Resonant Feedback

• Feedback is a loop of audio that grows upon itself, from guitar to amp to speaker to air to guitar and around again and again. With acoustic or semi-acoustic guitars, amplification will excite the resonant frequency of the instrument itself, which will in turn start to vibrate at that frequency, adding to the cycle. Resonant feedback can be addressed with reduced volume, increased distance from speakers or acoustic damping, such as covering the sound hole.
  
  

Microphonic Feedback

• Though guitar pickups are made to detect magnetic signals, inexpensive or defective pickups may act as microphones and pick up acoustic signals and feed these back. A pickup that was previously fine may have had its wax coating deteriorate enough so that microphonics occur. This type of feedback is distinguished by its high and random squealing, not unlike the feedback of a microphone. Reducing volume or increasing distance from speakers may help. If this is a new problem with a guitar that was previously fine, have it checked by a technician.
  
  

Tube Microphonics

• While tube amps are prized for the tonal qualities they impart, one of the flaws of the tubes themselves is that they can, with damage or age, become microphonic as well. For this reason it's possible that a faulty tube and a guitar in close proximity can start a feedback loop when otherwise the guitar is fine. The effect of tube microphonics is less predictable, but the issue may be accompanied by radio frequency interference.
  
  

Excess Gain Staging

• Foot pedals and multi-stage amplifiers are popular with some guitarists. Overdrive and distortion effects are particularly important in hard rock and heavy metal genres. Each of these devices adds a stage of gain with the express purpose of forcing an audio signal into distortion. An amp with both channel volume and master volume can accomplish a similar effect with the channel volume up high. The master volume is then used to control the overall volume. If too much gain staging is added anywhere, excessive feedback may be a symptom. Reduce the strength of each gain device in your guitar signal path.