ARP to Musitronics Mu-Tron III Modification
by: Whirl-A Sound
The Mu-Tron III changed my life. I’d just started college and was living in a small apartment in town when I started REALLY getting into the Grateful Dead. I did all the stuff deadheads do, and by the end of it, I’d begun the envelope filter tone search. My wonderful girlfriend allowed me to use the stove as a soldering surface as I learned electronics.
Long story short, It took many years of envelope filters, mods, comparisons, ect, before I finally bought a genuine vintage Mu-Tron III. By that point, my situation had changed and I was pursuing an electronics education full-time and could finally do some real analysis.
The ARP Mutron iii is very similar to the Musitronics model, the most noticeable difference being the power supply is internal. The power supply is different however, as noted by Mike Beigel in a statement put out about the ARP Mu-Tron III in 2011. You can find that statement at the bottom of this blog post.
When I got my ARP Mu-Tron III it honestly sounded really bad. I bought it from a well-known pedal seller on Reverb and unfortunately, the power supply was in pretty terrible shape. It had some poor-quality work performed that explained the sound (lighting fast sweep due to incorrect calibration & power supply modifications that gave voltages +14v and -8v, unusable for Garcia tone). I initially planned on rebuilding it to ARP spec as a very blurry schematic does actually exist. But then I had the simple idea to convert the ARP Mu-Tron iii to the Musitronics Mu-Tron III with a built in PS-1. The ultimate Mutron!
The Power Supply
The original Musitronics Mu-Tron III ran on a bipolar 9v supply fed by a half-wave rectifier from a 24VAC transformer. The ARP uses an 18VAC transformer and a bridge rectifier to deliver a 12v bipolar supply. I opted to stick with the original PS-1 design as I’m a cork sniffer.
The biggest problem I faced was needing a proper 24VAC 85ma power transformer that would fit into the case. Here's what I was able to find:
Hammond 166C24 24VAC CT @ 85ma. Just found this one as of 12/12/23. Seems to be the closest match, but the other two listed work fine as well.
The Stancor P-8394. 24VAC CT @ 85ma. This is the best-sounding one I've tried (yes, I said it, power transformers sound different) but they HAVE to be VINTAGE PRODUCTION. The top casing and laminations on the "Made in Thailand" version touch the bottom of the Mu-Tron iii chassis and bend the board. Too big. The older versions labeled "STANCOR" in block lettering fit like the stock transformer. They are harder to find, but sooo good when you do. This is the transformer I ended up with in my ARP Mu-Tron iii.
The Hammond 187A24. 24VAC CT @ 100ma. This transformer fits the casing but you need to ensure the leads are properly shrink-wrapped as the termination is solder lugs rather than lead wire. I built the first clone of my modded vintage Mu-Tron iii and used this power transformer. Probably could’ve just started this whole journey there.
The bad fitting P-8394 on the left, good fitting version on the right.
The rest of the mod is straightforward. You need to change the bridge rectifier to half-wave by removing the two relevant rectifier diodes. You then need to update the Zener's to 9.1v 1w (I used 1N4739A). The last power supply component is four .01 shunt capacitors, two for each rail that is already on the board as part of the old design, so don’t worry about those. You do NOT need to update the 220ohms to 2W if you are rebuilding an ARP. Read below:
I was initially puzzled by the presence of 220ohm 2W resistors in the PS-1, but I realized that it wasn’t directly clear because it serves two purposes. It not only sets the zener current, but it also serves as a way to protect the power supply. In the event the power supply outputs are shorted to ground, that resistor will bake a little dissipating a little over a watt, but the power supply will survive. An ingenious design that makes use of the otherwise undesirable source resistance.
This condition can exist because the PS-1 and Mutron are separate. In the ARP modification, this condition cannot exist as it’s a single unit, and the fuse takes the place of failure protection. You can upgrade the resistors if you’d like, I did it before I understood their dual purpose, but it’s not necessary.
Envelope Generator Response
You’ll need to change R19 from 240k to 180k as it forms the voltage divider that allows the filter to sweep downward at the correct depth and sensitivity. It’s tucked under one of the switches and can be identified by its color code. There are also two 1.5k’s tucked near the 240k, one is in series with the optocoupler’s cathode variable resistor (presumably to prevent it from blowing) and another for the “effect” led. The resistors are upgrades to the circuit and I choose to leave them in. You’ll also need to update the 68k decay resistor to 47k if you choose to….Read more below:
Let’s take a moment to remember how the optocoupler in the Mu-Tron affects the signal. Assume the effect is sweeping upwards. In parallel with each opto resistive element is a 220k resistor which sets the furthest “heel down” setting. When you pick, the LED in the optocoupler lights up based on how hard you pick via the envelope generator circuit. Inside the optocoupler are also two photoresistors that decrease in resistance based on light. This decrease in resistance causes the overall resistance of the filter to swing much lower than the 220ks (down to about 1k max depending on the opto) and creates the “upward” effect of the filter sweep.
Keeping that concept in mind, here’s an oscilloscope capture of the envelope generator circuit in action. This is three notes played on my guitar. It takes your wiggly AC sine wave and turns it into a DC representation that modulates the LED.
Both channel 1 and 2 are hooked to the same signal. I have channel 1 inverted to show the representation of the incoming signal as more of an “AC waveform.”
How that pertains to us is in the calibration of the opto current. The 25k trimmer on the pedal controls how much current is sent to the opto, and the envelope generator will trigger the led, but too much or too little opto current will leave us with a pedal that lacks dynamic range. Our goal is to find our sweet spot.
It’s important to keep in mind, the faster the sweep, the more high-end will be present as the photoresistor is saturated with light. The slower the sweep, the less high end will be present as the photoresistor doesn’t get low enough in its resistance range due to less light. I believe it’s important to understand this concept to fully understand how to dial in your own optical-based envelope filter.
It may be easier to understand if visualized. Here is a frequency response chart of my MuTron clone in Bandpass. Each line represents a frequency response measurement with the input level to the effect stepped in 2dB increments:
You can see that a higher input level changes the pedal's frequency response by shifting the Q to a higher or lower frequency.
Calibration
Here is a procedure to calibrate the Mu-Tron III based on the Big Box Q-Tron Calibration procedure. I feel this gets you back to a calibration that feels “stock.”
Envelope Generator Speed and Response Mods
The classic mod to slow down opto response in Q-Trons has been to adjust the R18 (47k) decay resistor to 68k. 68k is the stock value of the ARP Mutron as the later Hamamatsu were likely faster than the original 805A’s Musitronics used in the mid 70’s. That doesn’t really matter to you though, try 47k and see what you like.
The Q-Tron + that Mike Begiel designed in the 90s used a 2k in the “slow” position (replacing the 330ohm attack resistor).
What you need to do at this point is: play the pedal A LOT. This pedal is based on picking and touch sensitivity (as if that hasn’t been drilled into your head already, my apologies). My personal favorite combo with the P873-14 optocoupler is 330ohm and 68k. The opto is a whole other issue i’ll get into at a later time.
My Mu-Tron has also been updated for true bypass, but i’d like to revert it soon as i’ve been told the preamp in bypass is a “thing.”
Here's that blip from Mike Beigel:
"By the time ARP was making Mu-Tron IIIs, I was already out in my own (Beigel Sound Lab) consulting and product design business again, and i never paid much attention to the ARP Mu-Tron products. (They are the Mu-Tron III products WITH the AC line cords, and labeled as produced by Mu-Tron INC. (not Musitronics Corp. which made them from 1972 to 1978."
Last night (January 17) while working with my colleague Richard Lingenberg on some new product design work, we got to talking about the ARP (Mu-Tron INC) units, and I got a BIG and DISAPPOINTING surprise. I had always figured that they were competent enough to make the product the way I had originally designed it, but that was WRONG."
To make this brief, Richard (who gets all my Mu-Tron Vintage repair work and his own repair work from other sources) has fixed up and calibrated a lot of these ARP units.
Apparently, they changed the inside POWER SUPPLY VOLTAGE but in doing so, they apparently DID NOT UNDERSTAND to change some component values that were designed for the ORIGINAL power supply voltage.
What Richard noticed about (ALL?) the ARP units coming in for repair, was that the FREQUENCY SWEEP was WAY TOO HIGH.
And also that the DOWN DRIVE never really worked to drive the filter down "all the way" as it was supposed to.
Both of these symptoms had to do with changing the power supply voltage but not compensating for the change by changing ALSO some very critical component values.
So Richard (who knows what a Musitronics Mu-Tron III unit SHOULD sound like), changed the offending parts values and recalibrated the SWEEP range and DOWN DRIVE to what they were designed to be.
I never knew about this before, and am sharing it now. If you have an ARP Mu-Tron III and don't like the way it sounds or works, now you know why.
If you want it to be fixed (even though it's not officially "broken" and recalibrated, please send me a note at <info at mu-tron dot ORG (NOT .com), and I will forward the information to Richard, who will contact you and give you a quotation on fixing the parts values and re-calibrating it to the frequency sweep range of the original factory Musitronics Mu-Tron Calibration.
(But if you are perfectly happy with your ARP-Mu-Tron unit, then by all means USE IT AS IS. Richard also noticed enough variation in the calibration of the ARP units that it's just possible that YOUR vintage ARP-MuTron III unit is accidentally calibrated "just fine".
There weren't all that many ARP units made, but I thought it appropriate to inform the owners of these vintage (and AUTHORIZED by Musitronics Corp) i.e. LEGITIMATE reissues, that there might be issues with the units that (I) didn't know till last night.
Thanks,
Mike Beigel"
Last updated: 12/23/24