Hi Steve,
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There has been a bit of variation in the build of the Wellbrooks.
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The one I took apart had two 82pF capacitors, each one connected across half of the transformer primary. I suspect they were originally intended to provide better rejection of strong FM broadcast signals, but it was a "happy accident" that they also improved the performance on the upper HF bands.
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The core material was closer to N30 (u' 4,300) (Epcos ) rather than 73 mix (u' 2,500) , which would tie in with the FG100LN additional transformer, that used something like the higher permeability x38 material (u' 10,000). My thinking is that the Epcos N30 cores would also have possibly been easier to obtain in the UK, when the Wellbrook was initially designed, as RS were pretty much the only big electronic parts supplier at the time, and was used by nearly everyone.
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Using back to back diodes across the loop terminals is fine, however that is not true for other types of antenna, with higher feed point impedances.
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I have tried CPH3910 fets and they work, but a lot depends upon your requirements for frequency coverage.
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Regards,
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Martin
