Note
QST, Oct 1932, p. 2.
Lafayette catalog number 55, p. 69.
Radio News, Mar 1933, p. 551.
QST, Jul 1933, inside back cover. (first reference to AGSX or AGS-X)
QST, Oct 1933, p. 78. for technical details.
QST, Jun 1934, p. 97. technical details
And see National's notes: QST, Mar 1935, p. 77.
Basic information from Barry Williams, KD5VC (SK), used with the permission of his estate (with some light editing).
Additional information from a video via the Antique Wireless Association produced by Brian Harrison. KN4R.
The AGS, which replaced National's short-lived RHM receiver, was introduced in 1932, and was probably the world's best high performance shortwave super heterodyne at that time. Hammarlund, the other major radio manufacturer or the day involved in high performance super heterodynes, offered the Comet Pro. The Comet Pro did not have an RF amplifier or pre-selection stage, although external pre-selectors were available.
The National Company AGS, meaning Airport Ground Superhet, was used at air fields across the country, and was developed jointly by government and National engineers. Operating one of these receivers next to an HRO shows that the AGS has several shortcomings that were corrected in the HRO. The first of these was the coil plug in system. The AGS required the operator to find the matching three coils and put them individually in the receiver in the correct order. The lack of a B+ switch required the operator to either turn all power off to the receiver, or to try to change the coils with B+ applied. Under typical operating conditions this was probably not serious as quickly changing frequency bands was not usually required. The second problem was that the tuning system is very fast and quite touchy. The dial used a vernier scale allowing resetting to one part in 1500. The movement of the main tuning knob required to move the dial by one step is about .010 inches. The dial mechanism and the use of rack and pinions to drive the three variable capacitors meant that everything had to be freshly lubricated to allow smooth movement otherwise the system moves in small steps which are difficult to control. The last major problem was that the available improvement in performance due to electronic design, primarily driven by the new vacuum tubes and the interest in higher frequencies, was great enough that the AGS needed a major electronic design update after only 2 years of production. It is notable that the AGS went through at least 4 different tube line ups during these two years.
National determined that using different manufacturing techniques, state of the art electronics, and improvements in the mechanical design could best be addressed by a new receiver rather than an update of the AGS. National did attempt to address the coil system with a late modification which was called the AGU. The AGU attached all three coils to a single plate, forcing the matching coils to be inserted together in the correct order.
AGS, $161.70
AGS-X, $191.10
References
Technical description. Radio News, Jan 1933, p. 414.
Technical description. Radio News, Mar 1933, p. 546.
Brief description (AGS-X). Radio News, Jul 1934, p. 27.
