Discharging capacitors


When any mains-powered device is connected to the mains supply, there are many safety hazards to consider while you're working on it. In many cases, disconnecting the device from the mains supply (by removing the plug from the wall socket) makes the device safe to work on. Valve amplifiers are an exception: even when they are disconnected from the mains, some of the internal components can store potentially fatal electric charges. This added hazard may remain for days in some amps.
      The risk comes from the large capacitors which act to filter the high voltage supply. When the amplifier is switched on, these caps are charged up to the full working voltages of the amp - around 360V for the PP-18, and 500V or more for some designs. When the amplifier is switched off, these capacitors act as a reservoir of electricity. In effect they have stored some of energy supplied by the mains, but that has not yet been used by the rest of the amplifier circuit (see Hear for yourself, right). Inadvertently touching a charged-up filter capacitor can kill, so it's essential that you check for these stored charges before you work on any amplifier.

Check first

Whether you're checking an amplifier that you've built, servicing a commercially made amplifier or testing an old wreck bought from a car-boot sale or junk shop, your first test when going into the chassis must be to check for stored charges. Your multimeter is the right tool.
      Set the multimeter to its highest DC voltage range and carefully place the probes across the first filter capacitor. You must take care not to accidentally touch any part of the circuit with your hands as you move the probes into position; on Amp Maker kits, the turret board is laid out to make the filter caps easily accessible, but take extra special care when checking 'rats-nest' and more crowded chassis.
      If you get a reading of over 10V, it's advisable to discharge the cap (see below). Repeat the process for the other filter caps in the amplifier. Generally, if one filter cap has a voltage across it, they all will. However, you can take nothing for granted, so follow the circuit, looking for and checking all of the filter caps one-by-one.

How to safely discharge capacitors

In all Amp Maker kits, the circuit is designed so that the al of the amplifier's caps discharge by themselves. This happens because the circuit includes a bleed resistor specifically added for the job. If you read a voltage across one of the filter caps on these amplifiers, the voltage will drop quite quickly with each second. Leave the probes in position and you can monitor the safe discharge.
      But for amplifiers that don't have a bleed resistor, the filter caps discharge much more slowly. To accelerate the rate - so that you can safely work inside the chassis - you can use a discharge lead (right). This is a temporary bleed resistor - by attaching it across the charged-up filter cap (taking great care that you don't touch the cap's terminals with your fingers as you attach the croc clips), you provide a quick discharge route for the energy stored in the cap. Check the voltages again and then repeat for other caps if necessary.
Hear for yourself
If you want to hear evidence that there's a significant amount of energy stored in the filter capacitors when an amplifier is turned off, you can test it out. Play your amplifier for a little while and then switch off the amplifier's power switch.
      Carry on playing the guitar and you'll hear that the sound continues for a few moments, decreasing in volume and getting more fuzzy as it dies away into silence. This sound output is only possible because the energy stored in the filter capacitors 'reservoirs' is available to the amplifier circuit to use to amplify your guitar signal. Eventually the charge is used up and the amplifier falls silent.
      The sound output may not last long, and it may sound weak and fizzy, but don't underestimate it. The energy stored is more than enough to kill you if you work carelessly around charged-up capacitors.


Take two insulated croc clips, some hook-up wire, a 10K 2W resistor and some heatshrink sleeving and you can make a temporary bleed resistor for amplifiers that don't have them built in.


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