Nuvistor
Nuvistors are very small vacuum tubes. There are used in many Tektronix products of the 1960's. Nuvistors used in Tek gear include:
As high-impedance buffers (cathode follower), Nuvistors were replaced by JFETs. As voltage amplifiers, they were mostly replaced by BJTs. Some products such as the 1A1 and 453 started production with Nuvistors and then later switched to transistors.
Nuvistors, like all tubes, eventually need to be replaced. They are no longer manufactured but they are still (as of 2012) available online. Another repair option is to convert the circuit to use a transistor. In at least one case (the 422), JFETs can be plugged directly into the Nuvistor sockets with no further circuit modification, and proper function of the scope is restored.