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Dario is a brand name of the French company La Radiotechnique that was founded in Paris in 1919. Initially the valves were made under the Radio Micro name but changed to Dario from 1927.
Philips purchased La Radiotechnique in 1931. From this date onwards the Dario valves were all made to Philips designs.
From Tyne's1047 'Saga of the Vacuum Tube' page 398: 'La Radio-technique at first used the trademark R superimposed on the outline of a transmitting tube. In 1923 with the advent of the thoriated filament, the R was changed to RT. In 1927 the trademark became Dario, and some tubes had both RT and Dario markings'.
Post WWII the company issued valves under their own name. The box carried the RT logo.
References: Wikipedia & 1043.
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