I rebuilt my st-70 about six weeks ago and have had a fairly loud hum since then that starts when the tubes warm up and doesn't vary with the volume.
The rebuild consisted of:
VTA board with k-40's and the cap upgrade, parts all installed inside the chassis
new 80-40-30-20 quad cap w/80 oriented to the back of the amp
removed front octals and mono switch
put new bias tests near the OT's on top
moved the power switch to the front, shielded the cable and drained it to the chassis grd.
moved input jacks to the rear, shielded and drained to the rca grd
After about two weeks the rectifier tube started to arc so I replaced it.
voltage tests are good
gone through it and checked all the solder joints
pulled the power switch and inputs out of the chassis.
The only thing so far that stops the hum is pulling the center tube only vta board.
Any help would be greatly appreciated.