The 42EC4A is an efficiency diode for television receivers equivalent to the more common PY500A.
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.
The overprinted Westinghouse logo and company name.
The cathode connects to the top cap. Inside the wide cathode tube can be seen an insulated wire helix, this separates the heater from the cathode to provide protection from breakdown due to the large voltage difference present.
A closer view of the top of the cathode.
The two types of glass printing. The 42EC4A is referred to as etched but it is actually printed.
The wide glass tube envelope is 28 mm in diameter and, excluding the B9D base pins, is 105 mm tall.
Reference: Data-sheet. Type 42EC4A was first introduced in 1967. See also1967 adverts.
