The ThorpyFX Heavy Water Dual High Headroom is a dual boost pedal from UK pedal maker Adrian Thorpe, known as ThorpyFX. It puts two boosts in one compact enclosure: a clean, high-headroom boost on the right and a dirtier germanium-diode boost on the left, each with its own controls and footswitch, and each usable alone or stacked together12.
Thorpe designed it after many requests to put the boost side of the ThorpyFX Dane into a pedal of its own, then added a second boost with germanium diodes for a different character34. As Thorpy put it, the pedal can "absolutely smash the front end of a valve amp into submission"1.
Two boosts in one enclosure
The Heavy Water's clean side is the high-headroom boost from the Dane, a full-range booster that makes your guitar signal louder overall while giving you scope to control the low frequencies510. The second side pairs it with a boost that features germanium diodes in its circuitry for a grittier, more harmonic character34. Gearnews described the result as "a clean and a dirty boost, in one compact enclosure"5.
Each side has three controls:
| Control | Side | What it does |
|---|---|---|
| Boost | Harmonic Germanium side | Sets the boost level of the germanium side4 |
| Lows | Harmonic Germanium side | Shapes the low frequencies of that side4 |
| Engage footswitch | Harmonic Germanium side | Switches the germanium boost in or out4 |
| Boost | Smooth Dane side | Sets the boost level of the clean side4 |
| Lows | Smooth Dane side | Shapes the low frequencies of that side4 |
| Engage footswitch | Smooth Dane side | Switches the clean boost in or out4 |
The two sides can be used independently or combined to hit the front end of a valve amp hard12. Guitar World's reviewer Trevor Curwen found that "both boosts on together is capable of delivering a ridiculous amount of gain that will truly pummel your amp"3. On the same publication, Daniel Steinhardt of That Pedal Show described the pedal's character in use:
"the richness of the boost just has a clarity to it that is completely dynamic off the volume control"2
Steinhardt, speaking to Guitar World, also described stacking the boosts as giving a beautiful organic drive sound, and named the Heavy Water as his current choice for harmonic boost, with its two boost channels deployable individually or in combination211.
Where it came from
The Heavy Water's lineage runs through two earlier ThorpyFX pedals. It grew from the Chain Home Tremolo, whose volume drop prompted Thorpe to build a high-headroom Dane smooth boost circuit into it; requests for a standalone Dane boost then became the Heavy Water, with its smooth boost, harmonic boost, and both together11. Guitar.com's 2019 effects awards noted that "Adrian Thorpe came up with the Heavy Water" as a dual boost unit featuring both the Dane-derived clean boost and one that uses germanium diodes6.
The pedal debuted in August 2019 in a smaller enclosure than the wider Dane52. For 2022, ThorpyFX added a CUSTOM SHOP version with TRS inputs and outputs, for players using loopers who want to activate both sides independently9.
Versions: the MKII Artwork Edition
In May 2025, ThorpyFX released the MKII Artwork Edition of the Heavy Water (alongside the Field Marshal fuzz) as part of its artwork refresh series, with new main artwork, water droplets appropriately featured, and decorated side panels4.
The MKII Artwork Edition is a cosmetic update only. The sound, feel and function are identical to the predecessor, and the circuits are unchanged4.
The MKII carries a price of £219.99 and is available from the ThorpyFX Webstore and leading international dealers4.
What reviewers said
Guitar World's review lists the pedal as a dual boost pedal with true bypass, powered by a 9V DC adaptor (not supplied), measuring 65 (w) x 125 (d) x 53mm (h)3. Guitar.com's review likewise noted true bypass and 9V mains supply only8.
Guitar Pedal X's Stefan Karlsson, tracing the pedal's origin, wrote that the germanium diode solution delivers even more harmonic content than the Spaceman germanium transistor take, with harmonics that add depth of richness and make everything sound better, fuzzes in particular11. In a separate piece he described the Heavy Water as serving beautifully as a superb always-on tone enhancer delivering exceptional added harmonics and texture7.
Videos
Sound Affects
Questions
What is the difference between the two sides of the Heavy Water?
The right-hand side is the high-headroom clean boost taken from the ThorpyFX Dane, which makes your signal louder without adding grit. The left-hand side is a dirtier boost that uses germanium diodes in its circuitry for a different, more harmonic character. Each side has its own Boost and Lows controls and its own engage footswitch.
Can the Heavy Water's boosts be used at the same time?
Yes. Each side can be engaged independently, and both circuits can be combined to hit the front end of a valve amp hard. Guitar World's reviewer found both boosts on together deliver a very large amount of gain, and Daniel Steinhardt described stacking them as giving a beautiful organic drive sound.
What power does the ThorpyFX Heavy Water need?
It runs on a 9V DC power supply, which is not supplied with the pedal. There is no battery option, so an external 9-volt supply is required. It is true bypass, with a standard input and a standard output.
How big is the ThorpyFX Heavy Water?
It measures 65mm wide by 125mm long by 53mm high at the top of the knobs, a smaller enclosure than the wider Dane pedal it borrows its clean boost from. That makes it a compact, small-format fit for a pedalboard.
Is the Heavy Water based on the ThorpyFX Dane?
Yes. It started life as the boost side of the ThorpyFX Dane overdrive and boost, released as a standalone pedal after many requests, with a second germanium-diode boost added. Its lineage goes back further, to the high-headroom Dane smooth boost circuit built to fix the volume drop of ThorpyFX's Chain Home Tremolo.
How much does the ThorpyFX Heavy Water cost?
At launch in August 2019 it was available for $230.85. In April 2024 Guitar Pedal X listed it at £209.99 ($279), and the MKII Artwork Edition of May 2025 carried a price of £219.99. The MKII's circuits are unchanged from the original.
Sources (11)
- ThorpyFX Introduces the Heavy Water Dual High Headroom Boost, Guitar World, 2019-08-06
- Daniel Steinhardt reveals his top 5 pedals (from the past 5 years), Guitar World, 2025-05-29
- ThorpyFx Heavy Water review, Guitar World, 2020-05-07
- Guitar Pedal X, Guitar Pedal X, 2025-05-08
- Thorpy FX launches the Heavy Water Dual High Headroom Boost pedal, gearnews.com, 2019-08-05
- Gear Of The Year: Best effects pedals of 2019, Guitar.com, 2019-12-03
- Guitar Pedal X, Guitar Pedal X, 2024-04
- Review: ThorpyFX Heavy Water, guitar.com, 2019-10-21
- HEAVY WATER, ThorpyFX, 2022
- What does a guitar boost pedal actually do?, Guitar.com, 2024-04-03
- Guitar Pedal X, Guitar Pedal X, 2024-04-08
