I need to chime in here!
Disclaimer: By nature, I prefer quality equipment of all sorts. And, this generator head is no exception.
I power my ST-15 with a 20HP Listeroid twin. The power produced has been problematic. My house has modern appliances and some won't accept the listeroid-ST power at all.
Here are the issues, none of which are major.
First, the ST, as delivered, has low voltage under load. About 6KW load results in 109V each leg. So, Tom from GA GEN set me up with a voltage regulator that eliminates the Z winding, and uses 120V from one of the legs to excite the field. Much better idea.
Second, the (now eliminated Z winding) excitation power is, as depicted in the post above, nasty.
Third, After very little use, my bearings are howling.
Fourth, The doghouse box is utter-junk. Made of steel so strong you can bend it in your fingertips.
Fifth, the overall quality and wiring, and insulation rating is poor. Requiring some effort to "fix" flaws.
Now, the question on everybody's mind..... Would I do it again? Yes, if powered by the listeroid, as the ST is a good companion. The Listeroid has power/combustion-event pulses that affect the AC output, and so the AC output won't ever be 100% clean. Even with very heavy flywheels on both the engine and generator head.