This two-tube circuit, one being a double, was found in Radio Electronics magazine from July 1958. The distance to the microphone depends on noise and losses. The cable must be shielded.
This current-limited circuit was found in a Popular Electronics from the 90's. ...
In this interesting circuit, a common relay is used as a vibrator to obtain high voltage with a...
Nixie-type displays are no longer used except for assemblies of people who like old technology. The...