Insert tube in the socket of the probe. Insert probe. ” plate current ” on the function switch of the probe. Remove tube. Allow tube to warm up. For testing plate / anode current select. EVEN THOUGH YOU ARE DEALING WITH A FEW MILLIVOLTS’ BETWEEN THE BANANA PLUGS ,IF THE CATHODE ON YOUR AMP IS CONNECTED TO CHASSIS ( PROBABLY IS ) YOU WILL HAVE THE FULL B+ POTENTIAL BETWEEN EACH OF THE BANANA PLUGS AND THE CHASSIS OF YOUR AMP.