Larry,
The output power is affected by the input voltage, within limits. If you have a QMX+ that's built for 12 volts, then the highest voltage you should power it with is 12 volts. The transmitted output power is going to vary a little from band to band and from build to build. I get about 4.5 watts out on 30 meters, but about 3 watts on 10 meters. I can use a buck converter reduce my input voltage to as low as 7 volts. Maybe you could go a little lower than that, but it seems to operate reliably at that level. That significantly lowers my output power, but I can't remember now what I get on each band with that.
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73, Dan - W2DLC
Daniel Conklin