Humming V1 in Fender Pro Junior

Hi all,

I wonder if anyone might have experienced something similar to this...

I want to reduce the gain in the pre-amp of my Fender PJ, so I have tried a number of different tubes in the V1 but they ALL create a tremendous electrical buzz.

Please note the stock Sovtek 12AX7 does NOT hum or buzz in the V1 position, but I want to be able to adjust it and I thought I should be able to experiment without the buzz. I have tried a JAN GE 5751, EH 12AY7 but the sovtek is the only one that works properly. I wonder, is this related to tubes or related to the electrics?

Many thanks!