Unplug the amplifier from power and remove the amp chassis from its cabinet using screwdrivers. Discharge capacitors of stored voltage before working on any components. Remove and replace any faulty tubes.
Identify the filter capacitors you need to replace. Aged capacitors may have a bubble or powder residue. If your amp is over 10 years old, it is a good idea to replace all filter caps, even if some show no signs of aging. Clip the negative lead of one cap using side-cutting pliers and solder the negative lead of the new cap in place. Repeat for the positive side, and complete this process with each filter cap you are replacing. Use replacement capacitors of the same farad value and an equal or greater voltage rating.
Check the screen grid resistor that is wired across the tube socket, if a tube has blown. This resistor blocks excess voltage when the tube blows, and may be damaged. Check the resistor using the multimeter to ensure it is still providing a resistance value within 10 percent of its rated value. If outside this value, de-solder both ends of the resistor and replace with a resistor of the same value in ohms and an equal rating in watts.