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. At position 10, completely transparent. Turn the tone down and it doesn’t get muddy.
Every solder joint numbered.
What you get
Three controls. Two modes. Nine core sounds before you touch an EQ.
| Blend left (neck) | Blend center | Blend right (bridge) | |
|---|---|---|---|
| Push (parallel) | Classic warm J-neck | Scooped, the J-Bass sound | Punchy, mid-forward |
| Pull (series) | Fat, P-Bass territory | Hollow, experimental | Bridge solo |
Tone sculpts each from full bright to warm. No wool, thanks to the Greasebucket.
Parts
All passive. No battery, no preamp.
| Part | Spec | What it does |
|---|---|---|
| Blend pot | MN250K dual-gang, center detent | Mixes neck and bridge pickups |
| Volume pot | CTS 250K push-pull DPDT | Master volume + series/parallel switch |
| Tone pot | CTS 250K audio taper | Tone control (Greasebucket circuit) |
| Tone cap | 22 nF film capacitor | Sets the tone rolloff frequency |
| Bass-control cap | 100 nF film capacitor (WIMA MKS2) | Prevents bass buildup (Greasebucket) |
| Series resistor | 4.7 kΩ metal film, 1/4W | Limits rolloff depth (Greasebucket) |
| Load pad resistor | 220 kΩ metal film, 1/4W | Tames series-mode resonant peak |
| Output jack | Switchcraft mono | Standard 1/4" output |
| Control plate | Chrome J-Bass plate | Build the harness on this |
| Ground bus wire | Silver wire, 1 mm | Single bus connecting all grounds |
| Signal wire | Solid-core hookup wire | For signal runs between pots |
| Shielded cable | 2-conductor + braid, ~20 cm | For the output jack run |
| Solder | Lead-free, rosin core, 0.5 mm | Stannol Kristall 611 or similar |
| Heat shrink | Assorted thin gauge | Insulating joints |
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 250K-AUD, 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.
- Set multimeter to continuity mode
- Push shaft down (default). Probe all 6 switch lugs to find connected pairs.
- Pull shaft up. Probe again.
- Map your results:
| Position | Expected (common CTS) |
|---|---|
| Push (down) | SW1↔SW2, SW4↔SW5 |
| Pull (up) | SW2↔SW3, SW5↔SW6 |
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
- Set multimeter to resistance (Ω)
- Measure Gang A: Lug 1 to Lug 2 while turning the shaft
- Full counterclockwise should give minimum resistance (full signal)
- 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.
- Set iron to 380°C
- Hold the pot in a vise or third hand
- Touch the iron tip to the back of the pot casing, feed solder into the joint
- Build a small puddle of solder, ~5 mm across
- 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
Eight 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. ●
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 controls the auto-engaging load pad:
5.4 SW4: leave unconnected. Nothing soldered here.
5.5 Solder one leg of the 220 kΩ resistor to SW5. Solder the other leg to the ground bus. ●
5.6 Jumper from SW6 to Volume Pot Lug 2 (wiper). ●
What this does: Push (down) connects SW4↔SW5, so the pad goes nowhere (disconnected). Pull (up) connects SW5↔SW6, so the 220kΩ resistor loads the signal, taming the resonant peak that series mode creates.
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. ●
⚠️ Do NOT connect Tone Lug 1 directly to ground. In a standard tone circuit, Lug 1 goes straight to ground. In the Greasebucket, Lug 1 connects to ground through the 22 nF + 4.7 kΩ series network. This is what prevents mud. If you wire Lug 1 to ground directly, you’ve bypassed the Greasebucket and built a standard tone circuit.
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: Wire the output jack
7.1 Cut ~15–20 cm of shielded cable. Strip both ends.
7.2 At the plate end: solder inner conductor 1 to Tone Pot Lug 2 (wiper). This is the signal. ●
7.3 If there’s a second inner conductor, cut it short and insulate with heat shrink. Not used.
7.4 Twist the braided shield tight, tin it as a bundle. Solder it to the ground bus. ●
7.5 At the jack end: solder inner conductor 1 to the Tip lug. ●
7.6 Solder the braided shield to the Sleeve lug. ●
Step 8: Final connections and check
These connections happen at installation time, when the harness goes into the bass.
8.1 Bridge ground wire → ground bus (leave long, trim at install)
8.2 Copper shielding → ground bus (one short jumper from control cavity shielding to nearest pot casing, one point only)
8.3 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, no click
- Series mode: Pull switch, noticeably fatter, more mids
- No hum: Touch strings/bridge, hum should stop
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)
| Wire | Color | Connects to |
|---|---|---|
| Hot | White (or colored) | Blend pot input (Neck → Gang A Lug 1, Bridge → Gang B Lug 6) |
| Ground | Black (or bare) | Neck ground → DPDT SW2. Bridge ground → ground bus |
Aguilar 4J-HC (3 wires per pickup)
| Wire | Color (Neck) | Color (Bridge) | Connects to |
|---|---|---|---|
| Hot | Gray | White | Blend pot input |
| Ground | Black | Black | Neck → DPDT SW2. Bridge → ground bus |
| Shield drain | Third wire | Third wire | Ground bus (at pot end only, do NOT ground at pickup end) |
Fralin Split Jazz (2 wires per pickup)
| Wire | Color | Connects to |
|---|---|---|
| Hot | White (typical) | Blend pot input |
| Ground | Black (typical) | Neck → DPDT SW2. Bridge → ground bus |
Verify colors against your actual pickups. Fralin is consistent but always confirm.
DiMarzio Ultra Jazz DP149 (4 wires + bare per pickup)
The Ultra Jazz has 4-conductor wiring. Default configuration is series (how DiMarzio rates the 250mV output).
| Wire | Color | What to do |
|---|---|---|
| Red | Hot (signal out) | Blend pot input |
| Green | Ground | Neck → DPDT SW2. Bridge → ground bus |
| Black + White | Series junction | Solder together, insulate with heat shrink |
| Bare | Shield drain | Ground bus (at pot end only) |
Verify colors against the documentation included with your pickups.
The 4-conductor wiring means you could later add per-pickup series/parallel switching (mini-toggle per pickup). That’s independent of the neck/bridge series/parallel on the volume pot. Not covered here, but the wiring supports it.
Greasebucket: what it actually does
At tone 10: Completely transparent. Identical to a standard tone circuit. The Greasebucket components are essentially out of circuit.
At tone 5: Nearly identical to standard. Barely engaged.
At tone 0 (full rolloff): This is where it matters.
| Standard circuit | Greasebucket | |
|---|---|---|
| Treble | Rolled off | Rolled off |
| Mids | Rolled off | Rolled off (less aggressively) |
| Bass | Boosted, woolly, muddy | Controlled, tight, musical |
The 4.7 kΩ resistor limits how much the cap can load the signal, keeping the low end from blooming. The 100 nF cap across the wiper stabilizes the response.
If you end up preferring a standard tone circuit: remove the 100 nF cap and the 4.7 kΩ resistor, connect Tone Lug 1 directly to ground, swap the 22 nF cap for a 33 nF. Five-minute conversion.
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.
Built for a Fender American Special Jazz Bass. Works with any standard J-Bass routing and passive pickups. Circuit validated with frequency response simulation across 13 pickup models.