A double diode triode is a type of electronic vacuum tube once widely used in radio receivers.
The tube has a triode for amplification, along with two diodes, one typically for use as a detector and the other as a rectifier for automatic gain control, in one envelope.
The commoner tube line-ups of an AM-only radio set with mains transformer having a double diode-triode were one of the following: ECH11+EF11+EBC11+EL11 Y8A Base -or- ECH42 (or 41)+EF42 (or 41)+ EBC41+ EL41 (or 42) Rimlock Base -or- ECH81+EF80 (or 85 or 89)+ EBC81 (or 91)+ EL84 (noval Socket) + rectifier and magic eye indicator (depending on the radio class and manufacturer).
This tube was designed for early AM/FM (MW/VHF) radio sets and was widely used until the end of the tube era; the double diode was used for FM demodulation, the third, independent diode for AM detection and/or automatic gain control (AGC).
These configurations were kept until semiconductor (germanium) diodes became available, making this type of tube obsolete.