I have a Marshall MR 2525c which started making weird noises. Can't really explain the nature of the noise. It has a tremolo kind of feel, not crackling and not really a hum either. Tried different cables and guitars. It doesn't change even when I unplug the guitar cable. Turned all the knobs both ways. Nothing changes except for the volume and tone of the noise which probably is normal. Besides the noise the clean, crunch, lead tones sound fine to me. Anyways, I asked my local music store where I got it what to do. They don't service Amps and I don't want to send it in if I can avoid it. They suggested I try to replace the tubes. I have my doubts that a tube went bad but I will give it a try. Now I'm inexperienced with exchanging tubes and I don't want to accidentally change the nature of the jubilee sound. I've searched the Internet about replacing tubes but it got me confused. From what I have understood is that the Amps need to be biased for the new tubes to work correctly. But apparently this is not the case if the amp is cathode biased? Some people suggested the amp is cathode biased but I haven't found any documents that confirms it. Does anyone have suggestions how I should try to solve this problem? First replace the pre amp tubes and if the problem still persists go for the power amp tubes? Or any other idea what could be the problem?