[ARC5] Receiver Selectivity

[Kenneth G. Gordon]

On 20 May 2013 at 10:01, Mark K3MSB wrote:

> A while back there were a few posts about adding a gimmick capacitor
> to the arc receivers to increase their selectivity;  I can't find
> those posts. Does anybody have them?

Yes. You connect your gimmick between the grid and plate terminals of the 
1st IF amp tube, then connect a 5K pot between the ground-end of the 
cathode bias resistor and ground.

Then, you alternately adjust the pot for minimum resistance and the gimmick 
so that the IF stage JUST goes into oscillation at the very end of the pot's 
range, as close to zero resistance as you can manage it.

I use two pieces of #12 solid THHN (house wiring wire) for the gimmick. It 
doesn't take much.

I have used this mod with several receivers. It works fairly well on weak 
signals, but not so well on strong ones (of course).

Also, in order to get the full effect out of this regenerative IF mod, you have 
to remove the suppressor grid connection from the cathode and connect the 
grid connection directly to ground.

This mod also doesn't really help much with the 2830 KHz IF of the 40 meter 
receivers either. That IF is simply too wide as it is originally to do much with 
unless you take all three IF cans apart and "adjust" the spacing between the 
coils much further apart. I am working on an article for ER Magazine on this.

The problem is that if you get the coils too far apart, gain suffers, and if you 
don't get them far enough apart, you won't get the best IF selectivity. Spacing 
is fairly critical.

Ken W7EKB