A bad PT blow the fuse if it's shorted. No fuse in the PT.
Start from the wall plug make sure you have 120AC there then check for 120ac where the cord goes into the amp before and after the switch and the fuse. If you have 120ac going into the PT you should be getting ac out the other side. Only the B+ coming off of the rectifier is dc pin 8. That then feeds DC to the rest of the amp.
Did this work before the 3 wire cord was added?
And if you could post some pics it may help.