I just received an old enlarger, and used it as a light source for contact printing my 4x5 negatives. Using the same set up, I.E. frame, paper, etc, I got very noticeable Newton's rings in my prints. I've never had them before, using the same components, other than the light source.

The enlarger is a Durst F30 condenser for 35mm film.

Is there something I should be doing different with the enlarger, as opposed to a more diffuse light source?

On the plus side, I can now dodge and burn prints, whereas I couldn't previously.