Ned Carlson wrote:

From what I can discern, what this thing does is
put a neon bulb in series with the plate or cathode,
if the tube is drawing current, the bulb lights up.

It's just a neon lamp in series with the filament! If the filament is
good, it lights up! It does not even do real emission testing. Since
the current through the neon lamp is minimal, it does not even illuminate
the filament.

It's a simple go/no-go test, and doesn't tell you
if the tube has defects other than lack of
emission. So its usefulness basically extends to
ferreting out truly dead tubes.

Not even that. But if you have a TV with a series string, most of your
failures will be open filaments anyway.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."