Quote: Originally Posted by Ryuichi /img/forum/go_quote.gif I bought a starving student amp a while back, and it does something a bit strange. Sometimes with a fairly low level input, I hear humming that grows in volume until it pops (sounds like an arc) and the humming goes away. It will also 'pop' when I tap the amp with my finger and the humming goes away. Any idea what is happening? I don't know much about tube amplifiers. The starving student can build up a charge on the signal ground for some reason - we suspect it has something to do with the power supply or the heater-biased mosfets. The problem is worse when the ground "sink" is not very big/good. Whichever it is, the pop is the sound of the charge dissipating - which makes the hum go away. You might check all of your connections and the connectors on the case - make sure they're all tight and have good ground connections.