Overview & Features
The Small Time is Merlin Blencowe's take on a PT2399-based delay pedal — well known for being one of the few simple DIY delay designs with proper tails on bypass. When you stomp the pedal off, the last few repeats are still audible as they decay naturally, instead of being abruptly cut.
Features
- PT2399 digital echo processor with two TL072 op-amp stages for buffering, mixing and tails generation
- J201 JFET source follower at the input — high input impedance, very low loading on guitar pickups
- Three controls: MIX (wet/dry blend), REPEATS (feedback), DELAY (time)
- Approximately 30–600 ms delay range over the 50 kΩ DELAY pot
- 78L05 onboard regulator — runs from any standard 9 V centre-negative supply
- Buffered bypass with tails — single SPST/SPDT footswitch, no 3PDT required
- Compact PCB designed for a 1590B-class enclosure
Populated TH Custom Effects Small Time V1.2 PCB. PT2399 (right, larger DIP-16), TL072 (left), J201 input buffer.
Circuit Theory
Full schematic — Merlin's Small Time, TH Custom Effects V1.2 (2016)
Signal architecture
The circuit splits into three functional blocks: an input buffer + dry path (IC2A), the delay engine (PT2399 with its surrounding R/C network and a J201 source follower feeding it), and the output mixer + tails feedback (IC2B). The clever part is how the wet signal is mixed back in: REPEATS taps the delay output and feeds it back into the PT2399 input, while MIX blends wet and dry at IC2B's summing junction. When the footswitch grounds the dry path, the wet signal keeps recirculating through the delay loop and decays naturally — that's the "tails" effect.
Stage 1 — Input buffer (IC2A)
Signal enters at IN through R7 (100 Ω, series), then through C6 (100 n, DC-blocking) into IC2A's non-inverting input. R3 (10 M) is a high-value input pull-down that prevents pop on plug-in. R11 (1 M) biases the +input to VR (virtual ground at +V/2). IC2A is wired as a unity-gain non-inverting buffer (output tied straight to −input). Its job is to present a clean high-impedance load to your guitar and provide a low-impedance source for everything downstream.
Stage 2 — Delay engine (J201 + PT2399)
The IC2A output drives two things in parallel: the dry path (through R16 to IC2B) and the delay input. The delay input first hits a J201 JFET source follower (Q1), which is the recirculating sum point — REPEATS feedback comes in here too via the wiper of the REPEATS pot. R8 (10 k) is the source resistor; the gate sees the signal through the SW switching network (D1 + R2 mute the input when the pedal is off, killing the dry signal but leaving the wet feedback loop intact — that's how tails work).
From Q1's source, the signal goes through C8 (100 n) into the PT2399's first internal anti-aliasing low-pass filter (LPF1), built around R5, R9, C2, C5. After the PT2399's digital delay line, the output passes through LPF2 (built around R17, R18, C11, C15) to smooth the sample-and-hold artefacts. The DELAY pot (50 kΩ + R25 1 kΩ in series) sets the PT2399's clock frequency and therefore the delay time — roughly 30 ms (pot at minimum) to 600 ms (pot at maximum), with the higher times getting progressively darker and grittier as the PT2399 leaves its design range. R24 (1 M) is the standard pull-up to the VCO input.
Stage 3 — Output mixer (IC2B)
IC2B sums the dry signal (through R16) and the wet signal (through R19, fed from the MIX pot wiper) into an inverting summer with R20 (10 k) as the feedback resistor. C16 (1 n) across R20 forms a single-pole roll-off above audio band — good RFI rejection. The output passes through R22 (100 Ω, series) and C24 (10 µF, DC-blocking) to the OUT jack. R23 (100 k) is the output pull-down.
Power supply
9 V DC enters through D2 (1N4001, series — reverse polarity protection) and is filtered by C4 (47 µF). The 9 V rail (VA) feeds the TL072. A separate +5 V rail (VC) generated by REG1 (78L05) and filtered by C13 (22 µF) powers the PT2399. R10/R12 (both 100 k) form the virtual ground divider VR = 4.5 V, decoupled by C9 (10 µF). LED current is set by R1 (4k7) — adjust to taste for your chosen LED.
Filter Analysis
The PT2399 needs anti-aliasing low-pass filtering both before and after its internal delay line. Merlin's design uses two 2-pole low-pass stages around the chip — they're built into the PT2399's internal op-amp pins, but the R/C values that set the cutoff are your components on the board. Their cutoffs determine how dark the repeats sound.
LPF1 — Input to delay
Sets the bandwidth that enters the PT2399's delay line. Cuts content above ~2.8 kHz before sampling — necessary to prevent aliasing at the chip's sample rate.
LPF2 — Delay to output
Reconstruction filter on the wet output — smooths sample-and-hold steps from the delay line. The combined ~−80 dB/dec roll-off above ~3 kHz gives the wet repeats a warm, slightly murky character — which is most of the charm.
Output mixer roll-off (IC2B)
C16 (1 nF) across R20 (10 kΩ) gives the summing op-amp a single-pole roll-off at f₀ ≈ 15.9 kHz. This is well above audio — its job is RFI rejection at the output, not tone shaping. Leave as designed unless you have a specific reason to change it (the badge in the BOM marks it as a tunable component for the curious).
PT2399 delay range
Delay time is set by the resistance between the PT2399's CC0/CC1 pins. With R25 (1 kΩ) in series with the 50 kΩ DELAY pot, the resistance sweeps from 1 kΩ (clean, ~30 ms) to 51 kΩ (long and progressively darker, ~500–600 ms). Beyond around 35 ms the chip is operating outside its specified range — the repeats get gritty, lo-fi and self-modulated. This is part of the PT2399's character and what gives it its tape-like vibe at long settings.
Bill of Materials
Standard 5% / 1% values throughout. The BOM here is authoritative — it has been cross-checked against the V1.2 schematic. Where the schematic and BOM ever differ on values, the BOM wins.
| Ref | Qty | Value | Colour code | Notes |
|---|---|---|---|---|
| Resistors — metal film, ¼ W, 1% | ||||
| R1 | 1 | 4k7 | Yellow · Violet · Black | Brown · Brown | Metal film ¼ W |
| R2 | 1 | 1k | Brown · Black · Black | Brown · Brown | Metal film ¼ W |
| R3 | 1 | 10M | Brown · Black · Black | Green · Brown | Metal film ¼ W |
| R4 | 1 | 10k | Brown · Black · Black | Red · Brown | Metal film ¼ W |
| R5 | 1 | 10k | Brown · Black · Black | Red · Brown | Metal film ¼ W |
| R6 | 1 | 100R | Brown · Black · Black | Black · Brown | Metal film ¼ W |
| R7 | 1 | 100R | Brown · Black · Black | Black · Brown | Metal film ¼ W |
| R8 | 1 | 10k | Brown · Black · Black | Red · Brown | Metal film ¼ W |
| R9 | 1 | 15k | Brown · Green · Black | Red · Brown | Metal film ¼ W |
| R10 | 1 | 100k | Brown · Black · Black | Orange · Brown | Metal film ¼ W |
| R11 | 1 | 1M | Brown · Black · Black | Yellow · Brown | Metal film ¼ W |
| R12 | 1 | 100k | Brown · Black · Black | Orange · Brown | Metal film ¼ W |
| R14 | 1 | 1k | Brown · Black · Black | Brown · Brown | Metal film ¼ W |
| R15 | 1 | 1k | Brown · Black · Black | Brown · Brown | Metal film ¼ W |
| R16 | 1 | 10k | Brown · Black · Black | Red · Brown | Metal film ¼ W |
| R17 | 1 | 10k | Brown · Black · Black | Red · Brown | Metal film ¼ W |
| R18 | 1 | 10k | Brown · Black · Black | Red · Brown | Metal film ¼ W |
| R19 | 1 | 10k | Brown · Black · Black | Red · Brown | Metal film ¼ W |
| R20 | 1 | 10k | Brown · Black · Black | Red · Brown | Metal film ¼ W |
| R21 | 1 | 10k | Brown · Black · Black | Red · Brown | Metal film ¼ W |
| R22 | 1 | 100R | Brown · Black · Black | Black · Brown | Metal film ¼ W |
| R23 | 1 | 100k | Brown · Black · Black | Orange · Brown | Metal film ¼ W |
| R24 | 1 | 1M | Brown · Black · Black | Yellow · Brown | Metal film ¼ W |
| R25 | 1 | 1k | Brown · Black · Black | Brown · Brown | Metal film ¼ W |
| Capacitors — film (box-style) | ||||
| C2, C15 | 2 | 10n | Box film — PT2399 LPF stages | |
| C5, C11 | 2 | 2n2 | Box film — PT2399 LPF stages | |
| C16 | 1 | 1n | Box film — IC2B output low-pass tuning | |
| C7, C12 | 2 | 47n | Box film — PT2399 internal nodes | |
| C3, C6, C8, C17, C19, C21, C22, C23 | 8 | 100n | Box film — coupling and supply decoupling | |
| Capacitors — electrolytic (polarised) | ||||
| C1, C9, C10, C14, C24 | 5 | 10µF | Polarised electrolytic — observe polarity | |
| C18 | 1 | 10µF | Tantalum — observe polarity. Standard electrolytic also works. | |
| C13 | 1 | 22µF | Polarised electrolytic — 7805 output decoupling | |
| C4, C20 | 2 | 47µF | Polarised electrolytic — supply rail filtering | |
| Potentiometers — 16 mm PCB-mount | ||||
| MIX | 1 | 10k lin | B taper — wet/dry blend | |
| REPEATS | 1 | 10k lin | B taper — feedback amount | |
| DELAY | 1 | 50k lin | B taper — delay time | |
| Diodes | ||||
| D1 | 1 | 1N4148 | Small-signal — observe polarity | |
| D2 | 1 | 1N4001 | Reverse polarity protection — observe polarity | |
| LED | 1 | 3 mm | Indicator — colour and brightness to taste; pair with R1 | |
| Transistors | ||||
| Q1 | 1 | J201 | JFET source follower (input buffer). J107 2N5457 — verify pinout (GDS), most J-FETs work. | |
| Integrated circuits | ||||
| IC1 | 1 | PT2399 | Echo processor IC, DIP-16. Use a socket. | |
| IC2 | 1 | TL072 | Dual op-amp, DIP-8. Use a socket. JRC4580D OPA2134 TLC2272 | |
| REG1 | 1 | 78L05 | TO-92 +5V regulator. Powers PT2399. | |
Build Guide
The board is densely populated for its size — there's a lot squeezed into a 1590B footprint. Take your time, work from low components upward, and check polarity twice on every diode and electrolytic.
Component placement reference — top view. Pots mount on the back side.
Resistors
Lowest profile, do these first. All metal film, ¼ W. Match each reference designator on the silkscreen with the BOM value; the colour bands in the BOM make verification quick. Bend leads close to the body, insert flat against the board, and clip from the back after soldering.
Diodes (D1, D2)
Low-profile and polarity-sensitive. D1 (1N4148) is the small glass signal diode. D2 (1N4001) is the larger black plastic protection diode. The cathode (band) on each must match the silkscreen marking — getting D2 backwards prevents the pedal from powering up.
IC sockets
Fit sockets for both IC1 (DIP-16) and IC2 (DIP-8). Don't insert the chips yet. The notch on the socket should match the silkscreen — keeps you honest later. Optional but recommended: socket Q1 too (the J201 transistor sits in a 3-pin row), so you can experiment with alternative JFETs later.
Voltage regulator (REG1)
The 78L05 is a small TO-92 part — flat side faces the silkscreen flat. Check the pinout on the datasheet for your specific brand: most are Vin – GND – Vout, but some manufacturers use a different order. Wrong orientation may damage the regulator.
Transistor (Q1)
If you sockfeted Q1 in step 3, skip this. Otherwise solder the J201 directly. Pinout for J201 in TO-92 is D – S – G (drain, source, gate) when looking at the flat side with leads down — confirm from the silkscreen and the J201 datasheet before soldering.
Box film capacitors
Non-polarised, no orientation. Populate ascending: 1n (C16), 2n2 (C5, C11), 10n (C2, C15), 47n (C7, C12), then all the 100n parts. They sit flat to the board — push them in firmly so the body is against the PCB before soldering.
Electrolytic capacitors
Polarised — long lead is positive (+), the can has a stripe marking the negative side. Match the (+) on the silkscreen. C18 is specified as a tantalum but a regular electrolytic works equally well; observe polarity either way. Reversing any electrolytic risks a small explosion when powered.
Off-board pads & LED
Solder in the off-board wire pads or pin headers for IN, +9V, GND, OUT, SW. Mount the LED last from the back — the long leg (+) goes to the silkscreen-marked anode pad. Don't solder the LED yet if you're using an enclosure-mount LED bezel; just leave the leads long for now.
Pots — back-side mounting
The pots mount on the opposite side of the board to all other components. Stick a layer of double-sided tape (or insulating tape) on the metal pot bodies before pushing them through, so the case can't short any of the 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 mounting in your enclosure.
Insert ICs and Q1
With all soldering done and double-checked for shorts, insert IC1 (PT2399), IC2 (TL072), and (if socketed) Q1 (J201). Match the notch on the IC to the notch on the socket. Don't force anything — if the legs are splayed out, gently bend them inward against a flat surface first.
Wiring
This is a buffered bypass circuit. You only need a single-pole latching footswitch (SPST or SPDT) to turn the effect on and off — much simpler than the usual 3PDT true-bypass arrangement, and the tails feature only works with this kind of switch.
Merlin's buffered bypass wiring — SPST footswitch grounds the SW pad to mute the dry path, leaving the wet feedback loop intact for tails.
| Pad | Connect to |
|---|---|
| IN | Tip of input jack |
| OUT | Tip of output jack |
| +9V | Power jack — centre (or DC jack tip, depending on jack type) |
| GND | Sleeve of input jack, sleeve of output jack, power jack ring/sleeve, footswitch one side |
| SW | Other side of SPST footswitch (when grounded by the switch, the dry path mutes — effect ON) |
| LED | Anode (long leg) to LED+ pad, cathode (short leg) to GND. R1 on the board sets brightness. |
Wiring close-up — footswitch on the left, populated PCB on the right with all off-board wires soldered to their respective pads.
Setup & Usage
Controls
MIX — wet/dry blend. Fully counter-clockwise = dry only (no echo audible). Fully clockwise = mostly wet. Slap-back lives around 9 o'clock; long ambient delays use noon and beyond.
REPEATS — feedback amount. Counter-clockwise = single repeat. Mid = a few repeats that decay naturally. Clockwise (past about 3 o'clock) starts pushing the loop into self-oscillation — useful for swelling drones and noisy textures, but back off if it runs away.
DELAY — echo time, roughly 30 ms (CCW) to 600 ms (CW). The PT2399 loses fidelity beyond about 35 ms — embrace it. Long settings sound dark, noisy and beautifully lo-fi.
Suggested starting points
Slapback — MIX 9 o'clock, REPEATS 8 o'clock (one repeat), DELAY 8 o'clock (~80 ms).
Classic delay — MIX 11 o'clock, REPEATS noon, DELAY noon (~300 ms).
Ambient wash — MIX 1 o'clock, REPEATS 2 o'clock (just below self-oscillation), DELAY 3 o'clock (long, dark).
Self-oscillation drone — REPEATS past 3 o'clock and ride the DELAY knob for pitch shifts. Reduce REPEATS to settle.
This is an easy build with a great-sounding result
Merlin's circuit is one of the more elegant DIY PT2399 designs — credit goes to him (valvewizard.co.uk/smalltime) for the topology and to the tails trick that elevates it above most DIY delays. Enjoy.
Disclaimer & Licence
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.
Original Small Time circuit © Merlin Blencowe — valvewizard.co.uk/smalltime. PCB layout and build documentation © TH Custom Effects 2016–2026. Build documentation V1.2.