Overview & Features
The Ginger is a JFET-based emulation of the Ampeg SB-12 Portaflex bass amp, designed by the team at runoffgroove.com. It runs four 2N5457 JFETs to mimic the SB-12's tube preamp stages, uses a passive Baxandall tone stack between stages 1 and 2 for proper shelving bass / treble control, and finishes with a 2N5088 BJT output buffer. The result: ideal for bass players who want the Ampeg sound without lugging a valve amp around — and equally good for drop-tuned guitar where the tonal flexibility of the Baxandall stack really pays off.
Features
- Four 2N5457 JFET stages (Q1–Q4) emulating the SB-12 tube preamp — DC-coupled in places to mimic real valve interaction
- 2N5088 BJT output buffer (Q5) — provides clean low-impedance drive into whatever's downstream
- Baxandall passive tone stack with separate BASS and TREBLE shelving controls — much more flexibility than typical Big-Muff-style tilt controls
- Six 3 mm red LEDs as antiparallel clipping pairs at three of the JFET sources — soft, harmonically rich clipping at ≈ 1.7 V threshold per LED
- Two PCB trimmers (R7, R13) for biasing Q1 and Q2 — necessary because individual 2N5457 devices have varying pinch-off voltages
- 4 controls: BASS, TREBLE, GAIN, VOLUME — all 16 mm right-angle PCB-mount, 100 k log
- 9 V DC powered with reverse-polarity protection (D1) — fits a 1590B enclosure
Populated TH Custom Effects ROG Ginger V1.0 PCB. Six red LEDs (D2/D3 top-left as a pair, D4/D5 mid-right, D6/D7 mid-bottom) and the two 10 k blue trimmers (R7 top-left, R13 top-right) are clearly visible.
A finished Ginger build. Silver enclosure with purple borders and "Ampeg SB12 Portaflex Emulation" graphic — drill template available at diy.thcustom.com/drill-templates/.
Circuit Theory
Full schematic — runoffgroove's Ginger, TH Custom Effects layout V1.0 (03/2014). Original circuit and component values by runoffgroove.
Architecture overview
The signal path is: input → Q1 (gain stage 1) → Baxandall tone stack → Q2 (gain stage 2 with GAIN pot) → Q3 (DC-coupled to Q4) → Q4 → Q5 (BJT output buffer) → VOLUME → output. The clever part is the Q3-Q4-Q5 interaction: Q3 and Q4 are stacked / direct-coupled to mimic the way a tube cathodyne or cathode-follower interacts with a final triode stage in a real valve preamp. Q5 then provides the low-impedance drive to the outside world.
Stage 1 — Q1 input gain stage
Signal enters at IN and is coupled directly into Q1's gate via R4 (33 k) — the JFET's high gate impedance means no input cap is needed. R5 (1 M) is the gate pulldown, setting the input impedance the guitar pickup sees. C3 (220 p) shunts high-frequency noise to ground.
Q1 (2N5457) is a common-source amplifier: drain to VA via R7 (10 k bias trimmer) — adjustable for individual JFET variation; source via R6 (390 Ω) with D2 / D3 (antiparallel red LEDs) across the source resistor. The LEDs clip when the source signal exceeds ≈ 1.7 V, providing soft saturation that emulates a tube's grid current limiting. Output is taken at the drain, AC-coupled by C5 (220 n) into the tone stack.
Stage 2 — Baxandall tone stack
This is what gives the Ginger its tonal flexibility. Unlike the typical Big-Muff tilt control found in many DIY drives, this is a proper passive Baxandall with separate shelving filters for bass and treble. The BASS pot (100 k log) sits in a network with R8 (22 k), R9 (3 k9), C6 (22 n), C7 (100 n) — its action is dominant at low frequencies and tapers off above the upper bass region. The TREBLE pot (100 k log) sits in a parallel network with R10 (33 k), C8 (3 n3), C9 (10 n) — its action is dominant at high frequencies. The two networks combine through R11 (100 k) which presents a high-impedance load to the next stage. See §03 for the calculated shelving frequencies.
Stage 3 — Q2 recovery stage with GAIN pot
The Baxandall stack is passive — it has insertion loss. Q2 makes that loss back. C10 (100 p) shunts gate RFI; the rest of Q2 is similar topology to Q1 — drain via R13 (10 k trim) to VA, source via R12 (390 Ω) with D4 / D5 LED clipping pair. The drain output drives the GAIN pot (100 k log) directly with R21 (6 k8) below the pot setting the minimum gain limit, then on into Q3.
Stage 4 — Q3 / Q4 emulator core
This is where the SB-12 emulation really happens. Q3 (2N5457) has its drain tied to VA (via R16 internally) — a fairly ordinary common-source. The interesting bit: Q4 (2N5457) has its gate fed from Q3's source/drain region in a direct-coupled stack. R14 (1 M) biases Q3's gate to VR (the virtual reference, ≈ 4.5 V) — with C12 (2.2 µF) decoupling — which sets the bias point for the whole stack. R15 / R17 (390 Ω each) are the source resistors. D6 / D7 at Q4's source provide a third clipping pair — see the BOM note about the LEDs vs the original 1N5818 spec.
Stage 5 — Q5 output buffer
Q5 (2N5088 BJT) is a common-emitter output stage. R18 / R19 (both 100 k) form a feedback / impedance-setting network between Q4's output and Q5's input. C13 (470 p) provides a stability / HF rolloff in the inter-stage; C14 (220 p) does the same at Q5's output. The output is AC-coupled by C15 (220 n) through the VOLUME pot to the output jack, with R20 (10 k) as the pulldown.
Power supply
Simple, classic. 9 V DC enters through D1 (1N4001) — reverse polarity protection. R1 (68 Ω) plus C1 (220 µF) and C2 (100 n) form a supply RC filter that provides clean rail (called VA) to all the stages. R2 / R3 (10 k each) form a divider giving VR ≈ VA / 2 ≈ 4.5 V — the virtual reference for biasing Q3's gate — decoupled by the substantial C4 (100 µF). No charge pump or split-rail tricks here; the bootstrap-style trickery is all in the JFET stack topology.
Tone Stack Analysis
The Baxandall tone stack is the Ginger's headline feature — far more flexible than the single-knob tilt controls common in DIY drives. It has two independent shelving filters: a bass shelf controlled by the BASS pot, and a treble shelf controlled by the TREBLE pot. Each pot pivots around a transition band defined by RC corners — the calculated values below tell you where each shelf "lives" in the frequency domain.
BASS shelf
BASS pot shelves the spectrum below ~200 Hz, with the transition band stretching from ~72 Hz at the bottom to ~329 Hz at the top. Plenty of authority on bass guitar fundamentals (low E ≈ 41 Hz, low B ≈ 31 Hz on 5-strings). At centre detent the bass is approximately flat.
TREBLE shelf
TREBLE pot shelves the spectrum above ~1 kHz, with the transition band from ~480 Hz to ~1.5 kHz. Above the upper corner the boost/cut is essentially flat at the shelf level. Sits above the bass-guitar fundamental range, so it shapes presence and attack rather than fundamental energy.
Why a Baxandall is the right choice here
The Ampeg SB-12 the Ginger emulates uses a real Baxandall feedback tone stack in its preamp — separate, independent bass and treble shelving with relatively flat midrange. The runoffgroove design captures this faithfully in passive form. The practical result is that the BASS and TREBLE controls don't fight each other the way a Fender-style tone stack does: turning up the bass doesn't pull the treble down (much), and vice versa. For bass guitar this is exactly the right choice — you can dial in a fat low end and a clear top end without one neutering the other.
Other filter corners (fixed)
- Q1 input HF rolloff (C3 / R4): 1 / (2π · 33 k · 220 p) ≈ 22 kHz — RFI rejection, well above audio.
- Q2 input HF rolloff (C10 / impedance): ≈ several tens of kHz — same purpose at Q2's gate.
- Output HP (C15 / VOLUME pot): 1 / (2π · 100 k · 220 n) ≈ 7.2 Hz — well below audio, full bass passes through.
- Q5 stability network (C13 / R18, C14 / R19): roll-off in the few-kHz range — sets the buffer's high-frequency response and ensures stability.
LED clipping vs ROG's original 1N5818 spec
Three antiparallel diode pairs sit at the JFET sources to provide soft clipping: D2 / D3 at Q1 (red LEDs), D4 / D5 at Q2 (red LEDs), and D6 / D7 at Q4. The kit ships with all six positions populated as 3 mm red LEDs, which clip at ≈ 1.7 V each direction — moderate-headroom clipping that's perceived as "warm" rather than "fuzzy".
The original ROG schematic specifies 1N5818 Schottky diodes for D6 / D7 (the Q4 pair). Schottkys clip at ≈ 0.3 V — much earlier — so the original ROG voicing has more aggressive output-stage clipping. Thomas's experience: the all-LED variant sounds great too. If you want the strict ROG voicing, swap D6 / D7 for 1N5818s; if you like a slightly cleaner output stage, keep the LEDs.
Bill of Materials
The BOM has been cross-checked against the V1.0 schematic. The kit ships with these exact parts; substitution alternatives are tagged in the Notes column.
| Ref | Qty | Value | Colour code | Notes |
|---|---|---|---|---|
| Resistors — metal film, ¼ W, 1% | ||||
| R1 | 1 | 68R | Blue · Grey · Black | Gold · Brown | Power supply rail series resistor (with C1, C2 forms supply RC filter). |
| R2 | 1 | 10k | Brown · Black · Black | Red · Brown | VR (virtual reference) bias divider top — sets ≈ 4.5 V reference. |
| R3 | 1 | 10k | Brown · Black · Black | Red · Brown | VR bias divider bottom. |
| R4 | 1 | 33k | Orange · Orange · Black | Red · Brown | Q1 gate input series resistor. |
| R5 | 1 | 1M | Brown · Black · Black | Yellow · Brown | Q1 gate pulldown / input impedance setter. |
| R6 | 1 | 390R | Orange · White · Black | Black · Brown | Q1 source resistor — sets bias current with R7 trimmer. |
| R8 | 1 | 22k | Red · Red · Black | Red · Brown | Baxandall tone stack — BASS shelving network. |
| R9 | 1 | 3k9 | Orange · White · Black | Brown · Brown | Baxandall tone stack — BASS pot return. |
| R10 | 1 | 33k | Orange · Orange · Black | Red · Brown | Baxandall tone stack — TREBLE shelving network. |
| R11 | 1 | 100k | Brown · Black · Black | Orange · Brown | Tone stack output to Q2 gate — high impedance load. |
| R12 | 1 | 390R | Orange · White · Black | Black · Brown | Q2 source resistor — sets bias current with R13 trimmer. |
| R14 | 1 | 1M | Brown · Black · Black | Yellow · Brown | Q3 gate bias resistor — sets DC operating point from VR. |
| R15 | 1 | 390R | Orange · White · Black | Black · Brown | Q3 source resistor. |
| R16 | 1 | 10k | Brown · Black · Black | Red · Brown | GAIN pot supply / Q3 drain. |
| R17 | 1 | 390R | Orange · White · Black | Black · Brown | Q4 source resistor. |
| R18 | 1 | 100k | Brown · Black · Black | Orange · Brown | Q5 base series / coupling network — Q3-Q4-Q5 inter-stage. |
| R19 | 1 | 100k | Brown · Black · Black | Orange · Brown | Q5 base series — together with R18 sets HF response of output buffer. |
| R20 | 1 | 10k | Brown · Black · Black | Red · Brown | Output pulldown to ground at VOLUME pot. |
| R21 | 1 | 6k8 | Blue · Grey · Black | Brown · Brown | Below GAIN pot — sets minimum gain limit. |
| Trimmers — bias adjustment (PCB-mount) | ||||
| R7 | 1 | 10k | 6 mm ACP or Piher (5×5 footprint) — sets Q1 drain bias. See §05 biasing procedure. | |
| R13 | 1 | 10k | 6 mm ACP or Piher (5×5 footprint) — sets Q2 drain bias. See §05 biasing procedure. | |
| Capacitors — film (box-style) | ||||
| C2 | 1 | 100n | Box film — main supply rail RFI bypass. | |
| C3 | 1 | 220p | Box film — Q1 input shunt to GND (RFI rolloff above audio). | |
| C5 | 1 | 220n | Box film — Q1 drain to Baxandall tone stack coupling. | |
| C6 | 1 | 22n | Box film — BASS shelf high-frequency corner cap. | |
| C7 | 1 | 100n | Box film — BASS shelf low-frequency corner cap. | |
| C8 | 1 | 3n3 | Box film — TREBLE shelf high-frequency corner cap. | |
| C9 | 1 | 10n | Box film — TREBLE shelf low-frequency corner cap. | |
| C11 | 1 | 220n | Box film — Q2 drain to GAIN pot coupling. | |
| C14 | 1 | 220p | Box film — Q5 output stability / HF rolloff. | |
| C15 | 1 | 220n | Box film — output coupling to VOLUME pot. | |
| Capacitors — ceramic | ||||
| C10 | 1 | 100p | Ceramic — Q2 gate RFI rolloff. | |
| C13 | 1 | 470p | Ceramic — inter-stage HF rolloff in Q3-Q4-Q5 network. | |
| Capacitors — electrolytic (polarised) | ||||
| C1 | 1 | 220µF / 16 V | Polarised electrolytic — main supply rail filter cap. Observe polarity. | |
| C4 | 1 | 100µF / 16 V | Polarised electrolytic — VR (virtual reference) decoupling. Observe polarity. | |
| C12 | 1 | 2.2µF / 16 V | Polarised electrolytic — Q3 stage AC bypass. Observe polarity. | |
| Potentiometers — 16 mm right-angle PCB-mount | ||||
| BASS | 1 | 100k log | A taper (audio log) — Baxandall bass shelving (~72–329 Hz). | |
| TREBLE | 1 | 100k log | A taper (audio log) — Baxandall treble shelving (~480 Hz–1.5 kHz). | |
| GAIN | 1 | 100k log | A taper (audio log) — pre-amp drive amount. | |
| VOLUME | 1 | 100k log | A taper (audio log) — output level. | |
| Diodes | ||||
| D1 | 1 | 1N4001 | Reverse polarity protection — observe polarity, band toward circuit. | |
| D2 – D7 | 6 | LED 3 mm red | Six 3 mm red LEDs forming three antiparallel clipping pairs at the JFET sources (D2/D3 at Q1, D4/D5 at Q2, D6/D7 at Q4). Each LED clips at ≈ 1.7 V — moderate-headroom soft clipping. The original ROG schematic specifies 1N5818 Schottky for D6/D7 — clips much earlier (~0.3 V) for more aggressive output-stage clipping. The kit ships with all 6 LEDs — Thomas finds it sounds good with all-LED clipping; substitute 1N5818 at D6/D7 for the original ROG voicing. | |
| Transistors | ||||
| Q1 – Q4 | 4 | 2N5457 | N-channel JFET, TO-92. Pinout (flat side facing you, leads down): D – S – G. J201 MPF102 work but pinout may differ — verify before insertion. Sockets recommended (lets you swap and match for best biasing). | |
| Q5 | 1 | 2N5088 | NPN BJT, TO-92. Output buffer / recovery stage. Pinout: E – B – C (flat side facing you, leads down). 2N3904 works as a substitute (same pinout). | |
Build & Bias Setup
This is a moderate-density build. JFETs need biasing after assembly, but the procedure is simple — by ear, taking only a few minutes.
Component placement reference — top view. The two trimmers (R7, R13) are at the top corners; the four 16 mm pots mount on the back side.
Build order
Diode and resistors first
Lowest profile components, do these first. Start with D1 (1N4001) — polarity matters; the band must match the silkscreen line. Then all 19 resistors. Match each reference designator on the silkscreen against the BOM colour band — the colour-band column in the BOM is the fastest verification method.
Transistor sockets — recommended
Socket all five transistors. JFET parameters (especially VGS(off)) vary considerably between individual 2N5457 devices, so you may want to swap and try different parts to find a set with similar bias points. Use 3-pin SIP sockets or DIP-style sockets cut to length. Q1–Q4 (2N5457): pinout D – S – G (flat side, leads down). Q5 (2N5088): pinout E – B – C.
Trimmers and LEDs
Fit the two 10 k trimmers (R7, R13) — 6 mm ACP or Piher with 5 × 5 footprint. Use the silkscreen to confirm orientation; the wiper pin matters for adjustment direction. Set both trimmers to mid-rotation as a starting point before any further work — this is the safe bias-by-ear starting point.
Then fit the six clipping LEDs (D2 – D7) as antiparallel pairs. Long lead is the anode; the silkscreen shows which way each LED in a pair faces — the two LEDs in each pair point opposite directions. Get this wrong and you have a constant-on indicator instead of a clipping pair (you'll find out fast — the LEDs will glow continuously when powered).
Ceramic and box film capacitors
Non-polarised, no orientation. Populate ascending: 100 p (C10), 220 p (C3, C14), 470 p (C13), 3 n3 (C8), 10 n (C9), 22 n (C6), 100 n (C2, C7), 220 n (C5, C11, C15). Push them flat against the board.
Electrolytic capacitors
Polarised — long lead is positive, the can stripe marks negative. Match the (+) on the silkscreen for each: C12 (2.2 µF), C4 (100 µF), and the largest C1 (220 µF). Reversing any electrolytic risks a small explosion when powered.
Off-board pads
Solder the off-board wire pads or pin headers for IN, OUT, +9V, GND.
Pots — back-side mounting
The four 16 mm right-angle pots (BASS, TREBLE, GAIN, VOLUME) mount on the opposite side of the board from all other components. Use double-sided tape on the metal pot bodies to prevent shorts to any pin tips that protrude. Tack-solder the centre pin of each pot first, pull the pot back about 1 mm so it sits flat, let the solder set, then solder the rest of the pins. Don't forget to clip off the small anti-rotation bracket on each pot before final assembly.
Insert the transistors
With all soldering done and double-checked for shorts, insert the five transistors into their sockets. Match each flat side to the silkscreen flat. Don't force anything.
Bias setup — by ear
The two 2N5457 JFETs at Q1 and Q2 need their drain bias set correctly to give the best clean signal swing. Individual 2N5457 devices vary substantially — what works for one chip won't necessarily work for the next, hence the trimmers. The procedure below is what Thomas used during the prototype, and works fine without a multimeter:
| Step | Action |
|---|---|
| 1 | Both trimmers (R7, R13) at mid-rotation. All four front-panel pots at noon. Power up. |
| 2 | Plug guitar / bass into IN, output into amp at moderate volume. Play continuously while adjusting. |
| 3 | Adjust R7 first — slowly rotate until the signal is loudest without introducing distortion or harshness. There's a clear sweet spot; rotating past it on either side either kills the signal or pushes Q1 into hard clipping. |
| 4 | Then adjust R13 the same way — find the loudest, cleanest setting at Q2. |
| 5 | Re-check R7 once R13 is set — there can be slight interaction. Iterate once or twice if needed. |
| 6 | Done. Lock the trimmers in place if you have access to threadlocker — otherwise the standard ACP / Piher trimmers will hold their setting indefinitely. |
Wiring
Standard external-bypass wiring. Four pads off the board: IN, OUT, +9V, GND.
| Pad | Connect to |
|---|---|
| IN | Input side of bypass footswitch (or directly to input jack tip) |
| OUT | Output side of bypass footswitch (or directly to output jack tip) |
| +9V | Power jack — centre-negative DC jack tip (positive) |
| GND | Sleeve of input jack, sleeve of output jack, power jack ring/sleeve, footswitch ground tab if present |
Enclosure
This PCB fits a 1590B enclosure. A drill template / decal can be downloaded from diy.thcustom.com/drill-templates/.
Setup & Usage
Controls
BASS — Baxandall low-shelf boost / cut, ±~15 dB at full rotation. Audio log taper. Centre detent ≈ flat. Acts on frequencies below ~200 Hz with the transition band stretching from ~72 Hz to ~329 Hz.
TREBLE — Baxandall high-shelf boost / cut, ±~15 dB at full rotation. Audio log taper. Centre detent ≈ flat. Acts on frequencies above ~1 kHz with the transition band from ~480 Hz to ~1.5 kHz.
GAIN — drive amount into the Q3-Q4-Q5 stack. Audio log taper. Low settings = clean preamp; high settings = LED clipping at the JFET sources kicks in for moderate, harmonically rich saturation.
VOLUME — output level after the buffer. Audio log taper. Plenty of headroom — the Ginger is designed to drive a clean amp input cleanly, with adjustable bite from the GAIN side.
Suggested starting points
Clean Ampeg-style bass tone — BASS at 1 o'clock (slight boost), TREBLE at noon, GAIN at 9–10 o'clock (clean), VOLUME to taste. Ideal for active or passive bass — fat and clear without being woolly.
Drop-tuned guitar — BASS at 11 o'clock (small cut to keep things tight), TREBLE at 1 o'clock, GAIN at noon (just into the clipping region), VOLUME to taste. The Baxandall lets you keep the upper-mid presence while controlling the low-tuned mud.
Bass with grit — BASS at 1 o'clock, TREBLE at noon, GAIN at 2 o'clock, VOLUME to taste. The LEDs start clipping audibly — sounds like a vintage bass amp pushed hard, which is exactly the SB-12 voice.
Always-on clean buffer for guitar — Both tone controls at noon, GAIN at minimum, VOLUME at unity. The Ginger sits quietly at the front of your chain providing low-impedance drive into long cable runs.
About bass-vs-guitar use
The Ginger was designed to emulate a bass amp, but it's surprisingly good for guitar — particularly drop-tuned guitar where the Baxandall's low-shelf control gives you tight low-end management that a typical guitar overdrive can't match. For standard-tuned guitar it works as a clean preamp / mild drive with unusually flexible tone shaping; some players use it as an always-on tone-shaping front end before the rest of their chain.
This is a great-sounding circuit
Especially with the all-LED clipping and the well-judged Baxandall shelving frequencies, the Ginger covers a lot of ground. Have fun.
Disclaimer & Licence
PCBs based on runoffgroove circuits purchased from TH Custom Effects are intended for DIY / non-commercial use only. Any commercial use whatsoever is forbidden. Please contact runoffgroove for further information.
Ginger circuit design © runoffgroove.com. PCB layout and build documentation © TH Custom Effects 2014–2026. Build documentation V1.0.