Although the JFET is a semi-conductor that can be made to behave like a triode tube (and is, therefore, used quite commonly in electric guitar-related effects pedals and preamplifiers) it can only approximate the performance of a triode. Fortunately, the Vacuum Fluorescent Display (VFD) is a device that has both the structure and the vacuum of a triode tube. Invented and developed in the late 1960s by Noritake Itron in Japan, the VFD retains a niche presence in electronic musical instruments and other appliances (microwaves, car dashboards and audio equipment, for example) that are required to operate in a variety of environmental conditions and light levels. Being a true, directly heated triode, the VFD was seen to have audio potential, and a development collaboration between Korg and Noritake Itron resulted in the 2016 launch of the NuTube 6P1, a miniature, low-power (12mW per triode / 5-30 V DC), dual-triode, glass-envelope device, intended for use in audio applications and in musical instrument amplification. However, its low gain (14.5 as opposed to a 12AX7's 1250 ) rules it out as a direct 12AX7 replacement and, to my mind, positions it more as a signal-processing device that can add the characteristic tube harmonic distortion and overtones to audio passing through it.
(Page 2 of the web version).