I bought a 773 about a year and a half ago. It had been on my "wish list" for quite a while. Great condition and I got it (a 1950 version) for about 1/2 what they are currently listing for on the Bay. So, I was very pleased with it. I immediately did a "Super Tune" on it (see JustTrains.com) as well as replacing the brushes, springs, lube, clean etc. It ran very well after that, albeit at much slower top end speed than my other PW steamers and at much higher voltages. Lionel's guide.......12-20V is accurate for it. Recently, I found a High Stack motor for it and swapped it in. The speed didn't really change (that's an intentional gearing issue), but it does seem to be a stronger puller. In addition I decided to replace the worm cover. The original has 2 magnets on it. You can buy a ready made one......very expensive......or do what I did........ordered a cover and the 3 magnets and made my own. I have to say, though, I don't know if there's a measurable difference in the magnetraction strength. But it can't hurt and it's fun to tinker with. So, now it runs about a good as it will ever run. I do have a drawbar pull test track, but I can't seem to get an accurate measurement on it. But when I put a736 in front of a fairly long consist (for my layout), the 36 will spin it's wheels before it starts to pull the load. The 73 won't unless I floor it. So, from my subjective measurement and observations, I'd put it's pulling power somewhere between the 736 and my PW F3s (although the Fs are much stronger).
Bottom line, then.......if you can find one for a good price, it's a beauty and huge. And it's tender is as good as it gets (for PW).
Roger
P.S. The original motor (which runs flawlessly) is sitting on my workbench. If anyone needs to replace their 773 motor, you can email me (in my profile) and we'll work something out as I have no use for it now.