It doesn't ring a bell with me so I doubt I've ever heard of it. It features:
A kid who makes some secret room that is actually a giant musical... thing, with lots of electrical arcing on a huge rod. He shows it to some young girl. It was gated behind something like a fence.
The sorcerer's apprentice
I guess he also failed to mention he was talking about an animation. Are you sure you've got the right segment of Fantasia?
Assuming for a moment that we can call Mickey Mouse the "kid"...
What room did he build?
What fence?
What electrical arcing?
What huge rod?
What Tesla coil?
What Faraday cage?
There's a broom carrying buckets of water to a cistern and then a flood, some lightning, some giant waves.