replacing mosfets in power amps

[Simon]

I'm trying to find alternatives to the IRFP 240 and KTC 4370A power mosfets, now from what I can tell they are nothing special and they go into a power amp. I know that the IRF9540 and IRF540 are much the same but for voltage and higher voltage versions are available. How are mosfets selected for power amps ?

[Zero999]

I assume you're talking about a class AB amplfier, not a class D amplifier.

To be honest I've never designed a power MOSFET amplifier before but I suspect it's it's the same except being linear is more important and R-on is less important.

[qno]

One very important parameter is the Cgd.
If the gate drive circuit is incorrect your amp is prone to oscillations.
These oscillations will blow out your tweeter in no time.

After replacement check the output with a scope on a load resistor (not a speaker)  on zero to max power output
and look for oscillations on the signal. Yes, sweep the entire amplitude range.

These oscillations are difficult to prevent. This is why Mosfets are not so popular in class A or A-B audio amps.