I asked Andrew Ikin one time if he could share what core he used in his preamps for the transformers. He told me he uses the Fair-Rite 2873000302, same as I've shown on my schematic. (Amidon "BN73-302"). That's what Everett and I always use too. Note this is a conductive core, and with the small gauge magnet wire needed for the transformers, it's easy to abrade the coating when winding, so the wire touches the core. You do not want any wire to touch the core--the circuit's IMD will severely degrade, particularly for the T2 output transformer. Use a multimeter set to ohms with one probe to the core and the other probe to any of the (stripped) wire ends--there should be infinite ohms, otherwise you need to rewind it and try again! We always use a thin Teflon tubing spacer when we wind the transformers so the wire doesn't abrade when winding. We buy it on eBay, from China. The proper size is "9L 3.0 x 3.4 mm". Here's one seller of it:
(I have no experience with the cores that Martin mentioned.)
Steve
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On 7/21/2024 9:26 AM, Mike M via groups.io wrote:
Steve,
Schematic added:
/g/loopantennas/files/Wellbrook%20T1/schematic.pdf
Also, I notice that the 2SK715 had four ranks for IDSS and the 2SK932 has two ranks for IDSS.? Any idea of which rank was used by Wellbrook?
Thanks again.
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Mike M
