Jazz Bass Master Blend Wiring: Series/Parallel + Greasebucket + Buffer

A standard Jazz Bass has Volume / Volume / Tone. Two volume knobs is a 1960s compromise. You constantly adjust two controls to balance pickups, losing overall volume when you back one off.

Blend / Volume / Tone. One knob to mix pickups, one for overall volume, one for tone. Push-pull switch puts pickups in series for a fatter, mid-forward sound. Three knobs, nine tones.

The tone circuit is a Greasebucket, Fender’s design that prevents bass buildup when you roll off treble. 500K tone pot with a 22nF cap. Same value as the volume pot. Higher than stock 250K to preserve the brightness the buffer unlocks. Turn the tone down and it doesn’t get muddy.

Optimized for the Fralin Split Jazz, DiMarzio Model J, and Aguilar 4J-HC. Works with any passive J-Bass pickup.

Every solder joint numbered.

What you get

Three controls. Two modes. Nine core sounds before you touch an EQ.

Tone sculpts each from full bright to warm. No wool, thanks to the Greasebucket.

Companion articles: Pickup tonality guide (how pot values, cable, and circuit topology shape the frequency response). Pickup shootout (simulation of 30 pickups through this exact circuit, and why pot values matter more than cable).

Parts

All passive electronics except the onboard buffer. No battery. Power comes from the junction box via TRS ring.

Tools

• Soldering iron with temperature control (340°C for wire, 380°C for pot casings)
• Multimeter (continuity mode + resistance mode)
• Wire strippers
• Flush cutters
• Small pliers or hemostats (for holding wire against hot pot casings)
• Screwdriver (for mounting pots to plate)
• Lighter or heat gun (for heat shrink)

Before you start

Three checks and two prep tasks before touching the plate.

Know your lugs

The guide references lugs by number.

Blend pot (MN250K, 6 lugs in two rows):

• Top row = Gang A: Lug 1 (input), Lug 2 (wiper), Lug 3
• Bottom row = Gang B: Lug 4, Lug 5 (wiper), Lug 6 (input)
• Wipers are always the middle lug of each row

Volume pot (CTS push-pull, 3 pot lugs + 6 switch lugs):

• Pot lugs: Lug 1 (CCW), Lug 2 (wiper), Lug 3 (CW)
• Switch lugs (two rows of 3): SW1–SW3 (Pole 1), SW4–SW6 (Pole 2)

Tone pot (CTS 500K audio taper, 3 lugs):

• Lug 1 (CCW), Lug 2 (wiper), Lug 3 (CW)

Verify the push-pull switch

The DPDT switch on the back of the volume pot has 6 lugs. Which pairs connect in each position varies between manufacturers. You must check yours.

1. Set multimeter to continuity mode
2. Push shaft down (default). Probe all 6 switch lugs to find connected pairs.
3. Pull shaft up. Probe again.
4. Map your results:

If your switch connects different pairs, adjust the wiring in Step 5 accordingly. The principle is the same. You need to know which lug is the center (common) and which are the two throws.

Verify the blend pot rotation

1. Set multimeter to resistance (Ω)
2. Measure Gang A: Lug 1 to Lug 2 while turning the shaft
3. Full counterclockwise should give minimum resistance (full signal)
4. Do the same for Gang B. It should be the opposite (maximum resistance at full CCW)

Target: Turn left = neck only, center detent = 50/50, turn right = bridge only.

If it’s backwards, swap which pickup connects to which gang.

Pre-tin pot casings

This is the single hardest part of the build. Do it now while the pots are loose.

1. Set iron to 380°C
2. Hold the pot in a vise or third hand
3. Touch the iron tip to the back of the pot casing, feed solder into the joint
4. Build a small puddle of solder, ~5 mm across
5. Repeat for all three pots

The first one might take a few attempts. The casing is a big heat sink. Hold the iron on for 3–4 seconds before the solder flows. Do not exceed 5 seconds. Prolonged heat can damage the pot’s resistive element.

Pre-assemble the Greasebucket network

Solder one leg of the 22 nF cap to one leg of the 4.7 kΩ resistor. Slide heat shrink over the junction before soldering, then shrink it. You now have a two-legged sub-assembly:

[free cap leg] ——[22nF]——•——[4.7kΩ]—— [free resistor leg]
                          ↑
                    heat shrink here

This gets installed as one piece in Step 6. The free cap leg goes to Tone Lug 1. The free resistor leg goes to the ground bus.

The build

Nine steps. Each numbered sub-step is one solder joint or one action.

Step 1: Mount pots on the plate

1.1 Drop all three pots through the plate holes. From neck to bridge: Blend, Volume, Tone.

1.2 Finger-tighten the mounting nuts. Leave them slightly loose. You may need to rotate the pots to align lugs for clean wire runs.

1.3 Orient the lugs: pot lugs should face the same direction (toward one edge of the plate) so the wire runs between pots are short and tidy.

Step 2: Solder the ground bus

2.1 Cut a length of silver wire long enough to span all three pot casings with ~1 cm extra on each end.

2.2 Hold the wire against the pre-tinned spot on the Blend pot casing using pliers. Touch the iron to the existing solder puddle, let it reflow around the wire. Remove iron. Hold still for 2 seconds.

2.3 Repeat for the Volume pot casing. The wire now bridges blend and volume.

2.4 Repeat for the Tone pot casing. The wire now spans all three.

2.5 Tug-test each joint. The wire should not move.

Check: Multimeter continuity between all three pot casings. Should read < 1 Ω.

This is the hardest step. The pot casings absorb a lot of heat. Use 380°C, hold the iron on the joint (not the wire), and let the solder flow to the wire. If it doesn’t stick: let it cool completely, scrape the spot with a blade, re-tin, try again.

Step 3: Wire the blend pot

Seven connections. Start with the grounds, then the signal path.

3.1 Short jumper from Gang A Lug 3 to ground bus ●

3.2 Short jumper from Gang B Lug 4 to ground bus ●

3.3 Solder signal wire to Gang A Lug 1, the neck hot lead. Leave 20 cm long, don’t cut yet. ●

3.4 Solder signal wire to Gang B Lug 6, the bridge hot lead. Leave 20 cm long. ●

3.5 Solder a wire from Gang A Lug 2 (wiper), route toward volume pot. ●

3.6 Solder a wire from Gang B Lug 5 (wiper), route toward volume pot. ●

3.7 Join the two wiper wires (3.5 + 3.6) together at Junction X. Twist, solder, heat shrink. Run a single wire from Junction X to Volume Pot Lug 3. ●

Check: Continuity from Gang A Lug 3 to ground bus. Same for Gang B Lug 4. No continuity between any signal wire and ground.

Step 4: Wire the volume pot

Three connections. The simplest step.

4.1 Short jumper from Lug 1 (CCW) to ground bus ●

4.2 The wire from Junction X should already be on Lug 3 (CW) from step 3.7. If not, solder it now. ●

4.3 Solder a wire from Lug 2 (wiper), route toward tone pot. This carries the signal to the tone circuit. ●

500K is non-standard for bass. Most J-Bass harnesses use 250K. The higher value is the single biggest upgrade in this circuit. It loads the pickup less, letting the resonant peak ring freely. The simulation showed +5.8dB at 5kHz from the pot upgrade alone. Without the Greasebucket tone circuit to tame the top end, 500K could sound harsh. With it, you get brightness on tap with smooth rolloff.

No treble bleed. Lugs 2 and 3 are left clean. If you ever want to add one later, it’s a 2-minute retrofit.

Step 5: Wire the DPDT switch

This is the most confusing step visually, but each connection is simple. The switch is on the back of the volume pot. It has 6 lugs in two rows.

Pole 1 controls series/parallel switching:

5.1 Jumper from SW1 to ground bus ●

5.2 Solder a wire to SW2, the neck ground lead. Leave 20 cm long. ●

5.3 Solder a wire from SW3 that tees into the bridge hot lead (the same wire on Blend Gang B Lug 6). You can solder a separate jumper from SW3 to that lug, or splice into the existing wire. ●

What this does: Push (down) connects SW1↔SW2, so neck ground goes to ground bus (normal parallel operation). Pull (up) connects SW2↔SW3, so neck ground goes to bridge hot, putting the pickups in series.

Pole 2 is unused. Leave SW4, SW5, and SW6 unconnected. No solder on any of them.

Why unused? Earlier versions of this circuit used Pole 2 for a 220kΩ load pad that engaged in series mode to tame the resonant peak. The simulation work showed the optimized circuit already handles this. The pad was redundant and slightly dulled the series character. Removing it keeps series mode punchy.

Check: Set multimeter to resistance. Push the shaft down. Measure between the neck ground wire (SW2) and the ground bus. Should read < 1 Ω (connected through SW1↔SW2). Pull the shaft up. Same measurement should read open (no connection). This confirms Pole 1 is wired correctly.

Step 6: Wire the Greasebucket tone circuit

Four connections plus the pre-assembled Greasebucket network. This is NOT a standard tone circuit. Read carefully.

6.1 Solder the wire from Volume Pot Lug 2 (step 4.3) to Tone Lug 3 (CW/input). ●

6.2 Solder a wire from Tone Lug 2 (wiper), route toward the output jack. This is the final signal output. ●

6.3 Take the pre-assembled Greasebucket sub-assembly (22 nF + 4.7 kΩ). Solder the free cap leg to Tone Lug 1 (CCW). Solder the free resistor leg to the ground bus. ●

6.4 Solder the 100 nF WIMA cap from Tone Lug 1 to Tone Lug 2 (wiper), bridging both lugs. ●

Check: Measure resistance from Tone Lug 1 to the ground bus. It should read ~4.7 kΩ (the series resistor). If it reads 0 Ω, you’ve accidentally shorted Lug 1 to ground. Find and fix the short.

Step 7: Build the onboard buffer

The buffer sits between the tone circuit and the output jack. It converts the pickup’s high-impedance signal to low impedance at the source, defeating cable capacitance before it ever reaches the wire.

Build this on a scrap of perfboard (~10x10 mm). Six solder joints.

         +9V (from TRS ring)
          │
        10µF (power filter)
          │
   1MΩ ──┤ Gate ← from Tone Lug 2 (wiper)
  (bias)  │
        2N5457
          │
        Source ──┬── 10µF ── to TRS Tip
                 │
               4.7kΩ
                 │
               GND

7.1 Solder the 1 MΩ resistor from the JFET gate to ground. This sets the input impedance. ●

7.2 Solder the 4.7 kΩ resistor from the JFET source to ground. ●

7.3 Solder a 10 µF cap (power filter) between the JFET drain and ground. Observe polarity. The drain also connects to the +9V rail (TRS ring). ●

7.4 Solder a 10 µF cap (output coupling) from the JFET source to the output wire that will go to the TRS tip. Observe polarity (positive toward source). ●

7.5 Wire from Tone Pot Lug 2 (wiper) to the buffer gate. This is the signal input. ●

7.6 Mount the sub-board inside the control cavity with foam tape. Keep it close to the tone pot to minimize wire length.

TS cable fallback: When a standard TS cable is plugged in, the ring shorts to sleeve. The buffer gets no power and passes signal capacitively. For true passive bypass, use a TRS jack with a ring-normalled contact that hardwires tip to the harness output when no TRS plug is present.

Step 8: Wire the output jack (TRS)

The output jack is now a Switchcraft 12B (TRS stereo panel jack), not a mono jack.

8.1 Cut ~15–20 cm of shielded cable. Strip both ends.

8.2 At the plate end: solder inner conductor 1 to the buffer output (the 10 µF coupling cap from step 7.4). This is the buffered signal. ●

8.3 Solder inner conductor 2 to the +9V rail on the buffer sub-board (the drain side of the 10 µF power cap from step 7.3). This carries 9V from the TRS ring to the buffer. ●

8.4 Twist the braided shield tight, tin it as a bundle. Solder it to the ground bus. ●

8.5 At the jack end: solder conductor 1 to the Tip lug (audio). ●

8.6 Solder conductor 2 to the Ring lug (9V power from junction box). ●

8.7 Solder the braided shield to the Sleeve lug (ground). ●

Step 9: Final connections and check

These connections happen at installation time, when the harness goes into the bass.

9.1 Bridge ground wire → ground bus (leave long, trim at install)

9.2 Copper shielding → ground bus (one short jumper from control cavity shielding to nearest pot casing, one point only)

9.3 Connect pickup leads to the harness (see Pickup Wiring Reference below)

Full verification

Run through every check before you put the pickguard back on.

• Continuity: All ground points → jack sleeve, < 1 Ω
• No shorts: No continuity between any signal wire and ground
• Greasebucket: Tone Lug 1 to ground = ~4.7 kΩ (not 0 Ω)
• DPDT Push: Neck ground connects to ground bus
• DPDT Pull: Neck ground connects to bridge hot
• Blend rotation: Left = neck, center = both, right = bridge
• Volume sweep: Smooth, no crackle, full attenuation at 0
• Tone sweep: Smooth rolloff from 10 to 0, no dead spots or jumps
• Series mode: Pull switch, noticeably fatter, more mids
• No hum: Touch strings/bridge, hum should stop
• Buffer power: With TRS cable plugged in, measure ~9V between ring and sleeve at the jack
• Buffer output: Signal present at TRS tip with buffer powered. Unplug TRS, plug TS: signal still present (passive fallback)

Complete wiring reference

The full diagram with every connection, for when you need to trace a wire or debug.

Hover over any wire to isolate it. Click to explore connections.

Pickup wiring reference

The harness works with any passive J-Bass pickup. The only difference is how many wires each pickup has and what color they are. Connect them at install time.

Stock Fender (2 wires per pickup)

Aguilar 4J-HC (3 wires per pickup)

Fralin Split Jazz (2 wires per pickup)

Verify colors against your actual pickups.

DiMarzio Model J DP123 (4 wires + bare per pickup)

The Model J has 4-conductor wiring. Default configuration is series (how DiMarzio rates the output). This is the recommended configuration for this harness.

Verify colors against the documentation included with your pickups. DiMarzio color codes are consistent across models but always confirm.

The 4-conductor wiring also supports a coil-parallel mode that drops inductance from 4.7H to 1.175H. This produces the brightest sound of any pickup in the database but requires a different tone circuit (100K loading) to give the tone knob usable sweep. See Advanced: Model J coil-parallel below.

DiMarzio Area J / Ultra Jazz (4 wires + bare per pickup)

Same wiring as the Model J above. Red = hot, Green = ground, Black + White soldered together for series, Bare = shield drain. DiMarzio uses the same color code across all 4-conductor J-Bass pickups.

Greasebucket: what it actually does

The 4.7k resistor limits how much the cap can load the signal. The 100nF cap across the wiper stabilizes the response. Full comparison chart in the shootout.

No-load was in an earlier spec. Simulation showed a +6dB step function at the detent with 250K, and +3.7dB with 500K. Details in the shootout. The 500K pot recovers most of the brightness while keeping a smooth taper.

Standard tone fallback: Remove the 100nF cap and the 4.7k resistor, connect Tone Lug 1 directly to ground, swap the 22nF cap for a 47nF. Five-minute conversion.

Advanced: Model J coil-parallel

This section is only for the DiMarzio Model J with 4-conductor wiring. It adds a third coil mode (coil-parallel at 1.175H) to the two modes the main harness provides (standard at 4.7H and series at 9.4H).

The problem

At 1.175H, the tone knob barely works. A 500K tone pot with 22nF gives only 1.8dB of sweep at 1.175H. The pickup impedance is so low that the passive tone circuit can’t move the needle. The cap value doesn’t matter. It’s the pot resistance that needs to change.

The fix: switched 100K load

A 100K tone pot gives 6.7dB of sweep at 1.175H and tames the +15dB resonant peak at 6.4kHz down to +4.3dB. But 100K is too low for series mode, where it would overdamp the signal.

The solution: the 4PDT toggle that switches the coils also switches a 120K resistor into the tone circuit. In series mode, the tone pot sees the full 500K. In coil-parallel mode, the 120K resistor drops in parallel with the 500K pot, pulling the effective value down to ~97K. One extra resistor, already on the switch.

Parts (in addition to the main harness)

Wiring

The 4PDT toggle has 12 lugs in four rows (Poles 1-4). Poles 1-2 handle coil switching, Poles 3-4 handle the load resistor.

Poles 1-2: Coil mode switching. In one position, Black and White are soldered together (series, same as the standard harness). In the other position, White goes to Red (hot) and Black goes to Green (ground), putting the coils in parallel. This applies to BOTH pickups, so you need all four poles for the four wires.

Poles 3-4: Load resistor. In series position, the 150K resistor is disconnected. In coil-parallel position, the 150K connects between Tone Lug 3 (input) and ground, appearing in parallel with the 250K pot.

Toggle the diagram below between modes to see how the wiring changes:

The principle: one toggle, four poles, coil routing plus impedance matching in one click.

Default mode

If this is a studio bass going into amp sims, consider making coil-parallel the default (toggle down) and series the pull-up. The everyday sound is the one with the most spectral information. You reach for warmth when you want it.

If something’s wrong

No sound at all:

• Check continuity from blend pot wiper (Lug 2 of either gang) through to jack tip. Follow the signal path.
• Check that the jack tip is connected to Tone Lug 2 (wiper), not Lug 1 or Lug 3.

Sound from only one pickup:

• Check that both blend pot inputs have continuity to their respective pickup hot leads.
• Verify blend pot rotation direction.

Hum that stops when you touch strings/bridge:

• Normal for single-coil pickups. Hum-cancelling pickups should be quiet.
• If hum is excessive: check ground bus continuity, check that the bridge ground wire reaches the bus.

Hum that does NOT stop when you touch strings:

• Ground loop. Check that shielding connects to ground at only ONE point.
• Check for accidental ground connections (stray wire touching shielding, solder blob bridging lugs).

Series mode sounds the same as parallel:

• DPDT switch wired wrong. Re-verify which lugs connect in push vs pull.
• Check that SW3 is actually connected to bridge hot.

Tone knob does nothing:

• Check that the tone cap (22 nF) is connected between Lug 1 and the 4.7 kΩ resistor, and the resistor reaches ground.
• If you accidentally wired Lug 1 direct to ground, the tone will work but won’t be a Greasebucket, and it’ll get muddy at 0.

Tone Lug 1 to ground reads 0 Ω (should be ~4.7 kΩ):

• You’ve shorted Lug 1 to ground somewhere. Check for a stray jumper or solder bridge. Remember: Lug 1 does NOT connect directly to the ground bus in this circuit.

Series mode with blend centered is louder than either pickup solo:

• Normal. Series wiring sums the pickup voltages, and at center blend both pickups contribute fully. This is the topology working as designed, not a wiring error.

SPICE netlist

The full circuit is modeled as an ngspice netlist: pickup RLC, 500K volume, 500K Greasebucket tone, 2N5457 JFET buffer. Parameterized for easy pickup swapping.

The frequency response charts in the shootout use a Python transfer function model that isolates the pickup-to-tone interaction. The SPICE netlist includes the buffer and all parasitic loading, which damps the resonant peak relative to those charts. Both are valid views: the Python model shows how pickups compare, the SPICE model shows what comes out of the cable.

Built for a Fender American Special Jazz Bass. Optimized for Fralin Split Jazz, DiMarzio Model J, and Aguilar 4J-HC. Works with any standard J-Bass routing and passive pickups. Onboard buffer powered via TRS ring from a desktop junction box (The Link). Circuit validated with SPICE simulation and frequency response modeling across 30 pickup models.