any one had problems with x-force/magna force amps ?

[AudioShockwav]

the problems start when you over drive they're "B" BIASED circuit then you have what's known as thermal runaway but if you keep them at a respectible level of power they will work fairly well.

Agreed, keep the drive down, and it should be ok .... Also Agreed on the NO driver stage, and the 400-12 had Thermal protection to drop out the relay if heat sink temps got out of hand. It was not a bad amp for what it was, aside from a NON regulated power supply.
It now serves as a "4 x2879 B class Mobile" in a very large case, the power supply has been removed, and the parts used in another project.

73
Jeff

 

[mackmobile43]

Agreed, keep the drive down, and it should be ok .... Also Agreed on the NO driver stage, and the 400-12 had Thermal protection to drop out the relay if heat sink temps got out of hand. It was not a bad amp for what it was, aside from a NON regulated power supply.
It now serves as a "4 x2879 B class Mobile" in a very large case, the power supply has been removed, and the parts used in another project.

73
Jeff

ASW the thermal runaway I spoke of was in the bias of those amps as they have no thermal regulation in the bias circuit and the voltage applied to the base of the transistors will rise uncontrolled without such regulation and then the amp slips into some unknown class when totally heat saturated causing the transistors to bake to death.

 

[dudmuck]

The 2sc2879 is whats know as a "bipolar" transistor, also known as BJT.
All bipolar devices turn themselves on harder when they get hotter.
It is easy for them to burn up from thermal runaway in class A or B.
This is probably a good thing for class C since it should help keep the RF gain when getting hot.

MOSFET however is the opposite is true.
When a mosfet gets hot, it starts to turn itself off.
This prevents self destruction, but the RF gain can drop with heat.
There are mosfets made for the purpose of RF, like those from ST, microsemi and others.
But i think the same temperature drift would apply to the IRF520.