[HBR] HBR -- Part 13

[Peter Bertini]

Walt,

What is the first IF, and what is your mixing scheme for the 160 meter coil
set?

Pete



On Wed, Jan 28, 2009 at 8:40 PM, Walt Hutchens <waltah at earthlink.net> wrote:

> With changes in the plate and cathode resistors, the linearity of the
> plate detector (and sound of the receiver) is now okay.  It probably
> can be a bit better, with more study.  The acid test is listening:
> I'll know a more in a few days.
>
> There doesn't seem to be a crossmodulation problem because it sounds
> just as good during peak evening hours as it does in the middle of the
> night.
>
> The HBR-type coils are impressive: The Q's are WAY up there, and that
> means that off-frequency signals are attenuated before they can cause
> crossmod trouble in the front end.  I did use #22 wire for the RF and
> mixer coils rather than the #26 (I think?) recommended in the
> instructions.  The 160M coils will have to go to the smaller size,
> though.
>
> Further revisions to the detector stage will be needed when the BFO is
> wired, since there's no good place for BFO injection in the current
> circuit.
>
> It needed more gain. I figured the IFTs (remember, 100 turns of #40
> wire on a 9/32" form?) were a bit Q-challenged, so that was the first
> thing to be looked at. I had a pi wound coil that was self-resonant in
> the BC band: By removing about 2/3 of the turns I was able to adapt
> that to use with a 50 mmf shunt cap at 1700 kcs as a replacement for
> the 2nd IFT. In the original, the shunt cap was 150 mmf. Higher Z =
> more gain, even without the higher Q.
>
> That gave a good boost but considering that higher bands typically
> have less front end gain, even more would be useful. I'll try the
> same again, with either or both of the 1st IFT (which drives the
> half-lattice filter) and the 3rd IFT.  I can also optimize the mixer
> cathode resistor for slightly higher gain.
>
> For an HF receiver, you want no more gain in the RF stage than needed
> to cover up mixer noise -- very little when using reasonably modern
> tubes. (More of an issue if you're into using 6K8s ...) The big
> problem on these bands is overloading the mixer and/or RF stage,
> causing crossmodulation. Necessary total gain can be made up further
> along, usually in the IF stages.
>
> The ceramic socket I was planning to use for the oscillator coil
> turned out not to quite fit the hole -- the mounting screw holes are
> too far apart.  I found an Amphenol MIP socket that fits AND holds the
> coil solidly.
>
> When pulling the shield plates in the 1st IF grid and crystal filter
> area I discovered a joint that had never been soldered ... no wonder
> it was sometimes noisy if tapped. However, I wired the filaments of
> the three new tube sockets without a single mistake ...
>
> With the two crystals 1.5 Mcs apart (1700 kcs +/- 750 cps) the
> bandwidth is just about an ideal compromise for a receiver of this
> type.  It'll copy AM okay (though typical AM round tables require
> occasional retuning) and isn't much too wide for SSB.
>
> Total receiver plate current is around 40 mA.  The IF stages are about
> 7.5 mA plate+screen.  A few more mA will be needed when the BFO and
> marker oscillator are wired up.
>
> Walt
> KJ4KV
>
>
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>
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