Hi Mike, then the question becomes which loop?? They use/assume ?the chassis as if it has zero resistance and zero self inductance and zero mutual inductance. We just hope there was some method hidden in the madness when they designed it so critical areas are mostly isolated. It must be full of UNMARKED small loops It is a mystery to me how you don’t run into bad coupling. And like it was said I think, take a heater to a ground point, add a signal ground to the same point, and wait for corrosion to add some common resistance et voila a hum generator in the grid circuit. Don’t know if that is a real loop, but you can predict that those can go bad, and you can usually see them as possible hot spots.? Probably Jacques is talking about that sort of thing, but I have not gotten that far yet. ????

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