[HBR] The long, SLOW HBR project

[Walt Hutchens]

Bill said:

> I wasn't thinking of tighter selectivity with the 85 kc second I.F.
> except as it applies to something I have looked at that was labeled as
> "tail end" filtering. That filtering is applied just ahead of the
> detector and its purpose is to reduce broadband noise introduced in the
> I.F. chain after that crystal filter up front.  <SNIP>
> 
> With two I.F. "filters" at two different I.F. frequencies it will be
> like looking through the front window of a house and out the back
> window. If you take a few steps in either direction the view through the
> two windows narrows or is blocked completely.

Got it.   However the noise that is thus reduced is only that introduced
after the first filter, in this case, noise from the IF tubes themselves.
(Assuming that the IFs are well enough shielded and the frequency so chosen
that pickup of unwanted signals and atmospheric noise near the IF isn't an
issue.)   I believe that up to at least 20M (15?) the atmospheric noise that
is WITHIN the filter passband (and thus comes in via the antenna and
possibly from the first stage of the set) is a far greater issue.

Also the HBRs don't really have low noise front ends.   (Pentode RF stage
...)  They're excellent for this application but on the 'DC' bands, receiver
internal noise is rarely the issue.   (As long as you stay away from using a
6SA7 or 6BE6 mixer with no RF stage as in the very cheapest sets -- there is
a reason that typical AA 5 BC sets are spec'd as having sensitivity ~100
uV!) 

I think that in the HBR world the greater benefit would be from backing up
the rather poor skirts of a half lattice filter as in my set or the HR 10.
As I recall, these filters are relatively poor on one side and not
outstanding on the other.   They get some help from the antenna and mixer
coils -- Q's approaching 200, right? -- but that probably still leaves room
for detectable improvement.

Once you get to 10M and higher -- certainly in the 6M area and up and with a
low noise front end -- tube noise in the IF could be an issue so a back-end
filter could yield a meaningful improvement in sensitivity.   On the lower
bands I would leave room for a better crystal filter rather than convert to
another IF.   It looks like the HR 20 might be a source if you can locate a
junker set.

(The HR 20 specs should make clear what sort of filter it has.   It does
have the external diagram of something better than the HR 10's half
lattice.)

However, you have the parts for the double conversion approach ... that's a
consideration!

The HR 10 was poorly regarded as a ham receiver.   There's yet another
worthwhile project -- using the parts from that set to produce an HBR.
You've got a power supply, tuning cap, dial mechanism (already calibrated,
if you wind your coils to track it!), crystal filter, IFTs, most sockets and
a few other smaller parts, output transformer and speaker.   Transplant all
that to a 10" x 12" (or maybe larger) chassis, add /replace a few tubes and
sockets, add plug in coils and sockets, and I think you'd have a GOOD
receiver, rather than one on which opinions divide between "not too bad for
a novice in those days" and "not really good enough for a novice."

Interesting ... I'll keep my eyes open for a cheap HR 10.

Walt 
KJ4KV