"Greg Farris" wrote"
"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.
...

They all must have that function, as it's in the TSO! I'm sure it
was at the behest of ATC back when written, not for circuit design
considerations. Latter ain't FAA's problem, but the
manufacturers'.


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

ebay.com now and then. Shipping will far exceed winning bid
price.

Fred F.