National "AGS"
Latest & Finest "Communication Type"  H.F. Receiver
Copyright 2003, Alan R. Klase. All rights reserved.

The first ad for the AGS appeared in QST in October 1932.   "The AGS has been developed in conjunction with the Airways Division of the U.S. Department of Commerce to comply with the exceeding strict requirements of aviation ground-station service."  In fact,  the AGS name was derived from "Aviation Ground-Service."

Several major design features addressed short comings of exsisting receivers, e.g. the Hammarlund Comet "Pro"
  • A tuned RF stage to improve image rejection, signal-to-noise ratio, and weak-signal response. 
  • Single-dial tuning. 
  • Relay-rack mounting. 
  • Coil change from the front panel. 
  • Automatic volume control for hands-off operation.

The AGS is a nine-tube super-heterodyne receiver with one tuned RF stage and 2 IF stages.  Local oscillator and BFO are electon-couple for improved stability.   Four sets of plug-in coils covered 1.5 to 20MHz.  It was powered from an external power supply.

Technical Features:
  • Single-conversion superhet - 500 KHz IF
  • 1.5 - 20  MHz - plug-in coils
  • 1 RF stage
  • 2 IF stages
  • Crystal filter - on AGS-X
  • Frequency dial - 0 - 150
  • Bandspread:  bandspread coils
  • External power supply

Clean layout and extensive shielding are evident.  Coils plug into the cylindrical shields at the top.  The tuning asembly, below, contains three separate variable capacitors interconnected by a rack-and-pinion drive behind the front panel.  The upper picture shows the original short IF cans.  The lower image shows the taller air-tuned IF's.

Left:  AGS with coil rack and power supply.

Pricing from the manual:
AGS, with coils covering 1.5 - 20 MHz, less tubes.      $165.00
RFS, PM speaker in rack panel                                            17.50
CPR, coil rack                                                                         8.50
GRDPU, rack-mount supply for two receivers                    55.00
5886, table-top power supply                                                21.00

Bandspread coils for the ham band were introduced in February 1933 QST.  These used the same technique applied to bandspread coils for the SW-5 and SW-3.  The main tuning capacitor is tapped down on the coil, and each coil carries a trimmer to act as band set.

Air-tuned IF transformers were made available in April 1933 to improve stability.

1933 also saw the introduction of the crystal-filtered AGS-X.  Equipped with band-spread coils, this would have been a nearly ideal ham receiver, but it was too expensive for all but the most well-heeled depression-era amateur.  Fortunately, the boys at National knew this, and had a solution, the legendary FB-7.