There are tubes made which share one heater. For an exotic example with a working web page, see this tube where two triodes and one pentode share the same one h

There are tubes made which share one heater. For an exotic example with a working web page, see this tube where two triodes and one pentode share the same one heater filament. https://vinylsavor.blogspot.com/2021/11/tube-of-month-6bh11.html https://vinylsavor.blogspot.com/2021/11/tube-of-month-6bh11.... This one has two diodes and two triodes. https://vinylsavor.blogspot.com/search/label/6AY11 https://vinylsavor.blogspot.com/search/label/6AY11 And in this very design, they chose "6N3P valve contains 2 triodes around a single heater, halving the physical size and power requirements." There may have been tubes made where the triode function can be pretty rough (sufficient for a digital circuit) and several of them could share one enclosure. In the tubes shown above, apparently the limit was the number of pins on the socket - but also that all these active elements do not share any pin. Insulating the whole thing would run into issues like burning wire insulation.