Note
## Abstract
This article reviews the history, design evolution, and restoration of the distinctive 6-prong asymmetric plug-in coils manufactured by the National Company for its vintage 1930s shortwave receivers, most notably the SW-3 and SW-5. Developed in 1931 to overcome the thermal and environmental instabilities of earlier wood-flour bakelite forms, these high-Q inductors utilized a specialized mica-filled bakelite resin (R-39) and featured custom-machined wire grooves to maintain winding alignment over time.
The text provides a comprehensive guide to identifying the three primary production eras—the color-coded "10-20" series (1930), the hot-stamped "60-70" series (~1933), and the late "30-40" series (~1940)—cross-referencing them with compatible receiver tube configurations. Additionally, it clarifies the mechanics of National's unique tapped-inductance bandspread (BS) design, illustrates proper wiring configurations, and offers practical advice for collectors seeking to clean, rewind, or home-brew functional replacement forms to bring these classic regenerative receivers back into operation.
