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jcm800 2205 effects loop

Discussion in 'Marshall Amps' started by mike blemenschutz, Jul 27, 2021.

  1. mike blemenschutz

    mike blemenschutz New Member

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    hi, i am having an issue replacing the existing effects loop circuit with a metro zero loss circuit. i figured with the buffer in the zeroloss circuit it would take some of the noisy issues you get with the existing one. however there is no change...it actually seems worse... i guess its due to where the loop circuit taps the signal. i tried tapping at the master volume as i do on 99% of the marshalls and others.. but it grounds the signal ..... does anyone have an idea where to tap the signal for proper operation of the zero loss effects loop? thanks !
     
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  2. Kelia

    Kelia Well-Known Member

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    Curious on what year is yours ? asking because the 1988 I had was dead silent .
     
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  3. Gene Ballzz

    Gene Ballzz Well-Known Member

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    What are you attempting to use in that effects loop?
    Just Curious?
    Gene
     
  4. mike blemenschutz

    mike blemenschutz New Member

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    its a customer amp.... running four cable method with all modulation effects through the loop. delay, chorus, flanger, reverb, phaser. all these are completely quiet through the loop of any of the other amps i have modded with the Metro zero loss loop circuit.
     
  5. Gunner64

    Gunner64 Emotional Support Animal Gold Supporting Member

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    Isn't the loop in the 2205 tube buffered? Iirc half of one of the 12ax7's is for the the loop. That may be where the extra hiss is coming from. Maybe you simply have a noisy tube.
     
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  6. headcrash

    headcrash Well-Known Member

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    I consider placing buffered loops behind master volumes to not be the best practice. Gain of most guitar amp power stage (in fact the gain of the phase inverter stage...) is pretty high, which means it'll amplify any hiss coming from the loop.
    Problem with most buffered/active loops is: they incorporate quite a lot of signal padding, and reamplify it afterwards. This even worse for stomp FX level. So you have the buffer, then some signal padding (or first paddingm then buffer), then your FX device (which also may introduce noise or hiss, if it is not true bypass), then a gain stage to bring signal level up again. And THEN finally you'd have the phase inverter, which AGAIN amplifies hiss and noise.
    In most tube amplifiers the gain of the phase inverter stage is plenty and will drive the power stage early enough into saturation. This btw. is also why many master volumes don't leave much margin for dialing in queiter sound pressure levels, they may allow the power stage to get into saturation maybe on 3 or 4 out of 10 on the dial.
    Back to topic: I'd use the master of the 2205/2210 as a send level, and place a master volume after the recovery stage of the loop.
     
  7. headcrash

    headcrash Well-Known Member

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    There is a recovery stage consisting of 1/2 ECC83. But it is after the master volume... The OP might try and change the tube, maybe he's really having a bad one there, which may improve the noise situation, but it doesn't change what I wrote in my post above.
     
  8. mike blemenschutz

    mike blemenschutz New Member

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    thank you all for your responses. i tried changing the preamp tubes and no change .. the pedal board i use is one in which all effects are true bypass. i am thinking the issue is a flaw with the design of this particular model amp.... i build amps from schematics and wiring diagrams and do modifications of not my own design but from ones with great track records. i am learning every day.
     
  9. Gunner64

    Gunner64 Emotional Support Animal Gold Supporting Member

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    Fwiw my 87 2205 had a hissy loop as well..it wasn't as bad with rack effects compared to pedal effects, but imo was almost unusable either way. I would have to agree, a better design could have been used.

    Also if I remember correctly there is a fix floating around somewhere here on the forum, but I can't recall what was involved.
     
  10. Kelia

    Kelia Well-Known Member

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    Got a 1989 2205 a few days ago and this one has a noisy loop also , the 1988 I had previously
    didn't for some reason .:rolleyes:
     
  11. johan.b

    johan.b Well-Known Member

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    It's a 35 year old design.... put a modern loop in the loop to get levels and impedance match modern equipment and expectancy...
     
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  12. Kelia

    Kelia Well-Known Member

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    Yeah , great idea , I know it will be solved for sure !

    Issues aside, I like what Jason did on his 2205 , great channel and he linked a schematic of this mod
    on his web site and other mods also, very generous person as far of sharing his mods and ideas.

     

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