[Drake] R-4C problem

[djed1 at aol.com]

 I've been trying to track down a low level 60 Hz hum in the audio output.  I seem to have a fairly high level of 60Hz on the first filter capacitor in the 14V power supply.  I've replaced the filter cap before, but I noticed that the schematic has a 20 uF cap connected between this point and the filament supply.  Is this a typo on the schematic, or is this real, and if so, why would they do that?  It looks like the 20 uF cap is connected to ground, and not to the filament voltage, in my receiver.  Any other suggestions for what to look at?
Ed  W2EMN