The EY500A is a colour television efficiency diode. It is the PY500A with a 6.3 Volt heater for AC sets using a mains transformer. It would probably be found in colour monitors that would connect to other equipment. In professional applications a non-isolated chassis would be a hazard.
Thanks to Jonathan Marsh for the image.
The wide glass tube envelope is 30 mm in diameter, and excluding the B9D base pins is 105 mm tall.
Reference: Data-sheet. Type EY500A was first introduced in 1968. See also 1968 adverts.
Absolute Maximum Operating Conditions¶
Updated January 03, 2022.