From mgarvi–(at)–anix.com Sat Dec 2 09:27:48 CST 1995
Article: 6039 of alt.guitar.amps
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From: mgarvi–(at)–anix.com (Mark Garvin)
Newsgroups: alt.guitar.amps
Subject: Re: Tone circuits: part 3
Date: 2 Dec 1995 05:08:07 -0500
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In <49df1h$87--(at)--nrp1.news.primenet.com> mike–(at)–rimenet.com (Mike Rejsa) writes:
>*Many* thanks to Mark for his in-depth treatment!
>Minor question: Is there any effective difference between the 250pf/0.1/.047
>Fender setup and the 250pf/.022/022 Marshall version? (Pots are different
>too, of course…)
Hi Mike, If the ‘slope’ resistor (100k in the Fender circuit) and the
treble pot (250k) stay the same, then the center frequency of the:
250pf/.047 circuit is around 295 hz with about a 19.7 db mid cut
250pf/.022 circuit is around 430 hz with about a 16.8 db mid cut
The numbers above are derived using simplified versions of the tone
circuit which I outlined in a follow-up post. No guarantee on
accuracy, since I’m rushing this.
The .022 circuit gives the bass control wider range, because like
all simple R/C nets, these are limited to 6db per octave. Placing
the notch frequency a bit higher affords about 2-1/2 octaves down
to the guitar’s low E string (82.4 hz). The standard Fender .047
circuit’s mid notch is less than two octaves up from the low E
string, so the low E cannot be ‘boosted’ be even 12 db.
The other (.1) cap (at the top of the bass pot) is primarily DC-blocking,
since the reactance is still relatively low down to bass frequencies.
Changing to .022 doesn’t make that much difference, esp in the Marshall
circuit with a 1 meg bass pot.
Nor does the value of the bass pot enter into the center frequency
calculation very much. It DOES work as a voltage divider against the
100k slope resistor, so values over 250k don’t do a whole lot of good.
In fact, if a 1 meg pot is used there, best make sure that it’s a
true log taper (1/10th of the value at half-rotation) rather than
a semi-log (usually around 1/3rd at half-rot.). Otherwise all the
bass boost will happen in the low range of the pot…no audible
effect with further rotation.
Mark Garvin
