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

Stompboxes circuits published in magazines, books or on DIY electronics websites.

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

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  Thanks!
gigelmargel
Resistor Ronker

Posts: 260
Joined: 16 Jul 2007, 21:57
Has thanked: 17 times
Have thanks: 15 times

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

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

For this message the author ppluis0 has received thanks:
gigelmargel (09 Aug 2019, 09:01) ppluis0
Cap Cooler

Posts: 602
Joined: 14 Jul 2010, 19:33
Location: Argentina
Has thanked: 44 times
Have thanks: 90 times

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

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!
gigelmargel
Resistor Ronker

Posts: 260
Joined: 16 Jul 2007, 21:57
Has thanked: 17 times
Have thanks: 15 times

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

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.

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 ppluis0
Cap Cooler

Posts: 602
Joined: 14 Jul 2010, 19:33
Location: Argentina
Has thanked: 44 times
Have thanks: 90 times

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

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.

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,
You do not have the required permissions to view the files attached to this post.
gigelmargel
Resistor Ronker

Posts: 260
Joined: 16 Jul 2007, 21:57
Has thanked: 17 times
Have thanks: 15 times