GLASS AUDIO 28 (United States) Pete Millett By a some uniqueattributes. is manufactured differently than traditional vacuum tubes, which gives it vacuum fluorescent displays. Although it is a true directly heated triode, it instrument fame) and Noritake Itron, a Japanese manufacturer of dual directly heated triode device developed by Korg (of musical Learn more about the Korg Nutube 6P1—a new miniature, low-power The Korg Nutube x Table 1: Theseare theNutube6P1’s electricalcharacteristics. | Plate dissipation Plate voltage Filament voltage Maximum Ratings: Grid current Plate resistance Transconductance Amplification factor Plate current Grid voltage Plate voltage Filament current Filament voltage

May 2017 R&D Stories

| Nutube 6P1Typical Characteristics (eachtriode) audioxpress.com as low as 5 V. be can voltages mV, at 700 supply is rated plate and Ittubes. aid” on operates filament low very voltages—the “hearing battery-powered the to comparable Electrically, the Nutube 6P1 is somewhat somewhat is 6P1 Nutube the Electrically, If Vf Pa Va Vf Ig Rp gm µ (mu) Ia Vg Va 1.7 mW 80 V 0.8 V 6 µA 330 kΩ 54 µS(54A/V) 14.5 32 µA +2 V 12 V 17 mA 0.7V a somewhat low impedance source (e.g., a source source a (e.g., source impedance low and somewhat flow, a will current grid some that means bias. This grid positive a utilize will applications supplies, most power low-voltage on operate to ability A2. region—Class positive a low plate you voltage, the must operate grid in the is that to get the of much fact signal an with output which significantly constrains operation. Also notable voltage, plate permitted maximum and dissipation plate the maximum line,and load you observe must point quite operating an are selecting and when However, linear. triode a for typical are curves The characteristic curves will seem familiar (see common tube (e.g. the 12AX7), a first look at the plate for a summary of the 6P1’s electrical specifications. (gm).to Refer transconductance low has also it tubes, vacuum traditional most to compared that, is operation low-power such of consequence The mW. 1.7 only at rated and is triode, dissipation plate per the power of mW 12 only consumes Electrical Characteristics Since much of the Nutube’s advantage is its its is advantage Nutube’s the of much Since Even though the numbers are a bit different than a filament device—the power low a is Nutube The Figure Figure 1 Table 1 Table ). follower) must be used to drive the grid. Figure 2 shows the grid current. You can see that the grid current, although only tens of microamps, is significant when compared with the low plate current.

Mechanical and Physical Attributes The Nutube is manufactured by Noritake Itron in Japan, using a process similar to the one used to make vacuum fluorescent displays. It is, in fact, a one-pixel vacuum fluorescent display (VFD) device. The plate is coated with a phosphor just like a VFD, and it glows a blueish-white color when plate current is flowing. A close look at a 6P1 triode shows a fine coated wire running across the device (see Photo 1). This is AUDIO GLASS the filament. Even though it is heated enough to emit electrons, it does not get hot enough to show a visible glow. Below the filament is a fine, open mesh—this is the grid. Below that is a metal plate with the phosphor (glowing) applied—this is the plate. Two triodes are Figure 1: The curves are typical for a triode and quite linear. packaged together into one glass package, which is again borrowed from VFD manufacturing. It looks a bit like an old 40-pin DIP IC package, but only has pins along one edge (see Photo 2). The two filaments share a common center terminal. In most applications, this terminal is at ground potential. Because of this, certain circuits— such as cathode followers or differential amplifiers— can be more difficult to implement, since they would require a floating filament supply. One notable side effect of the construction is that the filament wire, under tension, is a lot like a piano string—when subjected to a shock or mechanical impact, it vibrates at an audible frequency (5 kHz or so). This microphonic behavior seems identical to that of the sub-miniature battery tubes and could cause problems when the Nutube is used in low signal applications. In some cases, it might be desirable to mechanically isolate or shock mount the tube. Figure 2: The grid current is significant when compared with the low plate current. The package is a glass sandwich, 45 mm × 16 mm, and is designed to mount to a PCB with pins spaced on 2 mm centers (see Figure 3). A small metal disc on the back of the package is used to seal the port where the air is exhausted.

Reference and Example Circuits The basic application circuit provided by Korg uses simple JFET source followers to buffer the signal in and out of the Nutube (see Figure 4). An input buffer is used to support the relatively low impedance looking into the grid and bias source. The output buffer is needed to drive anything but high impedance loads due to the low transconductance and high plate Photo 1: The 6P1 triode resistance of the tube. has a fine coated wire This circuit is intended to operate from a single running across the power supply between about 5 V to 30 V, plus it device.

audioxpress.com | May 2017 | 29 GLASS AUDIO 30 a with pinsspacedon2mmcenters. Figure 3:Thepackage measures 45mm×16andisdesignedtomountaPCB, one edge. Photo 2:TheNutuberesembles anold40-pinDIPICpackage, butonlyhaspinsalong x |

design and construction of vacuum tube audio equipment has been a hobby of his his of for many years. hobby a been has equipment audio tube vacuum of construction and design the in engineer design hardware board-level a as worked he Previously, ICs. power signal engineer, electrical an is Millett Pete About theAuthor Engineer at a semiconductor company, where he defines and supports and defines company, he where semiconductor a at Engineer May 2017

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mix ed- THDis lower, with 0.7% at 0.5 V in and 1.8 V out. times. 3.5 about to increased has gain the that see V.24You can from running circuit same the shows greatly improve at a higher supply voltage. for higher order products ( levels dropping with dB, -40 at level in equal nearly were and third second products harmonic distortion the that shows graph (FFT) Transform Fourier Fast a input, V 0.5 RMS with point, operating At this out. bend downward. The THD 5%reaches at about 3.1 V waveform at higher levels, causing the yellow trace to on the right scale. You can see the compression of the scale. The yellow line shows the output voltage, read voltage. The green line shows the THD, read on the left level as one would expect. the with increases THD 1%.The around was (THD) distortion harmonic total out, RMS V 1.35 and in, RMS V 0.5 With times. 2.6 about of gain a provided circuit the supply, V +12 single a With . Ω k 330 of grid bias voltage of +2.5 V, and a plate load resistance All were the run the tests audio with performance. a at the recommended bias and plate load, to determine an amplifier stage. of performance the in erences ff di enormous make loading plate and Bias point implementation. circuit gain and distortion in particular—are dependent on the resistor. dropping a with supply V 3.3 the from obtained is power Filament significant. be to appear doesn’t it level in this but with arrangement the values shown, no doubt there could be some bias shift with signal capacitor be used to couple the audio signal. There is the grid. This mandates that a large (10 F) coupling low µ to bias DC relatively provide to a used Ωis ) k (33 resistor value current, grid the support To V. +3.3 dropout (LDO) regulator. Bias from is 0 adjustable to needs a regulated 3.3 V supply, from typically an low Audio Performance with with low set input and ff o current noise is as needed, amplifier. an op-amp that Note headphone Sangaku output. the er ff bu to used is op-amp an and bias, JFET follower is used to directly drive the grid at zero to gm parallel, and increase lower the noise level. A DC-coupled in connected are sections triode Both V). 75 about case, this (in plate the for voltage B+ voltage a as higher well as used, are ±18 V supplies the op-amp input due to the 6P1’s high at Rp. necessary is case) this in Ω M (1 resistor large a Using the same operating point and load, things things load, and point operating same the Using In I ran some tests using the Korg application circuit, As with any vacuum tube, the audio performance— Figure Figure 5 I also used this as a gain stage in the Apex Apex the in stage gain a as this used also I Figure 6 Figure shows another example. In this circuit, example. another shows , the horizontal axis is the input input the is axis horizontal the , see see Figure 7 ). Figure Figure 8

A 5% THD isn’t reached until the output reaches 6.5 V and third-order products. Negative feedback can also RMS. The FFT at 24 V shows that the second harmonic be employed to trade lower gain for lower distortion. product has dropped, and now the third is the highest, at about -43 dB (see Figure 9). Applications By changing the bias and/or plate load, you can The Nutube 6P1 is intended to be used in tune the harmonics however you want. A small change various audio circuits, mostly in musical instrument in bias will change the relative levels of the second- applications. Its low power requirements enable its

Figure 4: The Korg application circuit uses simple JFET source followers to buffer the signal in and out of the Nutube. AUDIO GLASS

Figure 5: This is an example of a split-supply application.

Figure 6: With 12 V gain and distortion, the THD increases with the level. Figure 7: At this operating point, a Fast Fourier Transform (FFT) shows that the second and third harmonic distortion products were nearly equal in level at -40 dB, with dropping levels for higher order products.

audioxpress.com | May 2017 | 31 GLASS AUDIO 32 a from 24V. considerably atahighersupplyvoltage. Here isthesamecircuit running Figure 8:Usingthesameoperatingpointandload,thingsimprove Power Systems. in conjunctionwithaself-oscillating Class-D amplifierIC,theMP7770from Monolithic anexperiment,Idesigned andbuiltahybridPhoto 3:As amplifierusingtheNutube6P1 x |

May 2017 R&D Stories

| audioxpress.com distortion similardistortion to that of a conventional tube, with state. solid and tube by using both devices, other served are much better you goal, your is gain voltage voltage accurate if provide gain—but does it Yes, element. processing signal a as device amplifying active an as much so not Nutube the view I measurements, performance the Nutube can also find use in hi-fi audio equipment. cases, some In preamps). phono or microphone of to use in low-level applications (e.g., the input stages high noise, it low gm is and dicorrespondingly cult ffi its to due However, pedal. ects ff e guitar a as such for a ideal product systems, in use battery-powered In a guitar amplifier, it can be used to provide provide to used be can it amplifier, guitar a In youAsmight conclude, basedtheon audio dropped, andnowthethird isthehighest,atabout-43dB. Figure at24Vshowsthatthesecondharmonicproduct 9:TheFFT has you can vary the plate load resistor, bias voltage, and negative feedback. voltage, bias resistor, load plate the vary can as you want, you profile harmonic and gain have to whatever stage the tune to easy is it this With arrangement, distortion. reduces and point operating it can be used to emulate the harmonic characteristics much lower voltage and power requirements. In hi-fi, resistor (see a adding simply by feedback add to easy it’s bias, someplate-to-grid feedback.With positive a grid added also and parallel, in triodes both connected I erences. ff di of the couple a with to circuit, reference Korg similar circuit a used I a supply, V from 28 single powered is it Since splitter. phase JFET a and the used Nutube 6P1 as an input feeding stage, up for slightly lower gain than it would normally have than a big tube amplifier. PCB space, certainly smaller and much less expensive and each channel occupies only a few square inches of triode amplifier. single-ended It runs on a large single a 28 V like power supply much is it harmonically, and power, output W 50 about yields combination from Monolithic Power SystemsMP7770 (seethe IC, amplifier Class-D self-oscillating a with conjunction in 6P1 Nutube the using amplifier Not some interesting possibilities. amplifiers. power up opens it but sure, be to “cup tea” of triode everybody’s single-ended of level harmonics that extend to many times the the times many to fundamental. A theory extend of harmonic maskingthat says that harmonics level low- have does it amplifiers, Class-D most with as but W), 1 at 0.02% about measured (I itself by TheMP7770 amplifier hasquite low distortion DC the both stabilizes feedback of type This In In this amplifier, I circuit set the Class-D MP7770 hybrid a built and designed I experiment, an As Figure Figure 10 ). Photo 3 Photo ). This This ). Figure 10: I set the MP7770 Class-D circuit up higher amplitude, lower-order harmonics mask the for slightly lower gain than it would audibility of higher-order harmonics. So the hope here was that the sound of the resulting hybrid amplifier normally have and used the Nutube will be dominated by the Nutube input stage, with 6P1 as an input its relatively large amounts of low-order (mostly stage, feeding a second harmonic) distortion. It does, in fact, measure JFET phase splitter. similarly to a large single-ended tube amplifier. People who have heard it agree that it does indeed sound like a single-ended triode amplifier but has improved damping due to the low output impedance of the Class-D circuit. Conclusions Though it won’t make conventional small tubes AUDIO GLASS (e.g., the 12AX7) obsolete, the Korg Nutube can be a useful device for audio DIYers. I expect it will find many uses in musical instrument amps, guitar effects, and hi-fi equipment. Its low power and low operating voltage makes it a natural for experimentation by those who are not familiar or comfortable working with the high voltages typically used with tube circuits. For more information, visit www.korgNutube.com. The Nutube 6P1 and supporting evaluation hardware is available from www.Nutube.us. ax

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