From my reading in the old handbooks the principal of operation of this type of parasitic suppressor goes like this.. The coil is self resonant at the frequency the amplifier naturally wants to oscillate at. The self resonant circuit provides a high impedance to the parasitic frequency... The parallel resistor is simply a place for that energy to be dissipated. That's how I understand it... There have been arguments here and elsewhere in the past over the tube or tubes being the cause of the parasetic osciallations. That really isn't iomportant from out standpoint since nobody designs amps with those tubes anymore and the tried and true methods work.