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Capacitor     Recapping Guitar Amps


WHY REPLACE CAPACITORS?

 

A capacitor obviously needs replacement when it has failed and is causing problems.  Whether to "recap" more extensively, however, is sometimes a matter of personal preference dictated by art, science or religion!

Electrolytic capacitors, especially early paper types, dry out over time and are ordinarily good candidates for pre-emptive replacement in elderly amps like this Fender® Princeton 5F2-A.  Failing filter capacitors in an amp's power supply can cause 60-cycle AC hum, for example.

However, indiscriminate replacement of capacitors that aren't causing problems can waste money, instigate unexpected problems, or change an amplifier's tone.

"Modders" often replace capacitors specifically to modify an amp's performance or tone.  The capacitance is either increased, decreased, or removed entirely.

Let's look, one by one, at the four functions that capacitors perform in a guitar amp:  (i) power supply filtering, (ii) cathode bypassing, (iii) stage coupling, and (iv) tone shaping.

 

I.  POWER SUPPLY FILTER CAPACITORS

 

Filter capacitors are used in an amp's power supply.  They store electricity during the forward surge of current from your alternating house current, then feed that electricity to the amplifier when the current reverses.

The "primary" filter cap stores electrical charge for the amp's output stage.  It's the first capacitor in a line of caps fed by the "rectifier" (the vacuum tube in the schematic below).

The filter caps are outlined in blue.  Point "A" is the primary filtering point.  In this case, two 16μF capacitors are wired in parallel, making the total primary capacitance equal to 32μF.

Subsequent filter caps, at points "B" and "C" in the schematic, store charge for other circuit stages.  In between the filter caps, resistors are used to decouple the stages, so they don't interact.  They also prevent overloading of the rectifier tube.

Vacuum-Tube Power Supply

Power Transformer, Full-Wave Rectifier Tube, Filter Capacitors

 

All the filter caps are "electrolytic" types.  An electrolytic capacitor contains an electrolyte (an ionic conducting paste) that causes an oxide coating to form on the capacitor's positive plate.

This thin coating serves as the insulating layer between the two foil plates.  Electrolytic capacitors can provide large capacitance values in a small package.

Electrolytic caps are "polarized", meaning they can only be operated with a lower voltage on the terminal marked "-" without damaging the capacitor.

 

Decreasing the Filter Capacitance

 

If you decrease the capacitance of a power-supply filter capacitor, less charge is stored and available for the circuit it powers.  This causes the supplied voltage to sag during peak demands like a percussive bass note or sudden chord.

When the supply voltage in an amplifier circuit drops, the gain goes down.  Then, after the dip, the gain goes back up again.  This gain compression can be musically desirable, improving the "feel" of the amplifier as it self-adjusts to the player's attack.

But if you decrease the filter capacitance by too much, ripples from the alternating house current aren't fully filtered and can pass through to the amplifier circuit, producing an audible 60Hz hum.

 

Increasing the Filter Capacitance

 

If you increase the capacitance of a filter cap, a lot of charge flows into the capacitor instead of the amp, sending the energy into the electrostatic field surrounding the capacitor.  As a result, the supplied voltage is steadier but lower.

As mentioned above, a steadier voltage means less gain compression, yielding a truer, less spongy or marshmallow response.  On the other hand, a lower voltage supply will cause signals to clip sooner.

A steady voltage also improves a circuit's bass response.  That's because long, low-frequency waves need to draw more energy from a capacitor than short, high-frequency waves.  So a tighter, more prominent bass is theoretically possible with bigger filter caps.

If you want higher fidelity from your guitar amp, you might try larger filter caps for the preamp stage (point "C" in the above schematic).

Sometimes, however, an increased bass response can make an amp sound woofy, waste power on frequencies the amp can't produce, or make the amp unstable.

Too much primary filter capacitance can potentially damage an amplifier by drawing too much current through the amp's power transformer and rectifier tube.  For this reason, the primary filter caps aren't usually a target of modders.

 

II.  CATHODE BYPASS CAPACITORS

 

A "cathode-bypass" capacitor short-circuits signal current around the resistor that's attached to the cathode of a vacuum-tube.

A certain preset current flows through the cathode resistor in order to create a bias voltage between the cathode and the control grid of a tube.  This bias voltage lets the tube operate in a region where its plate current is in fairly direct proportion to its plate voltage.

If fluctuating signal currents aren't bypassed around the cathode resistor, then the bias of the tube and therefore the linearity of the amplification will fluctuate, distorting the signal's dynamics.

The signal's waveforms will also distort.  As the wave goes more positive, more bias is created, lowering the tube's gain.  Then, as the wave goes negative, less bias is created, raising the tube's gain.  In other words, wave peaks are flattened and valleys are deepened.

For these reasons, cathode bypass capacitors can be important.  In the schematic below, the cathode bypass caps are outlined in blue.  Points A, B, and C connect to the power supply schematic above.  Unless they're smaller than 1μF, cathode bypass caps re usually electrolytic types.

Functions of Capacitance in a Tube Amplifier

Cathode Bypass, Coupling, and Tone Capacitors

 

Changing the Bypass Capacitance

 

If you decrease the capacitance of a cathode bypass capacitor, lower frequencies can no longer pass through it.  Instead, they flow through the cathode resistor, increasing the bias and decreasing the gain of the low frequencies.

So, a smaller bypass capacitor gives you less bass and more apparent treble while a larger bypass capacitor gives you more bass with less distortion.

 

III.  COUPLING CAPACITORS

 

A coupling capacitor passes AC signal from one stage of a guitar amp to the next, while blocking any direct current from flowing.  In the schematic above, the coupling capacitors are outlined in red.

As you decrease the capacitance of a coupling capacitor, only the higher frequencies are passed along, and so the bass response decreases.  Conversely, increasing the coupling capacitance can yield more bass response.

 

IV.  TONE CAPACITORS

 

Tone capacitors are outlined in green in the above schematic.  While the previously mentioned uses of a capacitor influence tone as a side effect, the sole function of tone capacitors is to shape the tone.

The particular effect of changing a tone capacitor depends on its placement in the tone circuit.  There are many tone circuit designs, including the simple one in the above schematic.

Resistors also play a role in tone circuits.  Capacitance and resistance substitution boxes like those shown below can assist you in more quickly exploring the effects of altering tone circuit values in your amp. 

Capacitance and Resistance Substitution Boxes

Substitution Boxes for Capacitors and Resistors



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