Because the diodes need to have the exact same temperature as the transistor it is connected to. The circuit works kind of like a thermometer, which also tells the transistor to lower its temperature if it gets too hot. If there is no thermal connection between them, your amplifier will blow up.
If you use the Big Ones, we don't have to get into the endless debate over whether the C5200/A1943 have enough SOA to do 8 or 4 ohms with +/-45V rails, or whether you need to gang up four in parallel to be safe at 4 ohms....
Truth is that amp manufacturers will stick in the cheapest Jap TO264 they can find at the time, and it would be ok at 8 ohms and probably at 4 ohms if you don't go blasting it. The big On Semi's would pretty much user-proof it. They are slower and have more beta droop at 15A, but for an amplifier that had STK modules in it originally, who cares?
Thanks for all the help! Much appreciated!
I think that the transformer isn`t big enough to support higher power even if 4 ohm speakers are used.
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