>I'm not sure about this, but it seems to me that with Q6
either full on
>or full off that it's power dissipation must be fairly low. This is also
>reflected in the fact that Q6 has no heat sink. Operating my SW-40+ in a
>normal QSO, Q6 barely got warm to the touch.
Only reason Q6 gets warm is that it doesn't switch "on" instantly and
"off" instantly. Dave has designed the final for very high efficiency:
Driving with 50% duty-cycle helps.
Saturating Q6 helps too.
Keeping the load (that Q6 sees) as large as possible helps too.
Dave gets maximum power out using minimum DC current. Very important
for QRP rigs that might run from batteries. A Class C amp that followed
all the "class C" rules wouldn't do as well.