Korg 'Nutube' - next generation vacuum tubes

[rezzonics]

So they're selling a 50W amplifier paired up with a 25W speaker?

Recipe for success!

It's a solid state class d amp that puts out 25W into an 8 ohm speaker, and 50W into 4 ohms. So the speaker in question is most likely 8 ohms.

I saw this demo comparing Vox MV50 to AC15 using the same speaker and I was quite convinced by the results, at least with overdrive pedals, maybe not so much with the fuzz:

[thermionix]

So they're selling a 50W amplifier paired up with a 25W speaker?

Recipe for success!

It's a solid state class d amp that puts out 25W into an 8 ohm speaker, and 50W into 4 ohms. So the speaker in question is most likely 8 ohms.

Hadn't noticed that.  Still, a little safety margin is advisable.  Perhaps they're exaggerating the amp's output power a bit, a pretty common practice.

[PRR]

> I was expecting a huge degradation at +24VDC

Oh, OK.

Gain of a tube stage does not fall off very much with B+.

Here's 12AX7 data for 300V to 90V supply. Take the Fender-typical 100K Rp and 220K load (0.1Meg and 0.24Meg). At 300V, 50. At 90V, 38. A 70% reduction of battery voltage gives a 24% reduction of gain. Or 10.5dB less battery is only 2.4dB less gain.



(Maximum output level *does* decrease nearly with supply voltage, actually faster.)

This is true as far as it goes. There is around +/-1V uncertainty in effective grid bias due to "contact potential". With a tube of amplification factor of 100, supply voltages much under 100V may not bias consistently in mass production. The usual rule for no-trouble design was Mu < B+. 12AX7 types (as diode-triode radio tubes) were widely used near 90V, so the makers made some effort to be consistent in that application. Of course 24V is way lower (at 9V you are outside any B+ zone generally contemplated). The AX seems to be quite linear down to dozen-volts, with of course very low current and probably reduced gain.

Some extrapolation suggests that 12AX7 at 24V will be gain of 26dB, in sight of your modeled 24.6dB. (May only mean the sim extrapolates about the same as my figuring.)

> I added a current source in the simulations to model grid current

Grid current should rise with grid voltage. But not a linear rise like a resistor (or a resistor with a perfect diode). The amount and curvature of this current will influence distortion, unless very well buffered.

[rezzonics]

Thanks for your comments, they are very instructive. I plan to add JFET buffers/amplifiers before and after each triode section plus a tone stack. I am starting to have good results in simulations.
I was thinking that maybe I could use similar circuits with NuTube and a 12AU7 "starved" tube to get similar gain on both circuits so that I could compare them.

[vigilante397]

The store I used to work at is a Vox dealer and I said I would post on here as soon as we got the MV50 in and totally forgot I even opened one up and got gutshots (see below) for the sake of science. But frankly between the 3 different voices of amp the AC model hands down is the best. The "Rock" sounds really grainy overdriven and the clean just doesn't have much flexibility. I'm not a huge hybrid amp advocate but I really dig the sound and especially the flexibility the AC offers though. Plus a $200 price tag isn't bad

[GGBB]

The store I used to work at is a Vox dealer and I said I would post on here as soon as we got the MV50 in and totally forgot I even opened one up and got gutshots (see below) for the sake of science. But frankly between the 3 different voices of amp the AC model hands down is the best. The "Rock" sounds really grainy overdriven and the clean just doesn't have much flexibility. I'm not a huge hybrid amp advocate but I really dig the sound and especially the flexibility the AC offers though. Plus a $200 price tag isn't bad

Just curious about what speaker you used to compare the three, and if you think that makes a difference to each model. In other words - do you think the clean model would come out better in a review playing through a Fender cab as opposed to a Vox cab?

[vigilante397]

Just curious about what speaker you used to compare the three, and if you think that makes a difference to each model. In other words - do you think the clean model would come out better in a review playing through a Fender cab as opposed to a Vox cab?

I tried all 3 through the Vox 1x8 cab, 1x12 cab, and a Blackstar 2x12 with Vintage 30's. Don't get me wrong, none of them sounded bad, but the Clean had a little too much headroom so it was really hard to get it to drive well (desirable for some styles) and the Rock sounded like a 50W metal zone. Grainy high gain distortion with very little option for clean.

[GGBB]

I tried all 3 through the Vox 1x8 cab, 1x12 cab, and a Blackstar 2x12 with Vintage 30's. Don't get me wrong, none of them sounded bad, but the Clean had a little too much headroom so it was really hard to get it to drive well (desirable for some styles) and the Rock sounded like a 50W metal zone. Grainy high gain distortion with very little option for clean.

I get it - Clean is all clean and Rock is all rock, but AC gives you some flexibility.

Any thoughts about clean tone comparison between AC and Clean models? I am seriously thinking about one of these but don't have access to demo units yet. I'm partial to Fender tones vs. Vox, and mainly interested in clean settings since I get my dirt from pedals. The lack of ability to get driven tones with the amp alone wouldn't concern me.

[vigilante397]

I tried all 3 through the Vox 1x8 cab, 1x12 cab, and a Blackstar 2x12 with Vintage 30's. Don't get me wrong, none of them sounded bad, but the Clean had a little too much headroom so it was really hard to get it to drive well (desirable for some styles) and the Rock sounded like a 50W metal zone. Grainy high gain distortion with very little option for clean.

I get it - Clean is all clean and Rock is all rock, but AC gives you some flexibility.

Any thoughts about clean tone comparison between AC and Clean models? I am seriously thinking about one of these but don't have access to demo units yet. I'm partial to Fender tones vs. Vox, and mainly interested in clean settings since I get my dirt from pedals. The lack of ability to get driven tones with the amp alone wouldn't concern me.

One thing I did like about the Clean model was that you have a treble and bass control, whereas the AC just has "tone," so it's easier to dial in some beef on the clean where you need it. That being said the main difference between the Clean's clean and the AC's clean is the headroom. On the AC with the gain down low you can still get some hair with hot pickups and really digging into it.

[BubbaFet]

One thing I did like about the Clean model was that you have a treble and bass control, whereas the AC just has "tone," so it's easier to dial in some beef on the clean where you need it. That being said the main difference between the Clean's clean and the AC's clean is the headroom. On the AC with the gain down low you can still get some hair with hot pickups and really digging into it.

So... do they sound like tube amps to you?

[anotherjim]

Interesting that the Nutubes need some degree of shock protection - rubber grommet mounts. How microphonic are they I wonder?

[PRR]

Read the application notes.

The VFD panel mechanical design is TERRIBLE for stability. Big flat unsupported screens. Unsupported so they do not block-out the display in the original use. Not really something they can engineer-out on a dying product run on existing machinery.

[vigilante397]

So... do they sound like tube amps to you?

Kinda I'm not going to say they are a mind-blowing sweet tone that I would get to replace all of my tube amps, but for a hybrid it sounds spectacular, and it is absolutely a gig-worthy amp. It has good tone, a lot of power, and it weighs next to nothing. So final answer, it absolutely sounds good enough to keep around as a gigging/practice amp, but don't expect it to become your #1 favorite amp in the world.

[teemuk]

Interesting that the Nutubes need some degree of shock protection - rubber grommet mounts. How microphonic are they I wonder?

Enough that technical documentation keenly suggests somekind of mounting that does not transfer vibrations. At least one document suggests using a separate circuit board for the NuTube circuitry, which is then mounted on rubber grommets.

Don't know about the NuTube, but in general VFD's are notorious for being very, very microphonic. Obviously it becomes a much bigger concern when the tube is used for audio amplification than when it's used as a display.

[rezzonics]

That long thin heating filament must pick up a lot of vibrations.

[vigilante397]

That long thin heating filament must pick up a lot of vibrations.

I didn't even think to check that. Could be worth considering, I think I'll ask one of the guys still at the shop to bang on one a bit while he's playing.

[rezzonics]

Clash of the Titans:
Nutube 6P1 vs 12AU7/12AX7


Who will be the winner?

[rezzonics]

Same hybrid amplifier circuit with 2 different assembly options:
NuTube 6P1:


12AU7 or 12AX7:


Everything assembled in a 1590J box:



PCB has been ordered and I soon will order the components.

I actually moved the speaker output jack to the rear.

[rezzonics]

Another AC15 vs MV50 comparison, using a Jensen® Vintage 12", Alnico P12Q, 40 watts speaker

[bluegarou]

So been playing with 6sn7,6sl7,12au7,12ax7 and Raytheon micro tubes for couple of years and having lots of fun. Looking at pentodes now. Started out at +-24v but now everything is +-15v.

Target applications range from reverbs to compressors. Hifi too.

What is confusing is why the tubes do so well at low voltages.
Everything lines up with tube data sheets as far as plate current,bias and so on.

This Haffler dh100 stereo preamp been running for about two months and sounds sweet.

The "dual op amp" is in preamp active tone control, class A output from composite hybrid op-amp.
PP output values are above theoretical values.

The two wires are for heater.