[DaleHCook]

With the Isotap having switchable voltages in 5V steps, I’m curious why you would need to add a variac to it?

My Variac box provides continuous control of the output voltage. Its input and output are fused, and the output has a voltmeter and an ammeter.

Note that I may be the only person in this topic who is actually using a Variac - that is a brand of variable autotransformer whose trademark was long held by the original licensee of patent 2949592, General Radio Company, and is now held by ISE, Inc. Calling anything else a "variac" is like calling every brand of automobile a "cadillac." Everything not made by GR or ISE is merely a variable autotransformer.