Tag Archives: capacitor discharge

Replacing/discharging capacitors?

While searching for stuff about my Marshall 2204, I happened upon a fantastic source of information; The Hoffman Amplifier forum, hosted by EL34 World. I saw that there was an ‘introduce yourself section’ so I posted this: Hello from Chicagoland

A forum member was kind enough to point me in the right direction (the Hoffman Amp store) after suggesting I replace the ‘electrolytic capacitors’ rather than replace the output tubes.

So from Hoffman, I’ve ordered 3 50-50 500v ‘can’ capacitors. The logic being, the Daly capacitors on the amp are original and therefore very old. They might be the source of all the noise/hum when the amp is even slightly cranked (in the sweet spot, so to speak). I’ve also ordered parts to make a capacitor discharge probe. It seems there’s no consensus on how to do this. Techniques vary from taking a screw driver and shorting the caps to the chassis. This can’t be particularly safe, although it’d be dramatic. Other methods involve essentially the same thing, but using a resistor in series to slow the rate of discharge. But nobody can agree on the value of such a resistor. I have found these variants:

  • 100k Ohm, 5W
  • 470 Ohm – 2k Ohm, 2 Watt

I have ordered a 5k, 5W wirewound resistor. And I already have some clips and heat shrink with which to make the tool.

I might eventually make one of these though: Capacitor Discharge indicator. It implies that “finishing it off with the ol’ screwdriver” is still a good idea once voltage drops below lethal levels.


My good friend Tom Reid has some insights on this:

My standard discharge tool is a very large Craftsman screwdriver. It worked
well for picture tubes over the years. The added resistor to a discharge probe creates a current through to discharge procedure. For best results, discharge through a standard incandescent light bulb, as the filament is both resistive, and inductive so as to absorb the shock.


Finally I found a page where someone quantifies the value of the resistor one might use:

It’s smart, then, to discharge the unit deliberately, through a resistor equal to about one ohm per volt of charge.

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