Hartke 3500 MOSFET output transistor

[tekhed7]

I would agree w/ Mars, conway. Putting in a single 240A along w/ other 240s is a real roll of the dice. It may switch on at the same gate voltage as it's brothers, but its likely it will either be turning on somewhat earlier in the waveform... & idling warmer (Not good) or not turning on as quickly as it's mates, who will end up sharing most of the work. The Mosfets work as a team, either pulling the spkr cone (if it's 1 of the FETs in the bank connected to the negative rail) or pushing it (if in the bank connected between the positive rail & spkr out).
It's usually best to change them out as a match set, esp. since the others May've been exposed to excessive heat or voltage spikes. However, you can pull your meter out (?) & check the idle current of ea MOSFET & see if the "newbie" is running "hard". Not sure of the setup w/ the 3500. You might be able to measure & compare the small voltage across the "source resistors" or you may need to [carefully] break into each MOSFET source leg to measure milliamps.
Have you considered contacting Samson for another insulating wafer? Maybe they can mail you one, or maybe some guitar shop amp tech will sympathize & lay 1 on you. ;-)

[Steve Conner]

When looking for insulating washers, try TO-3P as well as TO-247.

With the amp under load, you can feel the source resistors to see how hot they get. Each resistor is connected to one MOSFET. If the new MOSFET's resistor gets much hotter than the others, you could have a problem.

Measuring the voltage across the source resistors at idle should also work. It should only be some tens of millivolts. Check with the amp cold and again once you've run it on a dummy load and really got it hot, to check for signs of runaway. If the voltage keeps on increasing once the amp is hot, you're in trouble!

If the new device is a bad match, it's probably best to buy half a dozen IRFP240 from Mouser etc. and match up a new set yourself.

As others have said, it was probably a bad idea to make power amps with IRFPxxx switching MOSFETs in the first place.