Is this a head or combo? I had problems with a squeal with my head. It only happened when I had it on top of the speaker cabinet. Turns out it was a microphonic tube. Tap on the 12ax7's . Especially V-1. If you hear the taps you can try swapping it into a position further down the line (V-2, V-3 )
What size zener did you use? I was dropping too many volts with a 6 volt. After changing to a 12 volt , I only dropped a few.
The hum went away for the most part , also.
The rest of the hum left when I did the hum fix:SCRATCHY POT LAYOUT - VRM INPUT MOD
viewtopic.php?t=1176&start=0&postdays=0&postorder=asc&highlight=
Good Luck with it!
LeeMo