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January 22, 2014

Amps › Repairs ›


Electrolytic Capacitors & Amp Sound

Like all things, instruments age with time. With age comes the need for certain upkeep in order for your instrument to perform to its utmost. In a guitar amplifier the electrolytic capacitors play a huge role in your amp’s sound and should be maintained to ensure they are helping your amp perform at its best.

electrolytic capacitor

A capacitor is essentially two conductive plates separated by an isolation layer, known as a dielectric. Electrolytic capacitors are polarized which means they have a positive and negative lead and must be positioned in a circuit correctly (most often with the positive lead supplying the voltage and the negative side connected to ground). They also have a much higher capacitance than non-electrolytic capacitors.

Electrolytic caps are an extremely important component of an amp. They are found most commonly in an amp’s power supply, as cathode caps in the preamp and in the bias circuit. In an amp’s power supply the AC voltage from the power transformer is rectified to DC and will have a great deal of ripple in the current. This ripple must be smoothed (filtered) into a more pure DC supply, or the amp would be quite noisy. The smoothing takes place in these electrolytic “filter” capacitors and gets sent off to various parts of an amplifier’s circuit as supplied voltage.

Electrolytic Filter Capacitors

The size (capacitance) of a capacitor has a huge impact on the way it performs in an amplifier.

Bass frequencies, for example, require more energy to produce than higher frequencies. Since electrolytic caps store voltage, caps with a higher capacitance are better suited for reproducing low notes. In an amp’s power supply, large filter caps result in a tighter, faster response while smaller capacitances in an amps power supply can help give an amp a softer response and more sag.

Electrolytic Capacitor Sizes

As a general rule, the lifespan of electrolytic caps in a guitar amp fall around 10 years for optimal performance. After this time the electrolytic inside the caps begins to dry out and the caps should be replaced. Worn capacitors will result in an amp with an increase in noise or hum, possible ghost notes and a definite deficiency of punch and low end. Often times the capacitor will bulge, or leak when they wear out. Other times an amplifier will have a pulsating-type oscillation known as “motorboating”. This is another sign that the time has some for the electrolytic caps to get replaced.

Electrolytic Caps Replacements

Occasionally, an amp owner will be hesitant when it comes to replacing anything on their vintage amplifier for fear it will drop the value on it, or will lose that vintage tone. The fact is when something is worn out, it needs replacing. Would you keep 50 year old brake pads on your vehicle in order to keep the car authentic? Sure the car might still stop, but it wouldn’t be performing its best, and safest. Much is the same with electrolytic capacitors in an amplifier. If the amp is being used as an instrument to play and not merely an investment piece, a recap is strongly encouraged. As a rule I always give the original parts back to the owner of any instrument that I work on. This way the owner has the original parts if they ever decided to put them back in.

Many amp manufacturers these days skimp on the quality of the capacitors they use. This can have a big impact on the performance of the amplifier. Although less expensive, the low quality caps will most likely age faster, sound worse and are much more prone to failure. I have seen many modern amps, where low quality capacitors were used, with cap leakage. When this happens it is always best to replace all the electrolytic caps, not only to upgrade their quality and the response of the amp, but also to better date how old the caps are in the amps future. If a tech sees three different caps in an amp it can become difficult to determine how old they all are.

Electrolytic Capacitors Old Caps

Although electrolytic capacitors have gotten smaller in size over the last few decades, sometimes they can still be too large to fit in a certain circuit, or you may not have the correct cap value. One solution to this issue is wiring the caps in parallel. Two 10uf capacitors connected in parallel, for example, will result in one 20uf cap of the same voltage.

Other times a higher voltage rating is needed over what the capacitor is rated for. In this case you can connect the caps in series. This method will give you half the capacitance, but double the voltage rating. In order to do this properly a resistor network should be wired in parallel with each capacitor so that the voltage flowing between the two caps is even.

When performing a cap job certain procedures are recommended. One very important step is discharging the caps before touching any part of the amp circuit. Since electrolytic caps store voltage, even with the amp off and unplugged, voltage may still be stored in the capacitors. Discharging the stored voltage will make the amp safe to work on. Another important thing to do when replacing electrolytic caps is that once the new caps are installed the amp voltage should be ramped up slowly, using a variac. This process allows the electrolytic inside the capacitors to form slowly and thus perform better.

We all get old and certain parts of our bodies wear out, but it behooves us to keep up with some regular maintenance in order to be at our best. In the same way, keeping up on the health of your amplifier and its electrolytic capacitors will help insure your instrument is sounding its best for years to come.

Brian Carstens
Brian Carstens