A high-pitched squeal generally indicates a bad preamp tube. Tubes start to wear out from the moment they are installed. You can check for a bad preamp tube by gently tapping on them with a wooden pencil or chopstick. Allow the amplifier sufficient time for the tubes to cool down and remove the metal tube shields by pushing them in and turning them counter-clockwise. Bad or microphonic tubes will tend to behave like a microphone . A quality brand replacement tube should be installed – take care to correctly line up the pins of the tube with the socket.