Forming Capacitors

From cign–(at)–elios.phy.OhioU.Edu Sun May 21 12:29:41 CDT 1995
Newsgroups: alt.guitar
From: cign–(at)–elios.phy.OhioU.Edu (Dave Cigna)
Subject: Re: Tube amp question
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Date: Sun, 21 May 1995 15:42:35 GMT

In article <3pmvda$cb--(at)--yra.csx.cam.ac.uk>,
J.K. Rabone <93jk--(at)--ng.cam.ac.uk> wrote:
>In article <3pmksj$hi--(at)--ewsbf02.news.aol.com>, JJW10 wrote:
>>It’s a device that allows you to control the amount of current going to
>>the amplifier. Some people use them like a power soak, but it’s hell on
>>the transformer and the tubes.
>
>A variac is a variable type of autotransformer – it allows you to
>transform the mains voltage your amp is running off to a (usually
>lower) value. The reasoning is that you’ll get more break-up &
>distortion without excessive volume at the lower supply voltage. You
>could also use one in the output of the amp, to reduce the signal
>going to the speakers (the power soak idea). However, this is likely
>to upset the power stage & output transformer as jjw10 pointed out.

I definitely do NOT recommend using a variac between your amp and speaker;
it drastically changes the impedence that the amp sees. Don’t do it.

>You shouldn’t _need_ one to alleviate switch-on surges, although it’s
>a good way of doing so. The filter caps should be rated to handle the
>maximum inrush current – usually the tube rectifier stage (if fitted)
>limits the inrush anyway.

Electrolytic capacitors need to be “formed” before they are used. Here’s a
look at what’s going on: if you remember your freshman physics you’ll
recall that a capacitor is just two conductors with a thin piece of dielectric
(insulator) between them. The capacity of the device is proportional to e*A/d
where e is the dielectric constant (depends on the insulator), A is the
area of the conductors, and d is the distance between them. When you charge
a capacitor you’re storing energy in the electric field between the conductors.
The capacity of a capacitor is the amount of energy per volt that can be
stored. For power supply filtering we want big capacity, so we need to choose
an insulator with a big dielectric constant (big e), make the area of the
conductors very large (big A), and make the thickness of the insulator
separating them very thin (tiny d).

In electrolytic capacitors the two conductors are different materials. One
is just a piece of metal – usually aluminum – and the other is a liquid
electrolyte. When a voltage is applied between them a layer of oxide forms
on the metal. This layer acts as the dielectric insulator and is extremely
thin (measured in molecules instead of millimeters. The extreme thinness is
the reason that electrolytics have capacities so much higher than other types.

New capacitors don’t have the layer of oxide: it’s formed the first time the
capacitor is charged. If it’s done too quickly the layer won’t form properly
and you’ll end up with a weak capacitor. I always bring them up to full
voltage very slowly – over a period of about two hours. I’ve heard of some
people taking six or more hours. I have no idea how long it really takes to
form the layer properly, but I’m quite certain that just plugging them in
isn’t long enough. Gerald Weber (_A_Desktop_Reference_To_Vintage_Guitar_Amps_)
suggest wiring a 100 watt light bulb in series with the amp (between the
wall plug and the amp). When you plug everything in the bulb reduces the
current to the amp. This is surely better than just plugging in, but I don’t
know if it’s good enough. What worries me about this method is that the
current is still very how at the moment that power is applied. This is
exactly when you want to go slow.

One last interesting fact is this: when the capacitor is not charged the
oxide layer slowly dissolves leaving aluminum hydroxide in the electrolyte.
A new layer is formed when the capacitor is charged again. This process is
slow, taking months or years, but if it is done repeatedly the electrolyte
becomes saturated with aluminum hydroxide. This is why the caps last longer
in amps that are used everyday, but are weak in those pristine but rarely
used vintage amps.

Hope this is interesting to somebody.

— Dave Cigna

 

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