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You beat me to this and it is the salient point of most in service part failures:
I've mentioned to people in the past (and likely forgotten) running the amp
Let’s remember that IRF510’s were designed to run in switching power supplies and as motor current switches with a duty cycle of 50% or less at their listed full ratings. The thermo-conductivity of the die with adequate heat transfer is what limits this part, and operating it at greater than 50% duty…. in a higher class… say AB2 or greater will cause more heat than design and possible part failure. While this is mostly a design concern, one can surely get into trouble by cranking the on bias without understanding all of the limiting parameters involved.
Dr. William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ
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Moderator – North American QRO Group at Groups.IO.
From: QRPLabs@groups.io [mailto:QRPLabs@groups.io] On Behalf Of ajparent1/KB1GMX
All was well until the current dropped and the power meter did likewise. Quite the surprise.