Power supply capacitor is 39,600. uf. = 12 caps each 3300 uf.
I already increased the power supply voltage from 15 to 21 with all the other changes temperature came up 100 degrees F.
My mosfets are rated 60v 50a my meter show RF voltage is 70v. I will need to change mosfets and the transformer to get a higher voltage. I am going to change the transformer anyway, to 4000 watts with 240 vac primary. Soon as I decide on a new mosfet I will know the secondary voltage. I am considering IPP075N15N3, 150v, 100a, .0075 ohms. A 48 vac secondary will give me 67.8 VDC.
Keep in mind that, regardless of what the drain-source voltage of the mosfets is, the typical maximum gate-source voltage is only 20. I know that you're driving the mosfets gates with a 12 volt pullup resistor, but I'm pretty sure you're getting switching transients on the gates higher than that. The only way to see how high those transient voltages are is with an oscilloscope.
Also, the fact that the mosfet gate turn on current is through nominally high value resistor, (yes, 240 ohms is high for this switching frequency) will cause the mosfet to turn on rather slowly relative to your switching frequency.
A rough simulation using the Ciss capacitance of ~2500pF for the P55NF06 shows that, depending on the actual turn-on point, you may be loosing as much as 20%. That puts more heat in the mosfets and less heat into the work. What is the actual rise time of the gate voltage? How does the gate timing relate to the drain timing? Again, a scope will be needed to see and measure those things.
With a Ciss of ~5500pF the IPP075N15N3 has more that twice the input capacitance. That simulation shows the gate voltage ramping up and is only up to about 80% when the pulse ends. With that timing you could easily be dumping more heat into the mosfets than you do into the load.
PS
Gary, let me make one request about your posting style. You start a new thread with each new question you have about the same project. That makes it very difficult for people who start reading a new thread to understand the foundation of the thread. Please do either of the following.
1) Keep everything on the same project in a single thread (preferred) or
2) Post a link to another thread that defines the project with the opening post of a new thread.