SP9 Preamp Hum - Only w/Phono Tubes

I built an SP9 Tubes4Hifi preamp back in 2013 and it has worked flawlessly until very recently. I started to get a hum and decided I was overdue to replace all the preamp tubes. It sounds great again, as long as I don't put tubes in the phono preamp board. I figured my new 12AY7 phono tubes must have been bad and ordered a new set, same issue.

I am wondering if the transformer is going bad but isn't noticeable until the added gain of phono stage is active? Before I go randomly replacing parts, any thoughts or diagnostics?

-Tyler

a couple of possibilities, the most likely is that all six tubes are too much of a load for the filament supply, you may need a transformer with higher current capability.
Or it could just be the power supply to the phono section isn't supplying enough current.
Or you have noisy tubes. Phono sections have a gain of almost 1millionX before the RIAA EQ is applied resulting in 10000X gain overall.