How to Discharge a Power Capacitor in Tube Amp

Larger capacitors are typically found in the power section of amplifiers.

To be able to properly test and troubleshoot your tube amplifier with a multimeter it is important to discharge the capacitors located in the amp's power section to avoid false readings. Capacitors act as short-term batteries, storing a charge that can be called on by other components. This charge will remain in the capacitor even after the amp has been powered off, and can produce misleading voltage readings along circuit paths. Because of the high level of the stored power in the capacitor, safe discharging is a must.

Things You'll Need

• High wattage resistor
• Alligator clip
• Copper wire
• PVC tube or dowel rod
• Soldering iron
• Solder
• Multimeter

Instructions

1. Strip about 1/4 inch of insulation off each end of a 2-foot length of insulated copper wire, and solder one end of the wire to one of the leads of a high wattage resistor. The resistance required can be determined by adding 5 to 50 ohms per volt of the capacitor's rating. For instance, a 100V capacitor would require a resistor rated anywhere from 500 to 5k ohms.

2. Attach the other end of the wire to an alligator clip.

3. Solder the free lead of the resistor to a short piece of bare copper wire.

4. Tape the resistor and bare copper wire to one end of a length of non-conductive material, such as PVC tubing or a dowel rod. Make sure enough of the copper wire is exposed to act as a contact point.

5. Attach the alligator clip to one of the capacitor's leads. Be sure the amplifier is completely powered off before doing so.

6. Touch the bare wire end of the PVC tube or dowel rod to the other lead of the capacitor. It is very important to not touch any of the exposed leads or wires at this time. Hold the bare wire on the lead for at least several seconds to fully discharge the capacitor.

7. Test the capacitor with a multimeter to be sure all voltage has been safely removed.

Tips & Warnings

• Never discharge a capacitor while an amp is powered on to avoid a serious electric shock and damage to your equipment.

References

• Repair FAQ: Capacitor Testing, Safe Discharging and Other Related Information