S4 Ellipsoidal lamp size

According to the wiki:

The only physical differences between the S4 750 and S4 575 are: the cable, an extra hole in the light baffle in the lamp housing assembly which accomodates the 750W lamp key pin, and the paint on the reflector housing. The reflector did not change.

So changing the cap assembly replaces all these components.

Unless the paint on the reflector housing cannot handle the heat?

Perhaps it was an issue, or maybe they just changed over the factory to a particular powder coat at around the same time. All the temperature data for the current line of products is in the wiki too. Unfortunately there is no surface temperature data for the 575W Source Four so it's hard to say if the paint is going to be an issue. I guess it also depends on operating conditions for each fixture.

That being said, the older model 575w source fours (the ones before they added the ring for a safety cable to the fixture) Will not take a 750w cap, there is an extra guide or pin on the outside of the fixture that prevents the cap from seating properly

After some research and reading responses I can see why the debate has been going on for so long.  I have yet to see ETC officially come out with a definite yes or no, but I also understand why.

I have some older style 575 fixtures without the safety cable ring and the 575 caps seat perfectly well on them without any modification whatsoever.  I've had 750 caps on them for a long while but always used 575 lamps but want now to go 750, even though there is no comparison to the number of hours a 575 is rated; 300 vs 2000.  That's definitely bad news.  Oh well.  Maybe technology will invent a longer lasting 750, but then again maybe not in this led world we live.

It seems to me that 750 lamps will be ok and I'm going for it.

Thank you all.

The difference in lamp life isn't about wattage.  There are standard life lamps(300hours), and extended life lamps (2000 hours) in both 575 and 750.  The extended life lamps have a lower color tempthan standard life lamps, generally around 3k.  From my experience, I no longer use extended life lamps because as you build lamp hours above 800 -1000, the filaments start to sag and makes bench focusing near impossible.