High power transistor

[David Hess]

That's a step to far for me, i will take my chances on what i measured, i know its far from perfect, but it will do for me. If i have over specified the components for a project (like this one) I'm less likley to run into trouble. Thanks for the info. Things in my world move slow....

I actually agree and was just pointing how a better measurement could be made.  Just measuring the case temperature at a convenient spot even if it is not optimal will be good enough; I would probably use the center top of the TO-3 package which should follow the base temperature pretty closely but the same temperature should be returned for both transistors unless the mountings are not equivalent which is a pretty big factor.

Assuming both transistors are mounted well thermally, the MJ4502 will have more operating margin than the MJ2955 but the MJ2955 may be more than adequate.