You can actually get -10V to +10V from Spooky2, that is 20Vpp. If you use the boost cable, you can get 40Vpp. If you use a waveform with a 100% offset, then you get 10Vpp from the generator, and 20Vpp from the boost cable.
I have used hundreds of times Spooky2 as a Hulda Clark zapper with the boost cable in contact mode with 20Vpp, without any issues. In fact, I don't remember she told not to use a higher voltage than 9.5V.
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