which port or RCA connector to interface an ameritron al 811 on the back of a 6400 flex

Hi Jon, you can use TX1, TX2 or TX3. Connect one of them to the relay jack on the amp (not to the ALC).

In TX Band Settings in SmartSDR, check the boxes for the TX line that you are using on a per band basis.

In this example, I am using TX1 to key my KPA 500 amp. I am using TX2 to key my 2 meter transverter, and TX3 to key my 2 meter amp. Pay attention to how much power you put out on 60 and 30 meters. You may want to uncheck the boxes for these two bands.

Be sure to turn down the RF output of your Flex so that you don't overdrive the 811 amp. Follow the instructions in the amp manual to tune up on each band and mode.

Note above that I do not use ALC (HWALC box) with my KPA 500. I don't believe that you need it with the Ameritron 811, but I don't have any experience with that particular amp. Any correction here would be welcome.