Hello friends. I recently bought a pair of Hammond 126C interstage transformers to couple my 6SN7/300B SET. Using the circuit attached, I am getting 63 Volts RMS drive for the 300Bs with a very clean looking waveform down to 40 Hz. This is way more than adequate for the 300B. My problem is that I am getting an ugly waveform on the amplifier output, which is originating in the 6SN7 driver stage. (I have proven the source by experimentation). The offending signal is about 15 millivolts P-P at 50Hz with a strong 3rd harmonic component. My line frequency is 50Hz, so I am tempted to think it is mains supply pickup. This ugly beast was not present in the output when I was using RC coupling. Has anyone else experienced this problem with the 126C? I'd very much appreciate any suggestions.