I was always intrigued with the way Ted Nugent/Adrian Belew/Jeff Beck/Jimi Hendrix/SRV were able to control feedback and make it musical - are there guitar/amp settings and techniques that can make this kind of control a part of a players repertoire?
The answer to a ton of the questions here so far are the same basics: practice and experiment.
In order to get feedback, all you really need is a loud amp and an electric guitar. Hollow or semi hollow guitars can be kind of hard to control the feedback, so definitely start with a solid body. The thing to experiment with is your guitar's position relative to the amp. Where you position yourself will change the frequency of the feedback you are getting. Moving the guitar around can change the note that's feeding back.
Some other hints:
- Use some vibrato to help keep the sound "going".
- Feedback is easier with a gain in addition to volume, so make it a bit dirty.
- It's easier with a guitar that sustains well.
- Compression will help you get some good feedback going, as it keeps your signal going for longer at higher volume (i.e. sustain).
- You can use a wah pedal to control your feedback and the note that you are getting. Just slowly sweep through the wah's range and see how your note breaks up / feeds back.
The way I usually do it on stage is as follows:
Run a guitar straight into a Line 6 Pod X3 Live and then take that into the DI input for the venue backline (this lets me use my preset amp simulators) and have a pretty loud front of stage monitor.
In soundcheck I take a good look at where I can get feedback and although I don't mark them on the floor, I note where I need to be for particular songs or sections. I also set the volume so feedback will only happen at full volume on my guitar. When I don't want feedback I slightly back off on my volume or tone.
If I get feedback slightly wrong, I can move forward or backward a very small amount to get it to sound 'in tune'
caveat - this works well for loud rock and metal. Not so good with an acoustic guitar mic'ed up. For that I always rather have my monitor behind me so feedback doesn't ring so harshly.
One way of "faking it" is to use a pedal which can self-oscillate, like the Zvex fuzz factory. At some settings it will produce feedback sounding noise even with no volume from the guitar. It can be set so the volume control controls the shrieking. To the untrained ear, this will sound similar to feedback (which in some sense it is, but not the classic amplifier type which is discussed here).
The benefit is that it becomes more controllable, independent of position and can be reproduced at lower volumes.
Assuming you mean feedback from the amp, it has a ton to do with the amp circuit used. It is quite easy to make an amp oscillate. Usually this isn't preferred because it can be hard to control but I suppose with a properly designed amp and a bit of practice it wouldn't be too hard. The part of the amp that seems to have the most effect on controlling feedback is the the negative feedback. This is added to reduce distortion(which also reduces gain).
So first things first is to make sure you have a good amp to work with.
Kind of reinforcing what others have said but with some disagreement in my experience : I use a Marshall 1970's amp with a nice warm full sound, a strat (unmolested) and a Boss BE-5 effects board which has distortion and compressor, whcih I set so that the volume is the same whether clean, compressed or distorted.
Feedback occurs when some magic happens between the resonant characteristics of your amp, your guitar the note being played and the room itself. Between them they gang up and at some locations in the room (mainly relative to the amp) cause some notes/strings to ring almost indefinitely.
I can get feedback easily with the compression and distortion on (I don't use much) by standing a little away from the amp, normally to one side a little, and letting a note/chord ring. It doens't have to be too loud but that sometimes helps. It;s more about the sensitivity (gain) on the effects and the position of the guitar.
However .. * Using the tremolo arm/vibrato to vary the pitch doesn't 'keep it going' in my experience - it actually mutes/disturbs the feedback a little, and adds a lot of depth to it, but I have to wait for the feedback to join in again if I do this too much.
Adding a bit of delay (echo) can reinforce the feedback (I guess it's re-playing the note that's already feeding back) and make it more controllable, and allows for more vibrato loveliness.
If you get too near the amp, you get a different kind of feedback: direct from the pickup (single coils especially) which isn't related to any notes being played, is a high pitched squeal and is a nasty sound.
Sometimes the feedback gets out of control and you can hear it's going to get way too loud too quickly. To control this, I reduce the guitar's volume knob just slightly, down to ten ;-) I tend to grab the volume control quite quickly as I know I won't need that hand for plucking strings once the feeedback has started, but don't want to go blowing people's ears with nasty piercing stuff. You can use this to
As soon as you mute or re-fret the string, the feedback is disturbed and will possibly stop so if you want to change the note during the feedback, maybe anticipate bending a string once struck & allow it to sing away at you :-)
Since feedback control is also an issue when you have microphones connected to a PA system, there are some tricks that might work or be interesting to try with a guitar and amp.
Some pre-amps have a phase inversion switch that can be used to reduce feedback. When you change your position relative to the amplifier, the length travelled by the sound changes, and so does the phase. You'd have phase reinforcement for ringing frequencies and phase cancellation for other frequencies.
Another way to deal with feedback is to modulate the sound. Slow FM and frequency shifting have been suggested, and it has been demonstrated that you can increase the gain substantially before feedback sets in using such modulation. Now, a vibrato on the guitar would be very close to the kind of FM proposed as a feedback remedy, which would explain why a vibrato should generally dampen the feedback. I suspect it's more complicated than that; a vibrato may also mechanically excite the string and keep it sounding longer.
There is a good paper about feedback control if you want to go deeper into the theoretical side of it:
A lot of folks get their pickups dipped in wax to help control feedback.
Here's a link to Premier Guitar that explains how to wax pot your own pickups: http://www.premierguitar.com/Magazine/Issue/2009/Apr/How_To_Pot_Your_Own_Pickups.aspx
Pressing the string with more force, gains more sustain, and helps the feedback process. Bending the Neck slightly but fast fx's the tone and pitch, or Bending the string elongates the tone/ Or using a vibrato on your bridge (already mentioned above). Silence + Precision! Due to positive or bad interference, it's recommended to play single notes in a precise way in order to achieve a powerful feedback. These are the analog inputs for feedback as far as I have got it.