12D4A

Half-Wave Vacuum Rectifier Diode, engineered to handle high inverse voltage pulses up to 5,000V peak. Operating with a 12.6V heater, it functions as a high-efficiency damper diode in television horizontal deflection circuits to suppress inductive ringing and recover wasted energy, housed in a standard large 8-pin Octal-style base glass envelope (T-9 bulb).