Heater to cathode short or leakage in one of the tubes, (oscillator) maybe. 73, Bill Riches, WA2DVU Cape May, NJ Hi Gang, I am in need of info I have long forgotten! What are the causes of tunable hum? Way back when I was young and handsome! Hi! I used to know the common problems with radios but my recollection ain't what it used to be!