I bought a couple of penta amps (bottom only).  The power cords were cut and they were sold to me AS IS.  So I took a chance.
I quickly taped up a power cord from the little pigtail that was left.  Hooked up the radio to the amp and then out to a speaker.
 Surprize!  It works!  I have another set in the living room (with the penta speakers also).  There is, however, a slight hum.  I turned the plug around to see if that was it.  Nope.  I switched the speaker + and - around.  Nope.
 Could it be that I just need to properly attach a new powercord?  Any advice, ideas?
 
Thanks!
Guy