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Tip: With the amplifier on and the rear panel removed, you may be able to identify a microphonic
tube by gently tapping on each tube with a pencil's eraser (for example, since anything hard may
potentially crack a hot tube) to see if the sound becomes worse or changes. All tubes will amplify
the sound a little, especially preamp tubes. For example, if you hear an intermittent ringing sound
and tap each tube to find V1 is causing the ringing to start and stop, you can be sure it's the
problematic tube!
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