The ProCo Sterling Vermin is a premium, boutique take on the traditional Pro Co RAT circuit, introduced by ACT Entertainment under the RAT brand in 202512. Where a standard RAT is a workhorse distortion pedal in a folded-steel box, the Sterling Vermin puts a refined version of the same formula into a polished stainless-steel enclosure with laser-annealed graphics, hand-soldered in small batches in Jackson, Missouri13.
Inside, it is an analog distortion built around an LM741 op-amp with a pair of clipping diodes selectable by a mini-toggle: traditional RAT silicon diodes for the familiar aggressive, heavy-midrange grind, or BAT41 diodes for a smoother, more articulate voice14.
A RAT circuit in a stainless-steel box
The RAT circuit itself is relatively simple: a power supply stage, an op-amp gain stage, hard-clipping diodes, a low-pass filter tone control and a unity-gain output buffer5. The original RAT's character came in part from a happy accident, when designer Scott Burnham mistakenly installed a 47 ohm resistor in place of the intended value6. Classic RATs are known for the LM308N chip, which is often credited with giving the pedal its bite7.
The Sterling Vermin departs from that chip, using the LM741 op-amp instead, and adds diode selection on top of the RAT formula14. The press release announcing the pedal describes it as having "the soul of a traditional RAT pedal"3, and the maker's product page says it "takes the legendary RAT formula and sharpens it for a new generation of players"4.
The enclosure is the other half of the story. Guitar World's reviewer, Paul Riario, describes "a polished mirror-finish on its stainless-steel housing, accented by laser-annealed decorative calligraphic swirls"1. In Premier Guitar's demo video, John Bohlinger notes that the enclosure "showcases the merging of the pedal's vintage flavor" with its premium build8.
The controls
The control layout is classic RAT: three knobs and a toggle1.
| Control | What it does |
|---|---|
| Distortion | Sets the amount of gain, from amp-like overdrive at low settings to full RAT grind1 |
| Filter | A low-pass tone control that works in reverse: clockwise filters out treble and darkens the tone1 |
| Volume | Sets the output level9 |
| Diode toggle (SI/BAT41) | Selects clipping diodes: SI for traditional RAT silicon grind, BAT41 for a smoother, more balanced response1 |
The Filter knob is the one that catches out players new to the RAT. As on the original, it is essentially a low-pass filter that gets darker the further you dial it clockwise10, so the brightest setting is fully counterclockwise, the opposite of most tone controls.
The knobs themselves are a step up from a standard RAT: the pedal uses CTS pots and oversized knobs, which the designers say give "even, responsive control that affords a satisfying smoothness to the rotation, with just the right amount of tension"8.
Power and connections
The Sterling Vermin runs on 9V DC, center negative, and also accepts a 9V battery in a thumb screw/allen key compartment underneath the pedal14. Input and output are top-mounted 1/4-inch mono jacks, which suits tight pedalboards where side jacks would crowd neighboring pedals1.
History and design
The Sterling Vermin was designed by Shawn Wells and Matt Gates, who refined the traditional RAT circuit into a boutique-quality stompbox111. It was unveiled in 2025 by ACT Entertainment, which had taken up the mantle of the RAT brand13. Each unit is built in small batches and hand-soldered at the company's Jackson, Missouri headquarters311.
What reviewers said
Guitar World's review, by Paul Riario, rated the build quality at five stars, citing the "hand-soldered, premium CTS pots and boutique-built in the USA" construction1. On sound, the reviewer found the two diode modes genuinely different: the SI setting "offers a familiar sound, delivering that aggressive, heavy-midrange grind similar to a standard RAT pedal," while in BAT41 mode "frequencies soften a bit and there's an ever-so-slight drop in output," with the distortion sounding "smoother, tighter, and more defined" even with the Distortion knob turned up1.
The verdict was positive but priced accordingly:
"Refined and redesigned, the Sterling Vermin is a pricey but worthy spin on the original RAT circuit1."
Paul Riario, Guitar World1
The review's listed reason to avoid was blunt: "It's undeniably expensive"1. At $349, it costs several times the price of a standard RAT, and the buyer is paying for the enclosure, the US hand-build and the diode switch rather than a fundamentally different circuit.
Videos
Premier Guitar
Performer Magazine
Performer Magazine
Questions
Is the ProCo Sterling Vermin true bypass?
Yes. Guitar World's review lists the Sterling Vermin's bypass as true bypass, so the signal passes through untouched when the pedal is off. It runs on 9V DC center-negative power or a 9V battery in a compartment under the housing.
What does the Filter knob do on the Sterling Vermin?
It works in reverse compared with most tone controls: turning it clockwise reduces, or filters out, treble frequencies from the signal, making the tone darker. Like the Filter control on a standard RAT, it is a low-pass filter, so full counterclockwise is the brightest setting.
Where is the ProCo Sterling Vermin made?
It is built in small batches and hand-soldered at ACT Entertainment's facility in Jackson, Missouri. Guitar World's review credits the hand-soldered construction, premium CTS pots and US boutique build for its top build-quality rating.
Is the Sterling Vermin the same circuit as a standard ProCo RAT?
It is based on the traditional RAT circuit but refined. It uses an LM741 op-amp rather than the LM308N chip of the classic RAT, and a mini-toggle selects between traditional RAT silicon diodes for the familiar aggressive grind and BAT41 diodes for a smoother, more articulate response.
Sources (11)
- Act Entertainment Sterling Vermin Rat Distortion Pedal review, Guitar World, 2026-04-06
- WATCH: Get to know the new RAT Sterling Vermin Distortion Pedal, Performer Mag, 2025-07-07
- Act Entertainment Launches Sterling Vermin Pedal, Premier Guitar, 2025-05-07
- Sterling Vermin RAT by Sterling Vermin, RockBoard PedalPedia, 2026
- RATs and RAT-alikes explained, Guitar.com, 2025-08-21
- The ProCo Rat went from basements to blockbuster albums, Guitar World, 2024-12-12
- 5 types of overdrive pedal every guitarist needs to try, Guitar World, 2020-02-07
- John Bohlinger Demos the Sterling Vermin Rat, Premier Guitar, 2025-06-05
- SIMPLE PEDALS FOR EVERYONE!, JHS Pedals
- The JHS PackRat swarms your tone with 9 Rat pedals in one, gearnews.com, 2021-10-29
- RAT Sterling Vermin Distortion, Delicious Audio, 2025-06-06
