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Thread: Some help with a 1974 Gibson Les Paul Signature (Low impedance pickups)

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    Some help with a 1974 Gibson Les Paul Signature (Low impedance pickups)

    Hello everyone, new guy here and I hope you all have a great New Years! I could use a bit of help please with repairing an old Gibson Les Paul Signature (The 70's thinline ones). Apologies if this is the wrong section in the forum; I had a look through everything and I thought this would be the best place to post.

    So, it seems that about 30 years ago someone decided that this guitars original electronics were not what they wanted and changed them to a typical Les Paul style configuration. If you're familiar with these instruments then you'll know that they are equipped with low-impedance pickups and I'd imagine that's why it hasn't worked since. Both the pickups are original, working and read 25-26 ohms so that's good news but unfortunately the rest of the original electronics are long-gone. After looking over the schematic I can tackle the wiring and replace/substitute most parts but I must admit that making the small transformer is beyond me. I'm aware that as a last resort I can still use a little line matching transformer to get sound but it would be nice to actually have the three position 'level control' to change the output impedance as well.

    Can anyone make something like this or point me towards someone who can please? I've given up on finding the original part (number: 70-478) its like looking for hen's teeth. Epiphone did a re-issue a while back and the Jack Casady bass still uses the 50-250-500 level control if anyone has Casady electronics they'd be willing to part with. Here's some pictures of the original part:


    [IMG][/IMG]


    [IMG][/IMG]

    Thank you for any assistance and have a great 2019 -Kev

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    ToneOholic! big_teee's Avatar
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    I think I would try Gibson's custom shop.
    See If they have any idea how to restore it.
    Or maybe they can source the parts?
    http://www.gibson.com/Support/Repair...storation.aspx
    T
    **Anyone have any ideas?

    Click image for larger version. 

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    The Epi Jack Casady part sounds like the best bet to me.
    I would think they must have that part, maybe they don't sell it without returning it to them for warranty (limited lifetime)?

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    Not all old guitars are desirable or even valuable. That low impedance pickup setup was never even remotely popular for a reason. If it were mine, I would consider mounting some mini humbuckers on it and calling it a day. It is difficult to do do to much with it because, if itís the one I think with the angled pickups, it makes standardizing it difficult. But if you have never tried to use one of those low impedance models.. you may be very disappointed after all of the trouble restoring it. Iím pretty sure you can kludge something together to test it and see if you like it. The one person I knew who owned one back in the day used a mic impedance adapter to plug into a normal guitar amp.

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    Quote Originally Posted by olddawg View Post
    The one person I knew who owned one back in the day used a mic impedance adapter to plug into a normal guitar amp.
    Was that Les Paul? I know he developed a low impedance balanced guitar, but the drawback was it needed a matching transformer at the amp end. OTOH he could plug direct into a mix console for recording. Les wasn't much interested in distortion. He had plenty of echo effects, overdubs & whatnot to dazzle the ears. Not to mention terrific playing technique.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by olddawg View Post
    Not all old guitars are desirable or even valuable. That low impedance pickup setup was never even remotely popular for a reason. If it were mine, I would consider mounting some mini humbuckers on it and calling it a day. It is difficult to do do to much with it because, if it’s the one I think with the angled pickups, it makes standardizing it difficult. But if you have never tried to use one of those low impedance models.. you may be very disappointed after all of the trouble restoring it. I’m pretty sure you can kludge something together to test it and see if you like it. The one person I knew who owned one back in the day used a mic impedance adapter to plug into a normal guitar amp.
    THIS!^!^!^!^!^!^!^!

    As guitars go, the "low impedance"/passive ideas that were occasionally kicked around literally NEVER worked out. Rendering such guitars in the "Mr. Microphone" category of collectibles. Novel, but sort of useless. I'd complete a conversion to standard type pickups and wiring. Save the original pickups in case a collectors market ever DOES develop for this model, or sell them to a true believer that MUST HAVE those pickups. At your discretion.

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    "Never bet your life on somebody else doing their job." SoulFetish's good friend

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    "Back to the amp. It makes horrible sounds when I play my guitar thru it... because I suck at playing guitar." Mike6158

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    Yep.. it was made to plug directly into a console using a balanced low impedance XLR cable. Had an XLR jack on the guitar. They were for “super clean” use. But for some reason not even the jazz guys liked them. They sound... “funny” imho.

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    If it is this guitar? IMO it is a beautiful guitar.
    I think you could rip it all out and install new pots, new pickups with pickup rings.
    Don't know if it has a control access plate on the rear or not?
    If you have to do it all through the f holes, it will be harder and more trouble.
    T
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    I suppose you could also make a little preamp and put it in there if there is room for the battery. That way you could stick with the original pickups for stock appearance, and it would be interesting to hear how they sound 'active.'

    Andy

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    Click image for larger version. 

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    This is the version I saw..

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    Quote Originally Posted by olddawg View Post
    Click image for larger version. 

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    This is the version I saw..
    Thats a Les Paul Recording II. The signature is hollow like teee posted above.
    The L5S was also a low impedance solid body.

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    Hey, thanks for the replies everyone it's much appreciated I'll attach a couple pictures of the actual guitar I've got here below to avoid any confusion!

    Cheers for the Gibson Restoration dept link big_teee; I've got a few contacts at Gibson here and in the USA that I've tried but I'll drop them a line as well just to be sure. The transformer in these is pretty unusual and would likely have only been manufactured for this model but I'll try pleading for a Casady board as well lol

    I'm pretty well versed in how these guitars work (and how they sound, which I know is not for everyone) which is basically a Master Volume knob, Master tone knob, A rotary on/on phase switch, a three position 'Level' selector (the 50,250,500 ohm switch) a mono high impedance jack on the front to go to an amp, and the stereo low impedance jack on the side to run to a desk etc.

    I know it's a possibility to change all of the electronics and pickups to something else but it would be a shame since I still have everything required except this one silly transformer. Plus, it's going to be very expensive to change everything including the pickups and it's not what my customer has asked for! If anyone's got a line on one of these little transformers or knows someone who can make one it really would be fantastic
    Cheers, Kev





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    Perhaps someone in the pickup maker section would be more familiar with low impedance line matching? It's an unusual request to be sure!

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    ToneOholic! big_teee's Avatar
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    Quote Originally Posted by stairwaytokevin View Post
    Perhaps someone in the pickup maker section would be more familiar with low impedance line matching? It's an unusual request to be sure!
    I'm a moderator in pickup makers.
    I moved it over here.
    Unless you are making pickups, more guitars are repaired over here in this area.
    Also a lot of these guys also work on amps using transformers and the likes.
    T
    ** I sent bbsailor a pm, he does low impedance pickups, maybe he will stop by, and can offer some ideas?

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    Last edited by big_teee; 01-04-2019 at 09:56 PM.
    It's only Rock and Roll, but I like it!

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    The Casady is a current Epi model. For sure they have them, it's just a matter of finding the right person at Gibson to get your hands on one.
    Good luck.

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    Quote Originally Posted by big_teee View Post
    I'm a moderator in pickup makers.
    I moved it over here.
    Unless you are making pickups, more guitars are repaired over here in this area.
    Also a lot of these guys also work on amps using transformers and the likes.
    T
    ** I sent bbsailor a pm, he does low impedance pickups, maybe he will stop by, and can offer some ideas?
    I just returned from my vacation.

    Here are the rules of thumb to make low impedance pickups.

    AWG42, typical of high impedance pickups, would need to be thicker for low impedance pickups to minimize restive losses. AWG32 has 10X less resistance than AWG42. Jack Casedy liked the sound of AWG 28 for his bass.

    If a typical single coil high impedance (high Z) pickup has about 6000 turns, then a low Z pickup would have about 600 turns or one tenth the high Z turn number. The impedance varies by the square of the turns difference. A volume pot for a high Z 6000 turn pickup would be 250K but 600 turns of AWG 32 would require a pot value 100 times lower in resistance or 2.5K ohms. For humbucker pickups, just double the values in this example.

    The Jack Casedy bass used a three position switch marked 50-250-500 to feed the pickup output into different taps of the input step up transformer mounted inside the bass. This technique uses matching and mismatching the pickup load impedance (50-250-500) to create tonal and volume differences. If you choose to minimize the coax cable losses, put the transformer at the amp using an XLR connector and 2 conductor shielded mic cable.

    To provide any more relevant input, please describe your design restrictions or ultimate goals.

    Joseph J. Rogowski

    PS Post the details of your pickup design and bobbin size to better consider your options.

    PPS: Read this: https://www.gearslutz.com/board/so-m...ansformer.html
    This reference will give you another input to consider.

    I believe that you need a mic input transformer that has multiple input primary taps of 50 ohms (possibly 75), 150 ohms, 300 ohms to 10K to 15K or even 20K ohms secondary.

    To determine the turns ratio of published impedance specifications do this: Divide the input impedance into the output impedance and take the square root of that number. Example: 150 ohms input to 20K output is: 20,000/150 = 133.33. Square root of 133.33 is 11.547 turns ratio or approximate voltage boost.

    I hope this helps?

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    Last edited by bbsailor; 01-05-2019 at 03:47 AM. Reason: added PS

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    Old Timer olddawg's Avatar
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    What a beauty.. kind of like a cross between an LP and an ES335! I still would drop some minis in it in a heartbeat!

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    Cheers Big-teee, I was wondering if I'd posted in the right place! It is pretty cool looking and I'm going to keep pursuing the Jack Casady angle with Gibson, as well as the transformer idea. I must say a thank you to bbsailor for the enlightening post about the transformer. I've found a small mic line matching transformer (600ohm : 10k) that I'm going to try first and I should be able to work backwards from there to find the proper turns ratio. From what I understand now that'll be pretty critical This one even has a center tap on the primary so I can take a reading there as well, wonderful. I can always try another with a higher secondary, up to 25k at least if this one doesn't work out. I'll be using the guitar's original pickups so let's see what we can do

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    Quote Originally Posted by stairwaytokevin View Post
    Cheers Big-teee, I was wondering if I'd posted in the right place! It is pretty cool looking and I'm going to keep pursuing the Jack Casady angle with Gibson, as well as the transformer idea. I must say a thank you to bbsailor for the enlightening post about the transformer. I've found a small mic line matching transformer (600ohm : 10k) that I'm going to try first and I should be able to work backwards from there to find the proper turns ratio. From what I understand now that'll be pretty critical This one even has a center tap on the primary so I can take a reading there as well, wonderful. I can always try another with a higher secondary, up to 25k at least if this one doesn't work out. I'll be using the guitar's original pickups so let's see what we can do
    The center tap of the 600 ohm impedance is actually rated at 150 ohms for calculating the turns ratio.

    I am glad I was able to help you.

    Joseph J. Rogowski

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    I am unfortunately in the same position as the OP -- I am the first and only owner of a 1974 LP Sig. and the electronics are not working. I'm no engineer, but per the drawing that three impedance selector looks like it's an "inductor?" Like the varitone on the LP Recording guitar . . . I think we are SOL with trying to replace the original parts. Mine has been converted to a 335 setup running through the high impedance output jack, but it needs an external transformer now.

    If anyone can get parts out of Epi/Gibson for the Casady bass I'd love to know about it.

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    Quote Originally Posted by agabinet View Post
    I am unfortunately in the same position as the OP -- I am the first and only owner of a 1974 LP Sig. and the electronics are not working. I'm no engineer, but per the drawing that three impedance selector looks like it's an "inductor?" Like the varitone on the LP Recording guitar . . . I think we are SOL with trying to replace the original parts. Mine has been converted to a 335 setup running through the high impedance output jack, but it needs an external transformer now.

    If anyone can get parts out of Epi/Gibson for the Casady bass I'd love to know about it.
    The Shure A95U series mic matching transformer is still available. Look it up on line. It uses an XLR connector for input and a quarter inch plug for output directly into the amp.

    To convert your guitar, use a stereo jack in your guitar. Make an adapter cable with a stereo quarter inch plug using 2 conductor shielded mic cable. XLR pins 2 and 3 go to the tip and ring. Ground goes to XLR pin 1 and the ground of the stereo plug. This set up eliminates the high impedance capacitance of coax guitar cables on high impedance pickups by just having the A95U transformer out of the guitar and located at the amp input. Make sure that your pickups are low impedance.

    I hope this helps?

    Joseph J. Rogowski

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    Quote Originally Posted by bbsailor View Post
    The Shure A95U series mic matching transformer is still available. Look it up on line. It uses an XLR connector for input and a quarter inch plug for output directly into the amp.

    To convert your guitar, use a stereo jack in your guitar. Make an adapter cable with a stereo quarter inch plug using 2 conductor shielded mic cable. XLR pins 2 and 3 go to the tip and ring. Ground goes to XLR pin 1 and the ground of the stereo plug. This set up eliminates the high impedance capacitance of coax guitar cables on high impedance pickups by just having the A95U transformer out of the guitar and located at the amp input. Make sure that your pickups are low impedance.

    I hope this helps?

    Joseph J. Rogowski
    Thanks Joseph, I have an inline matching transformer and it works very will in the current setup to make the Low impedance pickups sound fine. What I want to do, though, is restore my original electronics with the impedance selector . . . I'm working on it. It may be just a grounding issue.

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    Quote Originally Posted by agabinet View Post
    Thanks Joseph, I have an inline matching transformer and it works very will in the current setup to make the Low impedance pickups sound fine. What I want to do, though, is restore my original electronics with the impedance selector . . . I'm working on it. It may be just a grounding issue.
    See this link for the schematic. http://archive.gibson.com/Files/sche...PRecording.PDF All the tone changes are on the low impedance side of the transformer. Use a selector switch per the schematic to add various capacitance loads and the phase invert switch to put two pickups on but out of phase for a variety of thinner sounds.

    Joseph J. Rogowski

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    Les Paul Sig Schematic

    Quote Originally Posted by bbsailor View Post
    See this link for the schematic. http://archive.gibson.com/Files/sche...PRecording.PDF All the tone changes are on the low impedance side of the transformer. Use a selector switch per the schematic to add various capacitance loads and the phase invert switch to put two pickups on but out of phase for a variety of thinner sounds.

    Joseph J. Rogowski
    Here's the schematic (or one of them) for the signature. It's not as complex as the recording or professional -- no decade control! Click image for larger version. 

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    Looking at the photo of the original switch/transformer assembly, that looks like a UTC matching transformer. United Transformer Corp, New York.

    If there are other photos available, there will be a number on the transformer case that may lead you to specs from one of their old catalogs or data sheets. Perhaps there is someone with the original part that will look at theirs and tell you the number.

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    As a follow up, the UTC "ouncer" model O-1 would seem to fit the bill.

    There are a number of 1970's UTC catalogs scanned and online out there. Who knew?

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