'Soft' triode used as a detector in radios in the early 1920s. The electrode structure is the same as the UV 201 but the bulb is filled with argon at a pressure of 50 µm of mercury. Gas filled valves were referred to as 'soft', high vacuum valves were called 'hard'. Soft valves were more sensitive detectors at low signal levels but were very fiddly to use and went out of general use by about 1925. 5 volt directly heated cathode.

Physical Description

Conical clear glass bulb with 4 pin UV base with brass shell and ceramic insulation. Mounted on a display block.

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