FILTER CAP DISCHARGE PROCEDURE

WELCOME BACK, DIYERS!

Nearly every time we dive into an amp-related tech article, the topic of filter cap discharge comes up. And for good reason—knowing how to safely discharge filter caps is essential when working on an amplifier’s internal components.

Yes, amplifiers carry lethal voltages. Take a Music Man amp, for example—it can hold up to 700V DC! While that might be considered low current, it's still several times more than what’s needed to kill a polar bear. Fortunately, if you follow the proper steps to drain your filter caps, you can prevent a deadly (or, at the very least, highly embarrassing) mishap. Remember, amplifiers can hold lethal voltages for days after their last use, so never assume it's safe to handle without proper precautions.

To safely perform this procedure, you'll need:

• A multimeter capable of reading DC voltage up to 100V (just to be safe).
• A few basic components to build a voltage-shunt stick (a.k.a. "safety pen"—patent applied for).

We’ll cover the details on that in a moment, but first, gather the following:

• (1) 100k 1W resistor
  OR
• (2) 56K resistors run in series
  OR
• (2) 220k resistors run in parallel

The key is to achieve a final resistance of around 100k.

You'll also need:

• A small, hollow, non-conductive tube (optional)
• An alligator clip with some wire for a long lead (preferably 20” to 24")
• An old banana plug or screw that fits snugly into your non-conductive tube

Once you’ve gathered all of these materials, you’ll start by connecting the resistor(s) to the piece of wire above and soldering the resistor to the end of the wire. Now, insert this entire assembly into your non-conductive tube. You’ll want the tip to fit tightly into the tube so it does not come off when testing. Next, connect your alligator clip to the exposed wire coming out of the top of the tube.

It should look something like the images below…

If you notice, a Bic pen sleeve works excellently with most banana plug tips, hence the name “safety pen.” Cool, huh?

Now that we have a way to discharge the amp, we need to know where to discharge it. In almost all vintage guitar amps, there is something called a power supply. This is where the main high DC (B+) voltage travels and is distributed by the filter supply to various stages. This section is usually near the power transformer. If it’s not as obvious as in a Vintage Fender or Marshall amp, trace the DC supply from the rectifier to the main board—it typically passes through the filter stage first. You’ll see large capacitors, either axial (Fender) or large can caps (Marshall).

Below is a pic of the Deluxe Reverb voltage points for reference, with red arrows pointing out different high voltages in this amp. These leads come from the filter cap board underneath the chassis…

Now, set your multimeter to its highest DC reading (typically 100V or more). Apply the black lead to ground (the metal chassis). Affix this in place since you will need to use your hands for holding the safety pen and the red voltmeter probe. Then, ground the end of the safety pen to the ground as well.

For detailed discharge instructions, refer to the 5F1 Wiring Diagram and Schematic below—critical discharge points will be marked in red.

For the first discharge, make sure the amp is powered off and unplugged. Next, apply the red probe of the multimeter to the solder joint on the first filter cap positive side (refer to Point A on the diagram and schematic below). Once you have a voltage reading, touch the tip of the safety pen to that connection.

At this point, you will see the voltage reading begin to drop down. This may take a minute or so, depending on the amp. When you see the reading settle at less than a couple of volts, move on down the line to the next cap’s solder joint (Point B) and repeat the above procedure down the line to Point C. Depending upon your amp, you may have more filter cap drainage points to address.

After you’ve drained and checked all the caps, take a moment to check the voltage at various points around the amp to ensure no lingering voltages remain. If you do detect any lingering voltage, simply touch the circuit with the safety pen again.

As you can see, once you know where to look, the process is actually quite straightforward. However, if you're unsure about the discharge points in your specific amplifier, always refer to your amp’s user manual or reach out to the manufacturer before proceeding.

Above all, stay safe and always treat an amplifier as if it still holds a live charge—because the moment you let your guard down is when the real danger begins!

Thanks for tuning in, and we’ll see you next time!