The manual states the following on page 59 section 5, "In SSB mode, touch [MIC GAIN] and then adjust MULTI knob so that the ALC meter stays within the ALC zone of the meter (up to half scale deflection) on voice peaks."
Now the problem is that the manual is a little ambiguous in regards to what half scale deflection means and in fact the image supplied doesn't even show the ALC meter.
|Default image shown in the FT991 Manual.|
What is considered half scale deflection? If we look at the actual ALC meter, shown below, is half scale halfway along the blue ALC zone or halfway along the 0 - +60dB scale?
|ALC meter display|
Well consider this the 0 - +60dB scale is for the S meter not the ALC, the full range of the ALC meter is within the blue zone only. After a number of friendly arguments about this I called Yaesu UK and confirmed the blue zone is the full range of the ALC meter.
So if the ALC is peaking at the red line on the image below then you'll be over driving the amplifier.
|Red line showing excessive level|
If you keep voice peaks to within the green line on the image below you should be good to go, and will keep other operators within you vicinity from moaning at you.
|Green line showing correct level|
Just remember adjusting the drive level so you have maximum output isn't the right thing to do. The difference between output say 40W or 50W to the other station won't even be noticeable, so keep you transmissions clean.
One other point if possible adjust the microphone gain while transmitting into a dummy load and adjust the ALC so voice peaks actually come a little bit lower than the green line this will help take into account the fact your antenna is almost certainly not presenting an impedance of 50ohms across the entire band.
In my next post on the FT991 I'll take a look at setting the compression level to be within parameters.