You mean you added an output jack to the amp and it doesn't work?
Do you have some leads with alligator clips? These are the easiest way to test stuff before soldering. And they are cheap and last forever, if you don't have any you should get some.
I would start by just connecting the jack to the speaker wires to see if sound actually reaches the headphones. Normally the phones should be really loud with a direct connection like that.
Do you use a switched jack? these can be a bit tricky to figure out. If so, it could just be that the wires are on the wrong side of the switch. Flipping them would fix the problem. Normally the wire from the circuit board goes to the tip connector and the switch connector goes to the speaker. When not plugged, power flows from the tip to the switch to the speaker. When plugged-in, the speaker gets disconnected.
Output speaker wire from amp to jack and then to speaker.
If the amp's output wires are only one hot and one ground(mono), you'll have to run 2 wires to the single hot(splitting the signal as a faux stereo).
Post some detailed pics of the wiring so we might see what's going on.