El Toro Preamp taming/R7 mod?

Seeing as how we are fortunate to count Paul Gagon as a member here, it seemed like a good time to give him a home to stretch out in and relax a little. Maybe exercise the grey matter and present a little history of the guitar and bass (amps too!) from his perspective as one of the eminent designers of our time.

Got questions? Great! Good questions might be, "What kind of windings are on the S500 pickup and why?" Or, "What was the thought process behind the MJ-4?" Troubleshooting questions should remain in the regular forums (he reads those too). Finally, please be mindful of how I feel about tech questions via PM or email.

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El Toro Preamp taming/R7 mod?

Postby bigtone23 » Wed May 24, 2017 9:45 am

I have seen many discussions regarding the L1500 preamp mod (taming via a loading cap or the R7 mod). The R7 mod that I read about is perfect: it matches pretty much the same high frequency content as in passive mode.
I was wondering if there is something similar to the R7 mod for the EL Toro?
I have a July of 1983 El Toro and the active mode adds a very noticeable treble boost. Many say this bass is a slap machine and was extra bright for that reason. It is a great tone, but I'm no slapper! It seems that perhaps the treble boost is there for cabs that didn't have tweeters/horns as they were still just coming up in popularity at the time?
I would like to remove that treble boost without having to turn down the treble knob. In passive mode, I really like the tone of the bass with all knobs on 10. In active mode, the treble knob has to come down to about 6-7. I use this bass in a situation where I'm switching the tones quite a bit, active, passive, pickups in series, pickups in parallel, neck pickup soloed...
I have solder skills and like to perform reversable mods. I just don't have the electronics know how to look at the schematic and know which part of the circuit is equivalent to R7. Any suggestions will be helpful and appreciated.
July 1983 El Toro. Rosewood, black, alder? body. 10 pounds.
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Re: El Toro Preamp taming/R7 mod?

Postby bigtone23 » Wed May 24, 2017 10:55 am

Also, it appears that my pre is all stock. I will post a pic when I get it opened back up.
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Re: El Toro Preamp taming/R7 mod?

Postby Ken Baker » Wed May 24, 2017 3:44 pm

Being the owner of an El Toro, I too wouldn't mind taming the preamp. If I flip the bass into active, the tone gets really zingy and over-driven sounding. Consequently, I run it in passive and get GREAT sound. OTOH, it would be nice to be able to run active. I just don't want this huge tone difference.

You found my thread from last year where I talked about swapping out the preamp for something more modern, but I haven't done anything with that yet. Time.... Plus, anything I do to my Bull would need to be totally reversible because it has a bit of history to go along with the great sound.

Tell you what; I'll move this up to Paul's Technicalities. I'll give Paul a heads-up and see how it flies.

Ken...
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Re: El Toro Preamp taming/R7 mod?

Postby bigtone23 » Wed May 24, 2017 4:34 pm

Very cool, thank you! Was just playing The Bull today after tweaking the set up a touch. Love the tone, but it is zingy! There has to be a simple tweak to the board to simmer it down.
Last edited by bigtone23 on Thu May 25, 2017 6:56 am, edited 1 time in total.
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Re: El Toro Preamp taming/R7 mod?

Postby Deano » Thu May 25, 2017 6:41 am

My 1986 stock El Toro does not add excess treble or zing with the preamp engaged. The preamp ads girth and presence, but not too much higher end. In fact, the preamp is very quiet compared to my 1990/91 L2000.
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Re: El Toro Preamp taming/R7 mod?

Postby Paul Gagon » Thu May 25, 2017 10:17 am

Of all the basses that I wish would be re-issued, the G&L El Toro would be my #1. During my time at G&L
I was only ever given one chance to dig into a vintage unit. This is when I drew out this diagram......

Image

I also drew up a schematic of the circuit board that I was looking at. Here it is.

Image

In a previous post, I talked about ways to Tame the L-1500/Climax bass. While there are some similarities
between the the El Toro and these other two basses, the Taming process will be a bit different.
If you look at the schematic diagram you will see a .001uf capacitor connected to the clockwise (CW)
lug of the Treble control (the dot indicates the clockwise position). The center lug of the Treble control
connects to ground and the .047uf Treble cap connects to the counter clock wise (CCW) lug.
So.....here's what happens when you turn the Treble control. When the Treble control if fully
CW (Brightest sound) you have the .047uf Treble cap isolated from ground by the value of the
Treble pot (250K ohms). At the same time, you have the .001uf Treble boosting cap from the preamp
going directly to ground, giving the preamp it's maximum gain at high frequencies. So what happens
when we turn the Treble control in the opposite(CCW) direction? Well, the .047uf Treble cap is now
connected directly to ground which darkens the pickups a lot. At the same time you have the .001uf
Treble boosting cap from the preamp going through the entire value of the Treble pot (250K ohms)
before going to ground, which reduces it's treble boosting ability to almost nothing.

Now, lets say we want to soften the brightness of the preamp when the Treble control is at fully
CW position. The easiest way to do this is to increase the resistance in the connection to the
Treble control that comes from the preamp. If you look at the wiring diagram, you will see a yellow wire
that comes off the preamp board and connects to the CW lug of the Treble control. if you lift the
yellow wire off the control and connect a resistor between the yellow wire and the control lug
that the yellow wire used to be connected to, you have now increased the resistance that the .001uf cap
sees to ground which reduces the preamps treble boosting ability. What value of resistor you ask?
Depends on how much treble boosting you want to remove. You could start with something like 10K
and see how it sounds. The larger the resistor value the lower the treble boosting will be.
If you didn't want any treble boosting from the preamp you could just remove the yellow wire all together.
This resistance we are talking about could be compared to the "R7" mods of the L-series basses.......kind of.

One other interesting thing about the El Toro bass is that it has one of the largest pickup loading caps
that G&L has ever used. This cap is located on the back of the Bass control. Its the one that is connected
from the Bass control center lug and ground. The value is a .0022uf. If this cap is missing,
you will have a very bright sounding bass. If it's there and you want to deepen the bass a bit,
you can solder a .001uf cap in parallel with it. Anyway, just something to experiment with.

I hope this adds a bit of helpful info on the El Toro. Keep us all posted on your mods.
I really wish I had one of these basses.
Paul

Admin note: Click the images for a PDF version.
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Re: El Toro Preamp taming/R7 mod?

Postby bigtone23 » Thu May 25, 2017 2:25 pm

Thanks Paul!
I'll try out your suggestions and let you know how it goes.
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Re: El Toro Preamp taming/R7 mod?

Postby bigtone23 » Fri Oct 06, 2017 3:48 pm

I experimented with various resistors on the yellow wire. After trying 10K then 220K, 470K, 680K I finally landed on a 1 MEG! It had a pretty equivalent amount of treble when compared to passive.
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Re: El Toro Preamp taming/R7 mod?

Postby Paul Gagon » Sat Oct 07, 2017 1:30 am

bigtone23 wrote:I experimented with various resistors on the yellow wire. After trying 10K then 220K, 470K, 680K I finally landed on a 1 MEG! It had a pretty equivalent amount of treble when compared to passive.

Thanks for taking the time to experiment and sharing your findings. Hopefully this will help other El Toro fans.
Keep on Rocking!
Paul
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Re: El Toro Preamp taming/R7 mod?

Postby bigtone23 » Sat Oct 07, 2017 10:28 pm

For sure, thank you for the tips, it made a big difference in what I like about the instrument. I like being in active mode and not have to futz with the treble knob.

I have more time right now, so I'll elaborate on what I found.
When the yellow wire was disconnected from the treble pot, the tonality of the high end was the same in active and passive mode. No overhyped boost, as expected from your post. I was almost going to leave it disconnected, but still heard a touch more low end in active mode vs passive mode. The 1 MEG resistor gave a touch of treble boost that matches the bass boost in active mode, resulting in a hyped tonality that was balanced. Kind of like the loudness switch on a stereo, but a little more subtle.

My El Toro's layout is exactly like the drawing above. Since I already had the .0022uF loading cap on my bass pot and the bass sounded fine, I didn't experiment with it. Next time I have it open, I will try putting a small cap in parallel with it just to hear the difference.

I think it's safe to say that putting that resistor on the CW lug of the treble pot is the key to taming The Bull!
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