The CV5258 (U191) is a television efficiency diode. The function of the device is to enable the stored energy of the line scan coils to be returned to the supply rail during flyback.
The valve is rated at 5,000 Volts (PIV) and a peak current of 600 mA. The valve is unusual in that the top cap is taken to the cathode. This means a 5,000 Volt heater cathode insulation. See alsoPY301.
The anode seen from the side. The helix visible near the base is the spacer that increases the separation of the heater from the cathode tube.
Face view of the anode. Below the bottom mica can be seen the thick insulation on the heater wires and the helical spacer.
The wide glass tube envelope is 28 mm in diameter and excluding the IO base pins it is 76 mm tall.