The GY501 is a half wave rectifier, designed for EHT use in PAL colour television receivers. The heater would usually be powered by a winding on the line output transformer.
Rotated through 90 degrees.
Here the end of the cathode tube is visible as are the heater wires. One side of the heater connects to the cathode at the top end.
A slightly different angle reveals the oxide coating on the cathode.
The wide glass envelope is 29 mm in diameter and, excluding the B9D base pins, is 79 mm tall.
