A neat little item followed me home - no, I carried it tenderly - from the club field day site recently. It's a National SW-3, regenerative receiver from the 1930's. It came from Pete, K2KLE, who retired from up North & has lived here in New Bern for 13 yrs He's had it in the house for that time, and it really is in nice shape, the wrinkle is clean, only a little bit of splatters, tape marks, etc, which should clean up well. He had put a small power supply on the back of it, as it was originally the "All-A.C." version, vs. the battery only. Not much difference between the 2 models, as an external power supply was required if not using batteries.
It has 3 tubes, 2 ea. #58, 1 ea. #56. 2 plug-in coils (40m the only ones there). Thanks to John, K5MO, and Garey, K4OAH,, I have a copy of an 8-page article, written by James Millen, which appeared in the June 1931 QST magazine, which covers the rationale behind the design of this series of radio. The first one, probably the SW-1, used 35, 36 & 37 tubes. The article covers the following in some detail:
Hardly the kind of detail you find in magazine articles introducing new equipment these days.
The coils are 1-1/2" in dia., 2-1/4" high, with 6 pins. The detector coil has a grid leak resistor, grid condenser, and trimmer condenser mounted inside it. It and the RF coil each have a pigtail for direct connection to the tube grid. The receiver was originally supplied with 3 sets of coils, for the 3.5, 7, and 14 mc ham bands, and gave good band spread for these bands. Other coils could be wound for any frequency from 350kc to 33mc general coverage, according to James Millen.
I have had the receiver powered up and listened to the 40m band. CW signals were copyable, and even, with patience, SSB signals could be understood. I hope to develop the proper technique for the regen. controls, and have some more enjoyment.
SW-3 info file, Adobe .pdf
click HERE to view & download (1.1mb)
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And remember: "They don't make tubes nowadays like they used to..."