Vactrol's and guitar pedals

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Gravitas
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Vactrol's and guitar pedals

Post by Gravitas »

This is my first post so forgive me if this has been answered before.

I have an Award SessionMaster JD10 guitar pedal and I would like to replace the pots with CV inputs.
I've heard that vactrol's are the way forward in this regards.
The potentiometers are 3 pole side mounted 200k Ω which I have to replace anyway?
Any suggestions please.

Thank you very much in advance.
Gravitas
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Re: Vactrol's and guitar pedals

Post by Gravitas »

Here's the link to the schematics from the Award.

https://www.award-session.com/pdfs/jd10-mk1.pdf
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Kevin Mitchell
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Re: Vactrol's and guitar pedals

Post by Kevin Mitchell »

Hello and welcome!
I drew up a simulation for you on how to connect an opto to replace the pots that are used as simple variable resistors.
https://tinyurl.com/2ccv83o4
I've added a 220k resistor in series with what's realistically a photocell. The photocells have high off-resistance so this is needed to limit the max resistance.

For the output level pot you'll have to do it a little different. You can use two optos with the photocells in series but you must invert the voltage for one of the LEDs.
Gravitas
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Re: Vactrol's and guitar pedals

Post by Gravitas »

Thank you very much.
I've got a few questions.

Bear in mind I'm a newbie at this stuff as I normally do my stuff virtual.

From looking at the circuit on the right hand side of the simulation
where it shows a CV and ground symbol I presume this is the CV input?
and where it says "optocoupler" is this for the vactrol?
Is this a direct replacement for the potentiometers or can
this be wired in with the potentiometers on the guitar pedal itself.
Either way I don't mind.

Also the CV diagram on the left is that for reference?

In regards to the volume pot I'll leave that one till last. :)
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Kevin Mitchell
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Re: Vactrol's and guitar pedals

Post by Kevin Mitchell »

Gravitas wrote: Fri Jul 29, 2022 2:22 pm where it shows a CV and ground symbol I presume this is the CV input?
You got it.
Gravitas wrote: Fri Jul 29, 2022 2:22 pm and where it says "optocoupler" is this for the vactrol?
Yup. They're the same thing.
Gravitas wrote: Fri Jul 29, 2022 2:22 pm Is this a direct replacement for the potentiometers or can this be wired in with the potentiometers on the guitar pedal itself.
Yes, direct replacement. You don't want to use pots in parallel. If you look at the schematic you'll see most of them have the wiper tied to one end - making it a variable resistor. So you simply install each lead of the photocell onto the two pot connections.
Or even better, use switching jacks which is what we commonly use in synth modules. That way you can keep the pots - you only have to lift the wipers and wire them to the SW lug of the jack. This means when no CV input is plugged in you can use the original control but once you patch in your CV cable it disconnects the wiper and CV takes over.
Gravitas wrote: Fri Jul 29, 2022 2:22 pm Also the CV diagram on the left is that for reference?
Yes. I wanted to include a variable CV in the demonstration. You can actuate it using the slider within the menu on the right.


Additionally, I suggest you roll your own optocouplers and experiment. Make a learning experience of it!
Gravitas
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Re: Vactrol's and guitar pedals

Post by Gravitas »

Awesome stuff.

So basically I need

4 x mini jacks for the CV input
4 x vactrols
4 x 220k resistors ?
and I have breadboard wire floating around someplace.

and good advice in regards to making my own vactrols.
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Kevin Mitchell
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Re: Vactrol's and guitar pedals

Post by Kevin Mitchell »

Gravitas wrote: Fri Jul 29, 2022 2:43 pm and good advice in regards to making my own vactrols.
Get a few different photocells and LED colors (3mm LEDs by the way) so you can try them out. They all have different on-off resistance as well as sweep behavior. You may favor one more than the other. Also don't forget the heat shrink tube.
Tayda has everything you need.

I've updated my last post with a little info on the CV jacks and how they can be used to switch from pot control to CV control.
Gravitas
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Re: Vactrol's and guitar pedals

Post by Gravitas »

Kevin Mitchell wrote: Fri Jul 29, 2022 2:48 pm
Gravitas wrote: Fri Jul 29, 2022 2:43 pm and good advice in regards to making my own vactrols.
Get a few different photocells and LED colors (3mm LEDs by the way) so you can try them out. They all have different on-off resistance as well as sweep behavior. You may favor one more than the other. Also don't forget the heat shrink tube.
Tayda has everything you need.

This sounds like fun.

I've updated my last post with a little info on the CV jacks and how they can be used to switch from pot control to CV control.
Cool.
If I do it this way then I can still use it as a straightforward forward guitar pedal/preamp when needed.

Thank you.
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Re: Vactrol's and guitar pedals

Post by Gravitas »

Kevin Mitchell wrote: Fri Jul 29, 2022 2:48 pm
Gravitas wrote: Fri Jul 29, 2022 2:43 pm and good advice in regards to making my own vactrols.
Get a few different photocells and LED colors (3mm LEDs by the way) so you can try them out. They all have different on-off resistance as well as sweep behavior. You may favor one more than the other. Also don't forget the heat shrink tube.
Tayda has everything you need.

I've updated my last post with a little info on the CV jacks and how they can be used to switch from pot control to CV control.
Quick update.

I'm having the potentiometers rebuilt as the original company went bust.
I've still got another couple of weeks to go until they arrive meanwhile
do I need mono chassis switched jack sockets or stereo chassis mount jack sockets?
Bearing in mind I need to keep the functionality of the original potentiometers
and do I add the vactrol circuit you provided in series or in parallel when
using the jack sockets?
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Kevin Mitchell
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Re: Vactrol's and guitar pedals

Post by Kevin Mitchell »

There's a flaw in my explanation. The issue is if you were to wire things as I've suggested using a switching jack then CV would go directly to the circuit and not the optocoupler's LED.

Trying to think of a solution for you...
It's not possible with just a switching jack alone. Adding toggle switches is a solution but that's obviously more work.
If that's not a problem for you I can modify the schematic to elaborate.

Here's a sim.
https://tinyurl.com/2gxoufoy
This uses a SPDT toggle to select the pot or the opto. No switching jack needed, mono is fine.
Gravitas
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Re: Vactrol's and guitar pedals

Post by Gravitas »

Kevin Mitchell wrote: Fri Sep 16, 2022 10:10 am There's a flaw in my explanation. The issue is if you were to wire things as I've suggested using a switching jack then CV would go directly to the circuit and not the optocoupler's LED.

Trying to think of a solution for you...
It's not possible with just a switching jack alone. Adding toggle switches is a solution but that's obviously more work.
If that's not a problem for you I can modify the schematic to elaborate.

Here's a sim.
https://tinyurl.com/2gxoufoy
This uses a SPDT toggle to select the pot or the opto. No switching jack needed, mono is fine.
Thank you.

I was a bit puzzled with the wiring in regards to using a switching jack alone
as I had ordered mono switching jacks and using them to CV the vactrol
and bypassing the potentiometer at the same time didn't make sense hence why I asked.

I was hoping to keep the design to the switching jack sockets only but as the toggle switches
will get the desired effect between CV and traditional then that's the way to go.

Okay, I'll order some toggle switches and mono jack sockets.
Any recommendations for the toggle switches?
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Kevin Mitchell
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Re: Vactrol's and guitar pedals

Post by Kevin Mitchell »

When I need only switches, jacks and knobs I use lovemyswitches.com. But that's my personal preference per my location in the world. Plus everything is relatively inexpensive. Taiway and CK are what I've been confidently sticking with for toggle switches but use whatever gets the job done for ya.
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