D-12 Build Guide

• Separate the PCB from the panel by breaking the tabs gently — score the tabs with a blade first, then snap cleanly. File any rough edges.
• Inspect both sides under good light: verify no lifted pads, cold solder bridges from fabrication, or damaged traces.
• Read this entire guide and the Errata section before placing a single component.
• Confirm tools: temperature-controlled iron (25–40 W, fine tip, ~680 °F / 360 °C), 0.7–1.0 mm rosin-core solder, multimeter, 91 % IPA, clean acid brush, alligator-clip heat sink, nitrile gloves.
• Initial clean: wipe both PCB faces with 91 % IPA on a lint-free cloth. Air dry completely.

• Five turret positions: BD (Back Diaphragm), BG (Backplate 2), BP (Backplate 1), Grid, FD (Front Diaphragm).
• Press each turret fully into its PCB hole using a turret tool or drill press — never hammer. The flange must sit flat against the PCB surface.
• Solder from the component side only. Use minimum heat and minimum solder — these are high-impedance nodes.
• Inspect each turret for mechanical squareness. A tilted turret will cause wire-routing problems in later steps.

Install resistors in three groups, from lowest to highest impedance. The high-Z resistors in group 3 require the most care — clean iron tip, minimum flux, no excess heat.

• Group 1: R12 = 500 kΩ, R13 = 500 kΩ, R17 = 100 kΩ. Standard 1 % metal-film resistors. Install in either orientation. Trim leads flush after soldering.

• Group 2: R14 = 250 MΩ, R15 = 250 MΩ (backplate bias resistors). Nitrile gloves required. Install with minimum flux. Clean joints with IPA on a fine brush immediately after soldering.

• Group 3 — critical: R11 = 30 MΩ (grid leak), R16 = 30 MΩ (front-diaphragm bias). These are the most sensitive resistors in the circuit. Gloves on. Clean the iron tip immediately before soldering. Apply heat for the minimum time needed to achieve a shiny joint. Use the smallest possible amount of flux. Inspect under magnification.
• 3.4 — Back-side components: Populate any remaining components specified on the PCB reverse side (refer to the silkscreen). Follow the same high-Z precautions for any back-side pads near the turret area.

• C10 = 5000 pF, C11 = 5000 pF (both polystyrene / styroflex). Non-polarized — install in either orientation.
• Heat sink mandatory: clamp an alligator clip on each lead between the capacitor body and the PCB pad, component side. This absorbs heat before it reaches the dielectric.
• Solder from the underside. Touch the iron to the pad for no more than 2–3 seconds. Remove heat immediately when solder flows.
• C11 — turret-to-turret: C11 bridges the BG turret ↔ Grid turret directly. Keep the leads short and route flat against the PCB to avoid stress on the turret joints.
• After soldering, inspect the capacitor body. Any discoloration, shrinkage, or warping indicates heat damage — replace before continuing.

• Clean both sides of the PCB with 91 % IPA on a clean acid brush. Work systematically across the entire board surface.
• Pay particular attention to the high-impedance area: all turret bases, R11, R14, R15, R16 and their solder joints.
• Do not apply IPA directly to C10 or C11 — the solvent damages polystyrene dielectric. Work around them carefully.
• Air dry completely — minimum 10 minutes at room temperature. Do not use heat to speed drying on a board with styroflex caps installed.
• After drying, inspect all joints under a magnifier. Any residue or cloudiness in the high-Z area must be re-cleaned.

• Identify the 9-pin noval socket orientation — the key pin or locating tab must align with the silkscreen indicator.
• Insert the socket and tack-solder two opposite pins. Verify the socket sits flush and square against the PCB. Correct any tilt before proceeding.
• Solder all remaining pins. Use standard heat time — this area is not high-impedance. Trim any excess lead length flush.
• Verify all 9 pin positions are soldered. A missed pin can cause intermittent failures that are difficult to trace.

• Dry-fit the two PCB sub-assemblies together at exactly 90° before soldering. Verify all connector pins are fully seated and the mechanical alignment is correct.
• Tack-solder two pins first. Recheck the 90° angle — once all pins are soldered, correction is destructive.
• Solder all remaining connector pins from both accessible faces. Inspect each joint for good wetting and correct seating depth.
• After soldering, verify mechanical rigidity of the join. Any flex indicates an insufficiently soldered joint.

• Grid wire: Cut a 2.5 cm length of thin, unshielded wire. Solder one end to the Grid pin of the tube socket; the other end to the Grid turret. Keep the wire short and routed flat.
• BG↔BP link: Cut a 2 cm length of the same wire. Bridge the BG turret to the BP turret. This connects the two backplate nodes together as required by the circuit.
• Use only thin, unshielded wire for both connections. Shielded wire adds parasitic capacitance that attenuates capsule signal — see FAQ.
• Keep both wires as short as possible. Dress them flat against the PCB surface to minimize stray capacitance.

• Option A — 0.47 µF NOS K42Y-2 PIO (Paper-in-Oil): Vintage-voiced character. Slightly rolled-off low end, forward and present. Recommended for the classic C12 tonal balance.
• Option B — 1.0 µF polypropylene: Extended bass response, slightly slower transient character. Choose if maximum low-frequency extension is a priority.
• C12 is non-polarized — install in either orientation. Verify the value before soldering.
• C12 sits between the output transformer and the tube plate — it is in a moderate-impedance section of the circuit and does not require high-Z precautions.

• Before wiring: use a multimeter to measure DC resistance of both wire pairs. The high-resistance pair (~1.5 kΩ) is the primary — it connects to the PCB. The low-resistance pair (~300 Ω) is the secondary — it connects to the 7-pin XLR.
• Mount the transformer body with a zip tie through the PCB eyelet. The transformer must be mechanically secure before wiring.
• Primary connections: Red = Pin 1, Brown = Pin 2. Connect to the primary (high-resistance) PCB pads.
• Secondary connections: Orange and Yellow. These route to the back of the assembly for later connection to the 7-pin XLR (Step 13).

• Chassis ground first: solder a direct wire from Pin 7 to the XLR connector body lug. This is mandatory for hum rejection and pattern-switch voltage integrity.
• Pre-cut and strip wires to the lengths specified below before soldering to the connector pins.

• Insert the PCB assembly carefully into the microphone body, aligning the PCB edges with the body rails. Do not force — the assembly should slide in smoothly.
• Secure with the top two screws first. Verify the PCB is seated squarely before installing the bottom screws.
• Bottom screws: the short screw goes on the left; the long screw goes on the right. Using the wrong length will either strip the threads or leave a protruding post.
• Tighten to firm contact — do not overtighten. The PCB must be secure but the body must not deform.

• Connect each XLR wire to its corresponding PCB pad, following the wire colour table from Step 11.
• B+ (Pin 1, Red): connects to the node between the output transformer primary and C12 — not directly to the transformer. Locate this pad carefully before soldering.
• After all connections are made, dress the wires neatly inside the body. Ensure no wire is under mechanical stress or pinched by the body seam.
• Double-check that the Pin 4 (POL) wire is isolated from the Pin 7 (GND) wire. Contact between these two pins is a critical fault — see Errata E27.

14.1 — Mount the capsule.

• Orient the capsule correctly: for CK12-type capsules, the front is the side with three larger screws. For Tim Campbell CT12, follow the marked orientation on the supplied covers.
• Mount with blue wire to the left as you face the capsule from the front.
• Tighten mounting screws to firm contact only — do not overtighten. Overtightening distorts the capsule housing, permanently damaging the backplate geometry. See Errata E28.
• After mounting, recheck that all screws remain snug — soldering heat from the next step can loosen them.

14.2 — Wire the capsule.

• Capsule wiring assignment (standard CK12 / CT12 with blue wire to left):
  Front white → FD turret
  Rear white → BD turret
  Front blue → BP turret
  Rear blue → BG turret
• Use minimum heat and minimum time when soldering to capsule terminals. Pre-tin wires before bringing them to the capsule.
• Keep capsule wires as short as possible — but with enough slack to avoid mechanical stress. Dress wires so they cannot contact each other or the body.
• After wiring, check that all four mounting screws are still snug.

• The D-12 uses a 12AY7 or GE 6072A tube (9-pin noval, single-sided key).
• Locate the key pin gap on the tube base. Align it with the socket key slot before applying any downward pressure.
• Insert gently with a slow, even downward push — do not rock or force. The tube should seat with a soft click. If resistance is felt, re-check orientation before continuing.
• Verify the tube sits fully seated and level in the socket. A partially-seated pin causes intermittent contact, which can damage the PSU.

• Connect the fully assembled mic to the D-12 PSU with a 7-pin XLR cable. Power up the PSU.
• Warm-up: allow 2–3 minutes for the tube to reach operating temperature before making voltage measurements.
• Measure voltages at the 7-pin connector on the mic side (with mic connected):
  B+ (Pin 1 → Pin 7): ~120 V DC
  H+ (Pin 2 → Pin 7): ~6.3 V AC/DC
  Bias (Pin 3 → Pin 7): ~−1 V DC
• Measure polarization voltage at Pin 4 → Pin 7 in each pattern switch position:
  Omni: 0 V
  Cardioid: ~60 V
  Figure-8: ~120 V If all positions read 0 V, see Errata E27.
• Connect to a preamp. Set gain to 50–60 dB. Speak into the mic — expected output requires more preamp gain than a modern condenser; this is normal for this circuit design.
• Cycle through all three patterns. Verify each produces output. Any pattern locked in one position — see FAQ.
• Check for hum, buzz, or crackling. With the mic fully assembled, the body provides Faraday-cage shielding. Noise that disappears when you touch the body = grounding issue.