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Tip: If you think you have a microphonic tube, you can very gently tap on each tube with a pencil's
eraser to see if the sound becomes worse or changes. All tubes will amplify the sound a little,
especially preamp tubes. But for example, if you hear an intermittent ringing sound and tap each
tube to find V9 is causing the ringing to start and stop, you can be sure it's the problematic tube!
Tip: Buy "burned-in" tubes when possible for maximum stability and minimal bias drift.
8
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