This is designed to kill the thumps. A sudden decrease in bias of the input tubes results in positive thumps at the anodes. The split coil acts a s very low impedance because these thumps result in opposite flux in the inductor core. This inductor presents a very high impedance to voltages of opposite polarity and very low impedance to voltages of same polarity.vinyvamos said:One thing that is puzzling me however is the split inductor to ground preceeding the grids of the second stage. Can someone more knowledgeable shed some light on this and it's purpose?