5 ms·

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

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...

illicium 12y ago
Isn't non-linearity a desirable property for audio tubes?
- hamiltonkibbe 12y ago
  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. EDIT: If you look at the plate curves for e.g. a 12AX7 http://www.paleoelectronics.com/blog/wp-content/uploads/2009/01/12ax7-withpower.png http://www.paleoelectronics.com/blog/wp-content/uploads/2009... 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.
  - kazinator 12y ago
    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.
    - hamiltonkibbe 12y ago
      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
      - kazinator 12y ago
        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. :)
SwellJoe 12y ago
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.
- hamiltonkibbe 12y ago
  Nothing like cranking the volume and getting those power tubes really cooking! I love my 65 reissue twin.
- 6stringmerc 12y ago
  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 http://www.kemper-amps.com/profiling
grandalf 12y ago
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?
- hamiltonkibbe 12y ago
  Not necessarily bad, but something that breaks up easily at low signal levels limits your headroom which leads to more noise later on
  - kazinator 12y ago
    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.
    - grandalf 12y ago
      Why couldn't you just introduce attenuation after each stage to preserve headroom in the next stage (assuming you had sufficient dynamic range)?
      - kazinator 12y ago
        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.
    - 12y ago

illicium 12y ago

Isn't non-linearity a desirable property for audio tubes?

hamiltonkibbe 12y ago
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. EDIT: If you look at the plate curves for e.g. a 12AX7 http://www.paleoelectronics.com/blog/wp-content/uploads/2009/01/12ax7-withpower.png http://www.paleoelectronics.com/blog/wp-content/uploads/2009... 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.
- kazinator 12y ago
  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.
  - hamiltonkibbe 12y ago
    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
    - kazinator 12y ago
      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. :)

SwellJoe 12y ago

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.

hamiltonkibbe 12y ago
Nothing like cranking the volume and getting those power tubes really cooking! I love my 65 reissue twin.
6stringmerc 12y ago
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 http://www.kemper-amps.com/profiling

grandalf 12y ago

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?

hamiltonkibbe 12y ago
Not necessarily bad, but something that breaks up easily at low signal levels limits your headroom which leads to more noise later on
- kazinator 12y ago
  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.
  - grandalf 12y ago
    Why couldn't you just introduce attenuation after each stage to preserve headroom in the next stage (assuming you had sufficient dynamic range)?
    - kazinator 12y ago
      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.
  - 12y ago