Umble V1.5 — Build Documentation

Overview & Controls

The Umble is the runoffgroove circuit of the same name — four cascaded JFET common-source gain stages, a Dumble-style three-band tone stack with an additional VOLUME (gain trim) and MASTER, and a pair of two-pole RC output filters that smooth the high-frequency content from the JFET clipping. Of the runoffgroove amp simulators, the Umble has perhaps the most distinctive voicing and the widest range of useful tonal possibilities — once it is biased correctly.

PCB V1.5 — SMD JFET option: the V1.5 board adds dual footprints for every JFET. Each Q position now has both the original through-hole TO-92 pads and an SMD SOT-23 footprint next to it. Populate one or the other, never both. The SMD option lets you use the modern SST/MMBF series equivalents (SST201, MMBFJ201, etc.) when through-hole J201 is hard to source, or simply for a cleaner, more compact build.

Build variants

Low-gain (original ROG)

Leave C16, C17 and C18 unpopulated — the source resistors of Q2, Q3 and Q4 stay un-bypassed. This is the 100% original runoffgroove Umble voicing. C5 (on Q1) is always fitted.

High-gain (modified)

Populate all four 4.7 µF source-bypass caps (C5, C16, C17, C18). Each stage gains noticeably more drive across the audio band. This is the default suggested by the BOM and gives the wider tonal range described above.

Front-panel controls

Control	Type	Function
VOLUME (GAIN)	1 M log	Despite its name on the original ROG layout, this is the gain knob — it sets how hard the second, third and fourth JFET stages are driven. Crank it for more saturation
BASS	500 k log	Tone stack bass — interactive with TREBLE and MID
MID	100 k linear	Tone stack mid — central scoop/boost
TREBLE	250 k log	Tone stack treble — interactive with BASS
MASTER	100 k log	Master output volume — sits at the very end of the chain after the post-Q4 filter network

Pot taper note: the BOM specifies 500 k log for BASS and 250 k log for TREBLE — these specific tapers were chosen after iterating on the original linear tapers, which made the controls feel uneven. With the listed values the tone stack has a wide, evenly-spread sweep.

Circuit Theory

ROG Umble V1.4 schematic — the V1.5 PCB carries the same circuit with optional SMD JFET footprints added next to each TO-92 position

Signal flow: IN → R1+C3 input filter → Q1 (JFET stage 1) → Dumble tone stack (TREBLE/BASS/MID + R17, C8, C9, C10) → VOLUME pot → Q2 (JFET stage 2) → Q3 (JFET stage 3) → Q4 (JFET stage 4) → two-pole RC output filter (R14, C14, R13, C13) → MASTER pot → OUT. The whole circuit runs from a regulated 9 V rail; there is no virtual-ground reference and no charge pump.

Q1 — input stage

Q1 (J201) runs in common-source. Drain bias is set by TRIM1 (50 k cermet trimpot) from the +9 V rail, with TP1 testpoint on the drain for biasing measurement. The source has R3 (1.5 k) with C5 (4.7 µF) bypass — Q1's source is always bypassed, so it always runs at full unbypassed gain across the audio band. Input filtering is provided by R1 (33 k) + C3 (270 p) as a single-pole lowpass into the gate (corner ≈ 18 kHz, primarily an RFI reject), with R2 (1 M) as the gate pull-down.

Dumble-style tone stack

Q1's drain feeds a three-band interactive tone network. R17 (100 k) is the input series resistor; C8 (330 p) bridges the input directly to the TREBLE pot wiper for high-frequency content; C9 (100 n) is the bass-coupling cap into the BASS pot; C10 (47 n) couples to the MID pot. R15 (10 k) sets the bass pot's reference impedance to ground. The TREBLE (250 k log), BASS (500 k log) and MID (100 k linear) all share the same centre node — this is the typical Marshall/Dumble-style topology where each control loads the centre node and influences the others. The output of the tone stack passes through VOLUME (1 M log) with C11 (47 p) as a treble bridge across the upper half of the pot — a classic "bright cap" that preserves treble content as VOLUME is rolled down.

Q2, Q3, Q4 — gain stages

The three following JFET stages are nearly identical in topology. Each is a J201 in common-source: TRIM2/3/4 (50 k) sets the drain bias, with TP2/TP3/TP4 testpoints; R4/R5/R6 (1.5 k each) are the source resistors; C16/C17/C18 (4.7 µF, optional) are the source-bypass caps that toggle between low-gain and high-gain variants. The differences are in the inter-stage coupling networks:

VOLUME → Q2 input

Wiper of VOLUME → C11 (47 p, bridge) and direct path → C4 (330 p, HF shunt) and R10 (220 k pull-down) → Q2 gate. The 220 k pull-down sets the input impedance.

Q2 → Q3 input

Q2 drain → C7 (10 n) coupling cap → R9 (68 k) series → R8 (100 k pull-down) → Q3 gate. The 10 n coupling sets a HP corner around 159 Hz with R8 — a touch of bass tightening between stages.

Q3 → Q4 input

Q3 drain → C12 (22 n) coupling → R12 (180 k) series → R7 (100 k pull-down) → Q4 gate. HP corner around 72 Hz with R7 — full bass passes through to the last stage.

Q4 → output filter

Q4 drain → C6 (15 n) coupling into the two-pole RC lowpass network described below.

Output filter and MASTER

After Q4, the signal passes through a two-pole RC lowpass network: R14 (15 k) + C14 (2.2 n) shunting to ground gives one corner at ≈ 4.8 kHz, then R13 (15 k) + C13 (2.2 n) in the same configuration gives a second pole at the same corner. The cascaded result is a 12 dB/octave roll-off above ≈ 5 kHz — this is what tames the buzzy high-frequency content that four cascaded JFET stages naturally produce, and is a big part of why the Umble sounds amp-like rather than fizzy. The filter then drives the MASTER (100 k log) pot directly.

Power supply

The 9 V supply enters via the BATTERY/PWR pads. D1 (1N4001) provides reverse-polarity protection. R16 (100 R) + C15 (100 µF) form a series-RC supply lowpass — corner around 16 Hz, well below audio, which removes ripple and switching-noise from the JFET drain rails.

Stage Analysis

All four JFET stages share the same common-source topology: TRIM (50 k) drain pull-up, R (1.5 k) source resistor, optional C (4.7 µF) source bypass for the high-gain variant. Voltage gain of a common-source stage is approximately A_v ≈ −g_m·R_d/(1 + g_m·R_s); with the source bypass cap engaged, the R_s term effectively shorts at AC and the stage runs at full −g_m·R_d. For the J201 at this bias point (g_m ≈ 1–4 mA/V), each unbypassed stage gives modest gain; bypassed, every stage runs much harder and the chain hits saturation early.

Source bypass corner (high-gain variant)

When C5/C16/C17/C18 are all populated: each R_s = 1.5 k in parallel with C_byp = 4.7 µF gives a corner at ≈ 22.6 Hz. Above this frequency (i.e. across the entire audio band) the source is effectively grounded for AC and gain is at its maximum. Below 22 Hz the bypass cap drops out and gain falls back to the unbypassed value — this is purely sub-audio territory and has no audible effect.

Q1 — input stage

Drain biasTRIM1 (50 k)

Source / bypassR3 (1k5) ‖ C5 (4u7)

Bypass corner≈ 22.6 Hz (always fitted)

Input LP≈ 18 kHz (R1 + C3)

Q2 — gain stage

Drain biasTRIM2 (50 k)

Source / bypassR4 (1k5) ‖ C16 (4u7, optional)

Bypass corner≈ 22.6 Hz (when C16 fitted)

InputFrom VOLUME wiper

Q3 — gain stage

Drain biasTRIM3 (50 k)

Source / bypassR5 (1k5) ‖ C17 (4u7, optional)

Bypass corner≈ 22.6 Hz (when C17 fitted)

Input HP≈ 159 Hz (C7 + R8)

Q4 — output drive stage

Drain biasTRIM4 (50 k)

Source / bypassR6 (1k5) ‖ C18 (4u7, optional)

Bypass corner≈ 22.6 Hz (when C18 fitted)

Input HP≈ 72 Hz (C12 + R7)

Output filter

Stage	Components	Corner	Effect
First pole	R14 (15 k) + C14 (2.2 n)	≈ 4.8 kHz	First-order LP, drops 6 dB/octave above 5 kHz
Second pole	R13 (15 k) + C13 (2.2 n)	≈ 4.8 kHz	Stacks with first pole — combined 12 dB/oct above 5 kHz

Why this filter matters: four cascaded JFET stages produce a lot of harmonic content past 5 kHz when driven hard — without the post-Q4 filter the pedal would sound buzzy and harsh. The 12 dB/octave roll-off mirrors the natural high-end damping of a guitar speaker and is what makes the Umble sound like an amp rather than a clipper.

Tone stack — Dumble topology

The three-band tone stack is the standard interactive Marshall/Dumble topology — every control loads the same central node, so adjusting any one knob changes how the others respond. There is no clean way to hand a single corner frequency to each band. Functional behaviour:

Knob	Components	Behaviour
BASS	C9 (100 n) + 500 k pot + R15 (10 k)	Bass shelf — interaction with R15 keeps the pot from nulling the bass entirely at one extreme
MID	C10 (47 n) + 100 k linear pot	Mid scoop/boost depth around the centre frequency of the stack
TREBLE	C8 (330 p) + 250 k log pot + C11 (47 p) bright cap on VOLUME	Treble shelf and bright-cap combination — keeps high content present at low VOLUME settings

Bill of Materials

Note on numbering: the silkscreen reference designators have gaps — there is no R11, no C1 and no C2 on this board. Total active part counts are 16 resistors and 16 capacitors. Don't go looking for a missing R11 or C1; they were never assigned.

Ref	Qty	Value	Colour code	Notes
Resistors — 1% metal film, ¼ W
R1	1	33 k	Orange · Orange · Black \| Red · Brown	Input series — RFI filter with C3
R2	1	1 M	Brown · Black · Black \| Yellow · Brown	Input pull-down — Q1 gate DC reference to ground
R3, R4, R5, R6	4	1.5 k	Brown · Green · Black \| Brown · Brown	JFET source resistors — one per stage. R3 = Q1, R4 = Q2, R5 = Q3, R6 = Q4
R7, R8, R17	3	100 k	Brown · Black · Black \| Orange · Brown	R7 = Q4 gate pull-down. R8 = Q3 gate pull-down. R17 = tone stack input series
R9	1	68 k	Blue · Grey · Black \| Red · Brown	Series between Q2 → Q3 coupling cap and Q3 gate
R10	1	220 k	Red · Red · Black \| Orange · Brown	Q2 gate pull-down — sets input impedance from VOLUME wiper
R12	1	180 k	Brown · Grey · Black \| Orange · Brown	Series between Q3 → Q4 coupling cap and Q4 gate
R13, R14	2	15 k	Brown · Green · Black \| Red · Brown	Two-pole output filter series resistors — set the post-Q4 LP corner with C13/C14
R15	1	10 k	Brown · Black · Black \| Red · Brown	BASS pot reference to ground — limits maximum bass cut
R16	1	100 Ω	Brown · Black · Black \| Black · Brown	Power-supply RC filter resistor (with C15)
Trimpots — 6 mm cermet
TRIM1, TRIM2, TRIM3, TRIM4	4		50 k	JFET drain bias trimmers — one per stage, fed from +9 V rail. See §06 for biasing procedure using TP1–TP4
Capacitors — Ceramic / Mica
C3	1		270 p	Input HF shunt to ground — RFI filter with R1
C4, C8	2		330 p	C4 = HF shunt at Q2 gate. C8 = treble bridge in tone stack (Q1 drain to TREBLE wiper)
C11	1		47 p	VOLUME pot bright cap — bridges the upper half of the pot to keep treble at low settings
Capacitors — Film (non-polarised)
C6	1		15 n	Q4 → output filter coupling cap
C7	1		10 n	Q2 → Q3 inter-stage coupling cap
C9	1		100 n	Tone stack bass coupling — feeds the BASS pot
C10	1		47 n	Tone stack mid coupling — feeds the MID pot
C12	1		22 n	Q3 → Q4 inter-stage coupling cap
C13, C14	2		2.2 n	Output filter shunt caps — set the post-Q4 lowpass corner with R13/R14
Capacitors — Electrolytic (polarised)
C5	1		4.7 µF	Q1 source bypass — always fitted. Observe polarity (long leg = +, + side faces source)
C16, C17, C18	3		4.7 µF	Optional — leave off for low-gain (original ROG) build, populate for high-gain. Source bypass caps for Q2, Q3, Q4. Polarity matters — + side toward source
C15	1		100 µF	Power supply bulk decoupling cap — observe polarity
Diode
D1	1		1N4001	Reverse-polarity protection on the +9 V input. Observe cathode stripe
JFETs (×4) — choose ONE footprint per Q position
Q1, Q2, Q3, Q4 (TH)	4		J201 (TO-92)	Through-hole, D-S-G pinout in TO-92. PF5102 2N5457 are pin-compatible drop-ins if J201 is unobtainable — bias trimmers absorb the variation
Q1A, Q2A, Q3A, Q4A (SMD)	4		SST201 / MMBFJ201 (SOT-23)	SMD alternative for V1.5 boards — the SOT-23 footprints are next to the TH positions. MMBFJ201 SST201 MMBF5457. Verify pinout for your specific part code
Pots
VOLUME	1		1 M log (A)	16 mm right-angle PCB-mount. Functions as the gain knob despite the "VOLUME" label
BASS	1		500 k log (A)	16 mm right-angle PCB-mount. Log taper — different from the original ROG which used linear; the log taper gives a more even sweep
MID	1		100 k linear (B)	16 mm right-angle PCB-mount
TREBLE	1		250 k log (A)	16 mm right-angle PCB-mount. Log taper for the same reason as BASS
MASTER	1		100 k log (A)	16 mm right-angle PCB-mount — the actual output volume control

SMD vs through-hole — pick one per Q: the V1.5 PCB has dual footprints at every JFET position. Each through-hole TO-92 pad set has its own SMD SOT-23 pad set adjacent to it; both connect to the same drain, source and gate nets. Populate either the through-hole part OR the SMD part at each Q position, never both. Mixing within a single Q position will short the device. You can mix between positions (e.g. through-hole at Q1, SMD at Q2/Q3/Q4) if needed, but most builders pick one type and stick with it.

SMD JFET sourcing: common modern equivalents in SOT-23 — MMBFJ201 (Onsemi, direct J201 equivalent), SST201 (Vishay, J201 equivalent), MMBF5457 (Onsemi, 2N5457 equivalent). All are widely available at major distributors and use the standard SOT-23 G-S-D pinout. The TO-92 J201 itself is going End-of-Life at several manufacturers, which is why the SMD option was added for V1.5.

Build Guide

Umble PCB silkscreen — component placement

Component placement (V1.4 silkscreen). The V1.5 PCB carries the same component layout with an added SMD footprint adjacent to each Q position

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

Wire the off-board pads BEFORE mounting the pots: due to the small board size, several IN/OUT/GND/PWR pads sit underneath where the pots will end up. Solder the off-board wires first, route them out away from the board, then fit the pots on top. If you fit the pots first you will struggle to get a soldering iron onto the buried pads.

If building the SMD JFET variant — fit the JFETs first

SMD components are easier to solder to a bare board than to a populated one. If you are using SST201 / MMBFJ201 / MMBF5457 in SOT-23, place and solder all four QnA positions now: tin one pad, tack the part down, then solder the other two pads. Skip the through-hole Q1–Q4 sockets entirely — those positions stay empty for the SMD variant.

Diode and resistors

Solder D1 (1N4001) — observe the cathode stripe against the silkscreen. Then populate all 16 resistors. They lie flat against the board and have no polarity. Note the gap in numbering — there is no R11 in the BOM. Take care to distinguish the 100 Ω (Brown-Black-Black-Black) from the 100 k (Brown-Black-Black-Orange) — the only difference is one band.

Transistor sockets (through-hole variant only)

The docx recommends socketing the JFETs — useful if you plan to swap them or hand-match. Solder TO-92 sockets, or use single-row machined-pin headers cut to length. Skip this step if you fitted SMD JFETs in step 1.

Trimpots (TRIM1–TRIM4)

Four 50 k cermet trimpots, 6 mm body. They only fit one way around. Set each to roughly mid-position before soldering — fine adjustment comes after power-up.

Ceramic / mica capacitors

C3 (270 p), C4/C8 (2× 330 p), and C11 (47 p). No polarity. Push fully home, solder, clip leads.

Film capacitors

Box film capacitors next, in ascending value: C13/C14 (2× 2.2 n), C7 (10 n), C6 (15 n), C12 (22 n), C10 (47 n), C9 (100 n). No polarity.

Electrolytic capacitors

C5 (4.7 µF, always fitted) and C15 (100 µF). If building the high-gain variant also fit C16, C17, C18 (3× 4.7 µF). For the low-gain original-ROG variant, leave C16/C17/C18 empty. Polarity matters for all electros — long leg = positive, match the silkscreen + symbol and the stripe on the can.

Insert through-hole transistors (if applicable)

If using TO-92 J201 (or PF5102 / 2N5457): Q1, Q2, Q3, Q4 all into their respective sockets. D-S-G pinout — match the flat side of the package to the silkscreen flat. Verify the datasheet for whatever part you actually have in hand.

Inspect the front side

Before flipping the board for the pots, inspect every solder joint under good light. Measure resistor values one more time, check electrolytic and diode polarities, look for cold joints and bridges. If you went the SMD JFET route, check the SOT-23 joints particularly carefully — the legs are tiny and bridges are easy to miss.

Off-board wires

Solder the IN, OUT, GND, +9 V (and the BATTERY pad if used) wires now, before fitting the pots — see the notice above.

Pots — back side mounting

Flip the PCB. The five pots are right-angle 16 mm board-mount types and go onto the back face of the board. Clip off the small alignment bracket on the top of each pot if your enclosure does not have the matching slots. Cover the back of each pot body with insulation tape (kapton or double-sided tape) — this prevents the freshly-soldered pin tips on the front side from shorting against the metal pot housing. Push each pot firmly against the board so it seats flat, then solder.

Pot soldering technique

For the wide flat pot pins: apply solder to the middle pin first, then while the joint is still molten, pull the pot away from the board by about 1 mm and let it harden. Then solder the remaining pins. This lets the pot body sit flat against the PCB without the wide pins lifting it.

Pot mounting technique — kapton tape on back of pot before soldering

Pot soldering — solder the middle pin first, pull the pot back a millimetre while the joint is still molten, then solder the rest. Insulation tape on the pot back prevents shorts

JFET pinout — verify before soldering: the through-hole footprint is laid out for D-S-G in TO-92 (looking at the flat face); the SMD footprint is laid out for SOT-23 with the standard G-S-D pinout used by SST201 / MMBFJ201 / MMBF5457. Always check the datasheet for the exact part code printed on your transistors before populating — pinout variants do exist between manufacturers.

Biasing & Setup

The Umble has four trimmers, one per JFET stage, plus four matching test points (TP1–TP4) that bring out the drain voltage of each stage to a labelled pad next to the trimmer. With four stages this is too many adjustments to do reliably by ear — use a multimeter.

Bias targets

Stage	Trimmer	Test point	Target idle V_D
Q1 — J201	TRIM1 (50 k)	TP1	≈ 5 V
Q2 — J201	TRIM2 (50 k)	TP2	≈ 5 V
Q3 — J201	TRIM3 (50 k)	TP3	≈ 5 V
Q4 — J201	TRIM4 (50 k)	TP4	≈ 5 V

Setup procedure

Power up with no input signal

Plug in the 9 V supply. All four stages should be quiet. Do not connect a guitar or signal source for the bias step.

Bias each stage in turn

Set DMM to DC volts. Black probe on a GND pad. Touch the red probe to TP1. Adjust TRIM1 until the meter reads approximately 5 V. Move to TP2 and adjust TRIM2 to 5 V. Repeat for TP3/TRIM3 and TP4/TRIM4. The J201 is fairly forgiving — anywhere from 4 V to 6 V puts the stage solidly into its working area.

Listen and fine-tune by ear

Disconnect the supply, plug in a guitar and an amp, power back up, and play. All four stages should now be in their working range. The 5 V meter target gets you into the area; subtle fine-tuning by ear afterwards (turning each trimmer slightly while playing) often finds a slightly different sweet spot — fullest, most dynamic response, no fizz at the top of the swing.

Once it's biased, it stays biased: JFETs are stable parts. You should not need to re-bias unless you swap a transistor or the unit is exposed to a major temperature change. The Umble can sit in a 1590B for years and still measure the same numbers when you crack it open.

Wiring & Enclosure

The Umble uses standard true-bypass pedal wiring. Audio and power come off four PCB pads:

PCB pad	Wire to
IN	Tip (signal) of input jack — through the 3PDT footswitch in a true-bypass arrangement, if used
OUT	Output side of the 3PDT switch (or directly to output jack tip if always-on)
GND	Sleeve of both jacks (typically through the input jack's sleeve, which carries chassis ground), and the negative side of the DC jack
+9 V / BATTERY	Centre-negative DC jack tip — standard Boss-style polarity. D1 protects against reverse polarity

True-bypass with status LED: for a 3PDT true-bypass switch with a status LED, wire the LED+ to +9 V through a series 4.7 k or higher resistor (chosen for your LED's brightness) and the LED− to one of the spare poles of the 3PDT that ties to GND when the effect is engaged. The Umble PCB does not provide an onboard footswitch or LED — those go in the enclosure.

Enclosure — drilling template

Suggested enclosure: 1590BB — five 16 mm pots in a row need around 70 mm of width, and the deeper 1590BB gives more room for the footswitch and jacks below.

1590BB drilling template — 5 pots, footswitch, two side jacks

Drilling template — measurements in mm. Five pots at 17 mm spacing, 23 mm from the top edge; footswitch centred below; jacks on the short sides

Always check your printout: when printing this template, verify the printed dimensions with a ruler against the figures shown before drilling. PDF/HTML rendering can scale the page, and a 1 mm error per knob position adds up to a board that won't seat properly.

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Disclaimer, Licence & Credits

Circuit credit: the underlying Umble circuit was designed by runoffgroove.com — sound samples, the original schematic and design notes are at runoffgroove.com/umble.html. The PCB layout, the SMD footprint addition (V1.5), the test-point placement (V1.3+) and the optional source-bypass cap modification (V1.4+) are by TH Custom Effects.

PCBs based on runoffgroove circuits purchased from TH Custom Effects are intended for DIY and non-commercial use only. Any commercial use whatsoever is forbidden — please contact runoffgroove for further information if you intend to build commercially. Redistribution of the PCBs and artwork from this document is not permitted.