From mgarvi–(at)–anix.com Fri Aug 20 22:07:24 CDT 1999
Article: 342216 of rec.music.makers.guitar
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From: mgarvi–(at)–anix.com (Mark Garvin)
Newsgroups: rec.music.makers.guitar
Subject: Treble ‘bypass’ caps on guitar volume controls (was’99 Fender NOS..)
Date: 20 Aug 1999 17:53:09 -0400
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>> Jim Collins wrote in message:
>> > The original Relic Nocaster had wiring that was more or less modern.
>> > The three position switch was neck, neck+bridge in parallel, and
>> > bridge. The tone control was a real tone control that was effective in
>> > all positions. The volume and tone pots were 250K, as they should have
>> > been. But Fender saw fit to put a rather large treble boost cap

>> Dan Stanley wrote:
>> I’m sorry, but what do you mean by this? I’m assuming you mean “treble
>> bypass”. There can be no boost from a passive component.

> Jim Collins writes:
>Boost, bypass, whatever. I realize that you cannot get a boost from a
>passive component, but I’m not sure that the term “treble bypass” fills
>the bill, either.

Just terminology, I think, Jim. I’ve usually heard that referred
to as ‘treble bypass’, but other terms would work.

>I am not opposed to using one of these bypass capacitors, at all. In
>fact, several of my guitars have them. I always thought Fender’s choice
>of cap was wrong. In my opinion it had too large a value. When I

Consider this: I often put resistors in series with the ‘treble
bypass’ (sorry) cap. The cap alone can exhibit the same peaky
effect that you get with Fender’s “Bright switch”. The actual
value of resistor will depend on a lot of things, but there are
some rough guidelines:

Find a cap that you like. Then calculate its reactance at mid-
treble frequencies…say around 2Khz or so. The equation is:

160000 / (freq * capacitance)

This is a reduction of the old 2*pi stuff. The frequency is in
hertz. The cap value is in microfarads (so in other words, your
Barden 120pf cap–which seems very low–would be .000120

The value that you end up with will be the effective ‘resistance’
that the cap presents at that particular frequency.

Ex:

For a 500pf cap (.0005 microfarads), at 2000 hz, the equation is

160000 / (.0005 * 2000)

That reduces to 160000 ohms, so the effective impedance of a
.0005 cap (500pf) at 2Khz = 160K ohms.

So…I recommend trying a 160k (150k whatever) resistor IN SERIES
with the cap. It will reduce the peaks in extreme highs, starting
at the frequency that you use in the equation. Think of that freq
as the ‘shoulder’ of the curve. Whatever’s above that will round
off more gracefully rather than continuing steep boost.

I am NOT talking about paralleling a resistor, like in some
diagrams (I think Lindy or Dimarzio site has that). That will change
the effective taper of the pot, but will have a relatively small
overall effect on frequency response.

This is all a trial and error thing. Depends on type of pickups,
value of controls, length and capacitance of cable, and of course,
your ears.

I tend to use slightly larger values of caps with slightly higher
series resistor values.

MGarvin

 

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