Mojotone Deluxe Reverb kit

Discussion in 'Shock Brother's DIY Amps' started by heave1, May 17, 2021.

  1. heave1

    heave1 TDPRI Member

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    I have purchased a MT Deluxe Reverb AB763 amp kit and they don't provide instructions. Would following Stewmac's instructions work on the Mojotone kit? Or Tube Amp Doctor?
    I have checked their layouts and they appear to be almost identical.
    What problems could I possibly run into?
     
  2. OldPup

    OldPup Tele-Meister Ad Free Member

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    I think this is the first video in the Deluxe Reverb build. There are several. I have not watched the series because I have not built this amp. I saw a similar one on the 5E3, which I did build. I would recommend watching the 5E3 build because it is sped up and it covers the basic sequence of when to do what. Hope this helps.
     
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  3. David Barnett

    David Barnett Doctor of Teleocity Ad Free Member

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    I think the Stewmac kit IS the Mojo kit, so the same instructions should work for both.
     
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  4. warrent

    warrent Friend of Leo's

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    As far as I know stewmac just resells the mojotone kit and adds the instructions to the kit.
     
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  5. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Neither and both.
    TAD has the correct wiring for a three wire power cord with a DPDT power switch. Mojo probably supplied a SPST switch. ;)

    Stewmac power cord wiring is not to modern code standards.:(

    Rob Robinette does not give build instructions but his layouts show a correct power cord wiring.:cool:

    TAD and Stewmac do not have a good ground scheme ime. They both have ground loops which can be noisy. I wouldn't use either one. Rob shows a better ground scheme. (The Fender filter capacitor doghouse makes having a good ground scheme difficult.)
     
  6. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    The kit probably does not include shielded wire.
    To insure the sensitive signal wiring does not pick up noise it is beneficial to use shielded wire in several locations.
    Long runs across the chassis, the wires to V1 Pins2 and 7, and V2 Pins2 and 7 are good candidates. The shield should only be grounded at one end.
     
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  7. heave1

    heave1 TDPRI Member

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    I'm trying to decide the best method of grounding using a bus bar like Rob Robinette, or soldering a bus bar to back of pots, which Mojotone recommends. I haven't received the kit yet, but I also noticed the traces on the rear board are wired differently on the layout, than on the video. It also looks like Stewmac runs some of their traces on top of the board instead of on the back board side.
    Andy Johnson at MT told me to use the layout and that the video was only a guideline. He said the layout sheet has been upgraded so it should be easier to determine.
    I have completed 3 successful amp builds and I wanted to try a little more of a challenge with the AB763. I do know I need to get much better reading schematics, but I am working on it.
    Lowerleftcoast, I purchased some shielded coax wire and am planning on using it where possible. Steven at Trinity amp uses shielded wire in his amp builds.
    Is there any problems using standoffs on the eyelet board as regards to noise or tone of the amp?
     
  8. heave1

    heave1 TDPRI Member

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    I also am going to use the glass epoxy eyelet board from Hoffman amps. Any tricks to drilling the holes in the glass epoxy for the standoffs?
     
  9. djbDJB

    djbDJB TDPRI Member

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    Hello, I am sure you will get better/more accurate responses. BUT make sure you research proper safety procedures for drilling those boards!
     
  10. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Rob's way. Though thousands of amps do it, and Marshall did it, soldering the bus to the pots has drawbacks:
    1. It serves no actual purpose. To immobilize the pots, use star washers.
    2. It creates a little ground loop at each pot, which you may not hear but which isn't ideal.
    3. It is hard to do, solder to pots.
    4. Worst of all, it's super-hard to unsolder pots if you ever need to redo or replace.
    Standoffs with an eyelet board are fine, one might say even best practice. I stick my backside traces to the board with RTV silicone so they don't droop.

    The stuff drills like butter. A punch or nailset mark can help make sure the bit doesn't walk. I suppose fiber/epoxy dust is toxic -- wear a mask. (You should be able to find one somewhere.) :):):)
     
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  11. theprofessor

    theprofessor Poster Extraordinaire

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    Great questions and great answers, all. OP: start a build thread and you'll get a lot of guidance, bit-by-bit. You'll probably get more than you bargain for and will have to decide whose advice to take. But it's better than just trying to copy stuff from several layouts -- all of which have some kind of less-than-ideal stuff going on. And you'll learn a lot in the process. At the beginning of your build thread (maybe this is it?), post all the documentation you have at the moment. I am not an expert, but I recently built a Vibrolux Reverb successfully, which is pretty much the same as what you're building, with a few minor differences. It'll be fun. Just be careful and patient. And patient. It'll get done, and you'll miss the process. But the good news is, you'll have a great amp!
     
  12. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    I would use a standoff with just enough distance to keep the board safe from shorting to the chassis. Reducing the amount of wire and keeping wire from acting as an antenna should be in the back of your mind while assembling your amp. Tall standoffs will make for more wire and wire in the air.

    Imo a ground bus wire (or just a wire terminating at the input jack and each of the ground eyelets on the board) is adequate. Terminating on the back of the pots is not suggested as King Fan pointed out.

    Some Shock Brothers route all wire above the board because lifting the board is a pita when making mods or fixing problems. Others want it to look like an original amp. This is your amp and decisions have to be made. When I build an amp, my top priorities are reliability and reducing noise. I try to keep it looking similar to the original layout because I like vintage stuff. YMMV.

    I modified Robs layout to illustrate a ground scheme with a wire replacing the bus. I also show an alternate way of wiring the grounds from the pots. A further *improvement* could be made by lifting the chassis connection of *the power amp ground* terminal. The ground scheme would then have just one reference to ground at the input jacks.

    InkedAB763_Deluxe_Reverb_Layout_DIYLC_small_LI.jpg
     
  13. Greg70

    Greg70 Tele-Meister

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    I just finished a Mojotone Princeton Reverb and I soldered the ground bus between the pots like MT says to do. Their layout connects a lot of things to the bus so it made sense to me. Plus it was my first build so I didn't want to stray too much from their layout if I had to troubleshoot. I have some old school soldering flux that wets solder onto anything and it was relatively easy to solder to. I also left some slack wire, almost like a flat horseshoe, between each pot. That way if I ever have to change a pot I can just unsolder that pot and there will be enough slack to back it out of the chassis hole. My amp is super quiet, although it does have some hum on the reverb circuit.
     
  14. Greg70

    Greg70 Tele-Meister

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    The back of the board looks cleaner and more original Fenderish. I built my Princeton with it on the back of the board. I just made sure to double check all of the traces beforehand and took some high quality photos of them for reference before screwing the board down. We all know what a PITA it is to lift it up to access those!
     
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  15. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    If someone's already run the bus on the pots, no harm no foul. But he's asking beforehand, so lemme say it's not just my opinion. Merlin Blencowe lumps it in with a bunch of common things 'everybody does' that aren't best practice:

    "Vintage amps often inadvertently created ground loops by using non-insulating jack sockets, bending over the ground tab on a potentiometer and soldering it to the case, soldering a ground bus to the backs of the control pots, or using the chassis as a more-or-less ‘random ground’. Many amplifiers got away with this without gross hum, but it is still bad engineering no matter how convenient it may appear."

    But reading that gives us another reason not to feel bad; at least two of Merlin's other minor sins are somewhere from 'more common' to 'almost universal.'
     
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  16. Greg70

    Greg70 Tele-Meister

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    Okay, I see what you're saying now about the ground loops, and I agree that it's best to do it the best way from the start. Just because "that's how they used to make them" doesn't mean that they were doing it right at the time. My JCM800 has the PCB mounted pots on it and there is no ground bus between them. I'm guessing it was the earlier Marshalls that did this.

    Another example would be the "original sin" of Fender's SPST power switch. You gave me great advice on my build to upgrade the switch.
     
  17. heave1

    heave1 TDPRI Member

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  18. heave1

    heave1 TDPRI Member

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    What do you mean by "lifting" the chassis connection of the power amp ground terminal? Do you mean like using a terminal strip?
     
  19. schmee

    schmee Doctor of Teleocity Ad Free Member

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  20. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    A terminal strip would accomplish it. The terminal where the Center Taps, 6V6 Pin8's, Bias board, Bias pot resistor, and the negative side of the filter caps are gathered would not be electrically attached to the chassis. Then the only reference to ground would be made at the input jack.
    Theoretically this arrangement should result in less noise. The ground scheme of the Deluxe Reverb, with the long runs to the filter capacitor doghouse, is not optimal so the theory may not always pan out. Rob has found on *some* builds grounding at both the input jack and near the power transformer has resulted in less noise. It would be very easy to test both versions with a terminal strip in place. Just try it with a clip to chassis vs no clip to chassis.
     
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