The U191 is a television efficiency diode. The function of the device is to enable the stored energy of the line scan coils to be returned to the supply rail during flyback. Here the Mullard Type designation of PY301 takes second place to the U191 suggesting the U191 was the design copied by Mullard. Most popular valve type designs of this period were shared by BVA member companies as customers insisted on multiple sources of supply and the BVA wanted it to be UK made. The actual implementation of the design was up to the separate company and their preferred manufacturing techniques.
The valve is rated at 5,000 Volts (PIV) and a peak current of 600 mA. The valve is unusual in that the top cap is taken to the cathode. This means a 5,000 Volt heater cathode insulation.
The box anode has wide working faces, to facilitate heat dissipation, but a small front to back distance.
The broad face can be seen to be a central working area with side flanges formed where the two sides have been stitched together.
The wide glass tube envelope is 29 mm in diameter and excluding the IO base pins it is 75 mm in length.