With all of the Sunn amps, the output transformer has taps for 4 ohms, 8 ohms, and 16 ohms. It will have negative feedback tied to one of them inside the amp, and then one of the other ones will be connected to the back, either 8 ohms or 16 ohms, with another one connected to the extension speaker jack which is a switching jack. They utilized different transformer taps on the back of the amp from year to year, but you should be able to look at the back of the amp and see which one they were using...it was usually labeled.
I can look later when I get home and see if I have the schematic available for you. Maybe one of the others will chime in too.....
Greg
