@jbhiller You’ve got to use some homebrew cables that are speaker level (spades, bare wire, or whatever) at one end and RCA at the other... use a multi-meter to check whether black or red is tied to ground at your SET output. That will then connect to the outer jacket of the RCA plug. The other wire will be your signal wire going to the RCA pin.
If you’re driving only the F4 with your SET, you’ll also need to add a resistor across the SET’s outputs... probably somewhere between 8 and 16 ohms. When driving only the F4, your SET will essentially be unloaded, so you need to add some resistance to prevent damage to your tubes and transformers. Ideal resistance is whatever the SET was optimally designed to drive... and it should be easy to experiment to see if you get a cleaner signal when it’s seeing a particular load. If you’re using the F4 in a bi-amping setup, then you don’t need to worry about this aspect (as the SET will still be loaded by the midrange & tweeters).
Note that Mark at RenoHiFi used to make these cables for customers, so perhaps you could pitch him to buy a pair if you don’t feel comfortable building your own. Also, I had some trouble decoding this from the writing, but all the quotes you see in the 6moons article are directly from Nelson Pass. I think he’d probably provide a bit of input too if you wanted to get it directly from the source. Unfortunately I haven’t yet implemented any of this, so it’s all still theoretical on my end.
