There are some 1980’s articles about doing this sort of thing. They are not LM / BC-221 specific. They get more into the pretty significant differences between FET’s and tubes. Yes, it’s been a while … my recollection:
Even a pretty simple FET has way more Gm than the tubes used in these radios / instruments. It’s pretty easy to get way more gain than the design was targeted for. This is not generally a good thing ….
A tube has a *way* larger output resistance than any FET you can get ( so it’s much closer to an ideal current source). This can impact the Q of the output circuit. In combination with the higher Gm, it just might still seem like it all works ok.
A Pentode has a lot lower feedback capacitance that the typical FET. There used to be some mosfets that got closer. Even by the 1980’s then had become hard to find.
Yes, there was a lot more to it. I believe Ham Radio ran the articles. I’d be amazed if I got that all correct ….
Bob
You might have problems finding some of those fets.
I used BF244b and J310 fets when I was transistorizing a R392.
You could use a single dual gate mosfet for the mixer rather than the double fet shown.
On Saturday, August 30, 2025 at 04:42:05 PM EDT, Brooke via ARC5 <
[email protected]> wrote:
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