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 used as detectors, or Automatic gain control. Some common types are the 55, 12SQ7, 12AV6.

In the European market B stands for double diode and C for triode.

There are a lot of examples of double diode - triode tubes like EBC81 (6BD7), EBC91(6AV6), the older EBC41 (identical to EBC81 but Rimlock socket instead of noval), EBC33, EBC21, EBC11, EBC2, ABC 1(4 V heater). The series heater version were designed for 100 mA heaters and were named UBC81 , UBC 41 ,UBC11, etc. Usually a standard configuration of an AM radio 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(or42) 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). In case of series feeding without power transformer, the valves are no more with E but with U letter in the front. Example: UCH42+UF41+UBC41+UL41+UY41. This type of tube was designed in order to minimize the number of tubes in a radio.

There was also production of a triple diode triode, one diode having a separate cathode, named EABC80 and its versions for 300 mA and 100 mA named PABC80 (for TV Sets) and UABC80 for power transformerless radios. The American equivalent for this tube was 6AK8, 9AK8(PABC80), and 27AK8(UABC80). This tube was designed for early AM/FM/VHF radio sets and was widely used in many radios, almost until the end of the tube radio era. The tube has three diodes, one for AGC, one for FM/VHF, and one for AM (separate cathode).

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 or EL34), ECC85+EF80(85 or89)+ECH81+EABC80+EL84(or EL95 or EL34)+ rectifier (tube or solid state) and indicator, depending on the radio class and manufacturer. For series powered radios, UCC85+UCH81+UF80(85 or 89)+UABC80+UL84)+ rectifier and indicator. These configurations were kept until germanium diodes appeared on the market, making this type of tube obsolete.

References

RCA Receiving Tube Manual, Series RC-12, RC-19, RC 25 - Published by RCA.