Krzysiek Slychan wrote:

"I seriously think you advice would confuse our original poster,
and place his amp and himself at some risk."

Why?

Anyone farnarkling around with mains voltage without
considerable experience is likely to perhaps electrocute themselves or
burn something in the amp.

The fact that the original poster asked the question he did indicates
his knowledge/experience isn't high. If there is no obvious way to
voltage match from 120v to 100v,
say by rotating some voltage preset switch on the back of the amp,
then I assume that option is closed, so I thought it better to advise
some stranger
of the simplest and easiest and SAFEST way to achieve a wanted end
result,
and without the possibility of the variac being turned up to too high a
voltage
by accident, and not have that noticed by anyone.

Patrick Turner.