My last printing session with carbon produced prints that were sharp -- just not as sharp as previous carbons I have made. Everything was identical except for one thing. One of the merc vapor lamps seemed to be not as bright (in the visible spectrum), so I moved the printing frame around a lot during exposure to even out the exposure across the tissue. Normally I move it only once or twice during the exposure.
All that changing of position (and not using a vacumn frame) probably gave me the same results as using a diffused light source similar to a bank of BLB tubes.
I have the material/equipment for a vacumn frame system -- I need to get off my rear and figure out how to to put it all together. I remember years ago on the B&S forum reading about a glassless vacumn frame system that Stephen Luvick use -- I would like to try to duplicate that.