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.

Tagged , , , , ,

One thought on “Replacing/discharging capacitors?

  1. Roly says:

    Caps have a ripple current rating, and generally speaking no current through them should exceed this rating. These days I avoid, and advise against, screwdriver discharge a) because it isn’t good for the cap, b) because it isn’t good for the screwdriver, and c) because it normally sprays globs of molten metal which aren’t good for your eyes. It’s also a very bad habit to get in to if you work on big S.S. rack amps as these will happily chew the end off your favorite screwdriver. A standard 240V light globe of just about any wattage is a suitable discharge device (‘tho in 115V countries you may need two or three in series).

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: