[ARC5] FW: Way OT -- SX-28 - ARC5 selectivity

[Hubert Miller]

The first published regenerative IF circuit I recall is in Jones Radio Handbook from around 1936.
The circuit used a choke in the cathode of the IF tube, I think maybe paralleled by a pot. I used
to wonder how an impedance in the cathode could cause anything but degeneration, but the 
circuit depends on other capacitances not always obvious, tube interelectrode capacitance, so
you effectively have a Colpitts capacitance divider circuit. I wouldn't be surprised to see that the
regen IF circuit actually dated quite earlier, as during the 1920s people were trying every possible
permutation of circuit hookups and would likely try the wonder of positive feedback everywhere 
but in the power supply. 
I have a Navy MBM radio where the transmitter 6L6 PA has a choke in the cathode paralleled by
a 100 pF cap. I used to wonder what that was all about but I learned that this is the same kind of
circuit as the Jones, it provides a measure of regeneration to get a little more output on higher 
freqs, where the 6L6 operates as a doubler.  

Now re the S-38, if "I" were designing an entry level basic multiband receiver, I'd use:

dual triode - pentode as osc - mixer
one IF at 1700 kHz AND I would stack TWO IF cans before the IF tube, i.e. 4 tuned circuits at 1700 kHz 
triode - diode with triode as BFO as in the Collins circuit, and as 1st audio
output tube

I think this would be as cost - effective as any S-38 or Star Roamer and have better performance 
including image rejection.
-Hue Miller