The Free On-line Dictionary of Computing (19 January 2023):

Johnniac

    A mainframe computer based on a design by John
   von Neuman built at the Institute for Advanced Study, USA.
   The Johnniac went live in 1953 and was decommissioned in 1966.
   Its memory consisted of 80 special "Selectron" vacuum
   tubes, each of which held 256 bits of data.

   (2003-06-07)