Zero Loss FX Loop in JCM800 Kerry King

Discussion in 'The Workbench' started by Filipe Soares, Nov 2, 2019.

  1. Filipe Soares

    Filipe Soares Well-Known Member

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    I´ll start a new thread because I´ve already ordered the board and I think other people could benefit from the info.

    So, the fun part:
    1) drilling holes or putting it in a external box?
    2) I´d like to turn the loop on via footswitch and have a parallel switch in the box

    what are your thoughts about items 1 and 2?

    the ¨hard¨ part:

    I´ve disassembled the amp today and studied the boards with the schematics

    schematics
    https://drive.google.com/file/d/1hcEJdUD6A7ZWYQmGyzIC1o6l-vAuB3bM/view?usp=sharing
    https://drive.google.com/file/d/1ORubrsmFiDxaLrkzUP_yNq8Iai07jl5H/view?usp=sharing

    I think I have to acquire the signal from the R20 in the EQ section, @ampmadscientist confirmed me that I have to take the R20 out and put the loop there. I still don´t know where I have to take the voltage for the B+.


    I´m good with soldering stuff, but I´m new to amp modding. and I´m also super excited about doing this mod.

    Any info, inputs, 2 cents and opinions are welcome
     

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  2. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    Why don't you install the 47K first and measure the voltage on the loop board? (voltage with the tubes in and bias is set)
    The instructions specify a operating range for the voltage.
    The kit comes with several different resistors. I think I used 27K or 47K I can't remember which.
    Get in touch with New Religion and he might give you some pointers.

    I made a pot which adjusts the loop to bypass/parallel adjustable mix/series.
    I used an alpha 1 meg dual pot.
    This made the whole thing way more usable and I was happy with the sound quality compared to using the mix control on the effects pedal.
    (the effect is set all wet with no dry signal)
    However you probably want to do the basic loop first and then add any controls later on. (test if first and get the levels set)

    800PI.png
     
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  3. pedecamp

    pedecamp Well-Known Member

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    I'd be interested in how this can be done in an external box. :yesway:
     
  4. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    It's going to be difficult with an external box.
    The leads should be very short to avoid oscillations.
     
  5. pedecamp

    pedecamp Well-Known Member

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    Gimme some insight on this, can it be done? I have an extra speaker input on my 2204, can that become a plug to connect an outboard effects loop? I'd sit it right on top of the amp. And like Filipe said a footswitch for it.
     
  6. Lance Chambers

    Lance Chambers Well-Known Member

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  7. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    I wouldn't do that because there is a lot of stray signal which might cause problems.
    You would need to hook up audio and power through longer wires which makes it impractical.
     
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  8. South Park

    South Park Well-Known Member

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    You might want to be looking for a scope and signal gen.or some one who has one.
     
  9. Filipe Soares

    Filipe Soares Well-Known Member

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    My plan is to do that when the board arrives, find the lead, measure the voltage and do it. as the boards of the kk are very different from a standard 2003 or a plexi I have to study first...

    My idea was to drill a hole in the upper deck of the chassis and pass the shielded wiring to a hammond box. probably use a different grill behind the amp to acomodate the loop

    i have a real state problem here man, there´s no much space available in the back of the amp. the power amp board is very big (see pics). I have some space between one transformer and the board and just above the board, in offset , which does not pleases my eyes.

    if you put above where the lead came out it might be like 2in more of wiring.

    I´d love to buy a scope, but buying a scope to do this is kind of impractical, it would be cheaper taking it to the amp tech.
     

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  10. South Park

    South Park Well-Known Member

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    If you can put it in a box with plug in connections to the amp that would be nice. I like the idea of a fx loop. Take a look at the slo100 it has one of the best loops in the biz. The more research you do the better off you will be
     
  11. Filipe Soares

    Filipe Soares Well-Known Member

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    maybe all the board inside and just the wires for the jacks going up. I´ll take a look in the slo100
     
  12. Filipe Soares

    Filipe Soares Well-Known Member

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    Well the loop has arrived, and as I am a cheap motherfucker i´ve ordered the kit without the resistors. I did that because I already have all the parts, but the resistors... and I live 5 min away from a electronics store. So I sent to my favorite seller a message asking for the components, went to the store and bought it... I´ve checked the values at the store... and then after disassemble the amp my meter was showing a .001 value for the componentes.

    despite the resistors are 82k, 100k, and 5.6k they actually are 82, 100 and 5.6... fuck it. and they don´t have the resistors here. So, next saturday I´ll go to big store in downtown and buy it.

    for now I´ve started the wiring and trying to figure out where to install it, the lack of space is amazing.
     
  13. Filipe Soares

    Filipe Soares Well-Known Member

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    I´ve tried to install. I triple checked the voltage getting out of B+ and I got 300v to the board. I took out the R20 and placed the in/out leads, but the only thing I hear when theloop is on is an immense lost of volume and signal. I´ve screwed up something, still don´t know what.

    @ampmadscientist and @NewReligion any ideas???
     
  14. South Park

    South Park Well-Known Member

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    You might not have a drop resister. The signal need a ground to know where to go. Look a the schamitic
     
  15. South Park

    South Park Well-Known Member

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    Does it have a power adjustment
     
  16. Filipe Soares

    Filipe Soares Well-Known Member

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    The current is actually 450v in the resistor i´m using, then I drop it to 300v. I don´t think this is the issue, i think the issue is where i´m acquiring the signal. it was supposed to be in the R20 470k in the pic below, but nothing is happening.
     

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    Last edited: Nov 6, 2019 at 11:01 PM
  17. Lance Chambers

    Lance Chambers Well-Known Member

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  18. South Park

    South Park Well-Known Member

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    The pot below r 20 is the the tap you are looking for that is the output of the tone stack
     
  19. Filipe Soares

    Filipe Soares Well-Known Member

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    the installation suggests you to place the loop between the treble pot and the master pot, which is R20.

    this guy does it in a different 800 iteration
     
  20. South Park

    South Park Well-Known Member

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    The pot below r20 on the wiper that gos to the master
     

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