High Voltage Leakage
This is a bad tube socket , I removed from the chassis , from one of my amplifier project. This socket would leak dc current from the plate pin to the cathode pin. As you can see , this is an blank tube socket, and under high voltage , abouts (+) 300 volts , cause a great deal of distortion of the amplifier signal. This is an example of how contamination on high voltage parts causes the high voltage to leak currents on the equipment. Yes , this is fixable.