[R-390] Noise Floor question

[Mark Huss]

My understanding is that the 6DC6 in the R-390A was a compromise between 
low noise and overload performance.  Since the highest sensitivity 
applications at the time for R-390A were in conjunction with high-noise 
antennas, such as Rhombics, it was a good match.  A common MARS mod for 
R-390/R-390A/Sp-600's in the sixties were replacing the First RF 
Amplifier with a Cascode VHF dual triode.  This supposedly reduced the 
First RF noise (esp. in the SP-600), but dropped the overload 
performance.  As for me, after some research in this mod, I decided that 
unless you live more than five miles from the nearest house, the 
atmospheric QRM and QRN really does not drop below a level that would 
show any practical performance improvement.
     Also, don't forget that in tubes, shot noise from the electrons 
hitting the plate also generate a lot of noise (why triodes are quieter 
than pentodes).

Bill Hawkins wrote:

>What you're looking for is thermal or Johnson noise. Try this:
>
>http://www.dataforth.com/catalog/doc_1065.html
>
>The first RF tube contributes noise because it is hot. Wonder what
>you could do to the noise figure of a 390 class set just by FETing
>the first RF? Don't even think of it if you're a purist or concerned
>about overload capability. But if noise is your first concern ...
>
>Bill Hawkins
>
>
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