My new Miller Bobcat has low amperage on 120 volt outlet.

is the tig welder 120 or 240. If 120 no you can't

On a Bobcat, the big knob needs to be turned all the way up to 10. If it is, then the engine speed (RPM) might be too slow at fast speed.

have the alternator checked by an electrician
a t idle will be low as the exciter circuit will be producing low volts but it should go up when at operating rpms
45 volts at rpms indicates that the field windings of the alternator are not getting full current to produce the desired out put

Brother support site you can email direct to them

http://www.brother-usa.com/askus/default.aspx?PGID=3&R3ModelID=hc1850#.VR2S6eEZmQJ Contact Brother

Both sets of the 120? If both of those are there (all 4 outlets), then the machine is producing 220v. So get a flashlight and look into the holes on the 220 receptacle, or check the wires on the backside of the receptacle. All that 220 is, is two 120 volts out of phase.

The unit may need recalibrating the idle/run setting. Instructions found in manual at 8-6 (subaru engine) and 9-5 (kohler). Idle speed around 2200rpm. Simple screw adjustment. PDF manual here :- http://www.millerwelds.com/om/o4434l_mil.pdf

Can you manually adjust the voltage on the generator without load?? If you have 120/240 60hz output on it and you are not overloading the unit (amperage), then it may be as simple as adjusting whats called your gain setting for your voltage regulator. Obviously you are seeing what happens when it is set too low. Trying raising that setting until it becomes unstable then back it off. Unstable meaning voltage starts jumping all over.

how long has the welder ben sitting not used? if it has been sitting a wile you may have to flash the fields if this dosent do it check the brushes.

Make sure that the idle is set at 40Hz. You do this by placing a frequncy meter on one of the 120Vac outlets. If frequency is low, the engine will run rough at idle. There is a 3/8 nut next to carberator that needs to be turned to the right to increase the idle speed.

If the idle is not working, then it is either you idle solenoid attached to the carberator or your idle board is bad. If you apply 12Vdc to the idle solenoid is should move, if not then the solenoid is shot, no good. If it works then you idle board is bad. Also, check to see if you are 60Hertz by measuring at the outlet with a frequency meter. I recommend a Fluke meter. You should have 120Vac and 60HZ at the outlet. If the frequency is low then your engine speed needs to be increased. The idle solenoid could be causing all these problems.