6SN7 - 5692 - 6F8G - 6H8C
One of the most famous preamp tubes of all time. The 6SN7 and it's equivalents were once the most popular tube. These are octal based tubes and were released early on, far before the small 9 pin preamp tubes we are familiar with today.
Compatible equivalents & substitutes: 6SN7, 6SN7GT, 6SN7GTA, 6SN7GTB, 6SN7WGT, 6SN7W, 5692, CV1988, CV278, 6H8C (Russian), ECC32.
Please note, the 6F8G is essentially a 6SN7 with a grid-cap. They require an adapter to be ran as 6SN7.