RF amps and IRF540 vs IRF640

[Dr Zeus]

With RF amps, I always seem to have the opposite parts for the amps I'm working with; example, most older Punch amps use IRF540/IRF9540 for outputs. Power amps on the other hand I've seen use IRF640/IRF9640. Looking at the datasheets the IRF640s are rated for twice the voltage but approximately half the amperage of the IRF540. This would make sense as I've seen Punch amps with +-35v rail, and power amps with significantly more.

Now, Im in a situation where I have a whole bunch of IRF640/9640 outputs but need IRF540/9540 for the rebuild I am currently working on. Would there be any chance in using IRF640/9640 in place of IRF540/9540? I went ahead and hooked the IRF640/9640 up for testing purposes and they seem fine, loud enough, and stable.

The amp I'm rebuilding is an RF Punch 75.2 rated for 37 per channel.

I do have some IRF9540N and IRF540Z transistors. I dont know if IRF9540N would cause issues...

[Perry Babin]

I've never tried either of the 540/9540 that you have. You'll have to try them to see if there are any problems. The 640/9640 would likely fail if the amp is driven hard.