The instantaneous voltage on the gate (referenced to the source leg) determines whether they're on or off. When the voltage is above about 4v, they're fully on. When the voltage is below about 2v, they're fully off. The exact point where they're fully on or fully off varies by FET. These are approximate values. These voltages are not what you're reading with your multimeter. The meter averages the voltage. The instantaneous voltage would need to be viewed on a scope.
Thanks Perry. You really know a lot about what you are talking about. Also, I did not realize you were the founder of bcae1. I love that website. I use it for building speaker enclosures. Anyways I don't have a scope at the moment but I am going to put all the FETs back in the power supply and see how it does. I hate these cheap chinese FETs though. I know this thing is going to blow again... I am using IRFP064N purchased on Ebay. From china though... shall see if this thing lights up once more 
OK I put all the fets in and the amp is running great. Pushing 2 12" soundstream subwoofers wired at 2 ohms in paralell. It isn't getting hot or anything and I have a 60 amp fuse inline. Used a 30 while testing amp on bench without audio. Now audio is amazing like it was before the psu went. Hopefully these chinese junk fets can hold up. Will know in time... for now-- JAM!!!
Yeah me too. This amp produces some amazing bass response and the crosovers being 24db/octave are nice. Did you get your going Dr Zeus? Oh and the BD139 and BD140 are a little stronger than the originals at 1.5 amps collector current versus 1 amp collector current for the C3228 and A1275.
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