[Milsurplus] BC-455 selectivity again

[Hubert Miller]

The subject was improving the selectivity of the 6-9 MHz Command Sets
receiver and we mentioned the idea of a ~2380 kHz crystal in the cathode
lead.

I said i'd tried this quick trick and it hadn't worked in the least, and i
thought additional RFC in the cathode bias resistor leg was needed.

Well, looking at a the ARRL Hints and Kinks ( 1965 ) today, page 18 shows a
no-coil crystal filter where the crystal is fed from the cathode and the
phase

capacitor is connected from the plate to the same output. But that's not the
point; the point is, they coupled from the cathode. In this case, with a 

6AK5 valve, the cathode bias R is 270 but there's an additional 1200 in
series with it. The grid DC return is to the junction of these resistors, so
that the

grid only sees the original correct DC bias. I'm thinking that an RFC is
large to put in the circuit space and you might have a problem with an RFC
stray

inductive coupling. So now i'm thinking instead, lift the 620 R from the 1st
IF cathode from ground; insert a resistor substantially larger than this, to


taste; and reconnect the original grid return from the 1st IF grid to the
junction of these resistors. With an effective cathode to ground resistance
of

a few thousand ohms, the crystal is much more likely to act like a series
circuit. 

The video i saw sure looked like the tuning rate of the receiver had been
modified also. I would do that too, but not by pulling apart the capacitor,
by

putting some small micas or polystyrenes in series with the gangs to limit
the C. 

I'm eager to try this, BUT first things first....

-Hue Miller  

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