due to a different grid geometry relative to a traditional triode? --- PS, if tube amps for music are interesting to readers of HN, but you have no background, there is an exceptionally lucid book on the topic: "Designing Tube Amps for Guitar and Bass" by Merlin Blencowe.

wrycoder 8y ago

If the filament is the cathode, you’re going to have to feed it DC for heater voltage.

coldtea 8y ago

>"Tube sound" is nonlinearity. In balance. Too much nonlinearity is just noise. You want to have good balance, nonlinearity for warmth and linearity for fidelity.