Thanks PK! So much great info! I'll be on that McMaster-Carr web site for a while! But if I want to use a real DC motor speed controller, I might just get a golf cart one that belongs with the motor.
Jafe is lending me his 10 Amp variac so I can get started experimenting.
About the 110-220 transformers.... Too bad yours malfunctioned. These are no doubt made to sit still in a house, so vibration may have been a factor. But I'm thinking that a vibration break ... no, wait, I suppose it would arc when it broke, so it would look melted either way.
Oh... PK, any thoughts on omitting the rectifier? From what I have read, a series-wound DC motor will run just fine on AC. I suppose I will know in a few days!
New topic: I also own a giant AC motor, but know only what it says on the label -- which tells me very little. It has more wires coming out of it than the Borg Queen. I suppose I'll try Jafe's variac on that one also -- nothing to lose, since the thing is just a 100 lbs paperweight now.