[ARC5] Transistorized LM

[Tom Lee]

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> 
>> 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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