[Amps] 4 x 6KD6: total impedance ?

[G. La Parola, IT9BLB]

I'd like to convert an old Yaesu FL-2000 with a single russian GI7-b triode.
The FL-2000 has 4 x 6KD6 tubes and I need to know their total input
impedance, just to understand if the original input circuit can still be used
or not.
If someone has already done such conversion on the same amp, I'd like to
share some informations.

Thanks in advance for any help,

Joe, IT9BLB