Carefully tighten the pin contacts. Sometimes you have to replace the sockets themselves if you cant get a reliable contact. A dental or biology probe works great. If the tube banged around a lot it is suspect. They are mechanical devices with high tolerances for their internal construction. Also suspect are really old preamp and phase inverter tubes. You can only tell by replacement. You shoulld always have a spare anyway to swap in and out. They're not expensive. Before doing a lot though, try another cabinet. You may have a speaker going south.