Ken, use an AC transformer / controller to give you the variable voltage, then, on the output, use a bridge rectifier, available for Tandy and other electronic suppliers, ensure it has the current carrying capacity you need, and place an electrolytic capacitor, 1000uf across the DC output, ensuring correct +- polarity as marked on the device, and there you have it.

https://ogrforum.ogaugerr.com/topic/ac-to-dc-1?reply=15649413459145644