# Setting Q for Mercurial Boost?



## bn40 (Jun 8, 2020)

Looking at an ebay review of the new Chase Bliss super duper preamp, it occurred to me that some of the functionality--semi-parametric mids either pre-or post-gain and three Q settings--can probably be had very easily by modding the old Mercurial. The first thing, assuming a big enough enclosure and careful fitting, can be achieved with an extra couple of jacks and a 3PDT toggle--basically an FX loop for your other gain pedals and an order switch so that the Mercurial is either before or after them. I think this would be a very useful mod, since this pedal obviously has its uses at both ends of the signal chain. Since I currently have mine in a 1590B, I think I might be able to pull it off in a 125B.
But the second part--a switch to select different Q settings--I need some help with. Any thoughts?


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## Chuck D. Bones (Jun 16, 2020)

It's quite easy to change the Q on this pedal.  Make R5 a pot.  22K in series with a C50K pot will vary the Q from 1.3 to a little over 6.  Don't go under 22K unless you want an oscillator.


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## bn40 (Jun 16, 2020)

Thanks!


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## phi1 (Jun 16, 2020)

Chuck D. Bones said:


> It's quite easy to change the Q on this pedal.  Make R6 a pot.  22K in series with a C50K pot will vary the Q from 1.3 to a little over 6.  Don't go under 22K unless you want an oscillator.



cool, I’ve often considered building this and would be interested in the Q knob.  I’m confused though, you said don’t go below 22k... but the original R6 value is 10k, am I mis-understanding something?  Did you possibly mean R4 or R5? just speculating since they are originally 47k. Unfortunately I’m not really up to speed on how this filter circuit works.


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## Chuck D. Bones (Jun 20, 2020)

Yeah, I meant R5.  Hit the wrong key.  I have the same problem with guitars.  ?

I just corrected my original post, nor sure if that will reduce or increase confusion.


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