Inside a guitar amplifier are circuits, switching mechanisms, and connections that can hold potentially lethal voltage at several points in the critical path. A very important thing to understand with guitar amplifiers is that these components can hold lethal voltage even when the amplifier is turned off.
Capacitors store high voltages that can remain in the amp long after you’ve last played it or powered it on. If you don’t have a thorough understanding of what you are doing when interacting with these components, it is very likely that the voltage will be released through you – which could impede your future enjoyment of the amp due to being dead. Opening up an amp without following safety precautions is one of the more dangerous things you can do in life and should be taken very seriously.
If you have no prior experience handling the electrical components of an amplifier or do not have a thorough understanding of the proper ways to maintain safety and mitigate risk, you should not attempt to open up your amp, and any repairs should be handled by a professional. You can often google “tube amp repair” + (your city) and find a repair shop near you.
This article is not intended to provide any holistic or end-to-end instruction and should only be used for informational purposes.
The basic steps to discharge a capacitor involve attaching the leads of the capacitor to a resistor that will enable voltage to drain out.
Measure With A Multimeter
You have to start by understanding how much voltage is contained in the capacitor, which means busting out your multimeter.
Set the multimeter to its highest DC voltage setting.
Next connect the multimeter probes to the two posts sticking out of the top of the capacitor. Hold the leads on the post while the multimeter measures the voltage.
A general rule of thumb is that anything over 10 volts is dangerous enough to harm humans.
What Size Resistor To Use?
Ohms Law states that current is inversely proportional to resistance. You want to use a resistor that is several megaohms if you are dealing with a large amount of voltage – this way, the current resides within the bounds of the resistance. If you use too small of a value of resistor, then a dangerous current would be generated.
Connect the resistor to the capacitor
Attach the meter leads to the resistor so that you’re able to measure the voltage, and use alligator clips to attach one end of your connector wires to the resistor.
Then, take the other end of the wires and attach to the posts of the capacitor. Remember, the capacitor is still live and you should definitely not touch the posts with your bare hands.
Once you’ve connected the wires to the capacitor, you should see the display on the multimeter begin to go down. How long it will take to complete the discharge depends upon the amount of voltage stored in the capacitor.
When the multimeter gets down to zero, you can disconnect the capacitor from the resistor and then go on with your day.
Here are two videos that I recommend because of how simple to understand the instructions are. The first is from DIgiKey:
The second is from Blueglow Electronics: