[HBR] HB-65 or HB-67 experience?

[Brian Burns]

Hello Walt,

~ No experience, but -- this isn't going to surprise anyone -- an opinion. 

Well Walt, I for one really value your opinions!

~ As a way to use a mechanical filter that you have and want to use this is
good; otherwise it's a mixed bag.

Carl, KM1H says that selectivity gotten with tuned circuits sounds better
than with crystal filtering. Could this be because of the incredibly high Q
of crystals? Are mechanical filters prone to ringing as well? In any case,
my limited junque box doesn't contain one.

~ The problem in HF receivers is NOT low noise or hearing weak signals.
At least up to 20M the problem is rejecting the vast amount of garbage found
in and near the ham bands. The mechanical filter solves part of this problem
-- near channel interference -- but unless used with a great deal of
selectivity ahead of the filter, it does not solve the problem of images --
signals coming in on the other side of the LO frequency. That's because the
frequency is too low for a 1st IF.

The first IF on the HB-65 is at 1700 kHz, and the second is at 100 kHz.
Since it's a "band imaging" 80/40m receiver, it's not really a bandswitching
one, unless you count the switching of crystals in the converter section as
bandwsitching.

Of course, when you use the crystal controlled converter for the higher
bands, you have a triple conversion receiver. And switch on the BFO, and you
have four (count 'em) oscillators running all at the same time---a full
blown aviary!

~ That way you've got those high Q front end coils lopping off the image on
the lower bands where it'll be most troublesome.  

~ The HB-67 design (and relatives -- this design appeared at least
1965-69 in various flavors) could have been improved by the use of very high
Q front end coils a la W6TC, but since it's bandswitching and predates the
common use of toroids, not so.

So are you saying that toroidal coils in the front end can be made higher Q
than air core? That's really interesting, if that's what you mean.

The 7360's ability to handle strong adjacent channel signals is one of the
most attractive aspects of the HB designs.

~ I would probably also go with an RF stage run at close to unity gain in
order to simplify the coupling between the coils and allow AGC ahead of the
filter. The 6EH7 would be ideal for this. Of course that does complicate the
design.

Thanks very much for all your comments!

Cheers,

Brian



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