I'm not familiar with the 12SX7 tube, but it shares a common trait with many other tubes. And that is that the heater requires a higher voltage than the VacuTrace can provide.
In this case, testing is still possible, but you have to add an external heater supply. This is pretty easy for indirectly heated tubes (where cathode and heater are electrically separate). The hard part is getting an external filament transformer connected to the tube under test. For this, the Jumper Adapter Card is available, which allows you to connect a tube in any fashion using jumper cables.
Of course, not everyone has it. The other alternative is to build a Blank Adapter with heater terminals wired up to an external transformer. You can get elaborate and add a relay to copy the standby/operational modes, but it's not necessary. I wouldn't suggest going that far unless it is for production testing.
Once the adapter is wired up, apply filament voltage from the external source. It can be a filament transformer or dc lab supply. Once heater is warm, then you can exit standby mode and start sweeping curves.
Directly heated tubes takes a bit more effort. The external supply MUST be a floating secondary winding. It must be electrically isolated from ground or any other connection. Then connect the center tap to the IKA or IKB signal (cathode). This connection provides a path for the cathode current to flow through the sense resistor.