Have you tried the Vaca Mts. of Eastern Napa county and Yolo county? Lake Berryessa is rattlesnake cental around here, people are always encountering them while fishing, partying, ect. Also Cache Creek below Clear Lake has a lot.
Most of my exploring is done farther West. I see them at Lake Hennessey, Skyline Park in Napa, and Mt. St. Helena and the Palisades near Calistoga.
The Knoxville Berryessa road and the old mining roads near Mt. St. Helena (Ida Clayton, Oat Hill) should be top notch crotalus hunting roads.