The Pen20 was made from 1932 in both B5 and B7 bases. To achieve the modest 1.5 Watts output the anode load was 8,000 Ohms. In 1934/5 the Mullard price was 18/6 (18 shillings and six pence) - expensive.
The dome has the etched lettering. The grids are spiral wound and circular. The two clearly visible are the Suppressor and screen grids. The control grid is closer in to the cathode. The cathode is an oxide coated cylinder with a ceramic tube within to provide heater cathode insulation. The heater is a single strand passing through the centre of the insulator. The glass insulator outside the mesh anode provides support for the electrodes.
The pinch and lower connections. The suppressor grid extends below the anode.
