Not sure if I mentioned this before or not... I have two different ST-5 type heads.
One head I got about eight years ago, maybe. It's a steel cased unit. Good and bad with this design. Good, in that the main housing can't crack like an iron one. But, not so good in the fit and finish department. The welds don't look so great and it goes down hill from there.
The case is made up of pieces of welded steel and then machined "true". During the machining for the stator seats (those ribs that the stator is pressed into) they cut all the way through in a couple places. The stator seems to be tight but it looks like pretty sloppy work. Remarkably, the pole gap looks pretty uniform. The mounting base is made out of the same thin sheet metal and is actually a bit flexible if you yank on the end of the head.
This "tin can" head does seem to have copper windings and it does have iron ends. The harmonic winding is "weak" and the unit can't hold voltage at much of a load. I intended to play with capacitors on the harmonic to see if I could get the voltage up a bit but I haven't had the time to play with it. It came with a "western style" bridge rectifier - which I need to change out (didn't know those were suspect when I first bought the head).
Next, about a year ago, I bought an iron ST-5. The castings are a lot thinner than I'd like but do seem to be pretty good quality, maybe better than the Listeroid castings. The machining seems OK - bearings fit properly (not jammed tight but not sloppy). The mounting base is very solid and obviously has no detectable flex. The main housing has me a bit worried because it is clear there is some stress in it (trying to make it expand) due to the press fit of the stator. The bell-end at the pulley side seems to be very fragile. There is not much iron left where they have all the cooling slots on the bottom.
This iron head seems to have copper windings (the connections are welded, by the looks of them). It came with an AVR and a digital meter. The meter seemed to work OK so I kept it (though I moved it off the head). The AVR worries me (I bought two spares) BUT I assume its presence means that the harmonic winding is "hot" and could be adjusted with a resistor. I just hope it fails open and doesn't run away and fry my equipment.
I swapped out the rectifier on this head (it was similar to the other head). I used a nice quality one and put snubber caps across each diode. I put fuses on the output (though I don't think the Listeroid is capable of blowing up the generator).
I will say that the AVR is pretty stable and keeps the voltage nice and tight - even with good size load fluctuations. I have not put a scope on the output - someday... But first I need to fix the scope - or get one of those new ones. My scopes are all so old they have tubes (and I don't mean the CRT).
Strangely, the armatures of these two vastly different heads seem to be nearly the same. They both seem to have copper windings. They have the same plastic bobbins (which have a few cracked and broken ears) and they both have what look like cast aluminum fans. Both heads came with noisy (but sealed) bearings. The brush rigging is identical between the two.
Overall, I like the simplicity of these heads - you can take them apart in minutes - I just wish they had a little more quality. I gave the windings (stator and armature) in the iron head a good coating of Glyptal 1201. Also coated all the unpainted raw iron (basically the entire inside and the bottom). It hadn't rusted at all so I thought it was a good time. I need to do the other "tin can" head as well but it was partially rusted (inside) when I got it so not sure how good that will turn out.
-John (Boston)