I recently built a preamp using 12As loaded by a Lundahl 1680 transformers. However when poked at by the spectrum analyser there is an incredible amount of both 50Hz and 100Hz harmonics. I spent a long time trying to re-arrange earths, modify PSUs etc with no luck until, purely by chance, I placed a transformer cover over one of the tubes and saw the interference on that channel drop drastically. It would appear these tubes whilst being lovely sounding, are extremely prone to picking up hum fields. And I bought quite a few of them . . .
Does anyone had any experiences with 12As they can share?
