Cool concept, but I doubt the operating range of these tubes is large enough to get any sort of usable gain out. Unless there's a datasheet that says otherwise I'd imagine these are going to end up early in the gain chain as a "coloring" component.
With the low operating range and low bias point you don't get the same effect as you do with a tube biased to it's sweet spot. It seems like a marketing gimmick like the starved-plate 12ax7 stage they put in everything nowadays (with ~12V B+ as opposed to the 250-400V B+ you find in nearly all classic amp designs), along with a few LEDs and a window in front so you can see where your extra $100 is going.
If you plot (really any) load line on the plate curves for a 12AX7 with a < 50V B+ you're in a really poor, highly non-linear region. Looking at plate curves for some old VFD tubes the operating range for those is way down in the same region in the lower left corner if you plot them over the 12AX7 plate curves.
Given the size and the fact that it uses 2% of the power of conventional tubes, I doubt that these tubes could be used as an output stage. I would have to guess that they're running them cold as well. Preamp tube power draw is often dominated by the heaters
Of course this is all speculation until they publish a datasheet...
I find most cheap tube gear uninteresting, musically speaking. It's generally, as you note, a tiny low voltage 12AX7 preamp stage feeding a regular old transistor gain stage. There's a reason the old master volume Marshall amps are sought after, and why so many manufacturers have returned to all-tube designs.
Some of what goes on in audio is snake oil. In fact, a lot of it is. But, modeling amps and tube preamp stage gear still doesn't hold up well against a good tube amp. I bought a new amp a couple years ago, after having used the same amp for ~20 years (a 1984 JCM 800). I tried every amp in the small size range at two guitar stores. The clear winner was an all-tube Vox model. The modeling amps were way down the list, which was disappointing. I really believed we had the technology to replicate good amp tone (and some guitar amp sims on computer are reasonably good)...but we still aren't there.
And, the preamp tube with transistor volume stage amps were also clearly inferior to the master volume style all-tube amp I ended up buying.
Very nice write up and I'm in agreement. I've heard good things about Vox and their move back toward being the best consumer tube amp. I grew up with a 64 Fender Twin Silverface (Dad's gig amp) and it was a monster, just too much, but I'm glad we sold it to somebody who still has it. I became a solid-state guy but always lusted after the 5150 2x12" combo, and they're holding steady at about $500 used, so one of those is in my future.
I've used quite a few Line6 computer products and I like the bang-for-the-buck but would never, ever put them in the pro sound field. Fun to play with, cheaper than pedals, but takes me quite a bit in fiddling with settings + post production to make them worthwhile (same goes for my little MicroSpider amp, which is mostly for busking).
One thing that came across my radar was the Kemper, which seems to be that technological achievement you might have in mind. I've never played/heard one (that I know of), but I think it has a pretty good reputation thus far. Link: http://www.kemper-amps.com/profiling
I had a first generation 5150, as well, and loved it. Though when I down-sized it was the 5150 that got sold (while I kept the JCM 800). But, that was primarily a function of the size of the 5150 + 4x12" cabinet, while the JCM 800 was a 2x12" combo (which is clearly the ideal amp size for performance, though too big for recording); both amps were excellent. I consider it one of Peavey's crowning achievements, maybe the best thing they ever made. I haven't heard any of the new generation of them, once they stopped calling them 5150. They also moved most of their manufacturing out of the United States a few years back, which reportedly impacted quality...I don't know if the tube amps are still made in the US, or not.
Since I moved out of the motorhome (the reason I sold my JCM 800 and downsized to a tiny Vox was because I was living in ~200 square feet for four years, while traveling full-time), I've since bought another Marshall. It's a 50W JCM 2000 (I think...maybe 900) that I picked up for $600 (and a 4x12 cab that was a few hundred more), but it rarely gets used. The Vox sounds great and is right-sized for indoor use.
I'm really happy with my Vox, honestly; for most things it sounds as good as any amp I've ever owned, and I've owned some good as hell amps (I've also had some old Fenders). I wish it had a bigger speaker; a 10" would make it the perfect amp. It's got an 8", and sometimes low end stuff sounds pinched.
I was living in a motorhome, about 200 square feet, and traveling full-time. I held onto it for about a year, in the motorhome, but it just didn't make sense. I couldn't play it without being on the grid (too power hungry for the batteries), and had to move it every time I brought the slide in...and it was heavy as hell to be moving it around all the time. I sold it for a fair price, and once I settled into a fixed house again, I picked up another Marshall (a JCM 2000, this time, which is admittedly not in the same league as a six knob 800, but it's not a bad amp, at all). And, I have the little Vox, which is a great little amp. I play the Vox far more often than the Marshall, in fact.
I have owned a lot of classic gear over the years and have sold most of it. I view it as borrowing it from the universe rather than "owning it". If I buy it used, and I sell it for what I paid for it, or more, I find I have a healthier relationship with my things (I'm in charge, not the things). When moving into the motorhome, I sold a lot of great stuff. Antique furniture (my mom is an antique dealer, so my house was nicely furnished), synthesizers and guitars and amps, etc. The cost of keeping it was the cost of storing it, which would have added up to far more than its value over four years.
Anyway, these days, I have a good big amp (the JCM2000), a good little amp (the Vox AC4V), a good acoustic guitar (Gibson J45), a good electric guitar (Schecter from when Schecter made really good guitars by hand in California), a good bass (G&L L2000), and a good drum set (Roland TD4S). That's my minimum for musical happiness, and I try not to keep anything else, even if I acquire stuff temporarily to play with it.
Too bad where I live we can't buy good used gear. Nothing good ever comes by, people hoard and hold on to it for their lives, and when it does come by, it's overpriced to the point it justifies buying new.
99% of the stuff sold in stores it is cheap generic clones of Fenders (MOSFET amps and Stratos).
That would be hard. I live in Austin, TX, which is where half the broke musicians in the world live (and a number of famous ones). So, lots of really good stuff comes onto the used market here, and used gear sells fast, if it is priced fairly. So I can buy and sell good gear as often as I want, and my standards for what I buy can be quite high.
Why is early in the gain chain necessarily bad? Suppose the later stages of the chain introduce no distortion, would it be possible (in theory) to have low power devices like this create all the nonlinear effects considered desirable by audiophiles and then just amplify that later on?
That is nonsense. Headroom is inversely proportional to how much distortion you are getting, no matter where in the signal chain. A metal tone has very little headroom. You can't add headroom to it without gating tricks: measuring the level of the pre-distorted signal, and using that to control the volume of the distorted signal.
Headroom is bad anyway. The electric guitar has way too much dynamic range to be useful. For clean guitar, your best friend is compression, so that you don't disappear in the mix against other instruments when you articulate some passage of notes softly.
To clarify what I meant about headroom, I was specifically talking about headroom before the circuit starts to clip/break up/etc.
e.g. If you have a diode clipping OD/Distortion circuit made out of some diodes with a Vf of 5mV as opposed to the 700mV of a normal diode, you would need to have a smaller input signal for an equivalent amount of distortion. That smaller signal has a lower SNR given some constant noise floor from thermal noise etc, and the SNR can only stay the same or get worse after you start amplifying it.
That is well and good, but pointless. People use such a circuit to get a heavier distortion with the same input signal, not to get light distortion with a smaller signal.
Let's put it this way: if you have high gain metal distortion circuit gained up to a 5mV clipping threshold, and then you roll down your guitar volume knob to play the blues, you're kinda using it wrong.
It's pointless in the case that you mention, where you're going for a hard clipped metal distortion tone. I was envisioning more of an overdrive sort of application where hitting the rails is NOT the desired effect. The 5mV hard clipper was an extreme example, but you could conceivably use it for soft overdrive/compression with a small enough input signal which was what I had in mind.
If hitting the rails is not desired, then don't configure the effect to have so much gain.
The point still stands that you can get the distortion effect, at the desired amount, at various places in the signal chain.
The problem you're talking about is about using too weak a signal and then using make-up gain. That can also happen anywhere in the chain.
I can reproduce the problem without distortion at all, using a clean signal chain.
Repro 1: turn down the guitar volume to simulate an extremely weak pickup. Then turn up the input gain to compensate. Result: noticeable increase in noise.
Repro 2: turn down pre-amp volume, and crank up power amplifier volume to compensate. Result: noticeable increase in noise.
Gains/levels have to be evenly distributed in the chain.
Sure, you will lose headroom if your intent is to have some soft distortion from the back-end of the amplifier, but you put a heavier distortion earlier in the signal chain. The heavy distortion masks the later one; it evens out the level so the headroom from the back is not available. There is a fix for that: you can punch that distortion in and out with your foot as needed.
are SNR issues like that and the headroom an issue for a guitar amp? I mean why couldn't the solid state part be designed conservatively with extra headroom? Or do the desirable distortion products make that difficult even with a conservative design in later stages?
Generally you want to avoid noise early on in the gain chain just because it gets amplified later on. Headroom is entirely relative, you just need to design your gain stages such that the power amp isn't railing when you turn your guitar up halfway.
If you want to avoid noise early in the gain chain, do not rob your guitar of signal by attenuating the output of the pickups with the guitar's volume. Now though that is done, by some players (and works for them), it isn't a way to get headroom or minimize noise.
You get the least amount of noise by keeping the volume of the guitar at maximum, and switching tones later in the chain. A clean pre-amp channel has much better noise characteristics than a dirty channel, and rolled down guitar volume control.
The point I was making about turning the guitar up halfway was that you need to design amplifier gain stages so that there is not so much gain that the amplifier hits the rails with a reasonable input signal. E.g. If your preamp stage has a voltage gain of 100 and your expected input signal is 1v p-p but you only have +/-20v rails for the power amp stage, you objectively don't have enough headroom.
I see. Intersting. How do you think about the desirable distortion characteristics vs bad "noise"? Is there an industry standard definition for guitar amps?
Seems like the bad noise in the case we're discussing would be what the tube circuit generated when it was not being intentionally driven to distortion... is that a correct interpretation?
Noise is anything that is still present when you take away the desired input signal, excluding deliberate time-based effects like reverbs and delays.
Frequency components that are not in the original signal, but are only present when the signal is there are usually distortion.
(Why "usually": radio-frequency interference is noise. In audio, you may not notice RF when there is no signal. But when signal is present, it may mix with RF in a nonlinear way, causing additional distortion products attributable to the RF. I.e. the RF noise is "descends" into the signal's frequency range via intermodulation.)
Noise would be thermal noise generated in circuit components, or mains hum, anything that is there when you aren't playing any notes. I wasn't spraking exclusively about guitar amps, all this applies to analog electronics in general.
Of course, that's how it's done. Thus distortion is just an effect and not amplification. You can put a gain of thousands. The resulting may be swinging close to rail-to-rail (relative to the distortion effect's power supply). This is too much for the next thing in the chain, so it is turned down. No brainer.
Yes but not all non-linearity. With a high plate voltage (and a reasonable bias point) you get gentle compression as the tube starts to break up. That's the good (T00B) sound, with the even harmonics and everything.
When you run a low plate voltage you start to run into some of the undesirable things that happen when you get close to the point where the tube stops conducting entirely. you start to hard-clip the signal on one half-cycle and all those odd square-wave harmonics start to jump out, and that can sound even worse than some mediocre diode hard-clip/distortion stage designs.
The way you pick your operating point is you draw a diagonal line from the plate voltage at the bottom to the saturation current on the left, and the output current moves along that line as the input signal changes (the input voltage is the voltage at the top of each curved black line). A good operating point is one where the intersections of the curves with the load line (the diagonal line you drew) are evenly spaced along the load line. If they are perfectly spaced, the amplification is exactly linear. As an example, if you plot a load line for 350V plate voltage and 2mA max plate current the intersections with the curves will be closer together on the right and move farther apart as you move left along the load line. That is the asymmetrical compression that gives a nice tube amp distortion tone.
If you do the same thing with a 12V plate voltage (and any plate current) you start running into that 0V grid curve and it's hard to reason about what's happening in between the two data points (curves) you have, because at some point the curves change and start looking like the 0V one, but it's hard to tell where. Once you start driving the grid positive the tube will stop conducting entirely and hard-clip your signal, which is what I was referring to before.
The TL;DR; would be something like look at how the 0V curve goes the opposite way as all the others, that's not where you want to be.
Under-voltage is only bad if the tube is designed for a high voltage and you don't give it what it is designed for. If the tube needs 200V, you give it 200V. If it needs 30V, give it 30V.
Some snobs will say that nothing which runs at only 30V will have the proper headroom.
... and then they drive their amp with chip-and-diode based overdrive pedal (e.g. Ibanez Tubescreamer), running on 9V, haha.
I agree with your first point entirely, but there is precedent in industry for putting starved-plate tube stages in things just for marketing.
A "great" component in a shitty design can sound worse than a "shitty" component in a great design. I just have a feeling that the marketing push to "Just put a tube in it" will lead to more of these "token" tube applications because it's cheaper to run it at whatever voltage rail is already there than putting some hulking transformer just to run a 300V B+ rail for a single preamp stage.
The headroom thing is all relative, I don't think your amp would be too happy if you fed it a 300V P-P signal from your 3kV Audiophile-Grade-Headroom pedalboard...
I also can't wait for the 64-bit digital audio revolution. Nothing with less than 384dB of dynamic range is worth listening to! /s
If you're only getting 384 dB out of 64 bits, you're doing it wrong by using integers. 64 bit double floats go from, what, around E-308 to E308 in the exponent. So that is around 616 Bels, or 6160 dB. :)
With the low operating range and low bias point you don't get the same effect as you do with a tube biased to it's sweet spot. It seems like a marketing gimmick like the starved-plate 12ax7 stage they put in everything nowadays (with ~12V B+ as opposed to the 250-400V B+ you find in nearly all classic amp designs), along with a few LEDs and a window in front so you can see where your extra $100 is going.
If you plot (really any) load line on the plate curves for a 12AX7 with a < 50V B+ you're in a really poor, highly non-linear region. Looking at plate curves for some old VFD tubes the operating range for those is way down in the same region in the lower left corner if you plot them over the 12AX7 plate curves.
Given the size and the fact that it uses 2% of the power of conventional tubes, I doubt that these tubes could be used as an output stage. I would have to guess that they're running them cold as well. Preamp tube power draw is often dominated by the heaters
Of course this is all speculation until they publish a datasheet...