Overview & Features
The Tube Driver V2.0d is a faithful recreation and improvement of one of the most celebrated tube overdrive circuits ever made — used by David Gilmour, Eric Johnson, Joe Satriani and many others. Where the original had limitations, this design fixes them: the plate voltage is raised to ~60V (from a self-contained charge pump), the heater runs on 6V for compatibility with standard 9V pedalboard supplies, and the tone control has been upgraded to a Baxandall shelving network for much more musical bass and treble adjustment.
A relay true-bypass circuit with a PIC microcontroller provides clean, click-free switching. The whole circuit — tube, main PCB, pots, and switching — fits inside a standard Hammond 1590BB enclosure.
Real Tube Gain Stage
12AT7 (ECC81) dual triode running two cascaded common-cathode stages for genuine tube harmonic character.
60V Plate Voltage
On-board Cockcroft-Walton charge pump generates ~60V from 9V. No wall-wart required beyond a standard pedal supply.
Op-Amp Pre-Drive
TL072 buffer and variable-gain stage feeds a clipped, harmonically rich signal into the tube input.
Baxandall Tone Stack
Active-range bass and treble shelving filters. Far wider musical range than a single-cap tone control.
Relay True Bypass
PIC microcontroller and optocoupler drive a relay for quiet, reliable, click-free true bypass switching.
Tube Illumination
3mm super-bright LED in the tube socket lights the tube from below. Can be wired to the enclosure face.
Circuit Theory
Signal Path Overview
Signal enters the input jack → relay switching → op-amp buffer → variable-gain inverting amplifier (Gain pot) → two cascaded tube triode stages → Baxandall tone stack → Volume pot → relay switching → output jack.
Tube Driver V2.0d — full schematic
High-Voltage Power Supply — Cockcroft-Walton Multiplier
The 40106N hex Schmitt trigger (IC1) is wired as a self-oscillating inverter. Gate A forms an RC oscillator: R1 (2k7) and C7 (1n) set the frequency to approximately 309kHz. The remaining five gates (B–F) buffer and invert this square wave, driving a seven-stage Cockcroft-Walton diode-capacitor ladder (D1–D7, C1–C8, all 47n). Each stage adds approximately one supply-rail voltage peak, producing roughly 60V DC on the high-voltage rail. C9 (220µ) and C10 (47n) filter this rail. The 6V heater supply comes from IC2 (uA7806 linear regulator), which provides a clean, well-regulated voltage for the tube heaters — critical for low hum.
Op-Amp Section (IC3 — TL072)
The first op-amp half (IC3A) is wired as a unity-gain voltage follower: output fed back directly to the inverting input, with signal at the non-inverting input. C11 (47n) and R11 (10k) form a first-order high-pass coupling network (corner frequency ~339Hz) that blocks any DC offset from the input. R17 provides a DC reference to the virtual ground rail (VR, ~4.5V), setting the AC operating point for the op-amp.
The second half (IC3B) is an inverting gain stage. R14 (220Ω) sets the input impedance; the Gain pot (100k log) connects from the output back to the inverting input, varying the feedback resistance from nearly zero to 100kΩ. This gives a gain range of approximately 1× to 455× (+53dB). C17 (47p) in parallel with the feedback path rolls off very high frequencies, preventing oscillation at maximum gain. The IC3B output is AC-coupled through C19 (33n) and fed to the first triode grid via R15/R16 (both 10k), which form a voltage divider to keep the grid within safe operating range.
Tube Stage (V1 — 12AT7 / ECC81)
The 12AT7 contains two identical triodes. Both cathodes are wired together (pins 3 and 8) to a shared self-bias network: R9 (47k) in series with the BIAS trimmer (up to 10k), bypassed by C12 (22µ). Adjusting BIAS sets the quiescent current through both triodes simultaneously.
Triode A (grid at pin 2, plate at pin 1): the amplified signal at plate A passes through R10 (10k) and the coupling capacitor C16 (47n) to the grid of triode B (pin 7). This inter-stage RC forms a high-pass corner at ~339Hz, blocking the DC plate voltage while passing audio. R7 (22k) and C16 provide the bias reference and coupling path for grid B.
Triode B (grid at pin 7, plate at pin 6): plate B is loaded by R6 (47k) to the HV rail. The output is taken from plate B through R8 (1k) — acting as a source impedance — into the tone stack.
The 12AT7 can be substituted with a 12AX7 (ECC83) for higher gain and more harmonic richness. The designer's original preference is the 12AT7 for a less harsh character — feel free to experiment.
Baxandall Tone Stack
From the tube output, C14 (1µ) AC-couples into R21 (100k), the spine of the Baxandall network. The Bass pot (100k) sweeps through C28 (1n) and C29 (6n8), providing a low-frequency shelving control centred roughly around 200–1600Hz. The Treble pot (100k) works through C30 (1n), C31 (10n), and R24 (22k), with a corner range from approximately 700Hz to 7kHz. At their centre positions, bass and treble approximate flat response; rotating them clockwise boosts, anticlockwise cuts. The R21+C28 junction also functions as a gentle output low-pass shelf at around 1.6kHz, softening upper harmonics from the tube stage.
Volume Control and Output Stage
After the tone stack, the signal passes through the Volume pot (100k log) and C20 (10µ) coupling capacitor to the relay output contacts and the output jack. The R4 (1M) input resistor to ground ensures the op-amp input is referenced even with no cable connected.
Relay True-Bypass Switching
IC5 (PIC10F202 microcontroller) monitors a momentary footswitch and drives the relay coil via the optocoupler OC1 (CPC1017N) and NPN transistor Q1 (BC549). The optocoupler provides galvanic isolation between the low-voltage logic and the relay drive circuit. D8 (BAT85 Schottky) protects against back-EMF when the relay coil is de-energised. LED1 (3mm super-bright, current-limited by R2) indicates the effect is active; the LED is mounted in the tube socket PCB but can optionally be wired to the enclosure face.
Stage Analysis
Charge Pump Oscillator
40106 Ring Oscillator
Heater Supply
Op-Amp Gain Stage (IC3B)
Variable Gain Stage
Input Coupling
Tube Stage (V1 — 12AT7)
Triode A — Input Stage
Triode B — Output Stage
Baxandall Tone Control — Reference Frequencies
The Baxandall network uses two separate pot-controlled RC ladders. These figures are approximate due to component interaction — they indicate the shelving corner region, not a sharp cutoff.
Bass Control
Treble Control
Bill of Materials
| Ref | Qty | Value | Colour Code | Notes |
|---|---|---|---|---|
| Resistors — 5-band, 1% metal film | ||||
| R1 | 1 | 2k7 | Red · Violet · Black | Brown · Gold |
40106 oscillator timing |
| R2 (CLR) | 1 | 1k5 | Brown · Green · Black | Brown · Gold |
LED current limit — test for brightness with your LED first |
| R3, R8, R14 | 3 | 1k | Brown · Black · Black | Brown · Gold |
|
| R4, R17 | 2 | 1M | Brown · Black · Black | Yellow · Gold |
Input pull-down / VR reference |
| R5, R6, R9 | 3 | 47k | Yellow · Violet · Black | Red · Gold |
Tube plate loads + cathode bias |
| R7, R24 | 2 | 22k | Red · Red · Black | Red · Gold |
|
| R10, R11, R12, R13, R15, R16, R23 | 7 | 10k | Brown · Black · Black | Red · Gold |
|
| R18 | 1 | 2k2 | Red · Red · Black | Brown · Gold |
Optocoupler drive current limit |
| R21 | 1 | 100k | Brown · Black · Black | Orange · Gold |
Tone stack spine |
| Capacitors — Box film (unless noted) | ||||
| C1–C6, C8, C10, C11, C16 | 10 | 47n | Rated 80–100V minimum (HV multiplier caps C1–C8) | |
| C7, C28, C30 | 3 | 1n | Box film | |
| C9 | 1 | 220µ / 16V | Polarised electrolytic — HV rail filter | |
| C12 | 1 | 22µ / 50V | Polarised electrolytic — cathode bypass | |
| C13, C17 | 2 | 47p | Ceramic | |
| C14 | 1 | 1µ / 50V | Polarised electrolytic | |
| C15, C19 | 2 | 33n | Box film. C19 is the main tone character modifier — see Stage Analysis | |
| C18 | 1 | 47µ / 50V | Polarised electrolytic | |
| C19 (alt value) | — | 15n – 47n | Original: 10nF. See Stage Analysis note. | |
| C20, C21 | 2 | 10µ / 16V | Polarised electrolytic | |
| C22 | 1 | 100n | MLCC 5mm | |
| C23 | 1 | 4n7 | Box film | |
| C29 | 1 | 6n8 | Box film — Baxandall bass network | |
| C31 | 1 | 10n | Box film — Baxandall treble network | |
| Diodes & LEDs | ||||
| D1–D7 | 7 | 1N4148 | DO-35 — HV Cockcroft-Walton multiplier chain | |
| D8 | 1 | BAT85 / BAT43 | Schottky — relay flyback protection | |
| LED1 | 1 | 3mm super-bright | Tube illumination / bypass indicator | |
| Trimpots | ||||
| BIAS | 1 | 5k or 10k | 6mm ACP6 or Bourns 3306F — sets tube quiescent current. Can be mounted externally for tweaking. | |
| Potentiometers — 9mm Alpha | ||||
| Gain, Volume, Treble, Bass | 4 | 100k log (A) | Alpha 9mm. European convention: A taper = logarithmic. | |
| ICs | ||||
| IC1 | 1 | 40106N | DIP-14 — Hex Schmitt trigger. Use socket. | |
| IC2 | 1 | uA7806 | 6V linear regulator for tube heaters. Switched regulator module also supported — see build notes. Mount to enclosure as heatsink if using linear type. | |
| IC3 | 1 | TL072IP | DIP-8. Use socket. JRC4580D OPA2134 are pin-compatible alternatives. | |
| REG1 | 1 | 78L05ACZ | TO-92 — 5V regulator for PIC and relay logic | |
| Transistors & Optocouplers | ||||
| Q1 | 1 | BC549 | TO-92 NPN. Pin-compatible: BC547 2N3904. Verify CBE pinout per device. | |
| OC1 | 1 | CPC1017N | SO-4 SMT optocoupler — solder first, before any through-hole parts. See build guide. | |
| Other Active Components | ||||
| IC5 | 1 | PIC10F202 (pre-programmed) | DIP-8 PIC microcontroller — supplied pre-programmed for relay switching | |
| Tube & Socket | ||||
| V1 | 1 | 12AT7 | Noval 9-pin. ECC81 = direct equivalent. 12AX7/ECC83 is pin-compatible but higher gain — see Stage Analysis. | |
| S1 | 1 | Noval Socket (PCB mount) | 9-pin PCB-mount Noval socket on sub-PCB | |
| Electromechanical | ||||
| K1 | 1 | FTR-B4CA4.5Z | DPDT relay 4.5V coil. Omron G6K-2F-Y-DC5 is an alternative. | |
| Hardware | ||||
| — | 1 | IC Socket 14-pin | DIP/DIL — for IC1 | |
| — | 2 | IC Socket 8-pin | DIP/DIL — for IC3, IC5 | |
| — | 1 | Tube holder assembly | 3D-printed holder + sub-PCB (included in kit) | |
Build Guide
Tube Holder Assembly
Before starting on the main PCB, assemble the tube holder sub-assembly. This is the 3D-printed plastic holder with the Noval socket PCB.
Tube holder components
LED into socket PCB
LED (if used)
Insert the 3mm LED into the socket PCB from the correct side — long leg (anode) goes into the round hole. Do not solder yet.
Noval Socket
Place the 9-pin Noval socket on top of the PCB over the LED. Solder all socket pins. Then press the LED into its socket hole and solder the LED legs.
Insert Tube
Carefully press and wiggle the 12AT7 tube into the socket until all pins are fully engaged. It should sit squarely with no rocking.
Socket PCB with LED ready
Socket assembled
Pressing the 12AT7 into the Noval socket
Mount PCB to Holder
Insert the tube-and-socket assembly through the holder's opening, then fix the PCB to the plastic holder using 2.5mm or 3mm screws with a maximum length of 8mm. Set this sub-assembly aside until the main PCB is ready.
Completed tube holder sub-assembly
Main PCB Population Order
Main PCB — component placement overview
SMT optocoupler OC1 location
SMT Optocoupler OC1 First
Solder the CPC1017N optocoupler (SO-4 SMT package) before any through-hole parts. Locate the orientation dot and match it to the PCB marking. Tin one pad, hold the component in place with pliers, and reflow. Check alignment, then solder the remaining pins. This must be soldered before through-hole components make access awkward.
Small Signal Diodes D1–D7
Solder all 1N4148 diodes. Check polarity — cathode (banded end) towards the HV rail node. These form the voltage multiplier chain.
Diodes and rectifier components soldered
Schottky Diode D8
Fit the BAT85/BAT43 flyback diode. Check polarity.
Resistors
Solder all resistors. Use the colour code table in the BOM to verify each value before fitting. Low-profile components — solder now while the board is easy to work on.
IC Sockets
Fit sockets for IC1 (14-pin), IC3 (8-pin), and IC5 (8-pin). Solder all pins. Do not insert the ICs yet. Sockets allow later replacement if a device fails.
Ceramic and MLCC Capacitors (C13, C17, C22)
Fit the three small ceramic/MLCC capacitors. No polarity to worry about.
Film Capacitors
Solder all box-film capacitors. Note: C1–C8 (47n) are in the high-voltage multiplier chain and must be rated 80–100V minimum. Double-check the voltage rating on these before fitting. No polarity.
Transistor Q1 (BC549)
Fit the BC549 NPN transistor (TO-92). The flat face goes towards the PCB marking. Pinout is CBE (Collector-Base-Emitter) left to right when facing the flat side — verify against the datasheet if using a substitute.
REG1 (78L05, TO-92)
Fit the 5V regulator. Like the transistor, check the flat-face orientation against the PCB marking.
Relay K1
Fit and solder the FTR-B4CA4.5Z relay. It is polarised — it will only fit one way.
Electrolytic Capacitors
Fit all polarised electrolytics. Long leg = positive. C9 (220µ) is on the HV rail — ensure its 16V rating is sufficient for your HV output. If the charge pump generates above 16V under light load, use a 25V or 35V rated part for C9.
IC2 (uA7806 Heater Regulator)
Linear type: The TO-220 7806 can run warm under load. It is recommended to bolt it to the metal enclosure, which serves as a heatsink. Use a TO-220 mounting kit (insulating pad + nylon screw). Switched regulator module: If using the switched regulator sub-board, fit it last using angled pin headers as described in the build photos. No heatsinking needed.
Switched regulator PCB positioning
Soldering switched regulator pins
Regulator fitted as final component
Backside Components
The pots (Gain, Volume, Treble, Bass) and the tube socket connector are mounted on the backside of the main PCB. Do this after completing the top side.
Backside — pots and tube connector
Backside components mounted
Trim Pot Notches
Before mounting, use pliers to carefully cut off the plastic notches on top of each pot body. This ensures the PCB sits flat against the enclosure without gaps or rocking.
Fit Pots and Socket Connector
Mount all four pots and the tube sub-PCB connector from the backside. Insert all components before soldering to ensure co-planar seating.
Ribbon Cable to Tube Holder
Solder the ribbon wire to the main PCB first. Then insert the tube holder assembly into the main PCB cutout from the back, and solder the other end of the ribbon to the socket PCB. This order minimises awkward soldering in confined spaces.
Ribbon cable connecting tube socket PCB to main board
Insert ICs into Sockets
With all soldering complete, insert IC1 (40106N), IC3 (TL072IP), and IC5 (PIC10F202) into their sockets. Ensure all pins are aligned before pressing home. The PIC is supplied pre-programmed — do not attempt to reprogram it.
Fully populated main PCB
Wiring & Connections
PCB Connection Pads
Solder connection wires to the main PCB before fitting it into the enclosure. Leave the far ends of each wire un-soldered until the board is positioned, then solder the jacks, switch, and DC socket in-place to minimise wire length.
| Pad | Connect to | Notes |
|---|---|---|
| IN | Input mono jack tip | Guitar input signal |
| OUT | Output mono jack tip | Signal to amp |
| GND | All jack sleeves, DC jack negative | Short leg on DC barrel jack = Ground |
| +9V | DC jack positive (centre pin) | |
| S1 | Momentary footswitch one terminal | Other terminal to GND |
| S2 | GND (second switch contact, unused) | |
| LED+ | LED anode (if wired to enclosure) | Optional — LED can be left on tube board |
Tube Socket Noval Pinout
For reference — if you need to trace a fault to the tube socket board:
| Pin | 12AT7 Function | Circuit connection |
|---|---|---|
| 1 | Plate A | R5 (47k) to HV + R10/C16 to triode B grid |
| 2 | Grid A | R15/R16 voltage divider (op-amp stage output) |
| 3 | Cathode A | R9 + BIAS trimmer to GND (shared with cathode B) |
| 4, 5 | Heater | GND (heater end) |
| 6 | Plate B | R6 (47k) to HV + R8 to tone stack |
| 7 | Grid B | R7 + C16 coupling from plate A |
| 8 | Cathode B | Shared with cathode A (R9 + BIAS) |
| 9 | Heater centre tap | IC2 output (+6V regulated) |
Biasing
The BIAS trimmer (R9 in series) sets the shared quiescent current through both triode sections. This determines the operating point of the tube and has a significant effect on tone and headroom.
Power Up and Warm Up
With the pedal fully assembled and connected to a 9V supply, allow 1–2 minutes for the tube to warm up to operating temperature. The heater will glow faintly orange — this is normal.
Find the Sweet Spot
With guitar signal passing through the pedal, slowly adjust the BIAS trimmer while playing. You are listening for a lively, articulate, and responsive sound. Avoid the extreme positions at either end of the trimmer's travel.
Identify and Fix
If the sound is dull, compressed, or muffled, the bias is too far in one direction — adjust and listen again. Once you find the sweet spot, the bias can be left fixed. Optionally, the trimmer can be mounted to the enclosure exterior for ongoing adjustment.
Enclosure & Drilling
The circuit fits in a Hammond 1590BB enclosure. A drilling template can be downloaded from diy.thcustom.com/drill-templates/.
The panel layout places Volume and Gain pots on the left, Treble and Bass on the right. LED positions (tube glow, bypass indicator) are indicated on the template as small dots between the pots. The input and output jacks are on the sides.
Completed Tube Driver V2.0d in enclosure
Disclaimer & Licence
PCBs purchased from TH Custom Effects are intended for DIY / non-commercial use only. It is not permitted to redistribute the PCBs or artwork from this document. You may use these instructions and PCBs to build and sell your own product based on PCBs ordered from TH Custom Effects.
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