[ARC5] Transistorized LM

[Bob Camp]

Hi

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


> On Aug 30, 2025, at 6:44 PM, J Mcvey via ARC5 <arc5 at mailman.qth.net> wrote:
> 
> 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 <arc5 at mailman.qth.net> wrote:
> 
> 
> Hi Ken:
> 
> Thanks.  Here's some links:
> 
> https://archive.org/details/73-magazine-1971-05/page/60/mode/1up
> https://ia600403.us.archive.org/6/items/73-magazine-1971-05/05_May_1971.pdf#page=61
> https://prc68.com/I/USM159.html#LM_18_Frequency_Meter
> -- 
> Have Fun,
> 
> Brooke Clarke, N6GCE
> https://www.PRC68.com <https://www.prc68.com/>
> axioms:
> 1. The extent to which you can fix or improve something will be limited by how well you understand how it works.
> 2. Everybody, with no exceptions, holds false beliefs.
> -------- Original Message --------
>> 73 magazine for May 1971 has an article on this, for those interested.
>> 
>> Entitled, "The Transistorized LM Freq Meter", starting on page 60.
>> 
>> I was recently given two, supposedly non-working, LMs. They don't look all that bad to me.
>> 
>> I'll see if I can make them work.
>> 
>> Ken W7EKB
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