The PY33 is an HT rectifier for television receivers. It operates directly from the mains input and with a series resistance of 25 Ohms it will feed a reservoir capacitor of 200 µF. The 300 mA heater was the standard for TV receivers from the 1960s.
The advantage of the PY33 was a voltage drop across the valve of only 10 Volts made possible by an ultra smooth cathode coating.
The wide glass tube envelope is 30 mm in diameter, and excluding the IO base pins, is 92 mm tall.
References: Data-sheet & 1040. Type PY33 was first introduced in 1961. See also 1961 adverts.