Double diode triode
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 6.3V; "U" identifies tubes with heaters to be connected in series across the mains supply, drawing 100 mA; and "P" identifies similar 300 mA series-connected tubes. Following the voltage letter, "A" stands for a low-current (signal) 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 numbers straight, or sharp cut-off types.
American nomenclature, also used in Europe, used a number to identify the heater voltage, letters to identify the type, with no systematic coding system and a number specifying the total number of electrodes. The 6.3V EABC80 has 8 electrodes; the US equivalent is 6AK8, where the "AK" has no particular meaning (but the 6N8 is a dual diode+pentode, also with 8 electrodes, EBF80).
There are many double diode triode tubes, including EBC81 (6BD7), EBC91 (6AV6), the older EBC41 (identical to EBC81 but Rimlock (B8A) socket instead of noval), EBC33, EBC21, EBC11, EBC2, ABC 1(4 V heater). The commoner tube line-ups of an AM radio with main transformer set having a double diode-triode were one of the following: ECH11+EF11+EBC11+EL11 Y8A Base, ECH42(or 41)+EF42 (or 41)+ EBC41+ EL41 (or 42) Rimlock Base, ECH81+EF80(85 or 89)+ EBC81(or 91)+ EL84 (noval Socket) + rectifier and magic eye indicator (depending on the radio class and manufacturer). 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 single diode section, a double diode with common cathode and a triple diode triode, named EABC80 or 6AK8 and its versions for 300 mA and 100 mA named PABC80 (9AK8; for TV Sets) and UABC80 (27AK8) for transformerless radios with series heater chains. This tube was designed for early AM/FM (MW/VHF) radio sets and was widely used in many radios, almost until the end of the tube radio era. One diode is for automatic gain control (AGC), another for FM/VHF detection and the third for AM detection.
The main configurations for an early tube AM/FM/VHF set using EABC80 in the 1950s and '60s:
EC92+EF85(80 or 89)+ECH81+EF85(80 or 89)+EABC80+EL84(or EL95), ECC85+EF80(85 or89)+ECH81+EABC80+EL84(or EL95)+ rectifier (tube or solid state) and indicator, depending on the radio class and manufacturer. For AC/DC radios, UCC85+UCH81+UF80(85 or 89)+UABC80+UL84)+ rectifier and indicator. These configurations were kept until semiconductor (germanium) diodes became available, making this type of tube obsolete.
RCA Receiving Tube Manual, Series RC-12, RC-19, RC 25 - Published by RCA.