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Pro Electron took the popular European coding system in use from around 1934 for valves ( tubes ), the Mullard – Philips tube designation, and essentially re-allocated several of the rarely-used heater designations ( first letter of the part number ) for semiconductors.
The second letter was as " A " for signal diode, " C " for low-power bipolar transistor or triode, " D " for high power transistor ( or triode ), and " Y " for rectifier.
Additional letters did not follow the vacuum tube mode.
For example, " L " designated a high power pentode tubes, but " L " is used for RF power ( transmitting ) transistors ; " Z " is used for tube rectifiers but semiconductor Zener diodes.
Unlike the tube naming convention, if there are two transistors in a single envelope, the type letter was never repeated-so a dual NPN RF transistor might get a type " BFM505 " rather than something like " BFF505 " for instance.

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