6BK4C/6EL4A

High-Voltage Beam Triac/Triode Shunt Regulator Tube, engineered to control and stabilize the extreme high-voltage anode rail (up to 25,000V DC) in early color CRT television chassis. Operating as a closed-loop voltage regulator, it dissipates excess electrical energy directly as thermal output to prevent picture distortion, housed in an extended large-top-cap glass envelope (T-12 bulb).