Mutron III type Vactrol build.

[digi2t]

I decided to try my hand at recreating the Vactrol from the Mutron III. I've been working on the Foxx Guitar Synth, which is 75% Mutron, but I was wondering if I could save my VTL5C3's for another project.

I used two LRD's, a clear bright white LED, and the casing from a Bussman FRN type fuse, like this one . I cut the ends off, and squared it on a bench grinder to 7/8" long. It's about 3/8" in diameter.



I then used some foam that is used to ship IC's, to jig my LED in the casing. I sliced it to 1/8" thick. I used 5 minute epoxy to encapsulate. It sets slow enough to move things around a bit if need be, but quick enough not to require a big set up. It's also translucent enough not to mess with the LDR's too much. I punched the LED into the foam, and centered it in one end of the casing.







Then I poured in the epoxy, until the LED was just covered. I then set the LDR's on top of the LED, side by side, making sure to align all my pins. This would ensure that it lays flat on a board.  I added some more epoxy, until I was 1/8" below the top.



As the epoxy set, I gently ajusted the LED and LDR's to make sure that they set straight. After about 5 minutes things were stiff enough to put it down. I waited about 20 minutes, and then removed the foam that I used to jig the LED with a pair of small hemostats. Now I had a recess of 1/8" on both ends.





The recess on both ends is there so I can pour in some flat black Testors model paint, or I may try to just melt some black plastic into it. This will block out any light from entering the Vactrol. Not really a big deal when it's enclosed in a case, but would make it a bit of a pain to set up the range otherwise.



Before capping the ends, you can also test to see what kind of resistance you will need to set the range. By testing on a breadboard, you can swap different resistors, turn out the lights, and then see the LED light up through the epoxy. Once you've figured out the resistance range, you can cap it.

In the end, I found my resistance range, and temporarily capped the ends with some industrial double-sided tape. I pin-punched the holes for the leads, and slipped the tape on, effectively sealing off the ends temporarily. I tested the light seal, by listening to the auto-wah, and turning the room lights on and off. There was no change in tone, so far so good. I turned on the amp, and I tested it.

It soundly creams the twin VTL5C3's into the ground! The auto-wah wonk coming out of this sucker now is UN-EFFING-BELIEVABLE. I mean it was pretty decent before, but this baby took it to a new level. In all ranges, HP, BP, and LP, the tone is clearly much better than the twin VTL's. I'll try and post a video tomorrow.

There you go. The Binford 3000, Mutron replacement, Vactrol... at your service.

[mnordbye]

Great work! I eagerly await videoclips.

Though, I'm not so familiar with these circuits. Is it Auto Wah?

Magnus

[newperson]

what LDRs did you use?  Value/branding?

Good pictures and walk through.  A+

[Mark Hammer]

Great work! I eagerly await videoclips.

Though, I'm not so familiar with these circuits. Is it Auto Wah?

Magnus

The Foxx "Guitar Synth" is actually several different types.  It uses a basic state-variable filter, exactly like the Mu-Tron III, but includes a means for foot-controlled sweep, and two types of auto-sweep: envelope-control and LFO.  Think of it as the next step up from the EHX Queen Triggered Wah ( http://www.effectsdatabase.com/model/eh/queentrigwah ), which had envelope control and foot sweep, or the MXR Auto-Q ( http://www.effectsdatabase.com/model/mxr/dunlop/m120 ), which has envelope sweep and LFO sweep.

[R.G.]

Nice work.

You can also get pigments for epoxies at boat-parts places. A little black pigment in the end-cap epoxies would "dark enhance" the epoxy caps so it would be light sealed.

A similar dark tube and temporary foam seals makes a good LDR test rig. You can use the LED at a constant current as a standard light source. Then plug different LDRs into the other end of the tube, measuring resistance. All that's needed is some way to mechanically get every LDR oriented and placed in the same alignment to the LED and the same distance away. This tells you the resistance at that light level per LDR.

If you want, you can also measure the speed of the LDR by switching the LED on and off and watching the LDR resistance (perhaps as part of a potentiometer setup) on a 'scope.

[digi2t]

what LDRs did you use?  Value/branding?

Good pictures and walk through.  A+

The LDR's are made by Waitrony, model number KE-10720. I got them from here; http://www.ebay.ca/itm/270571011753?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

By a stroke of good luck, they also fit perfectly, side-by-side (leads parallel), into the FRN fuse casing.

One thing I highly recommend though, is matching your pair of LDR's. I simply just jigged each LDR under a lamp, keeping the same distance and angle from the light source, and measured the resistance of each. I then chose two that were within 10K of each other. I only had 10 to choose from, and surprisingly, they varied quite widely (+/- 200K). Since they're quite cheap, I recommend buying a bunch, and sorting them to get good pairs. Kind of like choosing jfets for a phaser  .

[digi2t]

A similar dark tube and temporary foam seals makes a good LDR test rig.

Truthfully, that's what I did to chose the LED. I just didn't write it up, because I did the write up twice, and lost it because IE crashed on me. This time around, I just wrote real fast, keeping to the important details.

As for the foam seals, DO NOT use the black IC packing foam. IT IS CONDUCTIVE!!! I tried it first, thinking that it would be the cure-all for capping the ends. Just epoxy them, and done. But... when I was testing, I was getting nothing but squealing coming from the rig. I almost abandoned the whole idea, thinking that the LDR's were not suitable. It then occured to me that the squealing sounded identical as when I would touch the Vactrol leads with my finger. That's when the bulb went on in my head, and I tested a piece of foam with the multi-meter. Yup... I was getting continuity. So, I switched to the pink foam. Thank God I'm stubborn, although my girlfriend doesn't get that  .

Gelcoat pigment. Absolutely smashing idea. I can get some at my auto parts dealer, in small tubes. Mix it into the epoxy. Dude... you're magic. Thanks!

[digi2t]

Alright, here is the video. Quick run through of the different passes, just to demonstrate the performance of the home-grown Vactrol.

[cjlectronics]

Great work!!  This is something that has been on my to do list for a while.  BTW, that list keeps being put on the back burner along with other projects.

You will want to find an LDR and LED that are within the same spectral wavelength.  With the LED at 540nm, for example, (which is a green LED) find an LDR with a spectral wavelength that peaks within this wavelength.  I think you'll find the response from the peak detector circuit in the Mutron III to be very favorable.  At least this is my theory and I feel strongly this will give you the optimum peak response from any envelope filter. 

CJ

[Ronan]

I finally realized that ebay seller of LDR's also has a website taydaelectronics.com... 24 cents each and a dollar for shipping. One has to sign in/register on the site though, otherwise the checkout process doesn't work properly resulting in Paypal not adding the shipping charge, which I reckon would cause all sorts of delays/problems.

I sure am glad those LDR's sound good in that circuit. I was a bit worried about response time, but it seems thats a non-issue. 

[DavenPaget]

I finally realized that ebay seller of LDR's also has a website taydaelectronics.com... 24 cents each and a dollar for shipping. One has to sign in/register on the site though, otherwise the checkout process doesn't work properly resulting in Paypal not adding the shipping charge, which I reckon would cause all sorts of delays/problems.

I sure am glad those LDR's sound good in that circuit. I was a bit worried about response time, but it seems thats a non-issue. 

Heh . Tayda was the first parts site i knew .

[petemoore]

  What R range does the circuit respond to ? I used a 1meg and a 10k in place of the LDR to figure that out.
  What type of LED [color etc.] makes the LDR 'jump' ?...how much and how fast [didn't worry about it].
  The low R range seemed to have effect, and the high range...for the circuit [neutron] seemed that any of the LDR's high-R was more than high enough, going easily past the 'high R' to which the circuit would respond.
  So it seemed that going for a low-R dark resistance photocell made sense, and for that, two low R LDRs get about twice as low as 1 does...the setup I had still went plenty high R and the circuit wide-swept like a charm...allowing E-adjustments to control to limit the 'wide-sweep' within 'usable' range.

[digi2t]

What R range does the circuit respond to ?

Whoa... Pete, you've lost me. Can you please explain, so I can wipe this lost expression off my face. Is the "R" for resistance, or the wave length sensitivity of the LDR? I've never come across that yet, so you just threw me curve here.

[charmonder]

cool! that saves like $5/6 each time. I'll have to give it a shot, I kinda gave up when it didn't work out so perfectly with duct tape.

[digi2t]

I've had a request to re-up the pictures for this tutorial, so here you go...

I decided to try my hand at recreating the Vactrol from the Mutron III. I've been working on the Foxx Guitar Synth, which is 75% Mutron, but I was wondering if I could save my VTL5C3's for another project.

I used two LRD's (Waitrony, model number KE-10720), a clear bright white LED, and the casing from a Bussman FRN type fuse, like this one;
I cut the ends off, and squared it on a bench grinder to 7/8" long. It's about 3/8" in diameter. One thing I highly recommend though, is matching your pair of LDR's. In a dark room, I simply just jigged each LDR under a lamp, keeping the same distance and angle from the light source, and measured the resistance of each lamp on/off. I then chose two that were within 10K of each other. I only had 10 to choose from, and surprisingly, they varied quite widely (+/- 200K). Since they're quite cheap, I recommend buying a bunch, and sorting them to get good pairs.



I then used some foam that is used to ship IC's - DO NOT use the black IC packing foam. IT IS CONDUCTIVE!!! -  to jig my LED in the casing. I sliced it to 1/8" thick. I used 5 minute epoxy to encapsulate. It sets slow enough to move things around a bit if need be, but quick enough not to require a big set up. It's also translucent enough not to mess with the LDR's too much. I punched the LED into the foam, and centered it in one end of the casing.







Then I poured in the epoxy, until the LED was just covered. I then set the LDR's on top of the LED, side by side, making sure to align all my pins. This would ensure that it lays flat on a board.  I added some more epoxy, until I was 1/8" below the top.



As the epoxy set, I gently ajusted the LED and LDR's to make sure that they set straight. After about 5 minutes things were stiff enough to put it down. I waited about 20 minutes, and then removed the foam that I used to jig the LED with a pair of small hemostats. Now I had a recess of 1/8" on both ends.





The recess on both ends is there so I can pour in some flat black Testors model paint, or I may try to just melt some black plastic into it. This will block out any light from entering the Vactrol. Not really a big deal when it's enclosed in a case, but would make it a bit of a pain to set up the range otherwise.



Before capping the ends, you can also test to see what kind of resistance you will need to set the range. By testing on a breadboard, you can swap different resistors, turn out the lights, and then see the LED light up through the epoxy. Once you've figured out the resistance range, you can cap it.

There you go. The Binford 3000, Mutron replacement, Vactrol... at your service.

[rankot]

If anyone's interested, very similar LDR can be bought from China (Aliexpress etc), model is GL5516. Fall time is a little bit longer (35 compared to 5ms), but everything else is almost the same.

[italianguy63]

I've had a request to re-up the pictures for this tutorial, so here you go...

I decided to try my hand at recreating the Vactrol from the Mutron III. I've been working on the Foxx Guitar Synth, which is 75% Mutron, but I was wondering if I could save my VTL5C3's for another project.

I used two LRD's (Waitrony, model number KE-10720), a clear bright white LED, and the casing from a Bussman FRN type fuse, like this one;
I cut the ends off, and squared it on a bench grinder to 7/8" long. It's about 3/8" in diameter. One thing I highly recommend though, is matching your pair of LDR's. In a dark room, I simply just jigged each LDR under a lamp, keeping the same distance and angle from the light source, and measured the resistance of each lamp on/off. I then chose two that were within 10K of each other. I only had 10 to choose from, and surprisingly, they varied quite widely (+/- 200K). Since they're quite cheap, I recommend buying a bunch, and sorting them to get good pairs.



I then used some foam that is used to ship IC's - DO NOT use the black IC packing foam. IT IS CONDUCTIVE!!! -  to jig my LED in the casing. I sliced it to 1/8" thick. I used 5 minute epoxy to encapsulate. It sets slow enough to move things around a bit if need be, but quick enough not to require a big set up. It's also translucent enough not to mess with the LDR's too much. I punched the LED into the foam, and centered it in one end of the casing.







Then I poured in the epoxy, until the LED was just covered. I then set the LDR's on top of the LED, side by side, making sure to align all my pins. This would ensure that it lays flat on a board.  I added some more epoxy, until I was 1/8" below the top.



As the epoxy set, I gently ajusted the LED and LDR's to make sure that they set straight. After about 5 minutes things were stiff enough to put it down. I waited about 20 minutes, and then removed the foam that I used to jig the LED with a pair of small hemostats. Now I had a recess of 1/8" on both ends.





The recess on both ends is there so I can pour in some flat black Testors model paint, or I may try to just melt some black plastic into it. This will block out any light from entering the Vactrol. Not really a big deal when it's enclosed in a case, but would make it a bit of a pain to set up the range otherwise.



Before capping the ends, you can also test to see what kind of resistance you will need to set the range. By testing on a breadboard, you can swap different resistors, turn out the lights, and then see the LED light up through the epoxy. Once you've figured out the resistance range, you can cap it.

There you go. The Binford 3000, Mutron replacement, Vactrol... at your service.

I too have done several Mutrons with these.  They sound great!  I had good luck with both white and green LEDs.

Instead of painting the ends, I get a piece of black heatshrink and cut it slightly longer than the barrel of the cylinder; the edges wrap over and seal the light out well enough.

MC

[italianguy63]

Initials or a medical condition?!



MC

[Soulchief]

Thanks for the pics!!

[nocentelli]

Thank you so much for re-upping pics for this particular post; I remember it clearly the first time around and your matter of fact and simple but detailed explanation inspired me to try to make a lovetone meatball after only having made a few basic fuzz pedals.