After searching AK on this subject, I have learned that a fair amount of people have experienced this problem and some brands seem to hum, others don't. My personal experience on the voltage amp/phase splitter 6u8 tubes in my ASR-433 has been that Westinghouse and RCA's were ok for a while then developed a hum. I tried a set NOS GE's that hummed right out of the box. When a hum in a tube develops, the hum follows that tube to the other channel when swapping L & R channel tubes. Apparently, some people have tried 6GH8A, 6BL8, and 6EA8's as substitutes with some success. My question is, does anyone have a theory as to what makes the 6U8 susceptible to hum or develop a hum?(not in my amp necessarily, just in general) Bad design of the tube?, the tube not being used in circuits for the purpose it was designed for? or? Would running DC on the heater make any difference? Thanks in advance for any discussion on this topic.