It is important to remember that there are significant differences in amps. Some amps are less susceptible to noise than others. And as leftside stated earlier, he has thrown out a number of noisy 68FG and kept the quiet ones. Quiet ones are just that: quiet. The top cap provides more isolation between the grids of the two triodes in an attempt to reduce RF-induced noise, and the use of a cap also freed up pin 1 to be connected to a metal shield, again to reduce RF-induced noise. But when you install a 68FG into an adapter allowing it to be used in a 6SN7 socket, these noise reducing measures are eliminated. The grids are no longer isolated in the socket and and there is no way to use a metal shield. So again, it is best to accept the fact that this tube is more susceptible to noise than a 6SN7 and it will often be necessary to roll through several in order to get a quiet one.