There are several ways to increase sustain and/or obtain controlled feedback. However, it must be stressed that feedback occurs in a system at a point of high gain and resonance, which does not always coincide with where you want it.

For example, high pitch whistles often occur when a guitar pickup is situated near speakers/horn drivers due to magnetic coupling, ie. between the coil in the pickup and the one in the speaker. The guitar pickup is usually electrically resonant at a high audio pitch so of course that is where it 'screeches. Acoustic feedback also occurs easily on hollow bodied instruments where the body is tuned, acoustic guitars for example feedaback at bass frequencies. That is why many 'acoustic' amplifiers incorporate 'anti-feedback' devices like tuned notch filters.

Anyway, here are a few ways to increase sustain/feedback starting with the most subtle. Use in combination and in moderation.

1. Use heavier gauge strings. They vibrate for longer so require less feedback to hold a note.

2. Make the guitar as rigid as possible. This particularly applies if you have a bolt on neck. Make sure the screws are tight or this will cause reduced sustain of the string.

3. Use hard materials for the Bridge and Nut parts that are in contact with the string. For my own strat, I made a new Nut out of an old brass tap. This replaced the plastic one supplied with the guitar I bought in 1972.

4. Lower the pickups to increase the distance between the pole pieces and strings. Some people raise the pickups to produce maximum drive to the amplifier. Unfortunately, pole pieces are magnetic, and close proximity to strings will dampen their vibration.

5. Use a high output pickup. Seymour Duncan have a strat single coil replacement that is very resonant. Easy to get feedback just above middle pitches but at the expense of losing the high 'top' you expect from strat's.

6. An old method is to use a treble boost before the amplifier (or effects unit) to overdrive it.

When an amplifier overloads it clips the signal, high overload simply causes the output waveform to eventually switch between two voltage levels of a value determined by the internal power supplies. This is known as going 'dc' and can easily damage Hi-Fi's. However, good guitar amplifiers are designed to work with this.

The audible effect is an increase in bass, going woolly and harmonic distortion. The addition of the treble boost counteracts the Bass and improves 'clarity' and rich harmonics. This in turn increases the likehood of feedback where you want it (if not overdone).

All the above can maintain the character of your guitar when used in moderation. However, if the priority is just a long note then obtrusive effects and other ways can be used. Sometimes the amplifier itself just needs more 'ooomph' to get attitude and sustain out of it. Ways forward are:

1. Add a couple of cheap piezo tweeters. Put these in a small box on top of the amplifier and parallel with the main speaker. This will cause a MAJOR difference in sound. Use two Maplin RT60Q or CJ83E wired in series, these are under fifteen pounds the pair and can really add life to an old amplifier.

2. An effect like the Line 6 POD can provide really good overdrive and simulates the sounds of different amplifiers. At low volume you can have long sharp sustain like Gary Moore, or select Marshall overdrive for the Status Quo sound. Unfortunately, it has several shortcomings. i) You get someone else's sound. (But you may not care) ii) There is no bypass switch. iii) It generally has a poor 'low drive' sound. iv) It has no high 'top'. A side effect of the way the internal technology is used. Sampled sounds in some keyboards also have this problem.

3. Try using an 'aural exciter' in conjunction with (or in place of) the amplifier treble control. You get a lot more harmonics that help induce feedback at 'higher' places on the guitar neck.

4. The ultimate is to use a guitar to midi convertor with synth module. Just press the hold button after hitting a note and it will continue until the neighbours start bashing at the front door. (Been there, done it, etc..)

Permission to reproduce this article from Bob Houlston:
http://www.houlston.freeserve.co.uk/feedback.htm