My Marshall Guitar amplifier Makes Noise From Dirty Nobs. How do I Take the amp apart and clean the nobs?

The "nobs" that have the noise is what we call a "dirty pot" Some can be cleaned others are to far gone to ever work well again. To clean a "dirty pot" you need to get into the internal area of the amplifier. Most amps have screws in the bottom that hold the unit into the wood frame. unplug everthing from the amp. turn the unit upside down and remove the retaining screws. that should allow the guts" of the unit to slide out. don't force anything. then you should be able to see the "pot" from the top. If you can you get electric contact cleaner (I use the NAPA brand) and the spray stick (red tube) that comes with the can of contact cleaner. In a safe area (no flame or heat) you spray the contact cleaner on the top and sides of the "pot" then work the knob back and forth all the w ay to each end of the allowable movement but never try to go beyond where the knob wants to stop. repeat this at least three times. take care not to let the contact cleaner drip onto anything as it could leave a stain. I usually try to use a lot of newspaper under the unit and be careful not to use any more spray than you need. You should feel the pot getting easy to move as it gets cleaner. Let everytthing dry off before you reinstall the amp into the outside frame. Move slowly and take your time. If it works it will save you a lot of repair cost as changing out the pots is not fun and could cost almost as much as the amp is worth