Hi Bertrand, do you mean A FETs drive a 300B? I don't think there is anyone who designs this way.
By the way, Audio Research did have some designs such as Classis60 with FET at the input, with tubes in the middle and back,
In my option, such circuits are too complex for DIY, I would prefer full tubes design.
FETs or MOSFETS may sound very "dry" but more Analytical, maybe you can adjust the sound by using paper in oil caps.
But FETs input is not my cup of tea.
By the way, Audio Research did have some designs such as Classis60 with FET at the input, with tubes in the middle and back,
In my option, such circuits are too complex for DIY, I would prefer full tubes design.
FETs or MOSFETS may sound very "dry" but more Analytical, maybe you can adjust the sound by using paper in oil caps.
But FETs input is not my cup of tea.