Hey this free app for my iPhone should keep anyone from asking if I’m sure it’s 120 and not 60 Hz. It’s 120 and interestingly, there’s a second harmonic thrown in there at 240 Hz just for fun. Anybody want to explain the harmonic hum? This is a single-ended (imagine that with 120 Hz hum) running a pair of 6V6’s in parallel. I’ve got solid state diodes feeding 66uF for a reservoir capacitor. I clipped in an additional 80uF for a total of 146uF and although the hum decreased, not as much as I thought it would. In the Champ copy I built, just going to 40uF seemed more than enough. Why would this beast require so much more filtering? Does filter size vary exponentially with number of power tubes? So, is something else going on here? I have seen other posts recommending to break in the power tubes. I certainly will do that, but I think I’m going to have to engineer a fix. Will a choke be more effective than a huge reservoir capacitor? Might I have to put in an RC filter before the plate supply to sacrifice some power for better filtering? Sorry for all the questions... last time I solved this by just adding more capacitance, but this time I’m not so sure that will work.