Quoting on my one entry, pfffff
Anyway, it's probably because of the heat development in the transistor.
It's possible to use only one transistor, however a big (active) cooler is necessary.
It think I will go with 3 transistors, I will get back to you with the results.
This is probably a very stupid question (maybe not :-) )
but why do I need 3 Transistors for a 5 amp supply?
I checked the datasheet for the MJ11015, and it states it can handle 200W.
From my calculations, I get : 30V * 5 Amps = 150 W...
I should say one transistor is enough...