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Eric Clapton Mid-Boost Help/Advice

Discussion in 'Stratocaster Discussion Forum' started by guildguy516, Mar 14, 2017.

  1. awasson

    awasson Poster Extraordinaire Silver Supporter

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    To reduce the bump in volume, you could get a 50k trimmer potentiometer and put it between the connection of the green and brown wires. That’s where the original 50k volume control goes. I haven tried it but I would think a little resistance there would lessen the initial volume bump when you engage it.

    I don’t know what to think about the TBX. I like the one I have but I know what you mean by the short range and difficulty dialling it in. I wonder is a standard tone control would give you more useful range.

    A dual concentric pot would be cool for bass on one and treble on the other but it would definitely change the aesthetic. It might work with a mat black pick guard and black Telecaster style knobs but it wouldn’t ever match Strat knobs.
     
    Last edited: Apr 4, 2019
  2. KelvinS1965

    KelvinS1965 Tele-Holic

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    I just fitted the mid boost kit yesterday to my Strat. I'm really impressed with how it gives such a range of tones and that it thickens up the Bareknuckle Apache pick ups I have. I wasn't expecting it to be so useful on clean tones for some reason, but I found that balancing the mid boost and TBX controls gives a huge range. I guess the TBX control is a bit harder to adjust in small amounts, but at least it stays put once set.

    I used it as an excuse to change the look of my Strat as I wired it all up on a new pickguard, which I wet sanded to dull it down to give a Bakelite look. Now I have two matching Fenders both with extra sounds (the Tele wiring is modded too, but no mid boost-yet).

    Pickguard 2.jpg

    The Fender Twins:
    Fender Twins.jpg
     
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  3. guildguy516

    guildguy516 TDPRI Member

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    Right now the green and brown go to lug 3 on a 250k push/pull. Could I throw a fixed resistor from the green to the lug to reduce that increase in volume?
     
  4. awasson

    awasson Poster Extraordinaire Silver Supporter

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    Yes, that’s the idea. I’m not sure how much resistance you’ll need to lower the output though. That’s why I was thinking of using a trimmer.
     
  5. guildguy516

    guildguy516 TDPRI Member

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    I purchased a 50k audio taper pot (as the schematics suggest) with switch off at 0. I'm going to wire that up and see how it impacts the volume bump. If it's still an issue I will experiment with resistors.

    Anyone have an idea of where to start with resistor values?
     
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  6. KelvinS1965

    KelvinS1965 Tele-Holic

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    Use a trim pot to adjust to how you want it, then measure the trim pot and replace it with the nearest value resistor you can buy.

    I'm really stoked with my Strat since I fitted this kit (standard wiring), but I get that you want to stop the change in level when switching on the boost circuit.
     
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  7. guildguy516

    guildguy516 TDPRI Member

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    Thank you for your guidance with this.

    I do love it as well. I play 2+ hour shows with one guitar - music spanning 60 years - and I can dial it in to the tone ballpark to capture the feeling of ~that~ song. I find that when I turn it on I end up backing the volume down a hair, so while I like a small increase in volume the 25db is far too much. I'm thinking +10-15 db would probably be right where I need it.

    It would be great if Fender offered this with a built in trim pot.
     
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  8. guildguy516

    guildguy516 TDPRI Member

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    Greetings again everyone! I just wanted to follow up on this thread as I know everyone has been waiting in anticipation (ha)! I finally got all the kinks worked out and have the circuit up and running exactly as I need.

    Long story boring, after playing with a few ways to include a resistor in the circuit...then different values...I attached the output of the preamp to a variable resistor. I sat in front of my amp playing, adjusting, playing, adjusting, until I got it where I wanted it. My intention was to then take out the variable resistor, measure it, and order a fixed resistor of that value. I plugged into another amp and this time I wanted to have slightly more gain so I adjusted it accordingly - then I thought "well couldn't I just leave the variable resistor in the circuit for minor adjustments in the future?" and this is exactly what I did.

    [​IMG]
    [​IMG]
    I drilled a very small hole in the guard, mounted the variable resistor on the rear side and now I have a trim pot I can adjust with the tip of a pick, if needed.

    Where it is currently set gives me a very smooth transition from stock pickups to adding the EC midboost, no more massive volume jump. When engaging the ECMB it picks up where the guitar leaves off. At 1 there is a slight boost in mid frequencies, no volume increase, still very Strat-like but more 60's Strat sounding - hotter and punchier (guitar is equipped with 1950's pickups). I find around 3/4 is absolutely perfect: just gritty enough, very dynamic, gives the pickups more body and gives me great control with pick attack, 5/6 is "humbucker rolled back", 7/8 is perfect P90 territory , and 9/10 is "Les Paul dimed".

    I would like emphasize how I didn't say " ECMB is like......" because I was significantly impressed with how I could replicate the tones of my other guitars - and I can be a cork sniffer. I A/Bed this next to a real 1950s P90s, a set of 1958 PAFs, a set of 1955 Guild Franz made P90s, and a set of 1966 Filtertrons. After spending just a little time dialing in the TBX/ECMB next to each instrument, the difference was nominal at best - and nothing you would notice in a live setting. The difference I heard was about 95% mental and the proof of that lies within my playing style.

    You know how you "play different guitars differently"? I started noticing my playing style adapted to the the tones that were coming out of the Strat: it's all mental.

    The differences I noticed were nominal. Next to the PAFs the ECMB wasn't as "airy", although it was that tone {it could have a lot to do with the PAFs being on a 335 style}. Next to the P90s the ECMB lacked just a little of that punchy P90 attack {it could be because the P90s are on a solidbody without a vibrato block/system}. Next to the Filtertrons the ECMB just needed a little twang on the attack {could be because it's not attached to a massive aluminum Bigsby}.

    I could pick apart the differences but it's comparing apples to cinnamon sticks, really.

    The take away is that with some minor modifications to the Clapton mid-boost preamp, and a little finesse in your settings, it is an extraordinarily versatile tool that allows you to clone many other pickups and their iconic tones: and this it does VERY well.

    Controls:
    5 way super switch
    1. Bridge
    2. Bridge+Middle
    3. Middle
    4. Bridge+Neck
    5. Neck

    250K master volume

    TBX control - resistor replaced with .001mF capacitor for bass roll off, completely removed from circuit at center indent

    250K push/pull, in - ECMB is out of circuit, out -engaged, about 60k resistance from output to ground gets you to start at the guitar's passive '10' and up from there.
     
    Last edited: Jul 10, 2019
  9. awasson

    awasson Poster Extraordinaire Silver Supporter

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    Awesome. That sounds like the best of all worlds in one tight package. I like the look of the trimmer too. Nice job!
     
  10. guildguy516

    guildguy516 TDPRI Member

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    What do you all think about making the preamp control a 250k no load pot?
     
  11. awasson

    awasson Poster Extraordinaire Silver Supporter

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    probably not a good use of a no-load because no load opens the potentiometer so that no signal passes through. If you put that into a place where there needs to be a variance of resistance for the circuit to balance something is going to be unhappy. My guess is that it will oscillate.
     
  12. psammetico

    psammetico TDPRI Member

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    Hello, I have installed the mid boost according your suggestions with a push pull to activate/deactivate and it works fine.
    The only issue I am facing is that the midboost control works in an opposite way: at 0, 1, it is a the maximum, close to 10 is at the minimum. It looks as some cable is inverted.
    I have also tried to swap the violet with the bown and viceversa, but the result is the same.
    The only difference is that I have grounded the boost pot even if in the sketch is not gorunded? Could be that reason?
    Thanks in advance
     
  13. awasson

    awasson Poster Extraordinaire Silver Supporter

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    When you switch brown and purple on the mid-boost pot, it will reverse the action so that should fix the problem.

    Maybe one of the legs of the mid-boost pot is connected to the housing of the potentiometer. If that's the case, then when you ground it, it is shorting that leg to ground. Otherwise, I don't know what they problem is.
     
  14. psammetico

    psammetico TDPRI Member

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    Thank Awasson, I already switched the brown with the purple, without any effect. I will investigate deeper. Thanks for now!
     
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  15. awasson

    awasson Poster Extraordinaire Silver Supporter

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    Check that none of the legs on your mid-boost potentiometer are shorted to the body of the pot. Thats the only thing I can think of that would cause such a problem unless the potentiometer is actually shorted or open internally. That can happen if it's overheated during soldering.
     
  16. psammetico

    psammetico TDPRI Member

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    Hello Aswasson, in the end it works.
    I resoldered it again.
    Now I am trying to find the right value of a resistor in order to have that huge difference when switching from the boost to the full bypass. At the same time I am trying to find the right capacitor, even when switching. at this moment I am arguing using a push pull for the tone control in order to change the capacitor value (a sort of varitone with two positions): I have to find the right values.
    In case someone else already did it, it would appreciate if could share.
    I let you know my experiments!
    THanks for now!
     
  17. awasson

    awasson Poster Extraordinaire Silver Supporter

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    Excellent! Glad to hear it's working properly. That's great.

    To account for the increase in volume when you switch the preamp into the signal path, I'd insert a resistor in the connection between the green and brown wires. It won't be a very large resistor. I recommended using a 50k trimmer potentiometer and dialling it in.

    Good luck and keep me posted.

    Cheers,
    Andrew
     
  18. franblanc

    franblanc TDPRI Member

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    Guitar Pre Amp – Fender Mid Tone Boost Kit Problems
    The featured Fender Mid Tone Boost Kit was installed in 2008 inside a 1992 Fender Stratocaster American Standard. However the result was lot of unpleasant distortion and noise. This is a common complaint with the pre amplifier which is marketed as a Fender product. It was purchased off of ebay from a reputable Electric Guitar Parts dealer who is still trading. Recently it was decided to examine the preamp to see if it could be improved.

    The preamp has a circuit board which is usually branded with “Fender” however this board does not have any markings. The second thing I noticed is that the AC bi polar (+ -) capacitors marked with yellow arrows are not audio caps. They are 10uf 25volt capacitors made by “United” which might be Chemi-Con. I decided to replace these with Nichicon UFW Audio Caps because these capacitors receive the Alternating Current signal directly from the pickups.

    There were also three NPN transistors marked MPS A18 701 in the positions marked with red arrows. These transistors have the job of boosting the gain of the guitar signal in three stages. On the layout diagram for this circuit board they are represented as 2N6429 TO-92 transistors but on the circuit board the components were MPS A18 701, both types are NPN. I replaced them with new transistors (MPS A18). The new transistors had a hFE (gain) value of 470 – 500, which is expected from this type of transistor. Whereas the old ones that I removed had a hFE of 150 to 170. This is alarming because it is possible that one transistor drifted over the years but it is unlikely that all three drifted to the same extent. It is possible the pre amp was made with a bad batch of transistors but it is equally possible that it was a counterfeit Fender product not subjected to the same testing or quality control. Because of this it was decided to replace the PNP transistor 2N5087 marked at the top of the image. The old one measured 400 hFE and its replacement measured 500 hFE so the old transistor was actually alright. I tested all the resistors and they appeared good. Four of them were lifted to test them out of circuit to make sure. Lastly I tested the yellow polyester DC caps with a multimeter and they too appeared to be well.

    The guitar was reassembled and tested and the sound was much improved. The gain is audible but the noise is gone and the clarity of the tone is much better. It should be noted that most of the grounding connections inside the guitar were re-soldered and the red power lead from the 9v battery was reattached more firmly. The battery itself was insulated and the entire preamp was placed inside a small anti static bag for safety. Lastly the guitar cavities were shielded with copper foil to eliminate hum. One conclusion is that there may be bad circuit boards on the market which might possibly be fixed by replacing the transistors and electrolytic capacitors. If you have a Mid Tone Boost kit and the sound is poor, then this might be the reason. I have included the only image I could find of the layout and component list for the Mid Tone Boost Kit as a guide. There are a number of compatible equivalent transistors for the 2N6429 (MPSA18) and the 2N5087. The only stipulation is that the former must be NPN and the latter PNP (Positive Negative Positive). Fender_Pre_Amp.jpg eclayout.jpg
     
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