Application notes for Nutube

Advantages

Low power consumption – Only 12mW per each channel
Low voltage – Operating from 5V
High quality – Made in Japan

STRUCTURE

Direct-heating twin triode

Electrical characteristics

Nutube delivers excellent linearity, close to that of an ideal twin triode.



Dimensions

Pin assignments & hole sizes

Pin assignments

Pin No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Assignment F1 F1 NP G1 NP NP A1 GND F2 A2 NP NP NP G2 NP F3 F3
F1 : Filament 1(Left) G1 : Grid 1
F2 : Filament 2(Center) G2 : Grid 2
F3 : Filament 3(Right) A1 : Anode 1
NP : No Pin A2 : Anode 2
GND : Internal shield, connect to GND

PCB hole arrangements. φ=1.0mm recommended.

Basic circuit of Nutube

Example of a single amplifier

Audio characteristics(1)

VCC=12V
Anode load=330kΩ
Gain 14dB
Maximum output 10dBV
S/N ratio(A-weighted) 102dB
Cut-off frequency 75kHz
Crosstalk 1kHz -79dB
Crosstalk 10kHz -70dB
Crosstalk 100kHz -55dB
Measurement circuit

Audio characteristics(2)

Frequency responses
THD+N

Power supply / Anode load

Power supply voltage(VCC) :5 – 80V

Please note: power supply exceeding the permitted limits will void any warranty.Recommended anode load resistances : 100k – 330kΩ

Audio characteristics on each VCC voltage

5V 9V 12V 30V
Gain 8dB 13dB 14dB 17dB
Max Output -2dBV 7dBV 10dBV 20dBV
S/N 91dB 99dB 102dB 110dB

Audio characteristics
on each anode load resistance(VCC=12V)

100k 220k 330k
Gain 9dB 13dB 14dB
Cut-off Freq. 380kHz 100kHz 75kHz

Biasing voltage / resistance

Grid biasing:2 – 3V(VCC=12V)
Use of trim pot is recommended for precise adjustment the of biasing voltage.
Biasing resistance:10k – 33kΩ
Considerable grid current will not allow large biasing resistance. Grid current is around 30μA at maximum.

Filament ratings

Filament circuits

Filament rating:0.7V 17mA(41Ω)
Direct current is recommended.
Inserting capacitor(10μF or more) between filament and GND improves residual noise.

8pin(GND) should be connected to GND to maintain shielding effect.


Recommended resistance values connecting to filament on each VCC voltage.

12V 680Ω
9V 470Ω
5V 240Ω
3.3V 150Ω

Using both cirucuits

Parallel connection
Series connection

Parallel connection of filaments enables to have each filament to have the same voltage.

In series connection, the filament current is half of that of parallel connection, which would improves battery life.

Warning

DO NOT exceed filament rating (0.7V 17mA).
Excess filament voltage will easily burn out the filament!
A red-heated filament shows excess voltage.

Normal state Filament 0.7V 17mA
Exceeded filament current(25mA)

To prevent microphonic noise

A Nutube is a direct-heating tube which means it can cause microphonic noise.
External vibration reaches the tube mainly through the mounted circuit board and also via the air around the tube.

1.To prevent any vibration from the circuit board (Fig.1)
insert some cushioning(a sponge or something equivalent and soft
) between Nutube(or the board mounting Nutube) and the main circuit board. The harness connecting Nutube and the main circuit board should be soft and thin enough.
2.Prevent any vibration from the air (Fig.2)
The vibration of the air (sound) reaching the surface glass of Nutube can cause the microphonic noise especially high frequencies (a metallic sound) .

Placing Nutube in a protective box/case helps prevent such vibrations.
Using acoustic material in the chassis further reduces the noise.
Placing a heavy metal plate (e.g. lead) on the surface of a Nutube can also reduce noise.