1987 tone stack voicing question

Discussion in 'The Workbench' started by black knight, Feb 26, 2020.

  1. black knight

    black knight Member

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    I have a '68 lead spec 1987 clone and the midrange is almost unbearable. The G string (784Hz range?) is audibly much louder and more pronounced than any other string, although not really limited to that range as the notes are still louder as the G string gets fretted higher and higher. It's as if the G string has a boost and the other 5 strings sit evenly in the background. It's not a guitar/pickup problem. It's definitely the amp. If I turn the mid pot to zero, it eliminates the string db descrepancy. But overall, the mid pot has little bearing on tone adjustment. Full sweep bears little change. All other controls behave normally. I typically about dime my mids on these plexi circuits. It does have a MV installed, however it behaves the same regardless if in use or not. Has anyone ever experienced this? Will changing the slope resistor and/or treble cap help alleviate the pronounced mids? Currently it is the stock 33K/500pF combination. Could it be just how this particular OT is filtering frequencies? It is a Merren OT, but I don't know the model # at the moment. I just assume the builder used the correct spec for this circuit. Any ideas?
     
  2. neikeel

    neikeel Well-Known Member

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    If it is a clone it is going to be someones interpretation of a 1987 so assume nothing. Please post pics of your amp with clear lighting and a decent camera with the ability to zoom and read cap and resistor values then we can work through it.
    Until you do that people will be guessing and shooting from the hip, not really knowing what they are talking about;)
     
  3. black knight

    black knight Member

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    Thanks Neikeel. Here's a couple to start with I can take more if necessary.

    Metro 2.jpg

    Metro 1.jpg
     
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  4. neikeel

    neikeel Well-Known Member

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    The build looks to be of good quality and has followed the Metro build instructions well. I see almost a 69 spec. with Classic tone PT (it has 0.022uF V1b coupler that should be 0.0022uF for lower cut on lead channel but that is what the Metro instructions included.)
    I presume that your output tubes are approximately matched and correctly biased and you are playing through a typical 1960 or 1982 Marshall cab?
    Other features you point out are the PPIMV and Loop.
    First thing to try would be a more mellow set of preamp tubes (you may not have) but a set of old Mullards would be ideal. This may not be realistic so you will aim to work with what you have.
    So next try unsoldering one leg of the silver mica cap on bright volume pot.
    After that desolder one leg of the big 0.68uF cap mid board. That will tame your upper mids (assuming the builder has used the correct 25k mid pot).
    After that, yes try 47k/390pF tone stack or then 56k/250pF tone stack but I am not sure that will do an awful lot, maybe a 50k Mid pot!
    More ideas from others awaited!
     
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  5. black knight

    black knight Member

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    Yes, 1960 type repro with UK G12M-25 greenbacks. This cab seems to mitigate the issue somewhat, but it is most prevalent when playing through an oversized bluesbreaker type 2x12 with Creamback G12M-65s. I will try your suggestions. Thanks for taking the time.
     
  6. Matthews Guitars

    Matthews Guitars Well-Known Member

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    Go over that with a fine tooth comb. Check every component value and if you have a cap checker, CHECK the values of all the capacitors in the tone stack and in the signal path. My suspicion is that you've got a component off value.
     
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  7. black knight

    black knight Member

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    Thanks Matthews. I will check what I can. I don't have a cap checker. I did just see that they're not expensive though so I will definitely get one. Honestly I am not all that techy and don't have the tools/experience necessary to fully evaluate/troubleshoot (still on the learning curve). Just mainly a garage tinkerer at this point. But I appreciate the advice and will follow it to the best of my ability.
     
  8. South Park

    South Park Well-Known Member

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    Try a different guitar one with single coils . I think some pick ups are mid strong
     
  9. Matthews Guitars

    Matthews Guitars Well-Known Member

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    Yeah, get one of those very inexpensive component testers you can find on ebay for under 20 bucks. Just remember, to read the value of a component correctly, you're going to have to at least disconnect one lead. In-circuit measurements can be very misleading when it comes to component values.
     
  10. ampmadscientist

    ampmadscientist Well-Known Member

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    It is really common for the tone controls to do very little.
    Especially mid control.
    There's probably nothing wrong...but you can do the jumper thing on the tone pots which may help a little.

    The overbearing mids can be from the bright cap on the volume control....
    There is a tone stack simulator which might help you:
    You can change the values of the circuit and see the results real time.
    http://www.duncanamps.com/tsc/download.html

    [​IMG]
     
  11. Gene Ballzz

    Gene Ballzz Well-Known Member

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    @ampmadscientist
    Hey my friend, a version of that calculator is now available for online use, for those who may be MAC-ish or otherwise can't run the regular Duncan.
    https://www.guitarscience.net/tsc/eseries.htm
    Slightly different GUI, but overall, the same results. I also think I read somewhere that this online unit ia approved and/or supported by Duncan?
    Just Sharon!
    Gene
     
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  12. ampmadscientist

    ampmadscientist Well-Known Member

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    I think that going thru every cap with a tester is a complete waste of time.
    People who are obsessed with measuring capacitor values in tube amps generally don't get very far...

    This is the complaint we hear about all the time: "ice pick" highs, screeching upper midrange.....most of it comes from the bright cap, and the 470pf bypass caps which are installed across the 470K resistors.

    The voicing isn't all in the tone stack....the EQ is built into the design of the preamp.
    The EQ is like this: "/ " it's deliberate.
    There is a reason why it was built that way.
     
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  13. mickeydg5

    mickeydg5 Well-Known Member

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    That sounds more like a guitar setup issue than an amplifier issue.
    The G string is always prominent especially if using larger string sets with solid G, low action and minimum clearance to pickup/pole. It is a physical attribute of the G string.
    Make sure the G action is higher that all the rest, back the tuning off a few cents and drop the pickup or pole(s) if possible to try and help the situation.
    Maybe also try wound G's if you have not already.
     
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  14. South Park

    South Park Well-Known Member

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    Go to a smaller size g string
     
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  15. mickeydg5

    mickeydg5 Well-Known Member

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    Yes, I do that also. :)
     
  16. Kinkless88

    Kinkless88 New Member

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    Have you tried blending your channels together ?
     
  17. NewReligion

    NewReligion Well-Known Member VIP Member

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    My $.02

    Do not be offended but the PPIMV's are Tone Stack killers. They effectively remove the Tone Stack and Presence to include NFB (Negative Feedback) unless turned to 10 which negates that idea anyway. In fact Ken Ficsher (Train Wreck Amplification - RIP) just removed the Presence & NFB when using any of the PPIMV's.

    That...That is your culprit. Now we can walk you through cutting your mids to be more pleasing but the PPIMV is causing your issue. Try turning down the Loudness on front, then turn the PPIMV up to 10, then bring up the loudness and listen to the difference in the entire Tone Stack. Bet it sounds like a Marshall Tone Stack. But it will not be quite right as the stock 220k/150k Bias Feed Resistors have been compensated for on the Dual Gang 250k/500k PPIMV pot that uses 2.2megs most likely.

    In addition:

    After eliminating the guitar as the culprit and still not pleased you might unsolder everything on the mid pot. You can test it for total resistance and sweep but I would just replace it with a NEW 25kL if it were not for the PPIMV which I am convinced is robbing your circuit.

    This for Certain: Get rid of that Carbon Composite 33k Slope resistor that WILL drift and install a 1/2 watt Carbon Film 33k. In fact I would change the remaining CC resistors to CF as there are few.

    As for the Caps. I am no fan of Sozo, (Muddy IMO). Side by side I find Mallory 150's are more "clear". Yes, it was one of my first experiments some 11 years ago on this exact circuit as I pulled the PCB and installed a Turret Board to change components like Legos.

    In addition and gee wizz info, the black bands on the caps are for the open end foil of the caps orientation (Signal to Noise). Some are installed incorrectly (orientation) assuming that the labeled end is correct which 50% likely are not as they are after all just labeled to shut up QA/QC. The only time I concern with this is when building a very high gain amp and using caps smaller than .002uf trust me it matters then IMO. Correct orientation can be done on a oscilloscope or by mannually ussing wires, alligator clips etc...(See YouTube).

    I believe the PPIMV is killing tone and disabling the pots from sweeping as designed. Hope I wasn't rude. Just trying to make a huge point and sprinkle some gee wizz while helping out.

    Best Regards, David...♫
     
    Last edited: Feb 29, 2020
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  18. MickeyJ

    MickeyJ Well-Known Member

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    my suggestion is that whenever New Religion starts talking tone stack, listen.
     
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  19. mickeydg5

    mickeydg5 Well-Known Member

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    I actually threw away all of my carbon composition resistors a long time ago. They just seemed inferior due to tolerance and drift.

    Also to mention is that none of my rigs/amplifiers have ever utilized a PPIMV. I have nothing againct them but have never used or employed one.
     
    Last edited: Feb 29, 2020
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  20. mickeydg5

    mickeydg5 Well-Known Member

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    I hope you guys realize there are all sorts of feedback circuits. Look into it. :D
     
    Last edited: Mar 1, 2020
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