Theres somthin wrong with the biasing system on my amp, i recently changed the tubes from 6l6's to EL34's at first they were runnin fine at around 38mV so i cranked the up to 42.5 mV and the sound just went all crappy and crackly. And when i checked what power they were runnin at they were right down to around 1.6mV. What does this mean? Thnks in advance