Obviously I am interested in the Drake 2-B. Steve KW4H restores rigs of this vintage (similar to our own vintage!). He has sent me some notes on his work on the Drake 2-Bs.
Steve writes:
Attached is a photo of a completely needless 455 kHz mechanical filter I found in this 2B. Never seen this before, but I found the info -- this was the Henry DMF-2, which was a modification kit sold for the Drake 2A and 2B. Yet more snake oil sold to the "I need a mechanical filter like a Collins" crowd. There was a REASON why Drake didn't filter on 455 kHz -- it wasn't nearly as effective as filtering on 50 kHz, which is exactly what the passband filter does. That, along with the 2-BQ Q-multiplier/speaker makes for a terrific receiver that rivals even many of the modern rigs of today.
That filter is clearly unnecessary, and Steve was completly right in getting rid of it. Still, it is a Kokusai. Pete N6QW gave me one of those. I think I have it in my Mate for the Mighty Midget receiver. One other note: As a kid, Steve was in Puerto Rico and operated from the navy base club station KP4USN. I have that station, during that period, in my log book (I worked KP4USN on April 12, 1974 on 15 CW). I have to dig up the QSL card to find out if it was Steve, or one of his friends.
In what way is an optional and quite decent 455kHz filter "snake oil"? Being mechanical or crystal in nature is irrelevant. Why is it unnecessary to increase overall selectivity with a good IF roofing filter in front of a not particularly good (passband ripple, inadequate shape factor), main 50kHz LC IF-filter? The obvious REASON Drake chose to downconvert to a 3rd IF was because they wanted continously variable selectivity which was not gonna happen by using a LC-filter at 455kHz.
ReplyDeleteRobert -- these things are, of course, a matter of opinion. Drake was very experienced in signal filtering, and if they felt a 455 kHz filter was all that necessary or profitable, they would have almost certainly designed one into the receiver or at least sold it separately themselves as an add-on. That filter really doesn't add much to an already terrific receiver.
ReplyDeleteI should have mentioned the technical reasons why that Henry filter wasn't a great idea for the 2-B (or, at best, a niche modification). The inherent disadvantage of the Henry DMF-2 in a Drake 2-B is that it creates a permanent 2 kHz funnel at 455 kHz ahead of the receiver’s 50 kHz IF system. Once the signal has been narrowed to about 2 kHz by the Henry mechanical filter, the 2-B’s wider selectivity settings can’t restore the audio and sideband information that has already been removed, so you give up much of the flexibility that Drake built into the receiver for wider SSB, AM, and general-coverage listening. The Henry unit also adds another 6BA6 IF stage, additional tuned circuits, extra alignment requirements, and another potential source of instability; Henry’s own instructions even mention correcting oscillation by adjusting its small IF transformers. On top of that, the installation is fairly invasive, requiring chassis drilling, wiring changes, and replacement of the audio-gain control with a switched control. It can provide very sharp SSB selectivity, but the tradeoff is that the 2-B becomes a much more permanently narrow and more complex receiver.
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