having trouble getting a formula to work with times

ok im trying to create a way of keeping all the hours i work in a excel spread sheet so i can work out what i should get paid.to what i actually get.

the trouble is anything over 39hours is paid at 1.5 normal pay. i thaught this mite solve / be my answer but seems to fail me somewhere i was hoping that a more experianced person could solve or guide me to solving my issue.

=IF(H20<39, (H20-39)*1.5*6.5, (H20*6.5))

using that i do get a number but its always minus.

if its not possible to do it this way and get the desired result or am i going about it all wrong??

thanks in advance