GLL2K

Thank you for your purchase of a Boscoe Music GLL2K preamp!

The GLL2K is a reproduction of the G&L version 1.3 bass preamp used in many G&L instruments, including:

• L•1500

• L•1505

• L•2000

• L•2500

• ASAT Bass

• Climax Bass

• Espada Guitar

The GLL2K reproduces the original G&L circuit using the original, no longer manufactured LM4250CN opamp and high-tolerance modern components. Each preamp is individually tested before shipping.

The GLL2K is designed primarily as a replacement for an existing G&L preamp. It uses screw terminals instead of permanently attached wires, making installation and future service easier.

The preamp measures approximately 40mm × 18mm × 11mm, making it considerably smaller than many original G&L preamp assemblies.

Understanding the G&L Tri-Tone System

The electronics used in G&L L-Series basses are somewhat different from the active bass and treble systems used in many other active instruments.

The Bass and Treble controls are part of G&L's passive P.T.B. (Passive Treble and Bass) tone circuit. They operate independently of the GLL2K preamp and continue to function when the instrument is in passive mode.

The GLL2K provides the active portion of the system.

On a typical L•2000, L•2500, or ASAT Bass, the controls include:

• Volume

• Passive Treble control

• Passive Bass control

• Pickup selector switch

• Pickup mode switch (series/parallel typically, though some have an additional split coil mode)

• Preamp selector switch

The preamp mode switch normally provides three operating modes:

Passive — the preamp is bypassed.

Active — the signal passes through the preamp.

Active with Treble Boost — the signal passes through the preamp with its additional high-frequency boost engaged.

The pickup selector and series/parallel switching are separate from the preamp. A problem with one pickup or one series/parallel setting is therefore not necessarily a preamp problem.

Single-pickup models such as the L•1500 and L•1505 use the same basic preamp but may use a different switch arrangement than an L•2000 or L•2500.

Tools and Materials

For a typical installation you will need:

• Precision flathead screwdriver, approximately 1.5–2mm (1/16"–5/64")

• Wire stripper suitable for approximately 22–26 AWG wire

• Small wire cutters

• Multimeter

• Suitable insulating foam or padding if the original G&L preamp insulation cannot be reused

A soldering iron is not normally required when the original preamp wires are cut and transferred to the GLL2K screw terminals. Soldering equipment may be required if you choose to desolder the wires from the original preamp or if repairs to the existing wiring harness are necessary.

Before You Begin

Before disconnecting anything, take several clear photographs of the original electronics.

Photograph:

• both sides of the original preamp,

• every wire connected to the preamp,

• the preamp mode switch,

• output jack and battery wiring,

• potentiometer connections,

• pickup switching,

• bridge ground,

• cavity shielding,

• and any ground wires between the controls.

G&L wiring has changed over the years, and wire colors are not always consistent. Photos of your particular instrument are more useful than relying on wire color alone.

If possible, label each wire according to its function before disconnecting it.

Disconnect the battery and remove the instrument cable before beginning installation.

Complete all wiring before reconnecting the battery.

Identify Your Installation Type

G&L has used several different physical control layouts over the years.

Many Fullerton Deluxe and Tribute basses use rear-routed control cavities with the potentiometers and switches mounted directly through the wooden body.

Many CLF Research models use a metal batwing-style control plate.

Older instruments, ASAT basses, and other models may use different layouts.

The electrical connections to the GLL2K are the same, but grounding and physical installation require slightly different attention depending on how the controls are mounted.

Preamp Version Compatibility

The GLL2K reproduces the G&L version 1.3 preamp.

One important wiring change occurred during the development of the original G&L preamp.

Older preamps used an external 10kΩ resistor in the wire between the preamp and the preamp mode switch. Beginning with version 1.2, this resistor was incorporated onto the preamp PCB itself. Version 1.3 retains the onboard resistor.

Therefore, if you are replacing an older preamp such as a version 1.0 or version 1.1, inspect the wire running between the original preamp and the preamp mode switch.

If a 10kΩ resistor is installed inline with the SWITCH wire, it must be removed or bypassed when installing the GLL2K.

The GLL2K already contains this resistor internally.

Do not assume the preamp version based only on the age of the instrument. Older basses may have been repaired or modified during their lifetime. Inspect the existing wiring before making changes.

Removing the Original Preamp

Disconnect the battery and remove the instrument cable.

Locate the existing G&L preamp. It may be wrapped in foam or other insulating material.

Carefully unwrap the preamp without damaging the surrounding wiring. The original insulating material may be reusable with the GLL2K.

Before cutting or desoldering any wires, compare the existing preamp connections with your photographs and identify each connection by function.

Whenever practical, transfer the wires one at a time. This greatly reduces the chance of accidentally exchanging two similarly colored wires.

If cutting the original wires:

• Leave as much usable wire length as possible.

• Cut close to the original preamp rather than shortening the harness unnecessarily.

• Strip approximately 5mm of insulation from the end of each wire.

• Make sure the exposed strands are clean and tightly grouped.

Do not permanently position the GLL2K until all connections have been made and tested.

GLL2K Connections

The GLL2K has five electrical connections.

The corresponding connection numbers used in G&L's version 1.3 schematic are also shown below.

9V+ / POWER — connection 1

Supplies positive battery voltage to the preamp.

INPUT — connection 2

Audio input from the instrument's passive control and switching network.

GND — connection 3

Circuit ground.

OUTPUT — connection 4

Audio output from the preamp to the instrument's preamp mode/output circuitry.

SWITCH — connection 5

Connects to the preamp mode switch and controls the additional active treble-boost function.

Original G&L Wire Colors

Wire colors have varied between instruments and production periods.

On many newer G&L basses, the following colors are commonly found:

9V+ — Red

INPUT — White

GND — Black

OUTPUT — Yellow

SWITCH — Red

Notice that both 9V+ and SWITCH may use red wire.

Do not identify these two connections by color alone.

Trace the wire to its destination, refer to your original photographs, or use a multimeter when necessary.

When replacing an otherwise working original preamp, moving the wires from the old preamp to the corresponding GLL2K terminals one at a time is usually the safest method.

Using the Screw Terminals

Before inserting a wire, make sure the terminal is sufficiently open by turning its screw counterclockwise.

Insert the stripped conductor fully into the terminal and tighten the screw until the conductor is securely clamped.

Make sure no stray strands remain outside the terminal where they could contact an adjacent connection.

After tightening each terminal, gently pull on the wire to make sure it is actually secured.

The screw terminals use an internal box clamp. It is possible to accidentally insert a wire underneath the box clamp instead of inside it. If this occurs, the terminal screw may feel tight even though the wire itself is not being clamped.

If the wire pulls free, loosen the screw, reposition the conductor inside the clamp, and tighten it again.

Do not overtighten the terminal screws.

When tightening the terminals, hold the PCB carefully by its edges. Do not use the tall capacitors or the socketed LM4250CN opamp as handles or supports. Excessive sideways force can damage the components or their connections.

Protecting the Preamp From Shorts

The underside of the GLL2K contains exposed electrical connections and must not contact conductive surfaces.

Possible sources of accidental shorts include:

• conductive shielding paint,

• copper shielding foil,

• metal control plates,

• potentiometer bodies,

• switch bodies,

• output jack hardware,

• and other exposed metal.

If the original G&L preamp was enclosed or wrapped in insulating foam, the existing material can normally be reused if it is in good condition.

Otherwise, use a suitable nonconductive foam, neoprene pad, or other insulating material to prevent the underside of the PCB from touching conductive surfaces.

Do not completely wrap the preamp in adhesive material or place pressure directly on the components.

The goal is simply to keep the PCB electrically insulated and mechanically secure inside the cavity.

Through-Body Installations

Many Fullerton Deluxe and Tribute basses use rear-routed cavities with their potentiometers and switches mounted directly through the wooden body.

In these installations, the wooden body does not always electrically connect the potentiometer and switch bodies together.

Preserve the original G&L grounding wires and shielding connections when replacing the preamp.

Do not remove or relocate ground wires simply because they do not connect directly to the GLL2K.

The GLL2K GND terminal only needs to be connected to the instrument's common circuit ground. The bridge, potentiometer bodies, switch hardware, cavity shielding, and output jack may connect to that common ground elsewhere in the original wiring harness.

Use a multimeter to verify continuity between grounded components when necessary.

Position the GLL2K so that:

• the underside is insulated from cavity shielding,

• the wiring is not pulled tight,

• no component is pressed against the cavity cover,

• the terminal screws remain accessible,

• and the PCB cannot move far enough to contact conductive hardware.

Metal Control-Plate Installations

CLF Research L-Series basses commonly use a metal batwing-style control plate.

Place the removed control plate on a clean towel or other soft surface while working to avoid scratching the finish.

A conductive metal control plate can electrically bond the bodies of the potentiometers and switches together, but do not assume that every mechanical connection provides a reliable electrical ground.

Paint, oxidation, washers, coatings, or other hardware can interfere with electrical continuity.

After installation, use a multimeter to verify continuity between the control plate, grounded potentiometer bodies, and circuit ground.

Make especially sure that the underside of the GLL2K cannot contact the metal control plate.

When reinstalling the plate, lower it carefully into position rather than rotating or forcing it into the cavity. Watch for wires becoming pinched between the control plate and the body.

Grounding and Shielding

The GLL2K GND terminal is the electrical reference point for the preamp.

When replacing only the preamp, it is normally best to preserve the original G&L grounding arrangement rather than redesigning the instrument's ground wiring.

The instrument's common ground should ultimately include:

• GLL2K GND

• pickup ground and shielding

• bridge/string ground

• output jack ground

• grounded potentiometer and switch bodies

• conductive cavity shielding, when present

These connections do not all need to terminate directly at the GLL2K GND terminal. They may be joined elsewhere in the instrument as part of the factory harness.

Some G&L MFD pickup versions include an additional wire connected to the pickup baseplate or shielding. If such a wire is present, preserve its original ground connection.

If you encounter excessive hum or noise after replacing the preamp, verify continuity between the instrument's common ground and all components that were grounded before the installation.

Power and Battery Wiring

The GLL2K is designed for normal operation to be powered by a 9V battery, but can be run safely up to 18V.

Connect the instrument's existing positive preamp supply wire to 9V+.

Connect the instrument's common circuit ground to GND.

For a normal preamp replacement, the existing battery and output-jack wiring should not need to be changed.

Many G&L basses use the output jack to switch battery power when an instrument cable is inserted or removed.

Some older G&L instruments—particularly some older L•1500, L•1505, and Climax basses—used different battery wiring and may leave the battery connected continuously rather than switching it from the output jack.

Replacing the preamp does not change the way the instrument's battery switching operates.

If you are working on an older instrument, document the original battery and output-jack wiring before making changes.

Do not assume that placing the bass in Passive mode necessarily disconnects the battery. Unless you know that your particular wiring does so, unplug the instrument cable when the bass is not being used.

About the SWITCH Connection

The SWITCH terminal is part of the G&L preamp-mode circuit.

It should not be confused with a simple power switch connection.

On instruments such as the L•2000, L•2500, and ASAT Bass, the preamp-mode switch selects between passive, active, and active-with-treble-boost operation.

The passive/active routing itself is handled by the external switch wiring. The GLL2K's SWITCH connection participates in the active treble-boost function.

If normal Active mode operates correctly but Active with Treble Boost does not, the SWITCH wire and preamp selector switch are important places to inspect.

When replacing an older preamp, also remember to check for the external 10kΩ resistor described earlier in this manual.

Pickup and Switching Wiring

Replacing the GLL2K normally does not require changing the pickup wiring.

The pickup selector and series/parallel circuitry are upstream of the preamp and should be left intact unless they are also being repaired.

On an L•2000, L•2500, or ASAT Bass:

• the pickup selector determines which pickup or pickups are used,

• the pickup mode switch changes the coil configuration (usually series/parallel),

• the Bass and Treble controls provide passive tone shaping,

• and the resulting signal is then routed through or around the GLL2K according to the preamp selector switch.

If only one pickup, one pickup selector position, or one series/parallel mode fails while the others work normally, inspect the pickup and switching circuitry before assuming that the GLL2K is at fault.

G&L pickup wiring has also changed over the years. Do not change pickup connections solely to match wire colors found in another bass.

Before Applying Power

Before reconnecting the battery, inspect the complete installation.

Verify:

1. The original wiring was documented before removal.

2. 9V+ is connected to the correct power wire.

3. INPUT is connected to the signal coming from the instrument's passive control network.

4. GND is connected to circuit ground.

5. OUTPUT is connected to the original preamp-output circuit.

6. SWITCH is connected to the preamp-mode switch.

7. Any external 10kΩ SWITCH resistor required by an older preamp has been removed or bypassed.

8. Every conductor is actually captured inside its screw-terminal box clamp.

9. No loose wire strands can contact adjacent terminals.

10. No wires are under excessive tension.

11. The underside of the GLL2K is insulated from conductive shielding or metal.

12. Existing bridge, shielding, and control grounding has been preserved.

13. The battery remains disconnected until all wiring checks are complete.

Testing the Installation

Do not immediately close the control cavity after installing the GLL2K.

Test the instrument while the electronics are still accessible.

Test Passive Operation First

If your instrument provides a Passive mode, test that mode first.

A typical L•2000, L•2500, or ASAT Bass should still produce output in Passive mode even if the battery is disconnected.

Verify that:

• the Volume control operates,

• the Bass and Treble controls operate,

• the pickup selector operates,

• and the pickup mode switch operates.

If these functions do not work correctly in Passive mode, the problem may be elsewhere in the wiring rather than in the GLL2K.

Test Active Operation

Reconnect the battery and switch the instrument to Active mode.

Verify that the preamp operates without excessive noise, distortion, or loss of signal.

The difference between Passive and standard Active mode is not necessarily intended to behave like a large conventional EQ boost. The G&L system uses passive Bass and Treble controls with switchable active electronics rather than a conventional active boost/cut tone stack.

Test Active With Treble Boost

Select the Active with Treble Boost mode.

Verify that the additional high-frequency response is present.

If standard Active mode works but the treble-boost mode does not, inspect the SWITCH connection and the preamp select switch wiring.

After all modes operate correctly, gently move the wiring while the bass is producing sound and listen for intermittent connections.

Only after the electronics have been tested should the preamp be secured in the cavity and the cover or control plate reinstalled.

Output Protection

The GLL2K includes the 1kΩ output resistor used in the later G&L preamp design.

This resistor helps isolate and protect the output of the LM4250CN opamp.

It is part of the normal GLL2K circuit and requires no additional wiring or modification.

Troubleshooting

No Output in Any Mode

If the bass has no output in Passive or Active modes, the problem is unlikely to be limited to the GLL2K.

Check:

• output jack wiring,

• Volume control,

• pickup selector,

• pickup wiring,

• common ground,

• and the signal path through the existing harness.

Verify that wires disturbed during installation have not broken or become disconnected.

Passive Works, but Active Does Not

This is one of the most useful diagnostic conditions in a G&L Tri-Tone system.

If Passive mode works normally but the Active modes do not, the pickups and much of the passive control wiring are already known to be functioning.

Check:

9V+ — confirm battery voltage is reaching the GLL2K.

GND — verify a good circuit-ground connection.

INPUT — verify the signal is reaching the preamp.

OUTPUT — verify the preamp output is correctly connected to the mode switch.

Also check the battery, battery connector, output-jack battery switching, and preamp-mode switch.

Active Works, but Active With Treble Boost Does Not

Check the SWITCH terminal first.

Make sure its wire is securely captured by the terminal clamp and trace it to the preamp-mode switch.

If the bass originally used an older preamp, check for an external 10kΩ resistor in this wire.

Also inspect the contacts and solder joints on the preamp-mode switch.

One Pickup or One Series/Parallel Mode Does Not Work

The pickup selector and pickup mode switching occur separately from the GLL2K.

If the problem occurs in the same pickup or coil mode in both Passive and Active operation, inspect:

• pickup wiring,

• pickup selector switch,

• pickup mode switch,

• and associated connections.

The preamp is unlikely to cause a failure that affects only one pickup selection or pickup mode position.

Bass or Treble Control Does Not Work

The G&L Bass and Treble controls are passive and are not generated by the GLL2K itself.

If one of these controls fails in both Passive and Active modes, inspect the potentiometer, capacitor, and associated wiring before replacing or troubleshooting the preamp.  

The Treble potentiometer should have a measured value of 250kΩ and the Bass potentiometer should measure 1MΩ.  Replacement potentiometers are available from our online stores should you need them.

Excessive Hum or Noise

Verify:

• GLL2K GND,

• bridge/string ground,

• pickup ground and shielding,

• potentiometer and switch grounding,

• output jack ground,

• and cavity shielding.

On a through-body installation, do not assume that the wooden body provides any electrical connection between the control bodies.

On a metal control-plate installation, verify continuity rather than assuming that mechanical contact with the plate guarantees grounding.

Also make sure the underside of the GLL2K is not touching conductive shielding.

Intermittent Output

Check the screw terminals first.

Gently pull each wire to make sure it is actually secured inside the internal box clamp.

A wire inserted underneath the clamp can appear correctly installed while making intermittent or unreliable electrical contact.

Also inspect for:

• broken wire strands,

• wires that are too short or under tension,

• cracked solder joints elsewhere in the original harness,

• dirty or worn switches,

• and output-jack contacts.

Active Mode Is Weak or Distorted

Install a known-good 9V battery.

Verify correct 9V+ and GND connections.

Check INPUT and OUTPUT to make sure they were not accidentally reversed.

Confirm that the preamp is not being shorted against shielding or other metal.

If replacing an older preamp, verify that an external 10kΩ resistor has not accidentally been left in the SWITCH connection.

Battery Drains Quickly

Check the battery and output-jack wiring.

Do not assume that selecting Passive mode disconnects battery power.

Unplug the instrument cable when the bass is not being used unless you have specifically verified that your instrument disconnects the battery by another method.

Remember that some older G&L models used battery wiring that did not automatically disconnect the battery when the instrument was unplugged.

The Bass Works Before the Cavity Is Closed but Stops After Assembly

This strongly suggests a mechanical short or pinched wire.

Inspect the underside of the GLL2K for contact with:

• conductive shielding paint,

• copper foil,

• a potentiometer,

• a switch body,

• the metal control plate,

• output-jack hardware,

• or another grounded component.

Also inspect for wires pinched beneath a control plate or cavity cover.

Reposition and insulate the preamp before testing again.

Final Installation

After testing all functions:

• Confirm that every terminal wire is secure.

• Position the GLL2K so that it cannot contact conductive surfaces.

• Reuse the original insulating foam or install suitable nonconductive protection.

• Arrange wires so they are not pinched or placed under tension.

• Make sure the cavity cover or control plate does not press directly against the PCB or components.

• Reinstall the cavity cover or control plate carefully.

Test the complete instrument once more after final assembly.

Need More Help?

Because G&L used several wiring variations over the long production history of these instruments, the wiring in an individual bass may differ from another example of the same model.

When working on an older instrument, identify connections by their function and destination, not by wire color alone.

If possible, consult the wiring diagram appropriate to the particular model and production period.

If you have any questions about installation or compatibility, please contact us!