[Mondial]

You can also try re-tensioning the tube socket contacts. Sometimes the metal contacts lose their spring tension and fail to grip the tube pins tightly. If the tube pulls out of the socket easily with little effort, the contacts are probably loose. After removing the tube, carefully squeeze each contact with needle nose pliers so that tension is restored.

Make sure all the tube shields are in place when you test the radio. The grid lead of the 75 audio stage is right near the 42 output tube and stray coupling can cause the circuit to oscillate and squeal, especially when the volume control is turned up.