Replacement transistor help needed

[circuitscircus]

I’m trying to fix an old amp that has a blown transistor but the original part is no longer available. I’ve seen some substitutes that are rated around 60–80V, 5A, and 40–60W but I’m not sure which one would be the safest choice. Has anyone else done a similar replacement? What worked out best for you?

 

[12Cronut]

hi @circuitscircus just wanna tell you that when dealing with power output stages, safety margin is everything.
My main lesson is to over spec the power rating where possible. If the original was 60W, look for a replacement rated at 80W or 100W (provided it fits the package). this gives you a much better safety margin against thermal runaway.
also, be sure to use new mica insulators and fresh thermal paste when installing the replacement! a tiny air gap or poor heat transfer is the fastest way to turn a replacement into another blown part

 

[circuitscircus]

This is some really solid advice especially the part about overspec’ing and redoing the thermal setup. I appreciate you laying it out so clearly; it’s going to be a huge help when I’m choosing a safe replacement.

 

[Alan]

hi @circuitscircus just wanna tell you that when dealing with power output stages, safety margin is everything.
My main lesson is to over spec the power rating where possible. If the original was 60W, look for a replacement rated at 80W or 100W (provided it fits the package). this gives you a much better safety margin against thermal runaway.
also, be sure to use new mica insulators and fresh thermal paste when installing the replacement! a tiny air gap or poor heat transfer is the fastest way to turn a replacement into another blown part

Great advice especially on over spec and ensuring proper heat transfer. Thanks for the solid tips.