Grid Feedback Coil
When the coil is initiated, there is no grid voltage and the tube is turned on with current flowing from the tank circuit to ground. The full on condition generates a pulse to the primary, and then the primary resonates the secondary. This grid coil picks up the pulse (resonates) of the secondary; furthermore, drives the tube to turn off like a switch and keeps current from passing from the plate to ground (Filament). As the grid coil pulses, it causes a voltage drop across the grid resistor. This voltage drop charges the grid leak cap driving a negative grid bias. This type of tube operation is identified as Class C operation, which is a very efficient way of running a vacuum tube.
25 turns on the grid coil seems to make this coil purr like a kitten. There is a red plexy ring between the top load and the grid coil to keep the top load from looking for a near ground as with the 833 coil flashing over. Tab on picture below to enlarge picture of grid coil.