TH Custom Effects Build Documentation · V1.2 · 2014–2026

Valv-e-tizer
Driver

12AX7 / 12AT7 Tube Distortion — V1.2

An op-amp pre-distortion stage driving a 12AX7 (or 12AT7) dual-triode running on an internally-generated ~60V plate rail, with a Baxandall tone stack on the output. Inspired by the well-known classics used by David Gilmour, Eric Johnson and Joe Satriani — runs from a single 9V supply.

12AX7 / 12AT7 dual triode ~60 V plate rail Baxandall tone stack 9 V DC, 300–500 mA 125B enclosure Tube outside

01

Overview & Features

The Valv-e-tizer Driver is a faithful take on the well-known circuit that runs a simple op-amp distortion stage straight into a vacuum tube. The result is the rich, dynamic harmonic content of a real triode pushed into compression — usable from polite overdrive to thick saturated lead tones depending on the gain setting and the tube fitted.

The clever part is the high-voltage rail. A 40106 Schmitt-trigger oscillator drives a 7-stage Cockcroft-Walton diode-capacitor multiplier that lifts the 9 V supply to roughly 55–65 V DC, giving the tube a proper plate voltage from a single pedalboard supply. A 7806 regulator drops the same 9 V down to 6 V for the heater — slightly under-running the 6.3 V filament for longer tube life. No external high-voltage supply is required.

Dual-triode tube

12AX7 (high gain, default) or 12AT7 (lower gain, more headroom). Both pin-compatible.

Internal HV rail

40106-driven 7-stage charge pump generates ~60 V from 9 V — no external PSU needed.

Op-amp pre-stage

TL072 buffer + variable-gain amplifier (×1 to ~×68) drives the tube grid.

Baxandall tone stack

True boost-and-cut bass and treble, smoother and more interactive than the original passive tone control.

BIAS trimmer

10 k trimmer sets the shared cathode bias of the tube — tune for the sweet spot of your specific valve.

"Glow" indicator

An LED under the tube socket lights the valve from below for the visual effect — optional.

Mechanical layout

The PCB is designed for a 125B enclosure with the tube mounted outside. A Noval print socket sits on the top of the board and the tube clips into it from above. The four pots, BIAS trimmer and 7806 regulator (which uses the enclosure as a heatsink) all fit in this footprint. Boxing it is straightforward — the pots hold the board in place, with optional mounting screws through the corners for a fully secure build.

Prototype in 125B — tube mounted outside via Noval socket on top of the board.

PCB component layout — tube socket centred between BIAS trimmer (top-left) and the four pots.

Current draw: the unit draws 300–500 mA at 9 V, mostly the heater. Use a beefy isolated supply output — a 100 mA or 200 mA pedalboard slot will not power this pedal. A dedicated isolated 500 mA+ slot is strongly recommended.

Two build variants in this document. Throughout the BOM and the analysis, parts are marked MyDrv (Thomas's tweaked voicing — fuller, more amp-like) and Drv (closer to the original Tube Driver — tighter upper-mid focus). Only two caps differ between the two builds (C11 and C19); everything else is identical.

02

Circuit Theory

Full schematic — Valv-e-tizer Driver V1.2 (TH Custom Effects, 07/2014).

Signal flow at a glance

Input → IC3A unity-gain buffer → IC3B variable-gain stage (×1–×68) → coupling cap → V1A common-cathode triode → coupling cap → V1B common-cathode triode → C14 coupling cap → Baxandall tone stack (BASS, TREBLE) → VOL pot → output. Power: +9 V → IC2 7806 → +6 V heater. +9 V → 40106 oscillator → 7-stage Cockcroft-Walton multiplier → ~60 V plate rail.

High-voltage charge pump (IC1, D1–D7, C1–C8)

IC1 is a 40106 hex Schmitt-trigger inverter. IC1A with R1 (2k7) and C7 (1n) forms a relaxation oscillator running at roughly 500 kHz on a 9 V supply. The remaining five inverters (IC1B–IC1F) buffer the square wave and drive a 7-stage Cockcroft-Walton diode-capacitor ladder built from 1N4148 small-signal diodes (D1–D7) and 47 nF film caps (C1–C6, C8). Each stage adds approximately one supply-voltage step minus two diode drops to the rail; the ideal output is N × (V_p − 2V_d) ≈ 53 V, and in practice a loaded rail of 55–65 V appears at the top of D7. C8 (47 n) is the final reservoir.

Working at hundreds of kHz keeps the multiplier caps small and the output ripple low. The 1N4148 is fast enough to switch cleanly at this rate, and the small-signal parts make the section physically tiny on the PCB.

Heater supply (IC2, C9, C10, C15)

IC2 is a 7806 linear regulator. The 9 V input feeds it through C9 (220 µF reservoir), C15 (33 n HF bypass on the input) and the regulator output drives the heater string of V1 with C10 (47 n) on the output for stability. The heater nominal voltage of the 12AX7 / 12AT7 is 6.3 V; running it at 6 V is gentler on the filament and helps prolong tube life. Mount the 7806 so its tab presses against the enclosure — the metal box becomes the heatsink and dissipates the ~1 W of heat the regulator produces while supplying ~300 mA of heater current.

Virtual ground (R12, R13, C21)

R12 and R13 (both 10 k) form a divider from +9 V to GND, generating +4.5 V virtual ground on the VR rail. C21 (10 µF) decouples this rail. All op-amp non-inverting inputs and signal midpoints are biased to VR, allowing the TL072 to handle AC audio on a single-supply 9 V rail.

Input front-end (R11, R4, C13, C11, R17, IC3A)

The guitar enters at the IN pad, through R11 (10 k) series, into a junction with R4 (1 M) pull-down to GND and C13 (47 p) shunt to GND. The R11/C13 combination forms a passive low-pass at ~340 kHz that suppresses RF and switching-noise pickup. From there C11 AC-couples the signal into IC3A's non-inverting input, which is biased to VR through R17 (1 M).

IC3A is wired as a unity-gain follower — its output ties directly back to its inverting input. This is purely an impedance-conversion stage: high input impedance for the guitar pickups, low output impedance to drive the gain stage that follows.

Variable-gain stage (IC3B, R14, GAIN pot, C17, C19)

IC3B is a non-inverting amplifier. Its non-inverting input sits at VR; the feedback path runs from the output back to the inverting input via the GAIN potentiometer (100 k log). R14 (1k5) sets the feedback divider's lower leg from IC3A's output. With the GAIN pot fully clockwise, feedback resistance is at maximum (~100 k) giving a gain of 1 + 100 k / 1k5 ≈ ×68 (≈ 36 dB). Fully counter-clockwise it falls to ×1 (unity).

C17 (47 p) sits in parallel with the feedback resistance, providing a high-frequency roll-off that prevents oscillation and tames any fizz from the op-amp before the tube — at maximum gain its corner sits around 34 kHz. C19 (MyDrv33 n / Drv10 n) is the output coupling cap that hands the signal off to the tube grid network.

Tube stage (V1A and V1B, R5, R6, R7, R10, R15, R16, C12, C16, BIAS)

V1A and V1B are wired as cascaded common-cathode amplifiers sharing a common cathode bias point. R7 (22 k) is the cathode resistor and C12 (22 µF) is its bypass cap; the BIAS trimmer (10 k) and R9 (47 k, populated only when an external bias pot is wanted) shift this operating point — the same DC voltage feeds both V1A.K and V1B.K, so BIAS sets the bias for both halves at once.

V1A: grid is driven via R15 (10 k grid stopper) with R16 (10 k) pulling the grid to GND for DC-blocking; plate uses R5 (47 k) into the +60 V rail. V1A → V1B coupling: V1A's plate connects through R10 (10 k plate-side resistor) and C16 (47 n) to V1B's grid, with R7 (22 k) — yes, the same value as the cathode but a separate part — acting as the V1B grid leak to GND. The C16 / R7 high-pass corner sits at ~154 Hz, which deliberately rolls off bass into the second stage to keep the tube saturation tight rather than mushy.

V1B: plate uses R6 (47 k) into the +60 V rail. R2 and C18 are PCB pads provided for an optional plate snubber (DC-blocking resistor in series and an HF roll-off cap to ground); both are not populated in either build variant. C14 (1 µF) is the output coupling cap to the tone stack.

Why two stages share a cathode: tying both cathodes to the same biased node is unusual but works well here — it saves a part, and because the two stages are cascaded the AC currents through the shared cathode resistor partially cancel. The BIAS trimmer therefore tunes the operating point of the cascade as a whole.

Baxandall tone stack (R21, R23, R24, C28–C31, BASS, TREBLE)

After C14 the signal hits a standard Baxandall active-style passive tone stack. R21 (100 k) is the input series resistor; the BASS pot (100 k log) sits between C28 (1 n) and C29 (6n8) with R24 (22 k) and R23 (10 k) defining the network slopes; the TREBLE pot (100 k log) sits between C30 (1 n) and C31 (10 n) at the output side. Each pot provides genuine boost and cut around its hinge frequency rather than the simple cut-only behaviour of a tone-stack-style EQ.

Treble hinges around ~1.6 kHz (set by 100 k × 1 n). Boosting raises the high-frequency shelf, cutting brings it down.
Bass hinges around ~160 Hz (set by 100 k × 10 n). Boosting raises the low-frequency shelf, cutting brings it down.

Output stage (C20, VOL)

C20 (10 µF) is the final output coupling cap; the VOL pot (100 k log) sets level into the OUT jack. The 10 µF / 100 k combination gives an effective high-pass at ~0.16 Hz — well below audio, so essentially flat.

03

Stage Analysis

The interesting numbers in this circuit are the op-amp gain range, the high-voltage rail, the inter-stage coupling cutoffs and the tone-stack hinge frequencies. The tube stages themselves are not analysed numerically — the gain of a real 12AX7 / 12AT7 depends on the actual valve, the loaded plate resistor and the cathode bias, and is usually set by ear via the BIAS trim.

Op-amp pre-distortion stage

A_v = 1 + R_FB / R14 (non-inverting), R_FB = 0 → 100 kΩ via GAIN pot

Setting	R_FB	Gain (×)	Gain (dB)
GAIN min (CCW)	0 Ω	×1	0 dB
GAIN ¼	~25 kΩ	×17.7	24.9 dB
GAIN ½	~50 kΩ	×34.3	30.7 dB
GAIN max (CW)	100 kΩ	×67.7	36.6 dB

The pot is logarithmic, so audible gain change is approximately linear across the rotation. Even at unity, the tube will still be driven into mild compression at hot signal levels — the GAIN pot sets how hard the front end pushes the tube, not whether it pushes at all.

High-pass corners along the signal path

The two build variants differ only in two coupling caps (C11 and C19). Their corner frequencies and the function of each:

Drv Driver build

Closer to the original

C11 input HP

33 n / 1 M = 4.8 Hz

C19 → V1A grid

10 n / 20 k = 796 Hz

C16 → V1B grid

47 n / 22 k = 154 Hz

C14 → tone stack

1 µ / ~100 k = 1.6 Hz

A high cutoff at C19 (~800 Hz) keeps the lows out of V1A — only mids and highs hit the first triode hard, giving a tight upper-midrange focus that's recognisably the original's voice.

MyDrv MyDriver build

Tweaked, fuller-sounding

C11 input HP

47 n / 1 M = 3.4 Hz

C19 → V1A grid

33 n / 20 k = 241 Hz

C16 → V1B grid

47 n / 22 k = 154 Hz

C14 → tone stack

1 µ / ~100 k = 1.6 Hz

Lowering the C19 corner to ~240 Hz lets the bass into V1A as well, giving a fuller, more amp-like saturation across the whole guitar range. Use this if you find the Driver build too thin or boxy.

Baxandall tone stack hinges

Bass shelf

BASS pot 100 kΩ · C31 = 10 nF

Hinge

160

Hz

Range

±12

dB approx

Slope

6

dB/oct

Boost and cut take effect below ~160 Hz. Centre detent is approximately flat. Cutting tightens the low end into a high-gain amp; boosting fills out the body for cleaner settings.

Treble shelf

TREBLE pot 100 kΩ · C28, C30 = 1 nF

Hinge

1.6

kHz

Range

±12

dB approx

Slope

6

dB/oct

Boost and cut take effect above ~1.6 kHz. The Baxandall topology means cutting treble does not also pull mids down (unlike a passive Marshall stack), so it stays musical at extreme settings.

Power rail summary

+9 V input — 300–500 mA, isolated PSU recommended.
+6 V heater rail — IC2 (7806) regulator, ~300 mA. Heatsink the IC against the enclosure.
VR (+4.5 V virtual ground) — R12 / R13 / C21, used as op-amp bias reference.
+55–65 V plate rail — 40106 oscillator (~500 kHz) + 7-stage Cockcroft-Walton multiplier from D1–D7 and C1–C6, C8.

04

Bill of Materials

The BOM is the source of truth — if the PCB silkscreen markings ever differ, follow the BOM. Two build variants are documented: only two caps (C11 and C19) actually change between them; everything else is shared.

MyDrvMyDriver — Thomas's tweaked voicing, fuller-sounding. DrvDriver — closer to the original Tube Driver.

Ref	Qty	Value	Colour code	Notes
Resistors — metal film, ¼ W, 1 %
R1	1	2k7	Red · Violet · Black \| Brown · Brown	40106 oscillator timing resistor.
R2	—	— DNP —		Do not populate. Optional V1B plate snubber series resistor.
R3	1	jumper		0 Ω resistor or wire link.
R4, R17	2	1M	Brown · Black · Black \| Yellow · Brown	Input pull-down (R4) and IC3A bias to VR (R17).
R5, R6, R9	3	47k	Yellow · Violet · Black \| Red · Brown	V1A & V1B plate loads (R5, R6). R9 is the optional external-BIAS series resistor — populate only if you want an external BIAS pot mounted on the enclosure.
R7, R24	2	22k	Red · Red · Black \| Red · Brown	R7 = shared cathode resistor + V1B grid leak. R24 = Baxandall network resistor.
R8	1	1k5–3k3	Red · Red · Black \| Brown · Brown	"Glow" LED current-limiting resistor under the tube. Test for brightness with your LED first; 2k2 is a sensible starting value. Do not populate if you do not want the under-tube glow.
R10–R13, R15, R16, R23	7	10k	Brown · Black · Black \| Red · Brown	R10 V1A plate-side coupling, R11 input series, R12/R13 VR divider, R15 V1A grid stopper, R16 V1A grid leak, R23 Baxandall.
R14	1	1k5	Brown · Green · Black \| Brown · Brown	IC3B feedback divider lower leg — sets gain range.
R21	1	100k	Brown · Black · Black \| Orange · Brown	Baxandall input series resistor.
Capacitors — film (box) unless noted
C1–C6, C8, C10, C16	9	47n		Box film, 80–100 V rating required — these caps see HV swings on the multiplier ladder.
C7, C28, C30	3	1n		Box film. C7 sets the 40106 oscillator frequency; C28, C30 are Baxandall treble caps.
C9	1	220 µ / 16 V		Polarised electrolytic — main input reservoir.
C11	1	MyDrv47nDrv33n		Box film. IC3A input coupling cap. tuning
C12	1	22 µ / 50 V		Polarised electrolytic — shared cathode bypass.
C13, C17	2	47p		Ceramic. C13 = input RFI shunt; C17 = IC3B feedback HF stability.
C14	1	1 µ / 50 V		Polarised electrolytic — V1B output coupling to tone stack. Voltage rating matters: this cap sees the full plate rail on one side.
C15	1	33n		Box film — 7806 input HF bypass.
C18	—	— DNP —		Do not populate. Optional V1B plate snubber HF cap (electrolytic footprint).
C19	1	MyDrv33nDrv10n		Box film. IC3B → V1A grid coupling cap. Sets the bass cutoff into the first triode. tuning
C20, C21	2	10 µ / 16 V		Polarised electrolytic — output coupling (C20) and VR decoupling (C21).
C29	1	6n8		Box film — Baxandall bass-mid network.
C31	1	10n		Box film — Baxandall bass shelf cap.
Diodes
D1–D7	7	1N4148		Charge-pump multiplier ladder. Mind orientation — the cathode band points toward the next stage.
LED	1	3 mm super-bright		"Tube glow" indicator under the socket — colour to taste (orange/red looks like a real heated filament).
Trimmer / pots
BIAS	1	10 k trim		6 mm ACP6 or Piher trimmer. Sets the shared cathode bias of V1A / V1B.
GAIN, VOL, BASS, TREBLE	4	100 k log (A)		9 mm Alpha (e.g. Tayda). All four pots are 100 kΩ log taper.
Integrated circuits
IC1	1	40106N		Hex Schmitt-trigger inverter, DIP-14. CD40106 MC14584 74HC14
IC2	1	µA7806		+6 V linear regulator, TO-220. Mount the tab against the enclosure as a heatsink. L7806 LM7806
IC3	1	TL072		Dual op-amp, DIP-8. Use a socket. JRC4580 OPA2134 TLC2272
Tube and socket
V1	1	MyDrv12AX7Drv12AT7		Dual triode, Noval (B9A) base. 12AX7 = high gain (default for MyDriver). 12AT7 = lower gain, more headroom (closer to the original Driver). Both are pin-compatible drop-ins. ECC83 (= 12AX7) ECC81 (= 12AT7)
S1	1	Noval print socket		PCB-mount Noval (9-pin) socket. Musikding part. Plastic sockets can be drilled to fit the 3 mm "glow" LED behind.

R8 ("glow" LED CLR): the BOM lists a range of 1k5 – 3k3 because LED brightness varies wildly between manufacturers. Test with your specific LED on a 6 V supply before soldering. If you don't want the under-tube glow at all, leave R8 out and either skip the LED or run a wire link in its place.

R9 (external BIAS): only populate this if you want to expose the BIAS as an external knob on the enclosure. For a standard build with the trimmer set once and forgotten, R9 stays out.

05

Build Guide

Standard through-hole population order applies — work from low-profile parts up to tall ones, leaving the tall pots and the tube socket for last so they reference final assembly height. The boards have the silkscreen labels on the top side; double-check polarised parts (electrolytic caps, LEDs, diodes, the 7806) against the silkscreen before soldering.

Populated board (top view) — early build with diodes, resistors, IC sockets and small caps in.

Backside view — pots and Noval socket fitted, board ready to box.

Small signal diodes

Populate D1–D7 (1N4148) first — they are the lowest-profile parts on the board. Match the cathode band on each diode to the silkscreen stripe.

Resistors

All metal-film resistors next, including R3 jumper (use a clipped resistor lead or a 0 Ω). Leave R2 and R8 for now if you want — R2 is unpopulated, R8 you may want to test with your LED first.

IC sockets

Fit the DIP-14 socket for IC1 (40106) and the DIP-8 socket for IC3 (TL072). Do not insert the ICs yet — they go in last after all soldering is done.

Ceramic and small film caps

C13, C17 (47p ceramic) and the 1n / 47n film caps, ascending by value. The 47n caps on the multiplier ladder must be the 80–100 V rated film type — these see the HV rail.

Larger film caps

C15 (33n), C19 (MyDrv33n / Drv10n depending on build variant), C29 (6n8), C31 (10n).

Electrolytic capacitors

C9 (220 µ), C12 (22 µ), C14 (1 µ), C20 / C21 (10 µ). Check polarity — the long lead is + and the silkscreen has a + marker. Pay particular attention to C14: the V1B plate side will sit at ~60 V, so the cap orientation must be correct.

BIAS trimmer and 7806 regulator

Mount the BIAS trimmer flat on the board. The 7806 mounts so its tab can press against the enclosure when the board is boxed — leave the leads slightly long so you can bend it into position later.

"Glow" LED and R8

Mount the 3 mm LED from the back of the board, with the long lead going into the round (anode) hole. The LED will sit underneath the tube socket and shine up through it — make sure your Noval socket has a clear hole in the centre. Some plastic sockets will need to be drilled; ceramic sockets can also be drilled but go slowly. Test brightness with the chosen R8 value before final solder.

Noval socket

Solder the Noval print socket on top of the board, directly above the LED. Leave a 1–2 mm gap between the socket body and the PCB so the socket sits slightly proud of the pots — this makes boxing easier.

Pots

Clip off the small anti-rotation tabs on each pot (they prevent flush mounting). Insert all four pots and the off-board jacks before soldering, so everything aligns flat. Solder the pots last so the board is held square by them.

Mount and insert ICs

Use the corner mounting holes to fix the board to the enclosure for a fully-secure build. Then carefully insert IC1 (40106) and IC3 (TL072) into their sockets — mind orientation by the notch.

Set BIAS

With the tube fitted and a guitar plugged in, slowly sweep the BIAS trimmer with the GAIN at noon. Find the position where the tube saturation sounds most musical — typically near the centre of the trimmer's range, but it depends on the individual valve. Once happy, leave it.

High-voltage warning: the V1A / V1B plate rail sits at 55–65 V DC. This is well below mains-voltage tube-amp territory but it can still bite, and it can damage the op-amp if a probe slips. Never touch the tube socket pins, the plate-side resistors, or the top of the diode-cap ladder while the pedal is powered. The HV rail discharges within a second or two of unplugging the supply, but verify with a meter before any rework on a powered board.

06

Wiring

Wire the board to the off-board jacks, 3PDT footswitch, DC jack and indicator LED as shown below.

Off-board wiring with 3PDT true bypass and (optional) external indicator LED.

Standard build (under-tube glow always on)

In a standard build, R8 is populated and the under-tube LED glows whenever the pedal has power, regardless of bypass state. To still have an on/off indicator on the enclosure, fit a second LED with its own current-limiting resistor (CLR) on the 3PDT switch as shown — that LED is the bypass indicator, while the under-tube LED is purely visual.

Alternative: under-tube LED switched with bypass

If you don't want the tube glowing while the effect is bypassed, do not populate R8 and instead run a wire link in its place — the dotted line in the diagram. Take the LED feed from the 3PDT switch lug that's only powered when the effect is engaged, and skip the separate switch-mounted indicator LED.

Wiring tip: the IN/OUT pads on the board carry low-level audio. Keep these wires away from the +9V and HV-rail traces, and as short as practical. The GND pad on the off-board side is the audio ground star.

07

Enclosure

The board is sized for a 125B enclosure with the tube mounted outside the box on the top of the PCB. A printable drilling decal that doubles as a drill template is available at diy.thcustom.com/drill-templates.

Tube cage / guard

If you intend to gig with this pedal, fit a tube guard — a U-shaped wire bail around the valve protects it from kicks and helps you grab a hot tube safely. The prototype shown uses a stainless steel guard tightened with two M3 nuts each side of the enclosure.

Prototype (1590B variant — not recommended)

Earlier prototype in a 1590B box — the 125B is recommended for production builds because it gives more breathing room around the IC2 heatsink and the tube socket.

Heat: the 7806 dissipates around 1 W of heat as it drops 9 V to 6 V at 300 mA. With its tab pressed against the enclosure, the box itself acts as the heatsink. Do not insulate the regulator from the box — it should be in good metal-to-metal contact (a thin smear of thermal paste is fine).

—

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PCBs purchased from TH Custom Effects are intended for DIY and non-commercial use only. Redistribution of PCBs and artwork from this document is not permitted. You may use these instructions and PCBs to build and sell your own product based on PCBs ordered from TH Custom Effects.