Art Todesco writes:
The room where the organ is located is
wired with aluminum wire, so the
resistance is higher, making the problem
worse.

Properly-functioning aluminum wiring should not have any effect.
Aluminum is less conductive than copper for equal cross-section area,
so the electrical code required use of aluminum 2 gauges larger than
the copper that would be used for the same load. This makes the
resistance per unit length about the same for the two wire types, and
you *shouldn't* see any worse dimming with aluminum.

On the other hand, aluminum is notorious for connections increasing
resistance over time, so this may be an indication that you have an
aluminum wire connection that is degrading. Excessive dimming is a sign
that something is wrong, not a normal side effect of aluminum wire.

Dave