- These things run at low voltages (5-80VDC), which is fantastic for a designer, because 'real' vacuum tubes need several hundred volts DC, which means a big power supply transformer/rectifier/filter network.
- These things don't appear to be able to source a lot of current, which makes them good for preamp and tone circuits, not so much for power amp circuits. Classic tube guitar amps got a lot of their tone from the phase splitter, power amp tubes, and power amp transformers. Don't expect an all-nutube-amp.
- Utility for guitar amps would be for "hybrid" amps with a tube preamp and tone circuit feeding a solid-state power amp. There's certainly room for experimentation by amp designers here, since most contemporary tube amps just copy old circuits (mostly Fender and Vox) and use the same tubes they used (12AX7 being the ubiquitous preamp tube). I doubt the NuTube sounds like a 12AX7 when dropped into the same circuit.
- Utility for hifi is questionable. An all-tube design allows for a very linear, low-parts-count amplifier. These will need to be mated to a solid-state power amp, which nullifies the advantage of a tube amp and enters "vacuum tube as marketing gimmick" territory.
100% this. A tube gain stage in isolation isn't especially difficult to imitate using solid-state electronics or DSP, but sounds nothing like a tube amp.
>Utility for guitar amps would be for "hybrid" amps with a tube preamp and tone circuit feeding a solid-state power amp.
The Vox MV50 amps make a decent attempt at this, pairing a Nutube with a class D power stage. It's tiny, it's reasonably loud and it sounds pretty good.
I suspect the lack of interest from other manufacturers is mainly due to cost; for the price of a single Nutube module, you could add a decent DSP chip and gain a whole bunch of extra functionality. An ornamental 12AX7 and an orange LED has better showroom appeal and costs significantly less. Purists will still want real tubes and real transformers, budget-oriented players will want an amp with all the DSP bells and whistles, leaving the Nutube caught in an uncomfortably narrow niche.
Also, a tube has absolutely no latency.
I think that's the primary key here. We can decently emulate a tube amp in software even (and that can improve indefinitely as our knowledge improves), but we'll never create drop-in circuit element emulators either in silicon or software without adding latency.
Subjectively, the one effect often missing from preamp-only tube amps I've tried is power stage sag. I've seen DSP software with it (or rather, a knob for it). In that respect, I'm not sure the solid state / preamp tube / all-tube segmentation can be justified anymore: modern modelling amps sound terrific (subjectively, to my ears), offer sonic variety (because why not), and have a completely different idea of value (comfort, power and size); With that, the NuTube just feels like it's going after an audience who won't be convinced that electrons flowing through a DSP are made of the same quantum stuff, which is unfortunate.
 Demo: https://www.youtube.com/watch?v=INi6lepBZt0
Compared to that, the dynamic of a tube are pretty simple, though creating a truthful emulation of a full amplifier is still a hard.
But listen to any Herb Alpert recording, and then listen to any synthesized trumpet. The gulf is as wide as the Atlantic. (Part of the reason is that Alpert plays with the notes and the transitions, while a synthesizer just emits a note.)
Well, maybe a crap one, or one not designed for that. Trumpets have some convenient physics that are (comparatively) simple to model, the problem you're thinking of may have more to do with the control system (i.e. not based on actuation of valves, but skipping right to the oscillators) than some inherent weakness of synthesis.
(I played a trumpet as a kid, so I suppose my ear for it is more discerning than most. My talent proved elusive, but the lasting value was I can appreciate how incredibly good Herb Alpert was. He could play like Fred Astaire could dance and Ernst Udet could fly.)
From my experience, overdriven guitar signal formed by ubiquitous FET-based approach driving a saturated tube power amp can sound gorgeous, tight and punchy, and feel very "natural" and responsive. 12AX7s into your ordinary solid-state power amps - not so much.
The real triode structure produces a warm, unique vacuum
tube sound, delivering excellent linearity.
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".
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.
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.
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.
Maybe it’s a stretch to apply this to audio. If you play a flat on piano, and IMD3 falls on a sharp, then that sounds bad. The distortion one octave higher sound fine. Even the sharp one octave higher sounds better than the adjacent sharp.
I bought a tube amp kit for Christmas, so it will be interesting to measure the two-tone IMD3. I only have measured audio THD once, and that was using a spectrum analyzer, so only looked at harmonics. Even if you used two-tones, the IMD would fall within the RBW of the spectrum analyzer.
The Child Langmuir law has a 3/2's exponent, so you get 2nd and 3rd for the single ended case. Further, virtually all guitar amps have a push pull amplifier output (of questionable symmetry) operating into the speaker, so the even order terms are cancelling to some degree here.
The general character of many tube amps is to make a smooth transition from predominantly 2nd order distortion to a mixture of odd order harmonics. This ends up as primarily the 3rd order as a guitar amp speaker makes a heck of an underdamped lowpass filter.
Between the cathode bias bypass caps, the dc blocking caps, and the RC behavior of grid stopper resistors combined with the Miller capacitance, guitar amplifiers have a lot of internal bandwidth filtering that reduces IMD amplitudes.
JFET current behavior in the saturation range is as you say, but not necessarily so in the ohmic region. Generally people using JFETs in these circuits use at least some of their ohmic range behavior.
Since you have skill and experience in this area, and also have a tube amp kit, I suggest googling for "Tubes 201." This is an excellent summary article for the behavior of vacuum tubes.
Also, Aiken amplification website, and Merlin Blencowe's book I mentioned yesterday you would find to be good reads.
Do the math, your distorted spectrum will contain sum and difference frequencies. (For me, what makes the math easy is breaking a real sine wave into a sum of its complex components, which are easier to work with.)
This is, as you say, what makes 3rd order bad in RF, because if I have two frequencies F1 and F2, F1+F2 or F1-F2 can be filtered out no problem, but 2F1-F2 or 2F2-F1 is a problem. But this intuition about distortion does not extend to music.
In music, it is not so much the individual frequencies that are relevant, but the harmonic spectra, and the spectrum is within spitting distance of baseband, so filtering won’t do you any good (at least not in eliminating distortion).
And since music contains harmonic spectra to begin with, for a given F in the signal, you probably already have 2F, 3F, 4F. So when you calculate the spectrum for 2nd order or 3rd order distortion, you get the SAME frequencies in the output either way, just in different quantities.
As a result, as electric guitarists know, you have to be very careful about what harmonic content you feed into distortion. If you feed in a note, you get the same note out with different texture. If you feed an octave, it sounds good. Perfect fifth, great, the distortion adds overtones of the octave below the root. A major third is iffy and something you’d use with less distortion. As you get farther away from simple intervals, like octave 2:1, fifth 3:2, fourth 4:3, or major third 5:4, it gets worse and worse. (Eddie Van Halen reportedly retuned his guitar, tuning the second string down 14 cents, to get the major third closer to a harmonic third.)
And this is why distortion for a single instrument sounds good, but distortion for an entire song with multiple instruments (some of which are usually inharmonic) sounds bad. Even with 2nd order distortion, do you really want sum and difference frequencies between overtones of the guitar and overtones of the snare drum? No.
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.
In balance. Too much nonlinearity is just noise. You want to have good balance, nonlinearity for warmth and linearity for fidelity.
Check out this video at around 3 minutes and you can see the lightning triggering as he plays guitar: https://www.youtube.com/watch?v=_AZj5tc8EmA
My 1970s HP oscilloscope contains several Nuvistors (https://en.wikipedia.org/wiki/Nuvistor), which are nearly as small. 17mm x 10mm dia.
And also, by the way, almost the only brand still manufacturing VFDs that are easy to find on the market (at least buying one by one).
And also, they make pretty tableware :D
I recently bought my first audio upgrade (Cambridge DAC 100, Little Dot MK III with Voshkod 6ZH1P-EV from the 70's on Beyerdynamic DT 1990 Pro headphones) and honestly, the first time I listened to it I was in tears. Coming from just plugging in some Sennheiser HD1 to my mobo audio out, the difference was staggering. There were sounds in tracks I was quite familiar with that I had never heard before. I bought a Schiit with a closed DT 1770 Pro for my office ... which wasn't nearly as good (and incidentally I think the filter caps might not handle the el cheapo DC-AC converter on the solar panels all that well which gives annoying (as in really audible) cracking and popping sounds even after tube rolling). Now for the downside: "audiophile" is a horrible niche to be in, there is so much cargo-culting and downright hostility that makes it a very "toxic" place to be in (not to mention stupidly expensive if you buy all the ahum "bs" ... a signal processing class would be a better investment...). But, I find the toxicity to be a little bit sad as well. Since it detracts people from a very real life improvement. Audio is one of the primary modalities of our sensory system, and even with mediocre hearing (like a male 30+ will likely have) you can, and do, hear an enjoyable difference for a modest investment. The difference between some second-grade Bluetooth thing and a nice "desktop" set /is/ real. My advice, get a nice open headphone (400-600$ range, I /really/ recommend something with a detachable cable since those fail first), something to replace your on board DAC for 150$, and an headphone amp (I like tubes, but for no other reason than that I like the way they look and "sound"). For something around 750-1000$ your life will be better and it'll last you a heck of a lot longer than any AirPod will (heirloom piece). That is, if you like listening to music, if not, disregard everything. Yes it's an investment, but this is HN, where I feel recommending 1k is not that big of a deal.
Now for the more on topic part: apart from some high voltage DC applications like transceivers or transmitters (like microwaves!) tubes have largely been replaced by solid state. And with good reason, they last much longer, and are less conductive to all sort of ailments ... so apart from /super/ specialized applications they have no place. Apart from maybe providing that nice warm glow on your desk, when you're lonely, at night, and just want to listen to that "one song" while staring out of the window.
That's technically true :)
Guitologist has a nice video review that could be interested to tinkerers and musicians: https://www.youtube.com/watch?v=raYGrKWSKRE
In contrast, a 12AX7 is indisputably a tube.
Is this purely used in audio or is there a display (video) usecase for this as well? The landing page seemed to imply the technology is coming from display tubes.
Ibanez has also released a Tub Screamer that uses them: https://www.ibanez.com/na/products/detail/nts_01.html
I had the "Clean" one for a while, which mimicks a Fender circuit. It sounded pretty darned close to a "real" tube amp. The only reason I got rid of it was I got a Fender Super Champ x2 with USB out for recording more easily.
the tech uses the same manufacturing line as a traditional vacuum fluorescent display. VFDs are technically vacuum triodes where the plate has been painted with a phosphor.
the device is probably not fast enough to be used as a video amplifier.
In an application like a guitar amplifier, some kind of effect/distortion pedal, etc the tube circuit is typically overdriven by design to get all of those warm harmonics. A tube that isn't being overdriven is every bit as capable of being a "wire with gain" as any kind of transistor or more modern circuit.
I get easily annoyed though when people start spewing pseudo-scientific BS justifying that deviations are objectively better than no deviations or that there are significant (or "obvious with good ears") differences between amplifiers where there simply are no such differences.
This isn't solely a defense of the position you challenged, more a call-out on your tactic here. You're not exactly representative.
The talk of transistor amplifiers being perfect and having no sound at all is of course nonsense: typically this kind of claim looks at frequency response and outright ignores slew rate, crossover distortion, behavior as the amp exits its passband on either extreme, and so on. There CAN be transistor amplifiers, both simple and incredibly complex, that are so good that they've got effectively no sound at all. It's even possible that it's easier to get there with a transistor amplifier given enough money and resources. But that's not the claim.
There are DSP effects that can simulate this as well, of course.
interesting that they break the typical vacuum tube naming convention--the first number usually refers to the filament voltage. as a 6P1, i'd expect it to have a 6.3V filament, but it actually has a 0.7V filament!
Electronic music exists as it does because the advent of digital synthesis/recording technology in the 1980s led to mass abandonment of analog gear by rock and pop musicians, which was eagerly snapped up by broke hippies and hip-hop DJs. Commercial musicians jumped on the digital bandwagon for reliability, range, and repeatability (sounds stored as easily-recallable software presents instead of requiring tedious rewiring and knob-twiddling or having 10 keyboards on stage). Also, commercial musicians had mostly used synthesizers to substitute for acoustic instruments; it's cheaper to hire a keyboard player than a string or a brass section, and digital synthesis and sampling offered more realistic imitations than was possible with analog gear. Electronic musicians are more interested - no, obsessed = by the timbral characteristics and weirdness of the instruments themselves. I know people who have dedicated years of their lives to analysing and reproducing the characteristics of discontinued instruments like the TB-303 bassline as if it were a Stradivarius. Synth circuit engineers are venerated like stars at music equipment trade shows, and the social hierarchy of electronic musicians is ordered by their known propensity for experimenting or modifying their gear rather than commercial success. It's an interesting ecosystem.
Anyway, rest assured that few if any of the products using this component will end up in a landfill.
I'm no way getting rid of it!
P.S.: I generally dispose any of my devices when they let their magic smoke out. Computers stay at least five years and upcycled. Mine -> Family -> Extended family -> so on.
Vacuum tubes are considered obsolete in pretty much any other field, but they're still the gold standard for guitar amplifiers. Despite huge advances in digital modelling, many guitarists still strongly prefer traditional tube amplifiers. Tube amplifiers are fragile, finicky, incredibly heavy and frequently too loud for domestic use, but they sound superb. The NuTube is by no means the first product to promise the sound of tubes in a lighter and more reliable package, nor will it be the last.
These Korg NuTubes were developed in conjunction with Noritake, the (only?) company that makes vacuum fluorescent displays.