Mojotone Effects Loop V2 sounds terrible

Discussion in 'Building the Classics' started by Bruce_W, Apr 30, 2020.

  1. Bruce_W

    Bruce_W New Member

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    Hi, I have a 2204 build that I am trying to install this loop in. I have seen one other thread about this loop and I have the same problem. Terrible clipping going on, it sounds bad. I have emailed Mojotone and they are telling me I can’t return it since I did install it. They had a tech send me a suggestion about lowering the supply voltage which didn’t change anything. Does anyone have any experience installing this loop?
     
  2. BadgerO

    BadgerO Active Member

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    I installed one on a bandmaster for a friend a few years ago, worked fine. They come with trimpots for input and output. I replaced them with chassis mounted pots. I assume it's before the phase inverter, getting what comes out of the treble pot.
    If you have a scope, see where is this clipping action happening. Is the LND150 transistors on the FX loop or is the FX loop pushing the phase inverter into overdrive?
    If it's the FX loop clipping, try a 1M pot on its input (from treble pot to the centre log, one log goes to ground, the other to the FX loop). See if lowering the input fixes it. You'd want those pots on the back permanently. You'll be adjusting them often.
    If I remember correctly, they also had a resistor in series with the signal right at the front, try increasing the value of that too if it's there.
    I used shielded wire all around (ground only one end of each wire's shield, heat shrink the other end so it doesn't touch anything) otherwise it picks up AC noise from the filament supply.

    I hope it helps
     
  3. Matt_Krush

    Matt_Krush Well-Known Member

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    Bruce, throw it away...or re-purpose the parts

    I have 2 in a box....since After I called them they just sent me second.
    I'm just going to send them back anyway...and, Yes...it was my thread you refer to.

    Get the one from Metropolis. Worked perfect..no messing around.
    Yep, we lost $70.
    Get over it and build your amp.

    I spent 2 months changing supply voltage, dropping input signal...all kinds of stuff.
    Nothing worked.

    Perhaps BadgerO has one of a different design from the current ones.
     
  4. Bruce_W

    Bruce_W New Member

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    thanks for the reply. Well maybe these 2 threads will keep others from making the same mistake. What about the holes you drilled? Did the metro loop fit in those holes?
     
  5. Matt_Krush

    Matt_Krush Well-Known Member

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    yes it did, except for the on/off switch that Mojo has.
    I ended up buying a little black plug to put in the hole.

    The Metro loop, has a dip switch used to go from -10db to +4db (instrument to line level).
    I ended up removing that from the board and jumpering to a mini-toggle switch that I used to fill the hole for the Mojo return level pot.
    I felt that I'd likely use that as I still have and use rack gear for effects.

    I did not use a return level pot for the Metro loop, I could have...but my uses dont require it.
    My delay has a gain and so does my EQ, so I can adjust levels there to give a solo boost when my lead effects are activated....same if I use rack gear.

    Its all what YOU need and how YOU use it that makes something right.
     
  6. Exojam

    Exojam Well-Known Member

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    What is your voltage on the B+ pad of the loop?
     
  7. Matt_Krush

    Matt_Krush Well-Known Member

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    I went from 230 to 320 (or whatever it's max recommended was).
    I tried all kinds.
    Signal voltage from treble pot started at 25vrms, I knocked all the way down to 1volt rms and it still sounded like it was clipped.

    Trust me, these are just bad design.
    Either they redesigned them at some point or had crapload of incorrectly assembled parts.

    my problem is that no one at Mojotone could answer any technical questions about the loop.
    Whats the min/max B+ voltage it can handle/
    Whats the min/max signal voltage it could handle before distortion?

    If they'd even have a schematic, I could calculate what it should be...but they don't even have that.
     
  8. Exojam

    Exojam Well-Known Member

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  9. 66 Kicks

    66 Kicks Member

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    Here is a schematic for version 1.5. The picture they have of version 2 looks like the same circuit to me.

    Mojo Fx Loop #2.gif
     
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  10. Exojam

    Exojam Well-Known Member

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    There are schematic’s in the pdf I linked to along with required voltages for the loop.
     
  11. 66 Kicks

    66 Kicks Member

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    Not of the actual loop itself.
     
  12. Exojam

    Exojam Well-Known Member

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    Ahhh, true!

    Sorry for the duck up
     
  13. Matt_Krush

    Matt_Krush Well-Known Member

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    funny, I asked Mojo and they wouldnt give me this.
     
    Last edited: May 4, 2020
  14. Bruce_W

    Bruce_W New Member

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    I did pretty much the same thing with the same results. It always adds the weird clipping that just makes it unusable. One thing I noticed is that once you go below a certain voltage (like 120v) the tone controls of the amp become unresponsive. Not that that has anything to do with it I was just curious as to why. Mojotone did do an RMA for me so that was cool. I bought the Metro loop.
     
    Last edited: May 2, 2020
  15. Matt_Krush

    Matt_Krush Well-Known Member

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    I do want to say that I like Mojotone and buy a fair amount of product from them.
    They've always helped and made things right (when they could).

    sent wrong face plate for an amp...no problem, sent another.
    The effects loop, I called them, they sent another....(it's apparent that they don't design or build these)

    am not trashing them as a company, the effects loop (or a batch of them) do not work.
     
  16. Bruce_W

    Bruce_W New Member

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    Yup, I would agree with all of that
     
  17. 66 Kicks

    66 Kicks Member

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    I don't know how such poorly designed loops came to be, but the earliest one I could find said it was drawn by some guy named Michael J. Soldano, Jr. and copyrighted 2011. This is hands down the stupidest commercially available FET loop I have ever seen. Jet City used to sell it for about the cost of shipping.

    Jet City Loop.gif
     
  18. 66 Kicks

    66 Kicks Member

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    I've been buying from Mojotone since the early nineties when they were in Napa, Ca. and I don't have anything bad to say about them. I don't think that Mojotone had anything to do with the design of that loop and they probably don't have anyone there that knows what is wrong or how to fix it.

    The Send portion of the loop should at least be able to swing the signal that is encountered at its intended use. The Source Follower can only swing 25Vp before clipping with 250V on its Drain. The signal at the Treble wiper can get far larger than this. You can put the optional Send trim in to avoid clipping the Source Follower, but I would have designed it to handle the largest voltage that the end user could possibly put on it including if they turn the amp clear up and also max the optional Send trim.
     
  19. 66 Kicks

    66 Kicks Member

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    The biggest problem is the Return circuit. The LND150 doesn't work right when using the same value components as a Common Cathode 12AX7 gain stage. In fact it doesn't work right when configured the same as a Common Cathode gain stage no matter what the component values are. It winds up either being biased too cold or having way too much gain for the Return circuit. Cutting the return signal down with a voltage divider brings up the next problem. The reverse transfer capacitance is way higher than advertised and the Miller effect becomes a problem with voltage dividers on the Gate. This includes variable voltage dividers such as a Return Level pot.

    The LND150 does work very well when used in a cascode configuration and the Miller problem goes away. This one has a 470K/470K voltage divider, but it could just as well have a Return Level trim.

    FX Loop 66K Cascode.gif
     
    Last edited: May 3, 2020
  20. 66 Kicks

    66 Kicks Member

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    Another approach to the bias problem is to use a fixed Gate voltage. This allows you to use a larger Source resistor for some degenerative feedback and a more linear response. The Source resistor and the fixed Gate voltage can be chosen for the desired gain so that you don't need a voltage divider at the Gate.

    FX Loop 66K Fixed Bias.gif
     

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