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The SD20 and Pen20 valves are Mullard versions of Philips-designed originals dating from the early 1930s. They might even be imported although all BVA valves were supposed to be British Made. (Note that the markings do not claim British manufacture.) The SG20 is a screen-grid type, the Philips pentode patent was exploited initially only in output valves, the SD20 is a diode- tetrode detector, and the SD20 is an output pentode. These three specimens could well have come from the same domestic radio (presumably a Philips).
Although they have indirectly-heated oxide-coated cathodes the internal structure (see Pen20) is still based on azide practice. Being basically designed to operate on Continental DC mains supplies, and since the available range of '20' valves did not include types necessary in a superhet, the market life of this range of types was limited. Heater and cathode life was poor and good working specimens are uncommon.
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