Light Bulb Cap Formers

From cign–(at)–lato.phy.OhioU.Edu Mon Aug 28 20:06:36 CDT 1995
Newsgroups: alt.guitar
From: cign–(at)–lato.phy.OhioU.Edu (Dave Cigna)
Subject: Re: Tube amp question…filter caps
Sender: postmaste–(at)–
X-Nntp-Posting-Date: Mon Aug 28 14:12:31 1995
Date: Mon, 28 Aug 1995 18:12:33 GMT

In article <41sk9l$k7--(at)>,
Computer Digest Limited wrote:
>Putman wrote:
>: Hi , i ‘m about to replace the filter capacitors in my Hiwatt
>: Also, i remember a post long long time ago about connecting
>: incandescant lights in series with the amp in order to “soften”
>: the blow to the new caps…is this right? if so could someone
>: post the exact procedure.
> The series connection you speak of is to reduce the voltage
> seen by the new filter caps. Electrolytic caps need to be
> brought up (the first time) at reduced voltage, so the
> electrolyte inside the cap will form correctly.

The problem with this method is that the resistance of a cold
light bulb is low and increases only after the filament heats up,
but by then it’s too late!

If you don’t have a variac and want to use this method to form
caps, then use bulbs with as low a wattage as you can find.
15 or 20 watt chandelier type would be good, or maybe those big old
Christmas tree lights that nobody uses any more, or even a whole
string of new Christmas tree lights. Wire the bulbs in series with
the power cord on your amp. Remove all tubes except rectifiers from
the amp and turn it on. Monitor the DC voltage across the caps to
make sure that you don’t exceed their ratings.

Leave it on for half an hour or so and measure the AC voltage at
the primary of the power transformer (The ‘auxiliary AC outlet’
is a good place to do this). If it’s no more than about 1/3 of
the wall voltage (40VAC in the US) then you can remove half of
the light bulbs and do it again for another half hour.

Note that if your amp has an international voltage selector switch
(110/220/240 VAC mains) and you’re in the US, then you might be able
to start with the switch in the 220/240 VAC position for lower
voltage on the caps, but I’ve never tried this.

— Dave Cigna


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