OK...I would like to get my wife a mixte/step through ladies bike that would be decent for some fire trail stuff. What I would like is something with a 26" size wheel like a 650A size that I could easily convert to 650B for a wider tire. Of course it would have to be able to take a wider wheel as well.

Basically something like what Rivendell offers new but on a budget. Like I said I can convert if needed so it does not need to be ready to go by my description.

If this is not doable I will have to look for a step through mountain bike frame but I would prefer to get her something fancier than that.

Any ideas?

Get a vintage mixte frame, and put 700c hybrid wheels (wider than regular road wheels) on it. Then by swapping tires you can dramatically change the personality of the bike, from trail to street.

You know...I never thought of that. I must be sloowwww. I suppose a 27" wheel would be easy to convert to a 700C and then by dropping size slightly I should be able to get some wider tires on there.

You will find many more mixtes in 27" than you will 650B or 26"

It seems that most bikes that take a 27 x 1-1/4" tire will fit at least a 700cx35

My wife's old Univega mountian bike is sort of a mixte. I never thought to take a picture of it. It's nuthin special to look at but she likes it better than any of the vintage bikes I've tried to get her on.
She don't care much for sailin either.

I just picked up a vintage 76 Raliegh for $75 on CL. Something like that could be easily built up with 650B wheels, especially since it was spec'd for long reach weinman centerpulls. The most affordable new frame option I find was the SOMA Buena Vista which is back in stock now.

Here's my wife's mixte. It originally had 27 inch wheels. I put 700s on it, no problem. I think it could easily handle hybrid wheels with wider tires.

Just checked a couple of older Peugeots - the space between the rear stays is about 52mm so I'd say you could put a pretty darn wide 700C tire on one of those. And Peugeot mixtes are some of the most common out there. I think the 60's - to mid 70's are the most stylish.

So if I found a Peugeot mixte which is very possible they show up alot would the Mafac centerpulls be able to adjust to a 700C size?

Thanks for the help guys!

Yes.

funny coincidence I just did that this morning, and the answer was as DirtDrop says.

If you look really close at the rear stays on a UO8 or similar Peugeot you will notice that the chain stays are crimped to allow tire clearance. If, like most North American market UO8's, yours has 27" wheels you will notice that the tire sits a few millimeters past the widest point in the crimp. That's because it was designed for 700C to begin with. The 27" is just an adaptation for markets where 27" wheels/tires were standard. 700C will bolt right in a the standard brakes will adjust to a perfect fit as this is how the bike was originally designed in France. They didn't modify the frame to fit 27" wheels/tires.