Intro Guide to Amplifier Capacitors

Welcome to the ultimate guide to amplifier capacitors!  Capacitors are an integral component to how guitar amplifiers work and sound.  In this article, we will cover several different aspects of capacitors including: types of capacitors, capacitor ratings, how to safely discharge a capacitor, capacitor tone, and how to identify and replace a bad capacitor.  Enjoy!

What is a Capacitor?

The best place to start is always at the beginning.  A capacitor is one of the many electronic components found inside your guitar amp.  They have two leads for signal to come in and out and are comprised of both a conductor and a dielectric.  A conductor will pass current flow while a dielectric will not.

In its most simple form, a capacitor is a component that holds a charge.  You can almost imagine it as a little passive battery.  Capacitors can also be constructed to only hold certain charges.  The ability to do so, has multiple uses within a guitar amplifier.  This ability to hold a charge, allows our circuit to complete different required tasks.

So how does a capacitor hold a charge?  This is where the combination on the conductor and dielectric come into play.  Imagine two conductors placed together with a dielectric in between.  By inducing a charge to one of the conductors, the charge will be inducted into the adjacent conductor.  This internal induction has the ability to hold a charge and discharge it as well.  There are many resources available if you want to get really in depth with how capacitors work, but some cursory knowledge is completely adequate for understanding guitar amplifiers.

Types of Capacitors

There are many different types of capacitors, and you will find two key varieties within an amplifier.  Signal capacitors, which are in the path of your guitar signal, and filter capacitors, which are in the power supply.  These capacitors are the most common, and your amp will likely have many of them.

Signal capacitors do not all look the same however.  Many are long and cylindrical in shape, while others are circular disks.  While these two types of signal caps are doing the same job, they have different compositions to work on different frequencies of your guitar signal.

The second main type of capacitors in your amp are the power caps.  These are also known as electrolytic capacitors.  These capacitors work a little differently from signal caps and you usually do have quite as many in your amp, though they are still very common.  These caps can be very large in size, while others are much smaller.  The main difference with power capacitors is that they work on the power signal from your wall outlet, not your guitar signal.

We now know that capacitors can hold a charge.  The electrolytic capacitors described above can hold an extremely large charge, that can be potentially lethal.  We will explain much of this later in the article.

Styles of Capacitors

Capacitors can be comprised of many different materials and be built in different ways.  It is most common to find them in three different styles.  The most common in guitar amplifiers are axial capacitors.  These caps have one lead connected to one side of the body, and another lead connected to the opposite.  I always think of a car axle to remember this orientation.

You will also find radial capacitors in guitar amps.  Radial caps have both leads attached to the same side of the body.  These capacitors are most commonly used on printed circuit boards due to their easy use with machinery.  There is no real advantage or disadvantage to either style, but it is important to be able to identify their differences.

Lastly are cap cans.  Cap cans look like very large radial capacitors with multiple leads all attached to the same side.  Cap cans are actually multiple capacitors that share a housing.  It is not just one large cap.  Often times these cap cans can have up to four capacitors stuffed inside.  These are also electrolytic capacitors and are used in the power supply.  Cap cans are quite common if vintage and boutique amps.

Capacitor Ratings

Like all electronic components, capacitors have rating that either describe a feature of the cap or a requirement that must be adhered to.  There three main ratings found on capacitors are farads, voltage, and polarity.  Farads are a unit of capacitance, much like how ohms are a unit of resistance.  Typically, amp caps are rated in microfarads, uF, or picofarads, pF.

Capacitors are also given a voltage rating.  This is the maximum amount of voltage that can be applied to the capacitor and still have it function within its specifications.  If voltage in excess of the rating given is applied to a capacitor, the cap may not function correctly or even completely fail.  It is important to always abide by the voltage ratings on capacitors.  It is ok to use a cap with a higher rating but it is never ok to use a cap with a lower rating that what the circuit calls for.

Some, but not all, capacitors will display polarity as well.  Typically, this is a feature of the electrolytic caps used in the power supply.  They have a positive and a negative terminal.  It is always important that these capacitors are installed with the correct polarity.  Usually, the positive lead will be attached to the power supply while the negative lead will attach to ground.  However, this is not always the case.  Some circuits, like the bias supply in your amp, actually have an electrolytic cap wired in reverse.  If electrolytic capacitors are installed with incorrect polarity, they can explode!

How to Safely Discharge a Capacitor

As I mentioned before, tube guitar amplifiers are capable of storing lethal voltages inside these capacitors well after the amp has been turned off.  If you are uncomfortable working on these devices, please consult a qualified amplifier technician.  However, if you are interested in working on and building your own amplifiers, here is how to properly and safely discharge capacitors.

First, insure you have turned off the amplifier and it is unplugged from the wall.  Then, double check that you have unplugged it again.  Yes, I’m serious.  Next, you must know which capacitors are storing the dangerous voltage.  It is in fact the electrolytic caps.  Now you need some equipment.  You will need an insulated jumper wire with alligator clips on both ends.  You will also need a metal screwdriver with a plastic handle.  Clip one end of the jumper to the screwdriver and the other directly to the amp chassis.

Now you are ready to discharge the capacitors.  First, take your non-dominant hand (usually your left) and put it in your pocket.  Amplifiers are much more dangerous when you offer them both hands and a path across your heart!  Then touch the positive terminal of the capacitor with the screwdriver.  This will short the positive side of the cap to ground.  Be prepared that this may produce a spark!  If you want to eliminate the chance of a spark, you will need to create a custom jumper cable with a resistor in series.  Go through each electrolytic capacitor one by one, and ensure they have all been discharged.  As I said above, if you are uncomfortable or uncertain in performing this, please take your guitar amplifier to a qualified technician.  It is not worth the risk!

Replacing Amplifier Capacitors

It is quite common to hear about people in the audio world talk about replacing capacitors or “re-capping.”  But why would you need to do this?  There are usually two reasons: a capacitor has failed and is no longer working properly.  Or, the owner is looking to upgrade the tonal qualities of said piece of gear.

It is usually fairly easy to spot a capacitor that has failed.  There can be a bubble forming under the surface of the cap, or you can actually see the capacitor leaking its electrolytic fluid.  Both of these are common for vintage amplifiers and will ultimately always happen.  It is also possible to see dark spots or burn marks if a capacitor has shorted.  Sometimes however, a capacitor can fail with no visible clues at all, so always be sure to perform thorough and accurate trouble shooting.

When upgrading capacitors there are two things that techs are looking to achieve, tone and feel.  Changing the signal caps will affect the tone while changing filter caps affect feel.  It is important to note that most of the time you want to always replace a capacitor with another of equal ratings.  There are some instances though when altering the ratings is appropriate.

Will New Capacitors Improve My Tone?

In some instances, yes, hands down it will improve your tone.  Other times just slightly, and other times not at all.  It really depends on what you have to begin with.  One, if your current capacitors are old and no longer functioning correctly, you will definitely see improvement.  Two, if your caps are fine, but maybe a little on the cheap side, you will likely see improvement, but it won’t be ground breaking.  Three, if you already have good quality caps, I wouldn’t waste my time.

Signal caps are where most people seem to turn first when swapping capacitors.  This can certainly help your tone but there is one important thing to consider.  Swapping out only one capacitor will have little to no effect.  You must replace all of the signal caps to hear a noticeable improvement.  Don’t let this deter you though, you can have a really killer sounding amp once completed!

Changing power filter caps is actually where you will probably see the most change.  These capacitors are responsible for how your amp feels.  By changing these caps, and even slightly increasing the rated capacitance, can really bring an amp’s tone to the forefront.

How to Replace a Capacitor

The first step when switching out any component is always the same: make sure your amp is powered off and unplugged.  Then check again.  After you have the chassis out and supported well, ensure you have correctly identified the capacitor you want to replace.  Get all your tools and materials ready such as pliers, solder, iron, etc.  Proper preparation is always a key step when performing amplifier maintenance.

Begin with one of the solder connections and apply heat with your iron.  In the other hand, have a pair of needle nose pliers ready.  Once heated, you will see the previous solder flow.  When you see this, you know the old component can be removed with the pliers.  Be sure to not tamper with any other components that might be soldered to this same joint.  Once the capacitor lead is removed, heat the joint again for solder removal.  When the solder flows, use a solder sucker or braid to remove the old solder.  Clean the joint afterwards with electronics cleaner spray and electronics swab.  Repeat on the opposite side.

Now we can solder in the new capacitor.  Proper mechanical fit is always the most important aspect of a quality solder joint.  Be sure the leads fit snug to terminal or turret, and ensure leads are properly trimmed.  From here, you can go ahead and solder in the new cap as usual.  For more information on how to properly solder, check out my soldering guide here.

Conclusion

I hope you have enjoyed this quick introductory guide to amplifier capacitors!  Here is a summary on what to remember.

Capacitors are designed to hold a charge, and they have multiple jobs throughout your amp.  The two main types are signal and electrolytic capacitors.  They come in three different styles: axial, radial, and cap cans.  Capacitors are marked and rated for farads, volts and polarity.  You must properly discharge capacitors to ensure you are safely working on your guitar amplifier.

It is common to change caps if a previous one has failed, or if you are looking to improve your tone.  Changing capacitors might improve your amp tone, or it may not.  Finally, replacing capacitors is a fairly straight forward process if you take your time to clean the joint and solder correctly.

Thank you for reading and be sure to check out some of my other amp articles!

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