I ground the main filter cap and the screen caps (40uF, 16uF and 8uF), AC ground to a lug on one of the PT bolts, ground the heater center tap there too.
I ground everything else (cathode, preamp filter, input jack, and pots to another ground point on the other side of the chassis, a brass ground plate in the case of my first two 2Strokes or a buss bar on others.
You have the center taps for the 6.3v pair and the B+ (high voltage) pair grounded, right? If the PT has any other center taps, they should be capped off and not grounded. Does your amp hum with nothing plugged in the the input jack?