I have a slight crackling noise on the 40W setting, not on the 20W setting.  I thought I would check the bias however when I try and test, I get no readings.  According to what I have seen on YouTube, you must have your amp on the 40W setting however I get no reading on  40W or the 20W setting.  Is there something I am not doing correctly?  Do you have to have an instrument plugged in with amp on?  The amp works when I use it, I'm just trying to get rid of this crackling noise on the 40W setting, 20W sounds great.