Random objects: the nuvistor

A vacuum tube like no other.

The transistor changed the world, but the revolution happened so quickly because we already knew how to use this new component. The fundamental circuits for communications and computing have been developed in the era of vacuum tubes; as luck would have it, the transistor behaved much like a vacuum tube.

On a physical level, the two devices are nothing alike. Vacuum tubes work by the principle of thermionic emission, a phenomenon that allows electrons escape into vacuum from an electrode that is heated to a glow. The simplest vacuum tube has a heated cathode and a room-temperature anode, letting the current flow in one direction and forming a simple rectifier, akin to a semiconductor diode. A more exciting variant — a triode — adds a metal grid in between. A negative voltage applied to the grid repels electrons and moderates the flow of current between the other two terminals. Electronically, this is quite similar to the behavior of the gate terminal of a field-effect transistor.

Interestingly, vacuum tubes went extinct despite being electronically superior to transistors; the tubes offered better linearity and switching characteristics than any semiconductor device back in the day. Yet, practicality prevailed: it sufficed that the transistor had a greatly reduced power footprint, thanks to not needing a heating coil.

At this point, some readers might object: surely, the transistor was also a more compact and more rugged device! Well… yes, but also no. Meet the nuvistor:

Nuvistor model 7586, assembled and removed from can. Photo by author.

Developed by the RCA corporation in 1959, this subminiature vacuum tube had a form factor similar to the early transistors. It ditched the fragile and bulky glass enclosure for a metal can with a ceramic base. Removing the can revealed a complex internal structure with three electrodes and a heater sandwiched inside a tubular structure:

Microscope photo of a nuvistor with can removed, by author.

Nuvistor design schematics from the RCA catalog.

Who knows — perhaps, had we discovered solid-state semiconductors a bit later or fusion power a bit earlier, our electronics would have been controlled by vacuum tube ICs.