I also use a slate call but it includes an
aluminum surface and a plexi surface also. I have
a lohman pump yelper, a box call, and a lohman mouth yelper (works
like a crow call where you just puff into the tube.)
I have used the mouth yelper in conjuntion with the
slate call to bring in gobblers that were hungup.
Sometimes a change in frequency or a combination
of different calls is all it takes to get that gobbler
motivated to come in.