[Milsurplus] Quick n dirty crystal selectivity - it no workee

[Hubert Miller]

Many years back, i had an idea for improving the selectivity of the 6 - 
9 MHz 'Command Sets' receiver.
The IF is 2830, IIRC.  I took a crystal from an old 2-MHz boat radio, 
2837 kHz, and put it across the
first IF cathode resistor, with the bypass cap formerly there, 
disconnected. It didn't do a darn thing,
no change in selectivity or sensitivity at all. The idea was to be 
something like the low-Z 'transfilters'
used in some transistor circuits. I thought later that maybe the cathode 
resistor needed an RFC in
series, otherwise the low resistance would shunt the crystal. But i 
never did that experiment, would
have taken more rewiring for my limited time. But - is this a workable 
idea, or is there a 'gotcha' -
something i'm not accounting for?

I did try the grid-to-plate gimmick capacitor to make the stage 
regenerative, but i didn't like that at
all. Too much like a poor implementation of  regenerative receiver. You 
don't want the selectivity
stage gain-controlled separately. In my experience a q-multiplier is 
much better. I did see some
'conversion' article probably in CQ Magazine that did build a 
Q-multiplier for the 2830 kHz IF.
I am curious how that really worked out. f/Q is the 3db selectivity and 
you can see 2830/Q is going
to be a LOT more than 455/Q. The crystal does have a very high Q and i 
thought in this low impedance
point, the cathode circuit, the parallel capacitance of the crystal 
holder would be insignificant.
Obviously i missed something. I am almost up for a rematch with this 
challenge.

While i'm at it: another question to the wisdom of the members. The 
USAAF radio receiver BC-348,
the last IF is overcoupled. I see all the other IF windings are 1.5 ohms 
DC with 250 pF resonating.
The secondary of the last IF can is 5 ohms DC resonated with 125 pF. My 
thinking was to simply
unwind turns from this winding and add capacitance to resonate. The 
winding is 2-pi and i still
don't know if i'd peel off the near winding or the more distant winding 
- to lower the coupling. BUT -
i had another idea: what if i just get another first or second IF can 
from a junker, and use that to
replace this entire original overcoupled IF can? I know in selecting IF 
transformers, there always
was one specified for 'diode load', but i have no idea what kind of 
impedance such a transformer
has.
Ideas - advice?
tnx
-Hue Miller
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