Note
The Model 58C was a component of an airport communications receiving system designed and manufactured by the National Company for the General Electric Company, under contract to the Civil Aeronautics Board .
It served as a companion or "stand-by" receiver alongside National's primary AGS receiver in standard airport ground-station installations . It was included to tune ranges not covered by the AGS .
Equipped with plug-in coils ranging from 13 1/2 to 115 meters (2600 kHz to 22 MHz (and advertised to amateurs with coils covering 150 kHz to 33 MHz), it was versatile enough for a variety of monitoring tasks .
The 58C was likely the last of National’s tuned radio frequency (TRF) family of receivers produced for serious communications work before superheterodyne designs fully took over .
The 58C was an updated, reworked version of the SW-58, which itself was the AC-powered equivalent of the DC-operated SW-34 "Thrill Box" .
Its model number was derived directly from its primary tube types.
The electrical circuit was identical to the SW-58, consisting of :
– A tuned RF stage (using a Type 58 tube)
– A regenerative detector (using a Type 58 tube)
– Two audio stages (utilizing a Type 56 tube and a pair of Type 245 tubes in push-pull) .
The 58C was noted for being sensitive and easily controlled, utilizing heavy shielding, filtering, and circuit isolation .
Unlike its predecessor, the 58C was specifically adapted for commercial rack mounting . Because of this, its two plug-in coils were moved directly to the front of the panel for quick user access .
To match the aesthetics and operational feel of the primary AGS receiver, the 58C's tuning mechanism was upgraded to a 4-inch Type N dial.
Note: the 58C plug-in coils are physically distinct from AGS coils; they are longer, have a smaller diameter, and are non-interchangeable .
