[ARC5] Transistorized LM

[Bob Camp]

Hi

There’s a *lot* more detail in the articles, if they can be found. They included a lot of “if you do this you run into that’ stuff. 

Bob 

> On Aug 30, 2025, at 7:54 PM, Tom Lee <tomlee at ee.stanford.edu> wrote:
> 
> I'd state it a little bit differently: A pentode generally has a higher output resistance than a JFET, but not a triode. Similarly JFETs certainly can have more or less gm than a tube; it depends on the JFET and on the bias current.
> 
> It is perhaps most helpful to think of a JFET as similar to a triode. A pentode is a lot like a cascoded pair of JFETs -- the output resistance will be high, and the Miller effect will be low. The "FETRON" was an attempt to replace pentodes using a (quasi) cascode pair and worked just fine in many cases.
> 
> As to bias sensitivity, a JFET's gm allegedly varies as the square root of drain current, while a vacuum tube's gm varies roughly as the cube-root of plate current. Not quite the same, but most circuits of the era were designed to tolerate a fair spread in vacuum tube characteristics and that has facilitated their conversion to JFETs (and sometimes even to junction transistors).
> 
> -- Cheers,
> Tom
> On 8/30/2025 4:21 PM, Bob Camp wrote:
>> 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> <mailto: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> <mailto: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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>> 
>> 
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