12v Power Supply

Many problems may occur with a heavy load. The transistor may fail to produce the given voltage at the output, by dissipating power in internal resistance, or the unregulated supply for the regulator may have high amplitude ripples that fall below the input/output dropout threshold. Good regulators need a negative feedback, and a properly designed rectifier.

BJT resistance almost doesn't matter here. They have to drop the voltage anyway, from lets say about 40V down to 12V. So dropping 28V must have e.g 10Ohm for 2.8A or 1Ohm for 28A! Mash rule Vin + Vce + Vload = 0V! Here the problem start at input voltage which suppose to be 170V peak and not 120Vpeak! Next, the transformators are wrong connected and have a much to low down step ratio. This cause an extreme high secondary output impedance because primary impedance/ratio²=output impedance which is here close the to 3kOhm @ 60Hz and this easily drops the transformer voltage with a weak load, beside an heavy ringing LC folter, caused by 1mF and transformer high inductivity ! Check this how to lower the impedance http://everycircuit.com/circuit/4925896545009664