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. In practice the two diodes usually share a common cathode. Multiple tube sections in one envelope minimized the number of tubes required in a radio or other apparatus. In European nomenclature a first letter "E" identifies tubes with heaters to be connected in parallel to a transformer winding of 6.3 V; "A" identifies similar 4 V; "U" identifies tubes with heaters to be connected in series across the mains supply, drawing 100 mA; "H" identifies similar 150 mA, "C" identifies similar 200 mA, and "P" identifies similar 300 mA series-connected tubes. Following the voltage letter, "A" stands for a low-current diode section, "B" for a double diode with common cathode section, "C" for a triode section, "F" for a pentode section, "H" for a hexode or heptode section, and "L" for a power tetrode or pentode section. The first number identified the base type, for example 3 for Octal base; 9 for B7G sub-miniature 7 pin. The remaining numbers identified a particular tube type; tubes with all characters except the first identical had identical electrodes but a different heater; e.g. the EBC81 and UBC81. Generally, odd numbers identified tubes / valves with variable mu characteristics and even numbersstraight, or sharp cut-off types. American nomenclature, also used in Europe, used a number to identify the heater voltage, then one or two sequentially assigned letters, then a number specifying the total number of electrodes plus one. The 6.3V EABC80 has 7 electrodes; the US equivalent are 6AK8 and 6T8, where the "AK" and "T" have no particular meaning; the 6N8 is a dual diode+pentode with 7 electrodes. There are many double diode triode tubes, including EBC81, EBC90, EBC91 and the older EBC1, EBC2, EBC11, EBC21, EBC33, EBC41, ABC1, CBC1. 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 +EF42 + EBC41+ EL41 Rimlock Base -or- ECH81+EF80 + EBC81 + EL84 + rectifier and magic eye indicator. AC/DC sets without mains transformer would use "U" tubes of the same types, e.g. UCH42+UF41+UBC41+UL41+UY41 rectifier. There was also a tube with a double diode and a triode sharing a common cathode, and an additional, independent single diode section, named EABC80 or 6AK8 or 6T8 and its versions for AC/DC transformerless receivers with series heater chains, named PABC80, HABC80 and UABC80. This tube was designed for early AM/FM 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. The main configurations for an early tube AM/FM set using EABC80 in the 1950s and '60s were: EC92+EF80 +ECH81+EF80 +EABC80+EL84 -or- ECC85+EF80 +ECH81+EABC80+EL84 + rectifier and indicator, depending on the radio class and manufacturer. For AC/DC radios, UCC85+UCH81+UF80 +UABC80+UL84+ rectifier and indicator. These configurations were kept until semiconductor diodes became available, making this type of tube obsolete.
