Pleximods.com » Safety!

Draining The Filter Caps

admin — Sat, 27 Feb 2010 22:31:46 +0000

DO NOT touch anything inside your amp chassis before doing this!

In your amplifier the filter capacitors (the large cans that stick on top of the chassis, sometimes the preamp one is board mounted) store voltages long after use, so this voltage needs to be drained, if not it can KILL.

This diagram shows the first preamp tube in a Marshall Plexi, in a normal Marshall Plexi it’s the one closest to the side edge of the chassis, and furthest from the power tubes.

Start by connecting an insulated crocodile clip to pin 1 of the tube socket, denoted by the grey line on the diagram. Connect the other end to a suitable grounding point, which in this case will be the chassis. Turn the mains and standby switches on. The voltage should drain in around a minute, but most people prefer to leave the crocodile clip connected to ground, as capacitors can re-grow voltage (like batteries)! Make sure you check that there’s no/negligible voltage left before you get to work on the amp.

Never short out the filter capacitors directly.

- Richard Baines

Bleeder Resistors

admin — Sat, 27 Feb 2010 22:19:17 +0000

One useful modification to add to an amp is to add 330k resistors in parallel to ground with all your can filter capacitors. By adding these the voltage automatically drains from the filter capacitors in minutes, and theoretically this reduces the need to drain the capacitors before working on the amp. These resistors have to be added to the amp anyway if you install Power Scaling, so if you plan on installing that at some point you can get it out of the way now and have a safer amp to boot.

<– here’s how to wire the resistors

However; don’t rely on the resistors to do their job – although it makes the amp much less likely to retain voltage, it’s still advised that you continue to drain the caps too with alligator clips just as an extra precautionary measure. If you’ve ever been shocked by an amp with residual voltage, you’ll agree that it’s worth spending the 30 seconds to do it.