G&L Bass Preamp 1.3 frequency plots n fun

[TDR1138]

The layout of the one that G&L sent looks to be the same as the one that Paul pictured, and notably different than what had come in the Tribute. Looks like a shift in board color and to smaller resistors (1/2 watt to 1/4 watt perhaps?). So, I'm just wondering what the concern is.

[Ken Baker]

The layout of the one that G&L sent looks to be the same as the one that Paul pictured, and notably different than what had come in the Tribute.

The layout of the new (beige) board is the same as the old (green) board. They are, however, functionally a bit different. His original board is probably 2 or 3 generations old, so it would look notably different than current boards.

Looks like a shift in board color and to smaller resistors (1/2 watt to 1/4 watt perhaps?).

Shift in board type, thickness, as well as color. Resistors are 1/4 watt as they have always been. 1/2 watt would be overkill in this application.

So, I'm just wondering what the concern is.

j&lbass' concern is that he was expecting a USA-style preamp, which is not what he received.

Ken...

[GeorgeB]

I don't get it. Both boards are identical, except for the minor issue that one uses 1% metal film resistors while the other uses 10% carbon films. j&lbass got what he ordered: a current production Rev1.3 Preamp. Btw, no way to identifiy the provenance -- plus it doesn't matter anyway.
I'd guess this a layman's concern, judging something by appearence which is actually irrelevant. Electrons don't mind about PCB colors etc.

[Ken Baker]

I don't get it. Both boards are identical, except for the minor issue that one uses 1% metal film resistors while the other uses 10% carbon films. j&lbass got what he ordered: a current production Rev1.3 Preamp. Btw, no way to identifiy the provenance -- plus it doesn't matter anyway.
I'd guess this a layman's concern, judging something by appearence which is actually irrelevant. Electrons don't mind about PCB colors etc.

It's not just a layman's concern thing, or an appearance thing. You're right that electrons don't care about the color of the board. They don't even care about the components we feed them through. However, in this application the lesser components (resistors, caps, and chip) cause the preamp as a whole to create noise that we perceive as hiss.

If you get a chance, give this a read. It spells out the differences in technical terms.

Ken...

[j&lbass]

Here's the back story. Earlier this year, I experienced increasing noise issues with my Trib and intermittent failure (no sound) when the preamp was on. I opened her up, tried a few new batteries, tired to sort out some of the wiring issues (there was a lot of extra long wires inside creating the appearance of a rat's nest). After trying a few things and reading around, I decided to purchase a new preamp from the online store. I hoped to fixed the problem and maybe realize some kind of improvement in the sound because the preamp they sell at the online store is the USA version (I didn't even realize up to this point that there was a difference).

I installed the new preamp and the signal returned. Unfortunately, it returned with a fair amount of noise. In the subsequent months, I tried to tame the noise in a variety of ways, but was never successful enough that I could return the Trib to the top of my rotation of two basses. At this point, I decided to sit down with Paul's schematic and bring some order to the rat's nest, in hopes that careful reassembly would clear up the noise. Some of the wiring was not connected consistent with the schematic and some of the wires were different colors, but those are easy problems to solve. I also began searching around this site for tips and ideas-- that's when I ran across Paul's excellent post. In the beginning, he states:

"Note: This PCB is manufactured by Cort and is used in the G&L Tribute line of basses. The Cort PCB normally uses
5% carbon film resistors, lower voltage capacitors and the TL061 chip. However, I modified this PCB to simulate
the USA preamp, which uses 1% metal film resistors, higher voltage capacitors and the LM4250 chip."

Curious, I checked the USA preamp that G&L sold me and it was clear that I had received the original Trib Cort preamp (that Paul has upgraded to USA specs by switching out components for his experiments). Finding the additional discussion that Ken mentions above, it seemed reasonable to me that my noise problem could be a function of having the Cort preamp rather than the upgraded USA preamp because the changes in resistor type and tolerances were originally done to address noise problems.

I only stumbled upon the difference in preamp after months of trying to sort out the noise problem. I reasoned that if this were a systemic problem at the online store (e.g., a mislabeled bin), then there may be more people out there currently experiencing the same problem and thought it would be helpful to let them know to check whether they have the low-noise USA version or the noisy Cort version.

G&L (Ken) was quick to address the issue and have responded very well. As far as I'm concerned, mistakes happen and excellent companies reach out to solve them. I'm happy to say that G&L has provided excellent customer service, which returns my faith in my favorite bass company.

John

[Ken Baker]

John, if you ever get a wild hair and want to dive back into the nest, we do have this retrofit goodie.

Thanks for the backstory and the feedback!

Ken...

[Bassman56]

I recently replaced the electronics assembly in my 2002 American L2500. All of the POTS and switches had become noisy. I installed the new electronics assembly and now the bass has a growl similar to a passive P-Bass. I contacted G&L and they advised me to check the wiring. The photo of the Tribute pre-amp looks just like the one I installed. Anyone have a similar issue? I want my G&L sound back!

[Paul Gagon]

I recently replaced the electronics assembly in my 2002 American L2500. All of the POTS and switches had become noisy. I installed the new electronics assembly and now the bass has a growl similar to a passive P-Bass. I contacted G&L and they advised me to check the wiring. The photo of the Tribute pre-amp looks just like the one I installed. Anyone have a similar issue? I want my G&L sound back!

Any chance you could post a few pictures of your bass control cavity so we can see the preamp board and control cavity? Close ups of the switches and pots would be very helpful too.

Thanks,
Paul

[jcburn]

OK, I thought this might be fun.

I've been experimenting with the G&L Bass Preamp (revision 1.3) and wanted to see what kind of modifications made sense.
What I have here is a series of Audio Precision plots showing the effect of component value changes. The PCB used for these
plots has the following components. LM4250 chip, 1% metal film resistors, 50v electrolytic caps and 100v poly caps.

Before we begin, here's the schematic of the bass preamp circuit. Revision 1.3

The above schematic will be used as a reference for the various plots. The component values shown here will be referred to
as the default values. The plots that follow will be showing the effects of altering the default component values to adjust
frequency response, gain levels and filter points of the treble boost.

But first, a picture of the PCB under test.

Note: This PCB is manufactured by Cort and is used in the G&L Tribute line of basses. The Cort PCB normally uses
5% carbon film resistors, lower voltage capacitors and the TL061 chip. However, I modified this PCB to simulate
the USA preamp, which uses 1% metal film resistors, higher voltage capacitors and the LM4250 chip.

Now, on with the plots.

Standard Preamp rev 1.3 as found in the current G&L L-Series basses

RED TRACE = Preamp Bypasses
BROWN TRACE = Preamp On
BLUE TRACE = Preamp On w/Treble Boost

Preamp plots adjusting the value of R7 (27.4K)

RED TRACE = Preamp Bypassed
MAGENTA TRACE = Preamp On w/R7 value at 121K
BROWN TRACE = Preamp On w/R7 value at 39.2K
BLUE TRACE = Preamp On w/R7 value at 27.4K (Default Value)
GREEN TRACE = Preamp On w/Treble Boost and R7 at 27.4K

Preamp plots adjusting the value of C3 (.01uf)

RED TRACE = Preamp Bypassed
BLUE TRACE = Preamp On
BROWN TRACE = C3 value changed from .01uf (Default Value) to .1uf

Preamp Plots adjusting the value of R5 (21.5K)

BROWN TRACE = Preamp On w/R5 value at 0 ohms (jumper wire)
BLUE TRACE = Preamp On w/R5 value at 21.5K (Default Value)
RED TRACE = Preamp On w/R5 value at 68.1K
MAGENTA TRACE = Preamp On w/R5 value at 100K

Preamp Plots adjusting the value of C6 (.015uf)

RED TRACE = Preamp Bypassed
GREEN TRACE = Preamp On
BROWN TRACE = Preamp On w/Treble Boost. C6 value at .015uf (Default Value)
BLUE TRACE = Preamp On w/Treble Boost. C6 value at .022uf

OK, I know this is a whole lotta crazy but I just thought people would want to know what the options are for modifying
this preamp. My hope is that this might get some interesting information out there and see if others find it interesting.
This is a very simple preamp but there's a lot of adjustments that can be made to tailor it to an individuals need.

Anyway, Enjoy
Paul

Thread resurrection time - Paul can you remember when you created these graphs - for the preamp bypassed reading were the tone controls completely dimed, completely rolled off etc? I find it interesting that the pickups have a completely flat response!

[Ken Baker]

Thread resurrection time - Paul can you remember when you created these graphs - for the preamp bypassed reading were the tone controls completely dimed, completely rolled off etc? I find it interesting that the pickups have a completely flat response!

The plots are dated 7/1-8/2016, as noted at the top of each one. All were posted here 7/14/2016.

These plots are only for the preamp and do not show the output of the pickups.

Ken...