Keeley Electronics PSI Fuzz

by Keeley Electronics

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  • Keeley Electronics PSI Fuzz


    “Make a Muffin Better”, Frank Zappa

    We set out to take a classic design with lots of history and make it obviously something new. We took the op-amp version of old PI and inverted one of the stages, brought out the midrange, tightened the bass, and most importantly, added a Germanium transistor output! Yes, this is a opamp-transistor hybrid. Don’t be miffed by op-amps in your pie folks, the germanium touch at the end makes everyone smile around the shop here.

    We wanted to celebrate our 12th pedal of the year by offering it in two forms. Two choices, 1) Custom Art in a late 1960’s style or 2) Greek Gold Deco. The first is hand drawn by Erin Lofties. Her stunning artwork looks out of the 60’s and was completed this week specifically for this pedal. Then then Gold Brick Greek God model. Stripped down, simple and clean.

    …This is how the first phase of our design went: (from Bob’s Blog)

    How do you make a design different? Put your own spin on it? You can make it distort differently, you can make the tone different, add some features, you might even be able to put a little lipstick on it. At the first audio/video repair shop I worked at in 1989, I remembered seeing clipping or limiting diodes, LEDs in fact, in certain types of gear where there were A/D converters. I noticed as I was working on the equipment that the LEDs were lighting up on the inside of the unit when the program material was high enough. No one could see them except the service tech. How cool would that be if everyone could see them on their distortion pedals?! So I’m pretty sure I was the first to make it a standard feature on a guitar effects pedal by putting the clipping LEDs on the outside of the pedal when I built the DS-1 Seeing Eye mod and later the Ultra mod. It’s done pretty regularly now on my pedals including the latest Red Dirt Pro and Stahlhammer. They might not light up very brightly, but that doesn’t matter, they are still doing their job. And so, I wanted to dive into the old Muff and make it mine. Rearrange things, add some bling, make it Psi Fuzz!

    It seemed like I could have fun doing an all germanium one, but that was done already. It seemslike there isn’t gross overpopulation of op-amp versions of the old gal. But what to do to make it mine, at least in some way? Well we can add to the circuit. Again, I was thinking germanium transistors since they always have a fun sound. So let’s add a common emitter amplifier to the end. That would put a germanium spin on the overall sound. But we have phase inversion then. Input and output are in phase on the vintage pedals and I wanted to maintain that. Great! I get to play with the stages! The first thing I wanted to alter was the 2nd “gain-stage” and filter. That’s a Sallen-Key filter and you can only make it using a non-inverting style op-amps. I got to play around with values and changed things up a bit. I wanted to know how it felt to swing that 1KHz 2nd order filter around. So I made it go up and down. In and out came the harmonics…and the noise. Very cool. I can set that filter to where I want it. I want it to have a bit of fire and life and not feel too cramped in the high end. I’ll use a lower noise, mildly more expensive IC to combat noise instead of kill the guitar tone. Okay, still have to invert the phase somewhere! Yup, the first stage is where it gets it. I changed a few things around to make it a non-inverting op amp and it works well. I’m still playing with those values but I dig it. Right now the low pass filter for the first stage is about 10Hz if I remember correctly and that’s okay with the girls I date. Phase is correct (with the soon to be added germanium gain stage) and I’ve changed the design so far, even if trivially.

    The Big Wazoo

    So I changed the 6 silicon diodes in the feedback of the third stage to 2 LEDs and yes, one of them will poke through the hole in the top. The “Keeley Mod” has been done to it, including blue as Neptune Psi Fuzz Blue LED. (Yes, another first from Keeley Electronics.) And finally, the pre-climax to this story, our phase is in crisis, the hero takes action, the germanium amplifier stage! Ahhh, fitting like a glove between the Voice control and the Voltage control is a common emitter amplifier using a AC187. I am pretty happy with the biasing of it. The collector sits around 4.5V and we get a gain of 10dB out of it. Oh wait! We don’t want this thing not putting out when it gets hot! That would be as wrong as a double negative! You know, when it’s a 113 degrees in the shade and some Rock God is on stage in Texas, just working it for all it’s worth, he don’t want things to blow ya know. Craighton and I get the heat gun out and sure enough, Mr. Semi-Conductor from the 60’s sags and wimps out in the heat. Just like your poor old fuzz face. When the mercury hit 115 the lights went out. Okay, off to the parts room. Out comes a real germanium diode. Not the Shenzhen China Schottky ones, but some good strong American ones. Ones that behave the same as the transistor when it gets hot. We used the germanium diode in a reverse polarity fashion across the base-emitter junction of the transistor to offset the temperature coefficient problem. Turned on the heat gun, just let it blow…it got hot..really hot…well over 140 degrees…and it never stopped pumping out the love.

    Come As You Are

    Our version has a single integrated circuit package, a quad op-amp, and a single transistor. Trim and Slick! I love it! Sounds terrific! We still need to fine tune this design a bit more but, there you have it; The Psi Fuzz by Keeley Electronics. Coming soon to a pedal board near you!

    Keeley Electronics PSI Fuzz Features:

    • Made in the USA

    • 9v Operation

    • True bypass

    • Rugged Construction

  • We want you to feel confident with your Prymaxe purchase, which is why when you purchase a new or demo (like new/gently used) product from us, you are automatically enrolled into our FREE Two Year Warranty.* 


    What is covered:

    • Brand new (non-clearance) and demo (like new, gently used) products that have been purchased within the last two years
    • Products that malfunction under normal use
    • Cost of repairs


    What is NOT covered:

    • Used (non-demo), vintage, and clearance products
    • Products purchased more than two years ago
    • Cost of shipping both ways
    • Items with clear signs of abuse (up to Prymaxe’s discretion)


    * We reserve the right to end this promotion at any time.


  • When you place an order at Prymaxe, you can rest assured that your item will be carefully packaged. Our shipping team is dedicated to making sure your item arrives in perfect condition and on time.

    All in stock orders placed before 2pm EST will ship same day and all orders placed after the cutoff will ship the next business day*. If you need an order immediately, please let us know and we’ll try our best to accommodate your request.

    * Our same day shipping policy may be temporarily suspended during a sale or if you pay with Affirm. Rest assured, we will do everything we can to get your order out as quickly as possible. Please call us at 1-855-PRYMAXE if you are placing an overnight order and need it to go out immediately.


    Domestic Shipping
    Most orders shipping within the US will ship via USPS or FedEx and are fully insured, we also offer express options if you need an item within 24 hours. All tracking numbers will be sent via email after the item is shipped. If you did not receive your tracking number please feel free to call or email us. 

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    Pre-Order Shipping: 

    Pre-Orders typically ship with 2 weeks. In the event a product is backordered we will contact you with an exact shipping timeframe. 

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  • If you would like to return an item, please use the return widget at the bottom of this page to look up your order and start the return process. If you need assistance, or if the widget is not working for you, please email us at with our order number and reason for return.

    45-Day Satisfaction Guarantee

    If you are not 100% satisfied with your purchase, you can return it within 45 days of the original purchase date for a full refund. If an item is not received by us within the 45-day window, the package will be refused. All new items returned must be in BRAND NEW condition with zero signs of wear or use and must be returned in the original packaging with ALL materials (free/bundled items -- must be unused/unopened, warranty cards, plastic, pouches, manuals etc.). Demo (Like New/Gently Used) items must come back in the same condition they were sent to you, with all included materials, if applicable.

    Our 45-Day Satisfaction Guarantee covers brand new and demo (like new/gently used) products only -- it does not cover used and clearance products (see below). It also does not cover items in the “What is NOT Returnable” list below. Please note: All actual shipping fees paid by Prymaxe will be deducted from your refund. This includes orders that qualified for free or subsidized shipping. Return shipping must be paid by the buyer. If you received a defective or incorrect item, please see our FAQ section below.  


    * Prymaxe reserves the right to define abuse of the return policy on a case by case basis and consequently refuse subsequent return requests by said abuser and place them "All Sales Final".


    Used and Vintage: 

    All Used and Vintage items come with a 48 hour approval period. Please contact us directly at within 48 hours of receiving your product for us to set up a manual return for you. Be sure to include your order number and reason for return in the email. All Used/Vintage items must be returned in the same condition they were sent to you in, as well as include all materials that came with it.

    What is NOT Returnable:

    • Accessories (this includes all pickups, strings, picks, pedalboards, tuners, expression/volume pedals, etc.)
      • However, if you received accessories with the product as part of a bundle, the accessories must be returned with the product, and they must be unused and unopened
    • Clearance Items (Clearance items are clearly marked with "CLEARANCE" in the title)
    • Special/Custom Orders

    Return Instructions:

    All returns should be insured by the buyer and sent via UPS, Fedex, or USPS with tracking. Please do not write on or attach tape or labels to the product box -- use a packing box to return the product. Attach a shipping label with the RA number onto the shipping box.



    When will I get my refund?

    Once your return is received by us, your refund will typically be processed within three business days. It typically takes a few business days for all credit card refunds to be processed and reflected on your statement. This means it will take a few extra days for your refund to show in your account after your return is processed. Please note: All actual shipping fees paid by Prymaxe will be deducted from your refund. This includes orders that qualified for free or subsidized shipping. Return shipping must be paid by the buyer. If you received a defective or incorrect item, please see the next question.  

    What if I receive a defective or incorrect item?

    If a product is defective upon arrival, or the wrong item was received, we will send you a replacement/the correct item, and all the shipping fees will be covered by us. For defective items, Prymaxe reserves the right to refuse the return if we determine the reason for the unit failing was not covered under the warranty (i.e., if the item was damaged due to user fault or error). If you feel your item was damaged in shipping, please take plenty of pictures of the damaged box and item and immediately contact us so we can open a claim with the shipping company.

    Please be very diligent with testing your defective item. Be sure to try other guitars, power supplies, cables, and amps. We also request that you take a video of the item not working. If we receive your item, test it, and find it to be working, we will give you a refund minus the cost of shipping both ways.

    When will I get my replacement item?

    If a defective or incorrect item was sent to you, we will send out the replacement as soon as we get the defective/incorrect item back and approve/process the return (typically within three business days). If we do not have the replacement in stock, we will issue a full refund. If you need the replacement to be sent out immediately, please place a new order, and when we get the defective/incorrect item back, we will issue a full refund of the original order.

    When must I ship my return?

    Your return must be received by us within 45 days of the original purchase date. This means you must ship your return with enough time for us to receive the package within 45 day deadline. If it does not arrive within 45 days, we will refuse the package. If you feel you sent the package within a reasonable time frame, but it arrived late due to the shipping company’s error, please immediately contact us to make an appeal before we refuse the package.

    What if I pre-ordered an item?

    Even if you pre-ordered an item, we must still receive it within 45 days of your order date in order to give you a refund, so please only pre-order items you plan to keep. If your pre-ordered item arrives defective, you will have a 48 hour period to contact us for a replacement.

    Why was my package refused?

    There are a number of reasons your package could be refused by us, including:

    • It was a new/demo item received outside of the 45-day window
    • It was a clearance, accessory, or other non-returnable item
    • The package is missing items that were included in the original shipment
    • The item shows signs of wear
    • The original product box was damaged or written on/the item was not packaged in a separate shipping box

    Why is the widget not letting me return my item?

    If you have a valid reason for returning/exchanging a product that is within our return policy, but the widget will not let you start the return process, please contact us directly at with your order number and reason for return.

    Returns Widget

    Use the widget below to look up your order.

    If your order is not found, please make sure you added the number sign (#) in front of it.

    If your order is still not found, please email us at with your order number and reason for returning so we can create a manual return for you.

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