Doctor Q "Quack Knob"

[Brushthrower]

Hey everyone,

I just picked up a second-hand EHX Doctor Q envelope pedal. After opening it up and tweaking the trimpot it sounded good. The only problem is, because of the way the board is mounted in the pedal, you have to take it out to access the trip pot. This gave me the idea of replacing the trimpot with an equal value potentiometer and mount it to the enclosure, in essence giving it a "quack knob". Has anyone tried this modification? Does it sound valid in theory? Just thought I'd ask for opinions before I start fiddling.

Thanks!

[Mark Hammer]

I've done it several times.  It works...within limits.  Only a modest portion of the trimpot is actually functional.  Ideally, I suspect the 25k trimpot should be replaced by a 10k pot with selected fixed resistors added to each outside lug to make up the full 25k.  You'd probably want to identify those values by taking measurements at the points in the trimpot's travel where usable changes to basic centre frequency occur.  So, for instance, the final arrangement might be something like a 5k1 on the ground side and a 10k on the other side of the 10k pot.  Or perhaps a 3k3 on the ground side and 12k on the other.  You get the idea.

But all of that aside, yes, it is worth pursuing.  You will find it particularly useful when switching modes between normal and bass.  I often find when one of the modes sweeps in a desirable way, the other one is not adjusted to my tastes when I flick the mode switch.  Being able to access the tuning pot means you're not forced to live with a compromise.

And while you're fiddling around, replace the 470k feedback resistor in the filter section with a 330k fixed resistor in series with a 1M pot.  This will adjust the Q of the filter and in the process move the sweep range around a bit.  You can also install a simple "fast/slow" switch to change the sweep rate.  Replace the 100R fixed resistor with a 220R resistor.  Wire up a DPDT or DPST toggle or slide switch so that the two common contacts are connected to the junction of the small resistor, 10uf cap and second diode.  Now connect one of the side lugs on the toggle to a 100R resistor that will be placed in parallel with the 220R resistor you've installed.  The other side lug of the toggle goes to a 100k resistor that will be placed in parallel with the 10uf cap.  So, in the "slow" position, you have 220R leading up to the 10uf averaging cap and no "bleed" resistor to ground.  In the "fast" position, the parallel 100R resistor drops the combined value down to about 68R (1/220 + 1/100 = 1/68), and places a 100k bleed resistor in parallel with the cap.  The difference is not huge, but is enough to produce discernibly different feels.

[Brushthrower]

But all of that aside, yes, it is worth pursuing.  You will find it particularly useful when switching modes between normal and bass.  I often find when one of the modes sweeps in a desirable way, the other one is not adjusted to my tastes when I flick the mode switch.  Being able to access the tuning pot means you're not forced to live with a compromise.

Yes, exactly what I found. Frustrating. Thanks a lot for the the detailed response...now on to fiddling!