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After shorting out the original MOSFET while attempting to take measurements, I installed a new one I got from Stephen (I grounded myself to eliminate static before handling). Initially the sound level was varying up and down on its own. I switched the amp off to try swapping tubes, and when powered up again the VRM was not functioning.

Any ideas?

Could I have over-heated the MOSFET leads when soldering?

Interested about this also. Just attempted to fire up my tramp yesterday and I'm not getting the 400V on the first filter cap so I'm thinking MOSFET damage. I haven't seen this mentioned before - what is the actual procedure for grounding your body? Is an anti-static wristband the way to go? Because I don't have a wristband, I just kept one hand on the chassis while touching the MOSFET, but I'm thinking the chassis wasn't really grounded since it wasn't plugged in to the mains.

I guess a better description would be "discharge any static" in your body by touching a ground, like a copper water pipe. You don't need to be continually grounded, in fact it's preferable to be isolated from ground when working with high voltage.

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We say "Note: Ground your body before handling the MOSFET.
Touch a waterpipe or some other such ground source.
These are tough industrial MOSFETS that can take a little beating and some mishandling.

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Stephen
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How can I test the MOSFET to see if it's dead or alive?