Seems like you can also buy them (nut very cheap) and make a DIY project. https://www.rs-online.com/designspark/design-of-a-korg-nutube-amplifier-part-1-tube-b

Seems like you can also buy them (nut very cheap) and make a DIY project. https://www.rs-online.com/designspark/design-of-a-korg-nutube-amplifier-part-1-tube-basics https://www.rs-online.com/designspark/design-of-a-korg-nutub...