Re-capping your Tube Amp

There is a lot of fist-shaking around the campfire about capacitors.
They are crucial elements in all electronics, and are commonly mis-understood.

Capacitors are passive electronic devices and, depending on their application,
can serve a number of useful electronic functions.
In the world of amplifiers, capacitors serve a few critical functions:
  • DC (direct current) coupling
    Coupling capacitors allow AC (alternating current) guitar
    signals to pass between isolated DC circuits (amplifier stages).
  • Tone / Frequency Filtering
    Capacitors can be used to filter frequencies throughout
    an electrical circuit (i.e. tone controls, amplifier "voicing")
  • Power-supply Filtering / "Power Smoothing"
    AC from the wall outlet must be converted to smooth DC to be
    effectively used in an amplifier.
    High value capacitors are placed after the rectifier circuit
    to help smooth out the DC rectification process, and are
    absolutely critical for proper amplifier function and performance.
When people refer to a "cap job" they are speaking about replacement of
certain, sometimes all, the capacitors in an amplifier circuit.
Faulty, "leaky" capacitors can cause poor amplifier performance, unwanted pops,
hums, hisses, and can even destroy tubes and transformers!
This is the power-supply filtering board from a 1971-ish Fender Bassman.
These caps were leaking DC causing poor amplifier performance.
Replacements are show in the picture on the right.


This vintage Marshall had a bad power-supply filter cap. Here is the aftermath of a
long-overdue cap job!
Capacitors degrade over time. Sometimes, they will show themselves as a hum or hiss when a certain control is used or the amplifier is run at
higher volumes, other times, they may produce an unpredictable popping sound often accompanied by a drop in volume and tone.
It is vitally important to the life of a tube amplifier to ensure that all the capacitors in the circuit are functioning properly.

Vintage amps are prone to capacitor problems. This is not the fault of the engineer(s) who designed the circuit, this is simply nature and physics at work.
As a general rule, most power-supply filter capacitors have an approximate "shelf-life" of about 15 years. Around this time they will frequently
begin to exhibit their last dying breaths, often heard by you and me as pops, unwanted hum (does anyone EVER want hum?), malfunctioning
tone controls and as general poor amplifier performance.

If your amplifier is exhibiting any of these symptoms, it may be time for a cap job (especially if your amplifier is over 15 years old!!).

PLEASE NOTE: Those large power filter capacitors can retain quite a charge for a surprisingly long time.
Do not believe the "internet gurus'" advice about pulling the plug while the amp is running and beginning work after five minutes. I have
personally observed a power-filter capacitor retain its charge for two days. If capacitors are not properly discharged before service,
serious injury --EVEN DEATH-- may result.
I'm not kidding.

If you have any doubts whatsoever about how to service your amplifier please do not opt to "learn the ol' fashioned way" as you can be
SERIOUSLY INJURED OR KILLED! Nobody wants that.


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