I use my FT-101ZD MK III regularly, and have refurbished a number of them. Actually, I have two of them here right now.
Hopefully you have an RF cage fan mounted... if not, as a novice/intermediate user, go buy one and install it.
Basically, the PRESELECT control has three tuneable slugs. They do work in harmony with the transmitter and the receiver. It's called resonance.
The receiver section uses two of them and one of those two is actually shared with the 12BY7 driver tube, and ALC circuit...
The two 6146B tube's cathodes are held above ground with a single precision resistor, which when switched in, is in parallel with the panel meter movement.
This resistor is the tube's cathode, screen and plate "current", shunt resistor for the IC meter.
It allows you to read the two power tube's total current flow under idle (with no signal, drive or "carrier"), and again, under driven conditions for actual power input. Not RF output.
The thing is that it is measuring DC cathode/plate current and DC screen current so it's not really a good indicator of any power output.
But very useful as a visual guide for tuning to resonance and max power out.
Set the PO-IC-ALC switch into IC mode (again, it's only measuring "total" DC current through the two 6146B).
TUNE mode
VOX GAIN control in MOX while only intermittently TX testing:
Check the idle current of the tubes is set to 50ma with no drive or signal.
This is adjustable too, but read the manual on how to do it, and don't exceed 50ma if you have to adjust this setting.
TUNE mode.
Tuning:
Pick you band and connect to a 50 ohm dummy load.
Read the manual for approximate settings, by bands/frequency, of the three controls, PRESELECT, LOAD and PLATE.
VOX GAIN control in MOX
Work your way up with the drive control and keep adjusting the PRESELECT, LOAD AND PLATE controls set for maximum output, while continuing the "dipping" of the PLATE current control as you go, finally stopping at a tuned output of around 100-110 watts.
Into my external wattmeter, the plate current on both radios show about 270ma to 280ma at this point.
If the two tubes were perfectly matched (they will not be), that is around 137ma each of DC current.
These tube are quite robust if you are careful with key down and transmit time cycles, you can run at 125ma-150ma each for years and years.
Now the final section is all tuned up....
To limit your output power on voice, when desired, turn the MIC GAIN control from zero, counter clockwise, to maybe as high as 11:00, depending on what kind of mic you are using.
So keep this in mind, in voice mods you'll be mostly using your MIC GAIN control to limit power output and voice peaks and in CW you'll be using the DRIVE control for power output.
I just tried mine and with the meter set to IC, USB or LB and while turning the MIC GAIN slowly up from zero, firmly saying FOUR, with the what meter in PEAK reading mode, the needle flicking forward to about 70ma-75ma, I'm getting about 10-15 watts output.
Same thing with MIC GAIN up to about 100ma is 50 watts.
150ma is peaking at over 100 watts peak output.
Looking at the mic gain control, the pointer is almost at the 9:00 position and the radio is making 100 watts PEAK and AVERAGE almost 45-50 watts!
So going a little further with MIC GAIN control to 175ma to 200ma, PEAK power is around 125-140 watts, AVERAGE is about 65-75 watts.
Read up on peak envelope power.
From this you might be able to see how easy it is to overdrive these things and sound like crap with the MIC GAIN set too high.
REMEMBER:
All of this VOICE mode stuff was done with the DRIVE control at ZERO!!
CW is a different thing and you need the DRIVE control for those power output settings.
When comfortable with all the above, now in USB or LSB you can mess with the COMP LEVEL PROC... but that does require you using the drive control for that extra punch.... just not anywhere as much as you think you need.
It's all in the YAESU FT-101ZD manual.