The PY500A is an efficiency diode for television receivers. Colomor was a UK reseller.
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 anode has four flanges set at right angles.
The heater runs from the normal heater chain but the cathode reaches great potentials that stress the heater/cathode insulation. To overcome the problem and to prevent breakdown of the insulation a second insulated helical wire is inserted into the cathode tube and the heater threaded through that thus increasing the distance between the heater and the cathode tube. The outer helix is connected to a stub descending from the lower mica.
The folded and stitched anode.
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. See also1967 adverts.
