The 2SK715 jfet used in the original has a typical yfs of 50 mS--which is much higher than any other jfet--somehow Andrew Ikin discovered them. When the circuit was first traced out I built a copy and verified it worked the same as an original Wellbrook ALA1530LN. I experimented with other jfets, none came close to working like the 2SK715. I also experimented with different paralleled jfets--I tried one, two, three in parallel (4 in parallel each side on the original). The input impedance steadily decreases as jfets are added--Andrew Ikin apparently decided 4 in parallel gave him the desired input impedance with the input transformer turns ratio he used, for the small one-meter diameter loop. 2SK715s were still available when I first traced the circuit out, though they were already listed as "obsolete", and I was fortunate to get some for my tests.
73,
Steve AA7U
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On 7/17/2024 5:44 PM, Steve Ratzlaff via groups.io wrote:
I made a folder in Photos just now and uploaded the schematics for ALA1530LN, ALA100LN, FLG100LN that I've traced out in the past. There should be complete info there for anyone to be able to make their own. The original leaded 2SK715 jfets are now unobtainable, but there are two SMD equivalents that work just as well, with the 2SK932 preferred since its typical yfs is the same as the 2SK715. One should be able to make one of these for around $50--maybe less. On eBay I see a Chinese seller claims to be selling an ALA1530LN or an ALA100LN for a very high price of around $250, for the loop amp alone! I have no idea if it's a direct clone of the Wellbrook.
/g/loopantennas/album?id=296460. The name of the folder is "Wellbrook official traced-out schematics ALA1530LN, ALA100LN, FLG100LN".
73,
Steve AA7U
