There has been far too much misinformation published online with suggested fixes for using Ten-Tec radios (esp. Omni 6/Omni 6+) with various non Ten-Tec amplifiers. My big discovery was that there was a big difference between early and late vintage Omni 6+'s in terms of interfacing well with non Ten-Tec amplifers. The late vintage Omni 6+ does a much better job. The keying loop would work with my Acom 2000A, although T/R transitions weren't totally silent. T/R WAS totally silent if I enabled the relay and used that instead of the keying loop. On the other hand, my early vintage Omni 6+ would NOT work reliably with an Alpha 99 or Acom 2000A using either the TX EN/TX OUT keying loop OR the relay output. For whatever reason, SSB seemed more problematic than cw with the Omni 6+ seeming to output SSB RF too early relative to the amp key line. I implemented several mods Acom published without total success. Bottom line is that the TX EN/TX OUT QSK keying loop in the Omni 6+ will only work properly with Ten-Tec amplifiers and there's little hope of fixing it since it is partially firmware driven ("TX ACK"). Also the "Y" fix that is often suggested for Alpha 87A/89 amplifier is NOT recommended and leads to cw xmsn truncation.
Paul W9AC proposed a simple mod that provides a solid state replacement for the Omni 6 relay output that should work with most/all modern amps. I have tested it and it seems to work quite well. It involves wiring a single MOSFET transistor and one resistor on the Control Board to the RELAY output. Existing circuitry is left intact so it is easily reversed.
The mod is easy to implement on the solder side of the control board. Remove all the well marked connectors from the control board, remove the 5 screws, and remove the board from the Omni 6 and turn it over so the solder side is up. See the diagrams below. A VN10LP MOSFET fits perfectly with source tack soldered to the relay jack ground pad and the drain tack soldered to the relay jack center pin pad. Solder a 10K 1/4w resistor to the center gate pin and then run a wire from the other end of the resistor to the Q8 collector pad. Check you work and reinstall the Control board. Power up the Omni 6 and be sure to turn off the output relay in the Omni 6+ Menu 1.
This ends up being my preferred fix for all vintages of Omni 6+.
The fix :
Implementation on Control Board solder side:
(for early vintage 564 with thru-hole components)
(this works because the solder side sits on spacers above the chassis)
(for late vintage 564 with SMT components)
Control board schematic: