Not sure I understand. The 2N6388 transistor is what it is - a 10 Amp 80 volt NPN Darlington pair ( a cascaded pair of transistors in one TO-220 housing ). It is a rectangular plastic block, with 3 flat leads and a metal tab.
Maybe the Hobart welder uses 3 of these transistors, and you are not sure which one to replace? That would depend on what the fault is. There are references on the web to wire feed problems on this welder ( always running at full speed ). The solution seems to be to replace transistor Q1 on the control board.
From what I've seen on the web, the 2N6388 Darlington pair transistor fixes most wire-feed problems on the Hobart 135 MIG welder.
I have not yet found a value for the speed control potentiometer. The part number will only be valid if you're buying a replacement from the original manufacturer. Otherwise it would just be a case of ordering a generic replacement potentiometer of the correct resistance, e.g. 1K Ohms, 5K Ohms, etc. However, a potentiometer rarely fails, and, if you already suspect the transistor, it is unlikely that two components have failed at the same time. I would suggest waiting for the result after you have replaced the transistor.