Power Amplifier ; the problem of breaking down

[m.ghafoori]

I have a problem using a v/uhf 100 Watt design from POLYFET. I have realized exactly the suggested TB167 design but the LB501 transistor breaks down without any specific reason.
The output load is 50 ohm and there is no input overdrive at all. Sometimes immediately after turning on the power supply, the transistor breaks down.
Is there any body who has the experience of working with power amplifier transistors and can kindly give me a help?

 

[stephen-7486]

post a copy of the schematic ind i will see what i can do