There are two probable causes for the ”crackling” sound and ”plasma” flickering..
Under the PA tube are located 2 small air capacitors. They are called C1 and C2. Over time they are attacked by acids and other air born chemicals. They need to be replaced every 2 to 4 years.
In the PA box there is a large air capacitor that the contacts wear down. This needs to be rebuilt/replaced every 2 to 4 years.
What is happening is the plasma is acting like a speaker. These parts are shorting out. Small electrical discharges are occurring between the plate and/or contact surfaces. This is then coming out of the plasma as ”Noise” and ”Plasma” flicker.
This condition will effect the performance of the instrument and should be remedied. This is not a normal user replacement part and should only be performed by trained personnel.