Mojotone 5F11 build thread

Discussion in 'Shock Brother's DIY Amps' started by AlbertaGriff, Dec 10, 2019.

  1. AlbertaGriff

    AlbertaGriff Tele-Afflicted

    Posts:
    1,236
    Joined:
    Mar 2, 2016
    Location:
    Canada
    Hi all. Before I even begin, I appreciate all the help. This is my first build and I already can tell I will have some dumb questions.

    Generally, I think I am understanding things. I was reading @robrob's site at work today.

    All I have done so far is mount the hardware, full well knowing I might have to remove some in order to make other things easier. I just wanted to get some sort of start.

    20191210_195304.jpg

    My first question relates to grounding. I plan to use a split grounding scheme, and I think it's actually laid out okay in the Mojotone layout provided (pictured).

    20191210_195316.jpg

    I am wondering what the best practice is as far as where to secure my grounds? The kit provides 1 3-lug terminal strip and 2 ground tabs (pictured). Are these worth using? If so, where specifically?

    20191210_195309.jpg

    Thanks again.
     
    Last edited: Dec 10, 2019
    savofenno and JohnnyCrash like this.
  2. AlbertaGriff

    AlbertaGriff Tele-Afflicted

    Posts:
    1,236
    Joined:
    Mar 2, 2016
    Location:
    Canada
    Also, I should note that I do plan to add the bias adjustment pot.
     
    Tonetele, JohnnyCrash and jsnwhite619 like this.
  3. JohnnyCrash

    JohnnyCrash Doctor of Teleocity Gold Supporter

    Posts:
    11,025
    Joined:
    Mar 12, 2005
    Location:
    Fullerton, CA
    I’ve used both the strips and lugs for grounds. As long as the nut is locking (the Mojo kits use locking nuts), I’ve felt comfortable with them.

    Did you get the entire kit, including cabinet and speaker?

    This should be a fun thread to watch!
     
    AlbertaGriff likes this.
  4. AlbertaGriff

    AlbertaGriff Tele-Afflicted

    Posts:
    1,236
    Joined:
    Mar 2, 2016
    Location:
    Canada
    Thanks for that - so where would you put the 3-lug and what would ground to it?

    I did not get the entire kit - I had a 'spare' tweed Vibrolux sized cabinet with a 12" baffle, and I just ordered an Eminence SC64 to go with it.
     
  5. Mark the Moose

    Mark the Moose Tele-Holic

    Age:
    44
    Posts:
    623
    Joined:
    Sep 4, 2018
    Location:
    Erie, CO
    This is a great kit for a fantastic amp, I built one last year. Please report back on the speaker when it’s all said and done!
     
  6. Tonetele

    Tonetele Poster Extraordinaire

    Posts:
    6,792
    Joined:
    Jun 2, 2009
    Location:
    South Australia
    Excuse me but nobody has answered AlbertaGrffi's question yet. That is, what/which to use and where to put them. Not being rude but having carefully reading his question no-one has told him.
    Mark the Moose -you'd be best qualified as you say you've recently made this amp. I could make guesses but won't as the schematic doesn't show the ground locations.
     
    AlbertaGriff likes this.
  7. Mark the Moose

    Mark the Moose Tele-Holic

    Age:
    44
    Posts:
    623
    Joined:
    Sep 4, 2018
    Location:
    Erie, CO
    As a total novice, I’m reticent to say what on SHOULD do but I can say what I did. Ground pin from the power chord goes to a small bolt I put on the chassis. I used the strip for my other grounds, cant remember exactly where I mounted it.
     
    AlbertaGriff likes this.
  8. sds1

    sds1 Tele-Afflicted

    Age:
    43
    Posts:
    1,107
    Joined:
    May 4, 2017
    Location:
    Orlando, FL, USA
    Here's my thoughts:

    - use one of the wire lugs over near the input jack. Run hookup wire to ground the V1A cathode there as well as the input jack. You may have to drill a hole for it. Their layout uses the input jack as the preamp grounding point, I just think this is a bad idea. Input jacks get loose. With my suggestion the input jack can fall out of the chassis and the amp still works.

    - use the other wire lug for a dedicated earth grounding point over near the PT. Give that green earth wire plenty of slack (all of it) so it is the last wire to break should that power cable get yanked out.

    - use the terminal strip to collect your power amp side grounds, PT center taps. Somewhere over there by the PT as well.

    Does that cover all your grounds?
     
  9. AlbertaGriff

    AlbertaGriff Tele-Afflicted

    Posts:
    1,236
    Joined:
    Mar 2, 2016
    Location:
    Canada
    Thanks @sds1 I believe that would work. I'm a little hesitant to drill an extra hole in the chassis, and there aren't any spare screws/bolts to fit.

    I had been planning on using an input jack as a ground, but if the general wisdom advises against that, then I will defer to general wisdom. But this sounds pretty good.
     
    sds1 likes this.
  10. jsnwhite619

    jsnwhite619 Friend of Leo's

    Age:
    35
    Posts:
    2,643
    Joined:
    Sep 10, 2013
    Location:
    Georgia
    Mojo now has a disclaimer not to drill their chassis because it can cause the chrome plating to flake off. There was a thread or two about that happening on here a while back. Which, it's really kind of a cop out for a $100 item -- I guess they got a bad batch of chassis, and now "don't drill or modify because we aren't responsible for the finish". Whatever you do with the terminal strip, remember that you have to connect the outside lugs to the center one if you use the entire strip for a grounding point.

    The two single tabs would be good to use on the power tube socket screws to attach 1 ohm resistors to measure bias.

    I ground the preamp to the input jack. Following Mojo's behind the board grounding scheme for the preamp works fine and is less cluttered inside than adding a ground bus bar.
     
    sds1 and AlbertaGriff like this.
  11. sds1

    sds1 Tele-Afflicted

    Age:
    43
    Posts:
    1,107
    Joined:
    May 4, 2017
    Location:
    Orlando, FL, USA
    Fender used the input jack, @jsnwhite619 uses it. There is plenty good precedent to ignore my advice, especially given the hesitancy to put a drill on that chassis.

    They do make a toothed lock washer for those jacks, it's a Fender part I believe. Another non-critical suggestion. :)
     
    Snfoilhat, King Fan and AlbertaGriff like this.
  12. jsnwhite619

    jsnwhite619 Friend of Leo's

    Age:
    35
    Posts:
    2,643
    Joined:
    Sep 10, 2013
    Location:
    Georgia
    I've definitely drilled and modded my share of chassis. I've been getting them from AmplifiedParts.com lately and haven't had any issues. Even drilling/punching new socket holes. My power cord ground usually is a #8 screw and lock washer drilled through the left side wall of the chassis. Kinda hesitant now though....

    Sent from my moto g(6) using Tapatalk
     
    sds1 and AlbertaGriff like this.
  13. King Fan

    King Fan Friend of Leo's

    Posts:
    4,489
    Joined:
    Jan 1, 2013
    Location:
    Salt Lake City
    This is the fun stuff -- so many choices, lots of ways to skin so many cats, and trying to figure best practices *you can achieve.* Best practice for grounds is to drill separate holes and use separate bolts / keps nuts (or tooth washers) plus loctite. I'd ask Mojo if they still advise against drilling -- there was some thought they'd got the chrome problem fixed.

    I agree with both @sds1 and @jsnwhite619. If you don't want to drill, you could use an input jack with a 3/8" star washer between jack body and chassis at the preamp end.

    The no-drill thing gets worse when we get to the power amp. Yes, many folks mount to a PT bolt, but many pros tell us those can get loose. If you had to do that, I guess I'd try a keps nut above *and* below the solder lug, with loctite and a cherry on top.

    But it gets even worse when you get to the household AC safety earth/ground -- the ground wire from your power cable. Again, some folks drop that to a PT bolt, but now we're not talking amp failure if it comes loose, we're talking possible failure to live.

    So if it's me, and if Mojo says not to drill, I might still drill. But I'd drill that safety ground hole in the side panel of the amp, where chrome peel should, we hope, not be visible. This is where Fender fastened their safety ground wire. I'd use the sharpest cobalt or at least titanium bit with a drop of cutting oil. (If you hate this advice, you have the option to mount to the PT bolt with all the cautions above.)

    79A90B0E-0D6F-4E7E-839F-2895FDFB221B.jpeg

    As a detail, you can see I repurposed ⅔ of a three-lug strip to act as tie point for my white power wire to a PT primary. Here the PT bolt isn't providing ground, and the grounded center lug will be cut off. We're coming back to that white wire in a sec.

    As for grounding, it looks like Mojo may be showing separate ground bolts. I had to look at the left end of their diagram.

    upload_2019-12-11_8-51-27.jpeg

    Ack. They still have the power wiring 'wrong', with the switch in the white / neutral limb. :eek:

    Also, Rob has upgraded his power amp by extending the PT CT to the first filter cap negative pole and running a ground wire back from there to a power amp ground bolt. Honestly, I'd ignore the exact number of input jacks and steal Rob's 5e3 diagram for grounding *and* household AC wiring. Note he does the hot and neutral wiring 'right.'

    Rob_5e3_Updated_Layout copy.png

    Final item: Is there a good hardware store near you? Just cuz you bought a kit doesn't mean you won't need any parts. :) The 'good' ones have a small-parts-bins section where you can get #4, 6, and 8 bolts, keps nuts, star washers (sometimes both inside and outside toothed), and often lots of other good stuff for 5 to 50 cents each.
     
    Last edited: Dec 11, 2019
    Snfoilhat and AlbertaGriff like this.
  14. AlbertaGriff

    AlbertaGriff Tele-Afflicted

    Posts:
    1,236
    Joined:
    Mar 2, 2016
    Location:
    Canada
    Thanks for that @King Fan! A lot of good info.

    From what I've seen, Fender just soldered the electrical ground to the side of the chassis, rather than to a bolt on the side? Am I wrong? Is that just not strong enough in general?

    I might opt to use a ground tab on the lower left transformer bolt for electrical ground, and the 3-lug terminal near the upper right bolt for the power amp ground, and the input jack for the pre amp ground as per the layout. It seems as though this might be an 'okay' solution rather than the absolute best. I do know to keep the electrical ground wire longer than the others.

    Also, I was planning on ignoring the layout in favour of the correct power wiring based on robrobs site.
     
    JohnnyCrash and King Fan like this.
  15. AlbertaGriff

    AlbertaGriff Tele-Afflicted

    Posts:
    1,236
    Joined:
    Mar 2, 2016
    Location:
    Canada
    And yes, there are hardware stores nearby where items like that could be available.

    Leads me to another quick question - on the Mojo 5f11 layout in your post, the two resistors coming off the pilot lamp meet a green wire at a black dot - what is that black dot supposed to represent? Simply a connection?
     
    JohnnyCrash likes this.
  16. King Fan

    King Fan Friend of Leo's

    Posts:
    4,489
    Joined:
    Jan 1, 2013
    Location:
    Salt Lake City
    Probably. Or that’d be another spot for a non-grounded lug on a tag strip to act as a solid tie point. Better yet, if the resistor leads are long enough, they might reach to a ground bolt. That lamp holder looms large in a tweed chassis.


    Sent from my iPad using Tapatalk
     
    JohnnyCrash and AlbertaGriff like this.
  17. King Fan

    King Fan Friend of Leo's

    Posts:
    4,489
    Joined:
    Jan 1, 2013
    Location:
    Salt Lake City
    Yeah, definitely not considered good practice any more. And not easy to do *well* without an industrial iron and a lot of experience.




    Sent from my iPad using Tapatalk
     
    JohnnyCrash and AlbertaGriff like this.
  18. JohnnyCrash

    JohnnyCrash Doctor of Teleocity Gold Supporter

    Posts:
    11,025
    Joined:
    Mar 12, 2005
    Location:
    Fullerton, CA


    Sorry, I’m dealing with a new four month old kid and grading papers for the end of the semester.

    Otherwise I would’ve had there wherewithal to answer the second part of the question.

    In any case, given how many amps we’ve all built here, it was only a matter of time before he got the answer he needed.

    Some of us are repairmen (“techs” according to barely gigging weekend warriors), others EEs, others bench-hardened amateurs... he’s in good hands here.
     
    AlbertaGriff likes this.
  19. JohnnyCrash

    JohnnyCrash Doctor of Teleocity Gold Supporter

    Posts:
    11,025
    Joined:
    Mar 12, 2005
    Location:
    Fullerton, CA


    Yes, it’s simply a connection.

    If I have enough star lug tags, or the lamp is close to a grounding terminal, I simply twist the ends of the resistors together, solder them, and then solder them directly to the ground lug.

    You could solder it to the lamp housing or chassis, but the housing may come loose, on the one hand; on the other hand, chassis soldering is a bad idea.

    It requires a lot of heat to bond the solder with the chassis. It’s common for most builders to not make a solid connection in this way.
     
    AlbertaGriff likes this.
  20. dan40

    dan40 Tele-Afflicted

    Posts:
    1,467
    Joined:
    Aug 19, 2015
    Location:
    Richmond Va
    When you wire the two PT primary wires, one will go to the power switch and the other wire will need to be joined to the neutral wire in your power cord. In many builds, folks will make this neutral connection with a wire nut to join the two wires together. I prefer to mount the 3 terminal tag strip to a PT bolt and then use one of the ungrounded lugs (left or right) as a soldering tie point for the neutral and PT primary wire connection. It looks better than a wire nut and is definitely more solid. The grounded terminal on the strip (center lug) can then be used as a grounding point for power grounds or a ground for the two 100r resistors on the pilot light. With the strip being mounted so close to the pilot light, the center tab makes an excellent tie point for those two resistors.
     
    JohnnyCrash and AlbertaGriff like this.
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.


  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.