RM Italy EL519 Röhre

GENERAL
EL509:
There are different versions of EL509. The old version of EL509 uses a 9-Pin Socket.
The new version of EL509 uses a 8-Pin Socket and connection.
So if you would like to fit the new EL509 to replace an old version of EL509, you will have to adjust the PCB, as the new EL509 has different socket and connection.
The old one uses a 9-pin socket and the new one uses an 8-pin socket.
Check datasheets about the differences.

EL519:
EL519 is the same with EL509 in most of applications. The only difference is that EL519 produces 2-3 Watts more than EL509.
The only application that they have a real difference is in Class A amplifiers usage.

EL509/EL519 and PL509/PL519:
EL509 and EL519 are operated with 6,3 Volt.
PL509 and PL519 are operated with 40 Volt.
When replacing EL509 by PL509 a different transformer is needed.