The PY500A is an efficiency diode for television receivers.
The function of an efficiency diode is to return energy to the anode rail from the collapsing field in the line output transformer. This generates high peak currents and the PY500A has a maximum peak current rating of 800 mA.
Note that the cathode is brought out to the top cap.
Considerable heat is generated and so the anode is generously sized.
The visible ceramic coated helical wire is a spacer. The heater sits inside this helix.
The base showing the insulated coiled heater wires and the extra coated wire of the spacer helix. The wire connects to a support but the support terminates in the lower mica without further connection. This construction allows a greater heater/cathode voltage than normal. This is essential as all the heaters would be connected in a chain across the mains supply.
The wide glass tube envelope is 28 mm in diameter and, excluding the B9D base pins, is 104 mm tall.
Reference: Data-sheet. Type PY500A was first introduced in 1967.