John Greenland
CVRS Member

Hey Ralph,
You are correct about the impedance according to the RCA tube manual RC20.
The plat to plate impedance is 10, 000 ohms.
The single ended amplifier impedance is listed as 7000 ohms.

I have attached to page from RC-20

Good luck

BTW there are other alternatives but it will require more mods to the radio.
For example replacing the 6F6’s with 6v6’s !!!!