Circuit Design

At a Glance

Ghost Spring Tank

One spring. One voice. Transformer-coupled for the organic bloom that no direct-drive circuit can replicate — the same mechanism that made the Fender 6G15 Reverb Unit sound like nothing else.

Schematic

Circuit Diagram

Generated from schematic.py — see schematic.py on GitHub

Signal Flow

Complete Path

Every stage in the signal chain with component values inline. Left to right: input from line source, through the spring and back out to the power amp.

Line In (from Lehle Parallel M)
  → Input Buffer     U1 OPA2134 · 1MΩ (R1) input · unity gain · R2 100Ω series output
  → Dwell Pot       RV1 10kΩ linear · C1 1µF film coupling cap · controls drive level
  → BD139 Driver    Q1 Class A · R3b 6.8k + R4 1k bias divider · R5 68Ω emitter degen
                   C2 100µF bypass · ~19mA quiescent
  → REB3S Xfmr     T2 primary (from Q1 collector) → 8Ω secondary
  → 9AB3C1B Tank   RT1 · 3 springs · long decay · 8Ω input / 2550Ω output
  → C3 Coupling     470nF film · blocks DC from tank output terminals
  → Recovery Preamp U2 OPA2134 · inverting · Ri=470Ω / Rf=100kΩ → gain=214× (~46dB)
                   Rbias=470Ω at non-inv input · 1–5mV → ~1Vrms
  → 300Hz Wet HPF  C4 100nF film + R6 5.6kΩ · f = 1/(2π×5600×100n) ≈ 284Hz
                   wet signal only — dry path bypasses this stage
  → Tone Pot        RV3 100kΩ · high-shelf on wet signal only
  → Mix Pot         RV2 100kΩ · Rdry=10kΩ summing · C_bright=47pF silver mica across wiper
                   dry path (from U1 via Rdry) + wet path blend here
  → Output Buffer  U3 OPA2134 · voltage follower · R7 100Ω series · <100Ω output Z
  → Line Out (to McIntosh MC100)

The Key Mechanism

Why Transformer Coupling

This is the part that makes the Ghost Spring sound like a spring reverb and not a plate sim. Most DIY spring reverb designs drive the tank directly from a transistor or op-amp output. That works — but it doesn't sound like a Fender 6G15.

The REB3S transformer's inductance forms a resonant circuit with the tank's input impedance (8Ω). This resonance creates:

• A peak at ~2–3kHz on attack transients — the "drip" character. This is the mechanism behind the distinctive splash on hard-picked notes.
• Soft low-frequency drive — the resonant circuit naturally rolls off very low frequencies into the tank, providing passive mud rejection without any filter in the signal path.
• Galvanic isolation — the transformer blocks DC, eliminating any risk of a DC offset reaching the tank's input coil.

The Accutronics REB3S is designed specifically for this application. It's used in boutique spring reverb units and is correctly matched to 8Ω tanks. Using a generic transformer (or no transformer) changes the resonant character fundamentally.

The 9AB3C1B — Why This Tank

Three springs give a denser, more uniform reverb tail than two-spring tanks. Two-spring designs can produce a metallic "ping" on hard attacks because the two resonant modes are more distinct. Three springs blend into a closer-to-continuous reverb tail — the same reason the original Fender 6G15 and most boutique spring reverbs use 3-spring tanks.

Circuit Decisions

What Makes This Design Different

Post-Recovery High-Pass Filter

Most DIY reverb designs filter before the driver — a HPF or shelving filter before the spring to prevent low-end boom. This unit takes the opposite approach: the full-bandwidth signal enters the spring, and the HPF is applied to the wet output after the recovery preamp.

The result: the physical attack transient (the full-frequency "thump") enters the spring intact, creating a realistic reverb bloom. The reverb tail is then filtered. Low-end boom clears up as the note decays, which is more natural than a pre-filtered attack.

Bright Cap on Mix Pot

The Mix pot acts as a voltage divider at audio frequencies. At low mix settings, it attenuates high frequencies more than low frequencies — the reverb tails lose their shimmer before they lose their body. A 47pF silver mica capacitor across the Mix pot wiper bypasses the pot for high frequencies, keeping the reverb "glassy" even when nearly blended out.

Emitter Bypass Capacitor (C2)

R5 (68Ω emitter resistor) provides thermal stability and sets quiescent current. Without C2, R5 degenerates the BD139's gain at audio frequencies — the spring is underdriven and loses high-frequency drive. C2 (100µF low-ESR Nichicon) bypasses R5 at audio frequencies, restoring full gain while keeping the DC stability of R5 intact.

Recovery Gain — 214× (46dB)

The spring tank's output is 1–5mV. Line level is ~1Vrms. The recovery preamp (U2 OPA2134 in inverting configuration) brings the signal up to line level with Ri=470Ω / Rf=100kΩ → gain = 1 + (100k/470) = 214×. This is not unusual — spring tanks are low-output transducers, and this gain is normal for the stage.

Component Rationale

Why Each Part

Every component was chosen for a specific engineering reason. These notes are the difference between a build that sounds right and one that doesn't.

Op-Amps — OPA2134PA

Transistor — BD139 (TO-126)

High-current NPN bipolar rated 1.5A / 80V. At 19mA quiescent current it runs well within ratings with negligible heat. The TO-126 package mounts flat against the chassis for passive heatsinking if needed. BD139 has excellent hFE linearity at low currents — critical for low distortion in the driver stage.

Resistors — Yageo MFR 1% 250mW Metal Film

All resistors are metal film, 1% tolerance. Carbon film resistors have higher noise (current noise 10–100× higher) and temperature drift that degrades the signal floor, especially in the 214× gain recovery stage. Order 5 of each value — metal film resistors are cheap and you want spares for the build.

Critical Resistor Values

Capacitors — Film Required in Signal Path

Potentiometers — Vishay/Spectrol 296 (MIL-PRF-39023)

Military-grade cermet element, gold-plated wiper, stainless shaft, rated 10,000+ cycles. These are overkill for a guitar rig but they will outlast everything else in the build and will never develop contact noise.

Output Stage

Low-Impedance Output Buffer

The output buffer (U3) drives the McIntosh MC100's RCA input and any cable between the unit and the power amp. Without a buffer, the Mix pot's output impedance varies with rotation — at center position it's at maximum (half of 100kΩ = 50kΩ), which would roll off high frequencies through cable capacitance and interact unpredictably with the MC100's input.

MC100 Interface

• MC100 has RCA unbalanced inputs with ~100kΩ input impedance
• Use a high-quality 1/4" TS to RCA cable, or install an RCA jack on the reverb unit's rear panel
• Keep cable under 6ft to minimize hum pickup — the output buffer makes the signal robust to cable capacitance, but length still matters for shielding
• If hum is present after all grounding steps, a ground-lift switch (10Ω resistor + 100nF cap between audio ground and chassis) can help

Power Supply

Internal ±15V Linear

The power supply is internal to the unit. No wall wart, no switching regulator — a toroidal transformer, bridge rectifier, and pair of linear regulators. This is the correct approach for a hi-fi reverb unit and the reason the circuit doesn't need external power management.

Enclosure

2U Rackmount Chassis

The 2U form factor is required — 1U is too tight to mount the 9AB3C1B tank horizontally with spring clearance. 2U gives comfortable room for the tank, PCB, power supply, and ventilation.

Chassis

Hammond 1455T2201 — 2U aluminum rackmount. Aluminum is mandatory: steel would interact magnetically with the toroidal transformer. Rack ears are included with the Hammond chassis.

Internal Layout

Rear Panel ─────────────────────────────────── Front Panel
│                                                  │
│  ┌──────────────────────────┐                   │
│  │                          │                   │
│  │  Spring Tank (9AB3C1B)   │     ┌──────────┐  │
│  │  horizontal, open down   │     │ Audio PCB │  │
│  │  grommet-isolated        │     │ (Vector   │  │
│  │                          │     │  T44)     │  │
│  └──────────────────────────┘     └──────────┘  │
│                                                  │
│  ┌──────────┐                                    │
│  │ PSU PCB  │ ← toroidal transformer here        │
│  │ (Triad   │   as far from tank as possible     │
│  │  F-219X) │                                    │
│  └──────────┘                                    │
│                                                  │
────────────────────────────────────────────────────
       ↑ IEC C14 inlet · Fuse · Rocker switch ↑

Front Panel Controls

Rear Panel Connectors

Tank Mounting

• Open-side down — springs hang below the transducers. If mounted open-side up, springs shift over time and reverb character drifts.
• Rubber grommets — 4× Shore 30A soft silicone/rubber. Hard rubber or plastic transmits vibration from the chassis (footsteps, rack fans) directly into the springs as noise.
• Away from the toroidal — orient the transformer's toroid axis perpendicular to the tank's spring axis and place them on opposite sides of the chassis to minimize inductive coupling (60Hz hum in the reverb tail).
• RCA cables short — keep tank input and output RCA cables under 12". Excess length adds capacitance that rolls off the HF drive to the tank and attenuates the high-gain recovery signal.

Front Panel — Custom via Front Panel Express

Custom 2U aluminum panel with laser-drilled holes and engraved labels. Holes required: 3× ¼" pot holes (DWELL / MIX / TONE), 2× ¼" jack holes (IN / OUT). IEC cutout and rocker switch hole on rear panel. Use Front Panel Express designer software to spec all hole positions to fit the Hammond 1455T2201 chassis dimensions.

Build Notes

Critical Rules for the Builder

These five rules are the difference between a working build and one that hums, oscillates, or drifts. Non-negotiable.

PCB — Vector T44 FR4 Perfboard

FR4 fibreglass is mandatory. The cheaper brown phenolic perfboard absorbs moisture and increases leakage current between pads — at the recovery stage gain of 214× this manifests as audible noise. Use 4× M3 nylon hex standoffs (not metal) to mount the board — metal standoffs can accidentally create a second chassis ground connection.

Phase Alignment — First Power-Up Check

Spring tank polarity varies between Accutronics batches. If the reverb sounds hollow, thin, or "phasey" when mixed in at 50/50, the tank output is out of phase with the dry signal. Fix: swap the two wires at the tank's output RCA connector (the 2550Ω side). No design change needed — this is a 10-second wire swap on first commissioning.

Solder

Kester 44, 63/37 tin/lead, 0.031". Kester 44 is the standard for hand-soldered audio. 63/37 eutectic alloy — snaps solid with no mushy semi-solid phase, reducing cold joints. 0.031" diameter is correct for through-hole perfboard work. Do not use lead-free — higher melting point increases heat stress on sensitive components and produces higher-resistance joints.