Korg Nutube 6P1 Double Triode

Simulation of a complete Korg Nutube 6P1 Double Triode device using a model developed for EasyEDA by signality.co.uk from the single Nutube12 model of the single triode, supplied by Korg.

The demonstration circuits are based on information in:

Supplier page:

https://uk.rs-online.com/web/p/double-triode-valves/1449016/

Supplier datasheet:

https://docs-emea.rs-online.com/webdocs/15db/0900766b815db1b5.pdf

Manufacturer datasheet:

https://form.k3r.jp/korg/nutubedatasheetrequest1e

Manufacturer apps notes:

https://korgnutube.com/en/guide/

Spice model:

Manufacturer supplied individual triode subckt model constructed for the complete double triode device and modified to include approximate heater load.

The heater load uses a simple resistive model.

In a device with a directly heated cathode, one end of the filament will be at a different voltage from the other. For example in the 6P1, with the heaters connected in parallel, one end may be at approximately 0.7V whilst the other may be at 0V ground.

This means that the effective grid to cathode voltage varies along the length of the heater filament.

Given the physical construction of the 6P1, it is estimated that the effective cathode voltage is midway between the voltage difference across each heater section. This means that for heaters connected in parallel as described above then the effective cathode DC voltage would be 0.35V. For series connected heaters supplied from 1.4V and 0V ground, this would imply that one cathode would be at approximately 1.4V*3/4 i.e. 1.05V and the other would be at approximately 1.4V/4 i.e. 0.35V.

The exact DC voltage also depends on the anode current of each triode but since this is small compared to the heater current, this may be swamped by the variation in heater resistance from device to device and due to the exact heater current supplied. Any AC contribution to the effective cathode voltage can be filtered out by appropriate cathode decoupling capacitors.

See also:
https://www.rs-online.com/designspark/design-of-a-korg-nutube-amplifier-part-1-tube-basics
and subsequent articles.