The U10 directly heated full wave rectifier has an internal coating on the upper part of the envelope. A close look shows that the valve has cathode tubes that are internally strapped to one side of the filament. Our other classic enveloped U10 has a hairpin of oxide coated ribbon as the emissive surface. The original U10 came with a balloon envelope.The maximum reservoir capacitor is not quoted, but at 60 mA maximum output current the U10 was clearly designed for broadcast radio reception in 1930s all mains sets. The 4.0 Volt heater is characteristic of the 1930s British heater standard.