My ZVS driver gets very hot on one Mosfet only

[arkon666]

Hi all, Can anyone help me please. I've built the ZVS driver from this page here http://www.electronics-lab.com/forum/index.php?topic=19386.0
and it sort of works BUT. One mosfet gets very hot and the other one doesn't. Any ideas why? Is this what it's supposed to do?
I've not used the IRFP240 or 50 but 2 x FDA59N30

Also currently I have no secondary on the flyback, just the 2 x 5 turns of wire on the primary side.

 

[audioguru2]

The simple circuit is completely symmetrical so each Mosfet conducts the same if the input squarewave is symmetrical.
What provides the input signal and is its waveform symmetrical? What is the frequency?