Contour plot for disersion curve not working

Questions
,
1

Hello,
I am trying to display a contour plot and vary some values to see how the Fermi surface would chance but for some reason the plot wont show up, any ideas of what I am missing?

ax = 2 ay;
ty = 2 tx;
co = 20;
Manipulate[
 ContourPlot[
  co - 2 tx Cos[kx ax] - 2 ty Cos[ky ay], {kx, 0, 2 Pi}, {ky, 0, 
   2 Pi}], {ay, 0, 10}, {tx, 0, 10}]
2

Try:

  Manipulate[
   With[{ax = 2 ay, ty = 2 tx, co = 2}, 
    ContourPlot[
     co - 2 tx Cos[kx ax] - 2 ty Cos[ky ay], {kx, 0, 2 Pi}, {ky, 0, 
      2 Pi}]], {ay, 0, 10}, {tx, 0, 10}]
3

Thanks that worked!