Tom,
If you are going through your 6/1 engine and checking and correcting all the little flaws inherant in the manufactureing, consider spending 1 hour taking the ST-5 apart. Replacing the bearings with sealed units will cost you about $67 dollars, and while you are at it get some machinist to put it in the lathe for the under 5 minute task of truing the rings. After this you will have a quiet, dependable unit which doesn't squeal from the brushes. Replacing the wierd rectifier with a Radio Shack full wave bridge rectifier is a no brainer as well and will cost $4.55 as you found out. I am sure with fresh quality bearings the parasitic load on the engine is less, as well. The sizes you need are bearing #6309 on shaft side, #6307 on the slip ring side.