Devil

Devil V2.0 schematic — three JFET common-source stages with inter-stage silicon clipping and mid-sweep tone stack

Signal flow: input → R1‖R2 pull-down → C1 DC block → D1 → Q1 (JFET stage 1) → GAIN pot → C4 → R6 → D2 → Q2 (JFET stage 2) → tone stack (MID + SW1 bass switch) → D3 → Q3 (JFET stage 3) → C8 → R16+R17 → LEVEL pot → output. Each gain stage is mechanically and electrically identical apart from the trimpot reference.

Identical JFET gain stages (×3)

Each of Q1, Q2, Q3 is a common-source amplifier built around the PF5102 (or J201/2N5457). The drain pulls up through a very large 10 M resistor (R3, R8, R13) in series with the trim network — the unusually-large drain resistor is what gives a Davisson-style stage its high voltage gain. The drain bias is set by the 20 k fixed resistor (R21, R4, R20) in series with the 50 k trimpot (R14, R9, R22), both feeding from the regulated supply rail. The source resistor is 47 k (R5, R10, R15) with a 1 µF electrolytic bypass (C3, C7, C9) across it — the bypass cap effectively shorts the source resistor for AC, so the stage runs at full unbypassed gain across the audio band (the bypass corner sits well below 4 Hz).

Inter-stage series clipping diodes (D1, D2, D3)

Between every gain stage sits a 1N914 silicon diode in series with the signal path, with the cathode toward the previous stage and the anode at the next stage's gate. Because the JFET gate is held at a high DC level by its 10 M drain resistor, the diode is reverse-biased at rest. AC signal swings from the previous stage modulate this bias; when the signal swings hard enough in one direction the diode is driven forward and conducts, in the other direction it stays cut off and blocks signal. The result is an asymmetric series clipper that reshapes the waveform between every stage — it is the single biggest contributor to the Devil's character. This is a Joe Davisson signature trick.

GAIN control

The GAIN pot (1 M log) sits between Q1's output and the second stage. It is wired as a variable shunt — wiper to ground via C3's lower end, top of element fed from Q1 source through C2 (100 n). This sets how much of Q1's output reaches the inter-stage network feeding Q2. Turn it up and you push Q2 (and consequently D2, then Q3 and D3) harder into clipping.

Mid-sweep tone stack and bass switch

After the second stage, an interactive RC network shapes the tone before the third stage. R11 (33 k) and R12 (33 k) form a series path between Q2's output and Q3's input, with C11 (1 n) bridging the same two points. C12 (1 n) shunts to ground at the mid-tap — and SW1 adds either C14 (47 n) or C15 (22 n) in parallel with C12 to deepen the low-frequency shunt for one of two character settings. The MID pot (100 k reverse-log) is a variable shunt-to-ground via C13 (100 n) at Q2's output node — sweep it through its range to scoop or boost mids. The reverse-log taper compensates for the perceptual non-linearity of mid response so the knob feels approximately linear by ear. R19 (1 M) at the third-stage input sets the input impedance and load presented to the tone stack.

Output stage

Q3's drain output is DC-blocked by C8 (100 n), then attenuated by the divider R16 (470 k) + R17 (470 k) before reaching the LEVEL pot (100 k log). This combination scales the very-high signal level coming out of the third clipping stage down to a usable output range and keeps the LEVEL pot operating in the most useful part of its travel.

Power supply

D4 (1N4001) provides reverse-polarity protection at the +9 V input. R18 (100 R) and C10 (100 µF) form a series-RC supply lowpass filter for the gain stages — corner around 16 Hz, which keeps power-rail ripple and switching artefacts well out of the audio band. The whole circuit runs straight off the filtered 9 V — there is no virtual-ground reference and no charge pump.

The Devil's three gain stages are all identical in topology and component values. Voltage gain of a common-source JFET stage is A_v ≈ −g_m × R_d, but the practical value depends strongly on the JFET's individual transconductance — PF5102 typically sits in the 1–4 mA/V range at the bias point used here. With 10 M sitting at the drain (R3/R8/R13), the small-signal voltage gain of an unbypassed stage is enormous on paper; in practice each stage is rapidly driven into JFET self-clipping and the inter-stage series diodes hard-clip what's left, so headline gain numbers are not particularly meaningful.

Inter-stage clipping (D1, D2, D3)

Tone stack approximate corners

The mid-control tone stack between Q2 and Q3 is a heavily-interacting RC network — the bass switch, the MID pot setting, R11+R12, and the cap stack all influence each other. The numbers below are approximate single-pole RC corners assuming the rest of the network is at high impedance:

Component placement (silkscreen)

Populated PCB — front side

Standard through-hole population order — lowest profile first, tallest last. The pots are board-mounted on the back side of the PCB and are fitted last, after the front side is fully populated and inspected.

Back side after fitting the pots and SW1 — cover the pot backs with tape before soldering to avoid shorts

Each JFET stage's drain bias is set by the 20 k fixed + 50 k trimpot network feeding the +9 V rail. The drain idle voltage is what the trimmer adjusts — the goal is to land each drain somewhere around the middle of the available headroom so the stage has symmetric room to swing in both directions before clipping into the rails. PF5102, J201 and 2N5457 vary considerably in I_DSS and pinch-off, so per-device bias adjustment can be useful for outliers.

Adjusting if needed

If a stage misbehaves: power the pedal up, leave the input disconnected, and measure the drain voltage of the misbehaving JFET to ground with a DMM. Aim for a drain voltage around 4–5 V with a 9 V supply — that gives the stage roughly equal swing room above and below the bias point. Turn the corresponding trimmer slowly to reach that range. Once all three stages sit in the 4–5 V drain range, reconnect the input and listen — the pedal should now be working normally.

JFET drain pin reference

Experimentation

Once it's working, the trimmers also become a tone-shaping tool. Pulling each stage's drain up toward the rail changes the symmetry of how the next inter-stage diode (D1/D2/D3) clips it — in some cases noticeably alters the harmonic content of the pedal. Try moving R9 (the middle stage) most aggressively if you're after voicing changes; Q2 sits between two clipping diodes and has the most influence on overall character.

The Devil uses standard true-bypass pedal wiring. Three connections come off the PCB at the IN, GND and OUT corner pads:

Suggested enclosure layout

1590B fits comfortably with three 16 mm pots and a small SPDT toggle, plus the footswitch and a status LED. Top row: GAIN — MID — LEVEL. SW1 toggle goes to the left or below the LED depending on enclosure. Top jacks (input on the right, output on the left when looking at the back of a top-mount-jack enclosure, mirroring the signal flow inside). See the prototype photo at the top of §01 for one workable layout.

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