You would use 40 amps. This is just a starting point. I would set the machine at 45 amps and use the foot pedal to control the penetration, no foot pedal set it at 40 amps and adjust travel speed to control penetration. Good luck.
With the added info you will need more than ten amps unless the piece that you are welding is like shim stock .040"x.500" and you would need a start that is about five amps and go up to ~ 40 because you still need to fuse the metal 100% for any strength. he older machines are hard to get to go to 5 amps you can help it by running the high freq. on continuous to stabilize the arc. You may still have an issue with not using filler because of "hot cracking" from no reinforcement.