Most of the changes from the 390 to the 390A, according to Signal Corps
documents, were made as cost-saving measures, not for the sake of improving
performance.
The mechanical filters result in tighter selectivity, but at the expense of
nonlinear phase shifts inherent to mechanical filters. That is what causes
the harsher audio quality in the A version, as well as in receivers like the
75A-4 that use mechanical filters.
I have both receivers, and find the 390 better suited for most amateur AM
reception. The mechanical filter selectivity of the 390-A can be wonderful
under congested conditions, but both receivers should have included an
additional intermediate selectivity position at 6 kHz. With normal phone
band congestion on 160-80-40, the 4 kHz filter is often too narrow for AM
reception while the 8 kHz filter is too wide. With the 390, although the
nominal 4 kHz selectivity seems to work a little better for amateur AM, the
8 kHz position is still often too wide.
I have added additional filters to my 75A-4 using an outboard adaptor, and
have selectivity positions available @ 300~, 3.1, 4, 6, 8, 9.7 and 16 kHz.
When the bands are lightly congested, I prefer the 8 kHz selectivity, but
most evenings when operating in the 3870-90 kHz Ghetto, I use the 6. When
there is heavy QRM I often switch to the 4 or even the 3.1.
Don k4kyv
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