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
              <[email protected]> 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 <[email protected]>
                      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
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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