Plexi 6v6 Sluckey/Huss

[Willabe]

Looks very nice! 

[EKDENTON]

nice good job!

[HommeMarrBuckley]

Perhaps, I celebrated too early but I was able to really play it this weekend and noticed something.
The amp is dead silent, however, when I push Normal volume past 6 or 7 a buzz/hum starts.
I maxed master volume and no problems.  With all pots to 0, and I turn normal the buzz will start as mentioned so looking at that area.
I'm thinking perhaps my cap there between normal and bright pot (470pf) is suspect.  One might be I misread the cap value (I hope I can read value on side but something tells me it was just labeled on the bag) or two, its polystyrene type and very fragile.
I dunno but going to open it up.
Any thoughts for troubleshooting?
TIA

[shooter]

Any thoughts for troubleshooting

swap out V1, check for DC on the grids of V1 n V2.  Is the buzz hum, like if you left the cable plugged into the amp, but no guitar, or more like filament 60-120hum?