As for the HC or KHZ (C) - JK contact preset, it should have a 100% offset set defined in the settings. What this means is that the 9.5v is all in the positive region. To have 4.75v on both the positive and negative region (adds up to 9.5v) means the offset has been lost.
When you use a 100% offset, 9.5v is equivalent to a 19v no offset setup, only you are limiting the output to the positive region. 19v is enough to penetrate the skin when we talk about amplitude with no offset, and thus the 9.5v positive offset is enough to do the same.
You can most certainly use 20v if you wish, which will result in 10v when using a positive offset.
9.5v positive offset was used as this closely models Hulda Clarks work, where she did not use the full amplitude on the devices she worked with -- a concept called headroom.
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