How to calculate the resonant frequency of a GCB-95?

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How to calculate the resonant frequency of a GCB-95?

Postby gigelmargel » 08 Aug 2019, 16:29

Hello!

I want to begin to modify a Dunlop GCB-95 wah, but I don't know how to calculate the resonant frequency. To be more clear, I read in the GCB-95 manual the following informations:
Filter Parameter
Low Pass 350 Hz ~ 450 Hz
High Pass 1.5 kHz ~ 2.5 kHz


Looking at the schematic below I have no idea where these frequencies are coming from...I tried different LC/RLC formulas, but with no luck :shock:

Image

Anyone can help me, please?

Thanks!
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Re: How to calculate the resonant frequency of a GCB-95?

Postby ppluis0 » 09 Aug 2019, 02:46

Hi gigelmargel,

The resonant circuit is comprised by the inductor and the 10nF condenser, named C7 in the previous schematic.

BUT the value of this condenser is constantly varied due a buffer action from transistor Q3 according to the potentiometer (hence: pedal) position.

Take a read at this link which explain in detail how a wah pedal works and suggest some mods also: https://www.electrosmash.com/crybaby-gcb-95

Another source of information belongs to RG Keen: http://www.geofex.com/article_folders/w ... wahped.htm

Cheers,
Jose

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gigelmargel (09 Aug 2019, 09:01)
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Re: How to calculate the resonant frequency of a GCB-95?

Postby gigelmargel » 09 Aug 2019, 09:01

ppluis0 wrote:Hi gigelmargel,

The resonant circuit is comprised by the inductor and the 10nF condenser, named C7 in the previous schematic.

BUT the value of this condenser is constantly varied due a buffer action from transistor Q3 according to the potentiometer (hence: pedal) position.

Take a read at this link which explain in detail how a wah pedal works and suggest some mods also: https://www.electrosmash.com/crybaby-gcb-95

Another source of information belongs to RG Keen: http://www.geofex.com/article_folders/w ... wahped.htm

Cheers,
Jose


Thank you very much, Jose! I've read both articles 3 times before ask here, but my theoretical knowledges in electronics are very subtle, so I didn't understood too much. So, I need a clear formula to calculate the resonant frequency.

To be more specific, I have a Zakk Wylde Wah which I want to modify in a Dimebag Wah. The main problem is that ZW Wah has a 500 mH inductor, while in DD Wah the inductor is 535 mH, so I must to recalculate all the 6 values of C7.

Thanks to all!
Radu
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Re: How to calculate the resonant frequency of a GCB-95?

Postby ppluis0 » 09 Aug 2019, 15:12

gigelmargel wrote:I have a Zakk Wylde Wah which I want to modify in a Dimebag Wah. The main problem is that ZW Wah has a 500 mH inductor, while in DD Wah the inductor is 535 mH, so I must to recalculate all the 6 values of C7.


Hi Radu,

I didin't know exactly the differences between these two models. Can you post the schematics ? Both models have the selector switch related to C7 ?

If this is the case: the 6 condensers have the same values in both wah versions ?

If we do the math, the quotient of 535mH/500mH is 1.07 so the condensers (if were of the same values only) will be multiplied by the same factor.

Cheers,
Jose
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Re: How to calculate the resonant frequency of a GCB-95?

Postby gigelmargel » 10 Aug 2019, 10:10

ppluis0 wrote:
gigelmargel wrote:I have a Zakk Wylde Wah which I want to modify in a Dimebag Wah. The main problem is that ZW Wah has a 500 mH inductor, while in DD Wah the inductor is 535 mH, so I must to recalculate all the 6 values of C7.


Hi Radu,

I didin't know exactly the differences between these two models. Can you post the schematics ? Both models have the selector switch related to C7 ?

If this is the case: the 6 condensers have the same values in both wah versions ?

If we do the math, the quotient of 535mH/500mH is 1.07 so the condensers (if were of the same values only) will be multiplied by the same factor.

Cheers,
Jose


Hello, Jose!
ZW Wah has no selector switch related to C7 (it has the fixed value of 22nF). The DD Wah has a 6 postions switch for C7 values. As you can see in the schematic, it's a little strange wired: at a fixed value of 1.2 nF are added the 6 given values.

Dunlop_Dimebag_wah.jpg


Anyway, your idea with the multiplication factor seems to sound good :)

Best regards,
Radu
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