[R-390] Please spare us

[Dan Merz]

Hi,  the crystal filtering in the radios you mention is of the type that
dates back to the early thirties and is not the passband type of filter with
a relatively flat top that passes a range of frequencies uniformily.  Rather
the top is very sharp and the passage of some range of frequencies is
further down on the skirts of the response.  This was early on called a
"signal signal response" filter because the bfo could be positioned relative
to the passband to produce only one side of the beat response and the other
side was suppressed.  One side of the filter has a very steep curve.  This
was very useful for cw and eliminated interfering signals that were
otherwise received.  Later, passband filters were made with multiple arrays
of crystals that produced flat-topped passbands but I don't think this type
of filter is used in either of the two radios you mentioned.  The 51J4 has
passband filters of the mechanical type that are similar to the mechanical
filters in the 390a,  though in different holders.  The 51J3 relies on tuned
i.f. transformers for filtering other than the sharp cw crystal filter.  Of
course the crystal filter can be used for ssb but the audio response is
somewhat restricted because of the sharp filtering characteristic.  There
are some vintage tube receivers with passband type crystal filters.  The
only one I have experience with is the Mackay 3010 which has a crystal
lattice filter of about 6 khz bandwidth in the 2nd i.f. stage and this is
backed up by selectable narrower mechanical passband filters in the 3rd i.f.
stage.   

The R388 has somewhat broader skirts when the transformer i.f. selectivity
is used,  compared to say 390a mechanical filters or 390 transformer i.f.
selectivity.  This difference is mostly important only when two signals are
very close.  I added a 500 khz ssb mechanical filter to my 388 to see the
difference;  it helped some but wasn't world shaking.  Though it was a
Collins filter,  it may not have been as good as the ones used in the 51J4.
I would think the 51J4 is capable of separating ssb signals better than a
51J3 but for cw there may be little difference using the crystal filter of
either.  Dan  

-----Original Message-----
From: r-390-bounces at mailman.qth.net [mailto:r-390-bounces at mailman.qth.net]
On Behalf Of Kenneth Arthur Crips
Sent: Monday, June 06, 2005 10:39 PM
To: R-390 at mailman.qth.net; Steve Hobensack
Subject: Re: [R-390] Please spare us

The R388/URR (51J3) has crystal filtering, and the R388A/URR (51J4) has
mechanical filters. I have never had the opportunity to compare these two
radios side by side. If anyone has been able to do this how do the two radio
compare.  My SP-600 has both kinds of filters but it needs a serious
tune-up, it is real deaf so it just isn't possible to compare using it. I
must say I like the phasing control the crystal filtering has.

                                Ken



Kenneth, Radio Station W7ITC,
with Beezie, Chic' Cowboy,
Members of the Irish Terrier
clan known as the Chew Crew.
 
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