I'm building an aleph 30 and i suppose that each Irfp 240 dissipate the same power (?)
Can i'mount one heatsink for each mos instead of one big heatsink or it needs to be mounted on the same heatsink to have a thermal coupling between Mos ?
No, not recommended. I've tried it with Aleph 60 and I experienced lots of instability problems like loud hum, unbalanced currents, distorted output, etc. I wired each fets in series. I'm trying to save on heatsinks, because a big heatsink costs a lot. And I mean a lot. So, instead I fixed all components in one pcb and stucked fans to keep those fets cool. I designed the pcb to accomodate all the parts and the fets spaced evenly and bolted to the heatsink.
But I hate the sound of fans, so I'm saving some money on big heatsinks.
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The original idea was to mount a Mos directly on the heatsink, without any mica's or silicone to optimize the thermal coupling between the mos and the heatsink. To avoid any short circuit each mos must be mounted on a separate heatsink...
But if all Irfp240 need thermal coupling between us, it doesn't work...
Your idea is sound, but (depending on your design -- reads local degeneration), you might be better off with tracking temperature and resulting heat efficiency reduction.
Besides, it might be hard to electrically isolate heatsinks.
Here is an alternative. Mount each transistor on a copper block. Use electrical isolation between this larger block and heatsink. This will probably take you very close to your goal of lower heat loss through isolator. You pay a small price in input capacitance.
Another alternative is to go with something like Bergquist Silpad 2000 or functionally equivalent. People are getting quite religious about Aluminum Oxide washers. The best, however is BeO washers (electrically, dielectrically and conductive). BeO is banned in many places though and not available in all sizes (TO3 IS available, TO3P/247 and 220 was not last time I checked).