"Standby" used to be a common function on vacuum tube equipment, keeping the
filaments hot, but cutting the plate current, so the device could be turned
"ON" and operate instantly, without having to wait for the filaments to heat
up and start emmitting. Some solid state equipment retained this function,
just because people were used to it, and operators had been taught to use it.


Is it possible that this is the case with the transponder? Anyone know of a
tube transponder?

I always learned that you are to use only two modes on the transponder -
"Standby" and "ALT" (or mode "C"). ON and OFF were never to be selected.

I was also instructed to IDENT after acknowledging a transponder code change,
even if not specifically requested to do so by ATC. The reasoning was that
they will see your ident immediately, while it may take them up to a minute
to see your code change.

G Faris