Calculating Capacitance / Rolloff Frequency for Across the Line RFI Supression

Question

I’m in the process of restoring my first All American Five (AA5) tube radio – in this case, a GE Model 115 from the late 1940s. As I’m getting ready to order replacement capacitors for it, one of them (.05uF, 600V rated) sits across the Live and Neutral mains lines for (I would assume) RFI suppression. OK, I say – and I go about looking to order a suitable X Class safety capacitor. .05uF is a bit more capacitance than seems usual for tube-based gear I’ve seen (usually it’s a .01uF) and it got me thinking: why the difference in value? And more broadly, what exactly does an “across the line” capacitor really do.

My assumption is that it is there to form a low-pass filter, shunting RFI above the 60 Hz mains. So I run a quick rolloff frequency calculation using 1/(2*piRC) where the load resistance of the tube filament chain is something akin to 480 Ohms (given that the radio draws about 30 watts on a 120V mains line). To my surprise, the rolloff frequency at .05uF is about 6600 Hz (and at .01uF 33,000 Hz). Much higher than I would have expected. Granted that either is still way below RF interference – but I would have thought that the rolloff would have been much closer to 60 Hz (esp. in the “old days” when perhaps motors and other low-frequency noise generators were more of the problem trying to be avoided).

So, my question is: what I am missing, either in my calculations or in the design intent of the capacitance values?

EDIT: changed old-style microfarad "mF" references to modern "uF"

Answer 1

Well, no, you don't want your filter to have a low impedance at the line frequency, because then you'd be wasting a lot of current through the filter. So cutoff numbers in the high audio range make sense in general.

It's much more important to keep stray RF (455 kHz and up) from the AC line from getting into the sensitive RF and IF amplifiers, and also keep RF from this receiver out of the AC line, to prevent transmitting interference to other receivers.

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Just a side note about notation: Old parts used "mF" for microfarads (and mmF for picofarads), but nowadays we now use that to denote millifarads, 1000× larger. You really should be using uF or µF here, just to avoid confusion.