I know it's an old tune, but who can tell me what effect Dusty Hill is using during the little breaks in the tune "Cheap Sunglasses". It's very synth-like and I'm having trouble replicating the sound...

I think the part you're describing is a clavinet. I was at Ardent Studios in Memphis recording while ZZ was doing that disc. They had a clavinet in the studio that they were using from time to time and I've always assumed they used it on that part.

Might very well be! Sure sounds more like keys (Clav, Wurlitzer with Overdrive, etc.) than it does a bass...
My guitarist read an interview with B.G. and he said it was some kind of synth/effect/blah, blah, blah..., triggered by the bass, but didn't mention what it was in particular...
Then again, it could be the ZZ Top "Shroud of Mystery". Always keep'em guessin'!!!
Thanks, GuitarG, for the help...

Maestro Ring Modulator on the guitar

it IS a ring modulator