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Design of a Korg Nutube Amplifier Part 1: Tube Basics

Karl is a design engineer with over a decade of experience in high speed digital design and technical project leadership in the commercial electronics sector.


July 27, 2020 11:43

An interesting little project for you, but of course it will not sound like the original valve amps nor in fact do many of the supposed valve sound amps that are poor imitations of the originals.
As far as transistors are concerned, you can say these are current amplifiers to discriminate with Voltage driven valve amplifiers.
The valve sound, unlike what some think are nostalgic sounds of an inefficient amplifier is misunderstood by the majority of people. It is the distortion that each amplifier produces that gives them their characteristic sounds.
A transistor amplifier will give distortion mainly derived from second harmonic, whereas a valve amplifier will generate a third harmonic distortion.
The biggest failure of many companies is to think the sound comes from just the devices used, whereas again the sound comes from the circuit as a whole. Be that the capacitors used (yes capacitors of different types alter sound) to the actual circuits that the likes of Marshall, Peavey and Vox used.
I worked with someone who designed and patented a true valve sound guitar amplifier, with many years of research going into the design.

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September 18, 2019 21:40

There are simulations of a single triode and of the whole 6P1 device here:

The whole 6P1 model is based on a single triode model from Korg.

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[Comment was deleted]

January 31, 2018 10:15

"A bipolar transistor is a current amplification device"
It is extraordinary how this myth continues to propagate. The fundamental equation of bipolar junction transistor operation is the Ebers-Moll equation which shows that collector current is a function of base-emitter voltage, The BJT is a voltage-controlled current source - this is based on the physics of the device and has been known since at least 1964 according to my $2 1964 copy of the GE transistor manual. Or see The Art of Electronics by Horowitz and Hill for a modern reference.
The myth of current control stems from the fact that in many transistors the base current is not negligible. This stems from manufacturing constraints, its not possible to make the base layer infinitely thin (although super-beta transistors get close at the expense of very low breakdown voltages).
An analogy would be making a triode using very thick wire for the grid. The grid current would then be non-negligible but the physics of the triode (anode current controlled by grid-cathode voltage) would be unchanged.
The BJT is not a current amplification device - the collector current is determined by the base-emitter voltage (physics of the device) and the base current is some small fraction of the collector current (manufacturing limitations). Collector current determines base current, not the other way round.

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January 22, 2018 10:00

If anybody is interested I tried to create Spice models for NuTube using Koren's model:

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[Comment was deleted]

January 5, 2018 11:33

A new tube, indeeed! Very interesting.
A couple comments about Part 1:
"Finally, unlike the circuit above, the grid needs a bias and so placing a pot to 3v3 will allow us to bias the tube accordingly."
Unlike the classic circuit, the NuTube requires a *positive* grid bias, which you can derive from 3V3, as stated. The classic triode (all the way back to Lee DeForest) requires a negative control grid bias, relative to the cathode. In the classic circuit quoted, that's derived by raising the cathode above ground, using RCATHODE.

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November 21, 2017 08:56

I'm just finding out about the Korg Nutube device today, and I'm excited. For a while now I've wanted to build something like a push-pull output stage using triodes like 12AX7s, in order to take advantage of the transfer curve of a triode in a distortion pedal. This device would certainly make it more likely any experiments like that will successfully fit into a pedal size box! All I see for purchasing these devices here in that advertisements on the design-spark pages ar in UK currency. Has Designspark made these availble for USA purchase?

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November 20, 2017 15:22

  • Moderated

I cannot help suspecting there is a lot of voodoo nonsense around valve amplifiers and vinyl recording ("angry electrons" indeed!). Is it not likely that the characteristic imperfections of these technologies simply trigger nostalgia? (Although personally, I prefer my music without hisses and pops.) I wonder has anyone tried measuring the transfer functions and noise characteristics and implementing them in a DSP? I bet that if you did that the audio snobs would be unable to tell the difference in a double-blind test! The feature could then simply be added at no cost as an optional software filter to any digital audio player.

I get the "steampunk" appeal of a traditional radio.

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November 14, 2017 08:32

Hi Karl, your basic guide to the transistor is stretching the truth somewhat. Transistors are ALSO voltage operated devices, BUT base current flows as a result of that voltage, beta of hFE vary wildly with many operating parameters but the voltage operation is much more predictable over a large range of parameters and will allow you to realise much higher gain if you design for this. Adding a base resistor will help stop parasitic oscillations, but for an amplifier circuit will almost always add distortion. (So for a guitar amp that might be a plus...) As far as I know circuit simulators based on the SPICE softare use the voltage operation to perform simulations and use the Ebbers Moll equation and perhaps more modern derivatives rather than just the beta value. Yes, I accept that there are those circuits whose design and operation are purely based on beta, but that is a design choice rather than a fundamental facet of transistor operation. Ok, that said, these new valves look really cool :) Thanks for the article!

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November 6, 2017 07:46

I'm very excited about this! I've experimented with other low-voltage tubes (Raytheon JAN-6418 and JAN-5636), with less than satisfactory results.

November 3, 2017 15:54

really looking forward to the next part of this Karl, thanks for sharing!

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