Guitar Amp Repair


Guitar Amps first appear in the 1930's after the invention of electrolytic capacitors and rectifier tubes allow built in power supplies to replace banks of batteries in electronic systems. They were first used to amplify acoustic guitars and Hawaiian and steel guitars which were the first instruments to be totally dependent on amplification for their sound. The quest for louder guitars, having spawned both steel and resonator guitars, eventually led to the development of pickups with with small magnet cores and guitars with solid bodies to reduce acoustic feedback. Just how this came about is subject to opposing claims and accusations and is better left for a different discussion forum. Needless to say the arrival of the solid body electric guitar in th 1940's with it's total dependence on amplification the development of amplifier technology and the market for guitar amplifiers took off.

Early guitar amps were designed around vacuum tube technology and that technology is still dominant today. The earliest amps average around 15 to 30 watts of output power and used large transformers in their output circuit and their power supply. The Fender Model 26 and later the Tweed series amps are representative of this era. While the large transformers are still a part current tube amp designs power output has climbed through use of higher plate voltages, and multiple parallel high power output tubes such as the Marshall Lead 100. In the late 60's and early 70's Experimenting with solid state began. Many of the early efforts were rejected for poor tonal qualities. They were especially bad sound if you tried to use them in an overdrive or distortion situation. Some solid state amps found a niche in jazz and country where a clean sound was combined with a tremolo or chorusing circuits Like Roland's JC-120 which featured a very popular chorusing circuit.

Starting in the late 90's digital circuitry began to find it's way into the guitar amp. Digital modeling circuits and optical feedback loops have allowed guitar amps to incorporate wide varieties of effects and emulated sounds of earlier generation amplifiers and miking techniques. This has greatly increased the versatility of individual amplifiers but has also increased the complexity of the circuitry and difficulty of guitar amp repair and troubleshooting.

The net result is that in order to be able to repair the full range of amplifier technology you must understand technology ranging from the earliest tube circuitry to the most current digital signal processing technology and highly integrated circuitry.

Equipment Needed for Guitar Amp Repair

Most standard tube amp guitar amplifiers can be troubleshot with a multimeter and a dummy load. A Variac with a built current meter is an extremely useful item to have. Some technicians prefer using a breakout box with a light bulb in series with the line cord. Both of these items aid in the trouble shooting of amplifiers with shorted capacitors or transformers. A good quality soldering iron is a must as well as good quality pliers, wire-cutters, wire-strippers, screw drivers and nut drivers. A tube tester is an invaluable piece of equipment if you do a lot of tube amp repair. They are no longer manufactured so you must acquire a used one from Ebay or some other second hand source. Biasing probes that insert in between the output tubes and their socket make the process of adjusting the bias on the output circuit much easier.

Many of the same tools will be necessary for solid state and digital amplifier repair. To this list a good 100MHz or better oscilloscope should be added. Also to your toolkit should add solderwick, a solder sucker or de-soldering station, some solder flux, and some dental picks. Your multi-meter should have a semiconductor junction tester or diode setting. Proper grounding and anti-static procedures should be used.

The most important item at times is a schematic diagram of the guitar amp model that you are trying to repair. This is the road map and is often the focal point of mental processes that try to match up errant behaviors with possible explanations. Schematics for older tube type guitar amplifiers can be found in "The Tube Amp Book" by Aspen Pitman, and "A Desktop Reference of Hip Vintage Guitar Amps", by Gerald Weber. Other schematics can be found on-line as free downloads.

Safety Precautions

It is very possible and at times almost easy to damage equipment or endanger your own health and well being from the moment that you open a guitar amplifier for repair. Tube type guitar amps are particularly dangerous due to the high DC voltages that are present to feed the various tube amplifier stages. Output tube plate voltages easily exceed 600 volts in some models with 350 to 500 volts being the norm. Wear rubber soled shoes and KNOW where the voltages are and how to avoid inadvertent contact with them. Insulated tool handles are another level of protection against shock. Tube amp circuitry is pretty hardy and hard to damage but reverse the voltage on an electrolytic capacitor can have explosive results and improperly biased output stages will at the very least shorten the life of the tubes if not resulting in a meltdown.

Solid state and digital guitar amps on the other hand rely on much lower voltages although 120-240 volts AC is almost always present. The danger of personal injury is lessened but the risk of damage to the equipment is much greater. You should know and use standard grounding and anti-static procedures at all times to avoid damage to semiconductors and integrated circuitry.

Can I Fix My Own Guitar Amp?

Unless you fully understand the terms and the implications of the entirety of the above discussion, you should not even be considering working on your guitar amp. Seek a repair professional to properly and safely repair your valuable and beloved piece of gear.

Where Can I Get My Guitar Amp Repaired?

MITA technicians are tops in the field, with the training and experience to reliably repair your guitar amplifier. Find the nearest MITA technician here for further consultation or a quality repair.