[50mhz] Preamp and receiver question

[John Geiger]

I have a receiver question for those on the list who know a lot more about
this stuff than I do, at least from the electronics standpoint.  I know more
about receiver performance and less about how we get there.

I have an Icom 706-had it over 3 months now which is approaching a record
for me HI HI.  On 6 meters I have some noise source to my southeast (right
at the Carribean) that really raises the noise level on it.  It hits S8 or
so if the beam is pointed in the right direction.  I didn't remember
the FT450 I had being that noisy, so I had a friend bring his FT450 over,
and it was showing a noise level of S2 or so in the same direction.

Now I know that different variables are in play.  Checking the QST review
for each radio, the 706 (preamp on) has a 3db advantage in sensitivity.  The
MDS for it is -139dbm vs -136dbm for the FT450 (preamp on).  That should be
1/2 S unit difference or so in picked up noise.  The S meter calibration is
very different.  For the 706 with the preamp on on 6 meters, a S9 signal
takes 3.3 microvolts, which is insanely liberal, but that is what it is.
For the FT450 under same conditions (preamp on, 6 meters) it takes a 33
microvolt signal for S9.  That is a factor of 10, which should show up as a
10db difference on the meter-correct?  If that is correct, we have a 13db
difference in readings between the 706 and 450 (3 db for increased MDS, 10db
for the meter difference).  That should be 2 S units or maybe 3-4 given the
non linear calibration of most meters.  But I am seeing a 7 S unit
difference or so.

So here is my big question-can the preamp device in the 706 (or any other
radio for that matter) become overloaded and start to generate increased
noise?  I think it can, correct?  If you put enough signal, local noise,
whatever into the receiver, won't the preamp at some point become swamped
and start to create noise on its own?  Could that be where this extra noise
is coming from?

You really can't run the 706 original on 6m without the preamp as it is
pretty deaf without it.  The noise blanker on either rig doesn't do much on
this noise, so it isn't that the NB in the FT450 is taking it out while the
706 isn't.

73s John AA5JG