Spaceman Mercury IV

The Spaceman Mercury IV is a germanium harmonic boost pedal from Spaceman Effects, a small builder working out of Portland, Oregon. Spaceman describes it as "a dynamic tone enhancer, created to bring out additional harmonic character from instruments and amplifiers"¹. It is a discrete analog circuit built around a meticulously selected vintage germanium transistor, and it launched in March 2018 as an update to the Mercury III circuit from eight years earlier¹,².

The idea is simple: a Boost knob gives you up to 35 dB of clean volume to push a tube amp, while a separate Harmonics knob blends in light, tube-like distortion. Because those even-order harmonics are generated independently of the boost, you control how much of each goes into the signal chain¹,³.

What it does: a germanium harmonic boost

Spaceman positions the Mercury IV as a boost first, but with a harmonic character that sets it apart from a plain volume pedal. The even-order harmonics the circuit generates are described by Delicious Audio as having "a sound and response like hot tubes on the edge of breakup, with subtle sparkle, complex textures, and octave-like overtones"⁴. Those harmonics are generated independently of the boost level, so you can add warmth and dimension at low volume or stack it with a full 35 dB push into an amplifier already near breakup¹,⁴.

The Harmonics knob is the heart of the pedal. RockBoard's PedalPedia entry describes it as blending in "light, tube-like harmonic distortion, even adding octave-like overtones to your solos and unique textures to chords"³. Pedal of the Day's reviewer noted that the knob "is very reactive to your playing, instrument and pickup choices, and reacts strictly to the input signal," which means the same setting behaves differently with different guitars⁷.

The pedal sits in a family of Spaceman harmonic boosters. Pedal of the Day describes the Mercury IV as "rooted firmly in the Boost department, but with a couple more options" than the earlier Saturn V Harmonic Boost⁷. The Mercury IV is itself an updated version of the Mercury III circuit, and the first run of White, Brass Metallic, Chrome and Copper editions used germanium transistors from the same vintage stock as the original Mercury III first run editions¹.

The controls

The control set is deliberately small: two knobs and two toggle switches.

The nine tone-shaping options from the two toggles are what separate the Mercury IV from a simpler booster like the Saturn V Harmonic Boost, which Pedal of the Day describes as having fewer options⁷.

Power and connections

Signal enters and leaves through quality open-frame Switchcraft jacks, and the pedal uses true-bypass switching through what Spaceman calls the heaviest duty mechanical footswitch available¹,⁴. Input resistance is 1 Meg Ohm, so it presents a normal load to passive pickups¹.

Build, editions and history

Spaceman builds the Mercury IV entirely in Portland, Oregon, describing it as "completely hand-soldered and handmade in Portland Oregon to the highest degree of quality possible"¹. The construction details include an immaculate hand-soldered PCB, detailed military-spec wiring, and a cast aluminum enclosure with a durable, engraved vinyl faceplate and jewel indicator light¹,⁴.

The pedal launched in March 2018 in five signed and numbered limited editions: Chrome, White, Brass, Black and Silver, with the Silver Edition priced at $299². Guitar.com reported the release as a Mach 4 version of the builder's boost pedal, priced from $299 to $349⁸. Later runs included a Pink Edition, a 1-of-1 with a hand-hammered number tag that Spaceman matched to a limited run of pink Sputnik III pedals, and an Olive Drab Edition limited to a batch of four, with the maker noting "We wanted to make more than just four, but are currently limited by available parts"⁵⁶,⁶. Each pedal number is hand-hammered on the nameplate, and each unit is signed and numbered on the inside¹.

What reviewers said

Published coverage of the Mercury IV has been positive. Delicious Audio called the Harmonics control the pedal's "real magic"⁴. Pedal of the Day's 2019 review highlighted the Harmonics knob's reactivity to playing and pickup choices, and pointed used-market buyers toward third-party listings⁷. Writing at Guitar Pedal X in 2022, Stefan Karlsson, who uses the Mercury IV in his own chain to further sculpt and enhance tone, called it "my ultimate Harmonics multiplier" for a long time⁹.

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