Ryobi SC-160 Scroll Saw Speed Control?

I have a single-speed SC-160 16" Ryobi scroll saw. The motor has only
a little information on its label as follows:
H/P: 1/10
Phase: 1
R.P.M: 1725
Amps: 1.6
Volts: 115
Hz: 60
Insulation Class: A
MFG:1991

I would like to add a speed control to it. So far my experiments with
2 different types of speed controls has produced the following:
1) Control from a lapidary machine: scroll saw runs at about 15 RPM
regardless of the setting on the speed control.
2) Solid-state light dimmer: Saw does nothing until the dimmer is set
to about half of the control's rotation range. Then it runs at full
speed after reaching midpoint.

Is there a speed controller available that will give me an infinite
speed range for this saw? I would like to cut metal with it.

Gary