[Hallicrafters] Short metal tube mystery

[Mike Everette]

Carl is right about the 717A.  The place I recall seeing them in their intended natural habitat was in the VHF ARC-5 receivers (the ones that don't look like any other Command Set, but fir the same racks).  Also in early radar IF strips.

More than likely the overload was due to a sharp cutoff characteristic.  The tube doesn't respond well to AVC action.

A better choice for the RF stages is a 6AB7 (aka 1851) which, of course, is what comes in many SX-28 receivers.  The 6AB7 is a remote-cutoff type that works well with AVC.  Its cousin the 6AC7 (aka 1852), however, is sharp cutoff.

There was a mod published in CQ, I think in the late 40s/early 50s  -- way before my time, but I have seen the article someplace, and have a "parts set" SX-28 with one of these in it -- about something called "The S-9er." This was a simple adaptor to allow plugging a miniature tube into the SX-28 (or whatever) first RF stage without changing the socket.  I forget which tube was used; I don't think it was a 6AK5, but perhaps a 6CG7 or 6CB6, both of which were popular TV-tuner types and are really hot tubes that when used in the proper circuit will allow one to hear a gnat break wind in Outer Mongolia with S-meter-slamming results.  (well maybe I exaggerate a little, but not much) Or maybe even a Nuvistor tube would work; Nuvistors were still in the future at the time of the article... in any case the mod is totally reversible if I remember right.  Results are largely dependent upon the care taken, particularly w.r.t. lead length, when constructing the
 adaptor socket.

73

Mike
W4DSE

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