[ARC5] ARC-5/274-N SAelectivity - separating IF coils

[Hubert Miller]

CQ had another article on the BC-348 which advised taking the DC off the IF coils. I am not sure I am convinced
that is a winning idea; I wouldn't have thought the detuning with varying plate current affects the coil
inductance that much; nor the Miller capacity effect from the grids of pentode tubes. Not only that, but it
requires a good stock of 2.5 mH  RFCs.
Maybe? I dunno. There was graph presented, before and after, but unhelpfully, and unconvincingly, there were
no numbers associated with the graph indexes.
The IF coils in the 348 have Q figures only around 100. With ferrite core coils like the long loopstick coils sold a
generation ago, the Q figures were in the 250 – 450 range. Possible even, I have thought, I the dynamotor space
a box with a series of such high Q coils would narrow up the selectivity. I have never liked the outboard Q5 route;
to me, it loses "integrity of the set", just my weird bias.
The Squires Sanders SS-IBS, the SW broadcast version of their innovative ham band receiver SS-1R, uses for the AM
selectivity nothing more than a bandpass filter out of high Q coils, and its IF is 1000 kHz.
-H M
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