Amp hums after lead ch mod and rev on ch one 74 Vibrolux

[rickc12]

Completed Rob Robinette mods which worked as described but they have introduced a bad hum problem . Hum is not affected by channel volume pots but disappears when V1 and V2 are pulled. Replaced V1,V2 and V3 with 12ax7EH tubes which seemed to induce much less hum to almost a usable level but doesn't sound as good as pre modded tone. High gain modified normal channel one is now also going through V4 in phase with Vibr channel ,could this have created ground loop problems.??

[R.G.]

[...] could this have created ground loop problems.??

Yes.

Possibilities also include a missed/erroneous wire; phase changes invalidating some hum cancellation, making residual hum more prominent; higher gain amplifying pre-existing hum by a lot.