[R-390] R-390a SS Conversion

[Bob kb8tq]

Hi

The risk in doing this is that (even with a JFET cascode) the output impedance of 
the solid state stage is much lower than tube it replaces. At the same time the Gm 
of the JFET stage is (potentially) much higher than the Gm of the tube. 

You can get the gain worked out and have the Q of the input or output tanks very
different than in the original design. 

Does any of that apply to this attempt? No idea ….

Back in the day, one could get HF compatible small signal MOSFET’s. Not so much 
these days. The JFET cascode is how it’s done ….

Bob

> On Oct 30, 2021, at 6:41 PM, Mack McCormick <w4ax.mack at gmail.com> wrote:
> 
> Alan,
> 
> I did not map out the impedance levels, perhaps Charles did. All stages are
> solid state. MPF102 JFETs. The original power supply is removed and the
> radio electronics are run on 24VDC. I did take several gain measurements.
> There was a real problem with getting the gain of the 1st IF correct.
> Coupling between stages is pretty critical.
> 
> Mack
> W4AX
> 
> On Sat, Oct 30, 2021 at 12:31 Alan Victor <amvictor at ncsu.edu> wrote:
> 
>> Hello Mack.
>> 
>> Curious if you and Charles mapped out the various impedance levels and
>> required gain from stage to stage? Did you convert every stage to SS or
>> targeted specific ones?
>> 
>> Apologize if this question is answered in one of your posts.
>> 
>> Thanks for the info!
>> 
>> Best73' Alan W4AMV
>> 
>> On Sat, Oct 30, 2021 at 6:55 AM Mack McCormick <w4ax.mack at gmail.com>
>> wrote:
>> 
>>> Perry,
>>> 
>>> Charles provides an excellent template to SS your R-390a. He is a
>>> wonderful gentleman and we collaborated often. I am the only other known
>>> person to complete the project. Charles and I went through four design
>>> changes as I SS's a R-390a. It is a rock solid design now.
>>> 
>>> Without knowing your technical/construction skills let me caution this is
>>> not a trivial project and requires at least intermediate level skills.
>>> 
>>> I posted about my SS conversion on this list within the past three months.
>>> You can search for the thread if interested.
>>> 
>>> Here are pictures of some of my work. Please note: The PC boards I made
>>> for
>>> the SS tube replacements are no longer the current design. I will provide
>>> you with the version 1 files but you will need to update them to the
>>> latest
>>> design using KiCad (free on DigiKey).
>>> 
>>> Good luck and I'll be interested in your results if you undertake this
>>> project.
>>> 
>>> https://photos.app.goo.gl/11AjyNCi2zV3T8LaA
>>> 
>>> 73,
>>> 
>>> Mack
>>> W4AX
>>> https://www.qrz.com/db/W4AX
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>> --
> 
> 73,
> 
> Mack
> W4AX
> https://www.qrz.com/db/W4AX
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