Interchange with a 5751 and a 6FQ7.

I'm hoping to upgrade and replace my 5751s. One RCA went bad and so have the GE Jan stock
CJ tube in place now. The 6FQ7s are RCA  Cleartops . The amp is a CJ PR12 with 5751 input tube and 2 6FQ7s as the driver.  12AX7?  12AU7? Thnx
Sorry to be opaque... Is there an equivalent tube to a 6FQ7 or a 5751 that would work in a CJ PR12 with the 5751 being the input tube and the 6FQ7 as the driver?  5751s seem to be scarce at Upscale Audio and thetubestore...thnx
Used mullard as drivers and ge jan 5751 in my 12's.  Tried many. This was one of the best combos.
The 5751 is a rough equivalent to the 12AT7.

The 6FQ7 is the same as a 6CG7; the 12AU7 is similar but with 1/2 the plate current capacity.

They use a different filament connections (the 6FQ7 is 6.3 volt filament with no centertap) so they are not interchangeable.