The flat lens board you are using for the 50mm lens is correct for the 75mm lens too. If you cannot detach the 50mm lens then you'll have to have a board manufactured by your local machine shop. This is pretty easy to do as you can obtain the dimensions by tracing the board on a piece of paper with detail about the thickness of the board. You want to measure the diameter of the 75mm lens from just outside the threads to just outside the threads on the other side of the lens. You'll have to have a retaining ring that fits the threads. The threaded end of the lens goes through the hole in the lens board and the retaining ring is screwed on from the other side to hold the lens.Originally Posted by Huram
regarding the condensers. The glass lens you see inside the silver bucket is the condenser. They made at least two sizes. The smaller of which should work for all negs up to 6cm x 6cm. The largest set of condenser lenses are about the same diameter of the silver bucket itself. The important thing is to be sure the condensers are wider than the negative so there is good light coverage.
Regarding the Besseler 75mm lens, I'd suggest looking on ebay or something for a lens. You will be able to pick up a lens (80mm is a better size) for a pretty reasonable price. A nasty dirty lens makes your prints muddy and a little soft