Stuttering amp

[Richard]

I installed a grounded plug on my Fender Princeton. Since I did this, I get a tremolo effect especially when I play an "A". On other notes, it's not particularly noticeable. I assume it's most noticeable on "A" because the of the frequencies (55hz and 60 cycle hum). The Vibrato works normally so I doubt it is involved. I checked the voltages and before I had 390 volts at the rectifier tube, now I have 363. I will replace the filter caps and rectifier tube soon (they are due), are there other things I should check. Also how do I turn the ground switch into a standby switch? I tried but misread the schematic and toasted a resistor, which may be the cause of the problem.