I have a few 500 series plug-ins with nuvistors, and
have found them to be
FWIW, late versions of the 1A1 (dual-trace 50MHz-ish plugin) have a pair
of nuvistors on the output PCB. Old versions, like mine, have those 2,
and anothter 2 on each of the range switches for the input amplifer. A
total of 6 therefore.
-tony