900VA SGTC

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.

Made mucho grid feedback coils trying to get this VTTC to provide the longest sparks achievable. A picture below shows all the grid feedback coils that were wound. Started with 22 turns and went all the way up to 29 turns. The more turns, the harder it drives the tubes off.. 29 turns was giving me 32 inch sparks; however, was getting flash overs on the grid coil and finally reduce the turns to 23. It is a fine line of achieving the longest sparks and keeping flash over down to a minimum.   Pictures below are of the gird coil on the primary form and a picture of the results of a 20 inch flash over from the top load (Toroid) to the grid coil. It was a nice light show; however, the flash over destroyed the grid coil and the primary form. Made a new primary form with a plexy ring between the top load and the grid coil to keep the top load from looking for a near ground. Tab on pictures below to enlarge for your viewing pleasure!

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