5e3 Mojotone Build - First Time Builder

Discussion in 'Shock Brother's DIY Amps' started by Pick_n_Strum, Nov 9, 2019.

  1. Pick_n_Strum

    Pick_n_Strum TDPRI Member

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    So just starting to put a Mojotone 5e3 kit together. I'm loving it so far but will have questions along the way. Thought I'd post here along the way for critique and learning experiences. The below photographs are pretty much week 1 - working on it when I can. I realize there is definitely an art to making the turret board look nice and clean with all the components - I have a lot to learn there.

    IMG_20191109_094341821.jpg

    I tried to get some better close-ups - definitely let me know if you see something wrong...Depending on when I get time, I'll try to take better pictures at various points in the build - unfortunately, these are just cell phone shots for now.


    IMG_20191109_093946162.jpg IMG_20191109_093935938.jpg


    I'm not the best solderer - especially on pots - but I'm definitely better than I was (coming over from DIYing pedals over the last few years). I will be checking continuity where possible throughout the build. As bad as these globs look, the ground bus is tight on all the pots and I have good continuity.


    IMG_20191109_094651033.jpg
     
  2. monkeybanana

    monkeybanana Tele-Meister

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    mmhmm
    I would reheat those blobs. Although there is continuity the bus bar may pop off. Heat up the pot real nice and pre tin then solder the component. On some pots a little sand paper helps. I think those Mojo pots work well without sanding.

    Also, are you using really thin solder wire? Getting slightly larger wire may help. And for me referencing gut shots of old Fenders gave me a good idea of what the solder joints should look like.
     
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  3. muchxs

    muchxs Doctor of Teleocity

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    My what large capacitors you have.

    :cool:

    716Ps @ 400v are a little more svelte.

    I don't solder anything to the back of the pots. If I did...

    Try my "two fisting" soldering method. I have two 35 watt soldering irons. I make a blob with Iron #1 and then poke the other side of the connection simultaneously with Iron #2. That gives me an instant 70 watts without having to warm up my soldering gun or my WMD I don't know how many watts it is but is sure is big soldering iron.
     
  4. King Fan

    King Fan Poster Extraordinaire

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    Great amp, enjoy the build.

    Be sure to read as many other build threads as you can, especially Mojo kit builds. They're good kits, but lots of specific little things come up during the build process.

    Your photos are decent. A modern cell phone will take fine pics. Try lighting 'em with at least two lamps, one from each side, plus diffuse overhead light -- flash is too harsh. Also board and chassis shots are most useful in horizontal / landscape, with PT left, inputs right. Consistent, easy to read, looks more like layout drawings.

    Speaking of layouts, Mojo's is OK, but Rob's is better in several important details. Also make sure to work as much as you can from the schematic -- much more specific about how the circuit actually connects.

    5e3_Deluxe_Amp_Layout.png

    Wise words. The pot-mounted bus seen in billions of vintage amps is electrically unneeded, unwise, (Google "Valve Wizard ground schemes"), a royal pain to solder well, and worse to unsolder.
     
    Last edited: Nov 9, 2019
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  5. boredguy6060

    boredguy6060 Poster Extraordinaire

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    Your soldering iron is way too cold, you need an iron that will heat the back of that pot in about 5-6 seconds, enough to melt enough solder to do what you’re trying to do there.
    The ideal method is to get in and out quickly. If your iron is too cold then you have to stay on the pot too long, allowing heat to spread all over. A nice hot iron will heat the desired area in a few seconds and be done with it. A hot iron isn’t expensive. I have an old Stahl that I bought at Radio Shack for about $12.00.
    A lot of these new soldering stations as their called won’t get hot enough to get in and out fast enough to keep from burning up the pot.
    Good luck,
     
  6. Tommyd55

    Tommyd55 Tele-Holic

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    ^^^^^^
    This ..
     
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  7. BobSmith

    BobSmith Tele-Meister

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    Agree with avoiding pot grounding bus bar. I just checked the Mojotone directions and it looks like they bend the pot terminal to the pot casing rather than using a bus wire. That’s definitely an improvement for serviceability. Many others here will do some kind of separate bus bar (avoids too many paths to ground as each pot is electrically grounded to chassis.)

    oh, side comment...your table top is too nice for amp building! :)
     
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  8. Pick_n_Strum

    Pick_n_Strum TDPRI Member

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    I'm definitely going to switch my pot wiring over to the Rob Rob setup. I've looked at that wiring diagram a lot but but didn't pay a lot of attention to the pot wiring oddly enough. That way I get rid of the globs. I'm definitely wiring my standby switch the Rob Rob method as well.

    I did sand the pots and used fairly thick solder wire but think I must need to turn up the heat on my soldering station like boredman said. I had plenty of room to increase heat on my station but was always under the impression to use as little heat as possible. These are all great tips however.

    These tips will help me get better at it for guitar wiring too. Side question: I assume you write to the back of pots in your guitars, right?
     
    Last edited: Nov 9, 2019
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  9. boredguy6060

    boredguy6060 Poster Extraordinaire

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    I’ve never grounded anything to a pot in an amp, I have done it many times when wiring guitars.
    The guy who build amps on StewMac videos solders a wire to the back of the pots, but I’m not sure if it’s for grounding.
    The only thing he says about it is that it’s to keep a pot from spinning around in the event that the mounting nut comes loose.
     
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  10. dan40

    dan40 Tele-Afflicted

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    I like to set my iron around 700 degrees for most jobs. It needs to be good and hot so that you can get on and off the joint quickly. Having a larger chisel tip also helps because a thinner pencil tip just can't transfer enough heat fast enough. It's also helpful to melt plenty of fresh solder onto the tip before starting the joint. You almost want to see a small blob of solder hanging from the tip when you go in to do a joint. This creates a solder bridge between the iron's tip and the item to be soldered and helps to transfer the iron's heat into the part much quicker.
     
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  11. Pick_n_Strum

    Pick_n_Strum TDPRI Member

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    Thanks for the tip. I was definitely too cold. I think initially my iron was in the low 600s when I soldered the pots.

    I got the pots unsoldered and wired up based on Rob Rob's method. I'll get more pics up when I get a bit further.
     
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  12. King Fan

    King Fan Poster Extraordinaire

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    Good job on wanting to 'do it right.' I see you've done some good stuff on your board -- all the resistor tolerance bands seem to point the same way, which (minor deal) looks nice but also (bigger deal) makes 'em easier to read. And 2 of 3 filter caps are rotated so you can read their values (same deals).

    We don't want leads to be bowstring taut (a right-angle bend at the ends will prevent that) but some straightening and alignment never hurt. Mojo's own example is close enough for government work...

    upload_2019-11-10_8-46-41.jpeg
     
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  13. BobSmith

    BobSmith Tele-Meister

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    Looks like you are having some problems with chrome peeling? That is not only a cosmetic problem but potentially a grounding problem.

    I think you said this was a Mojotone kit. When did you get it? I was having some problems with them, but they got me a new one in July timeframe that was much better.
     
  14. Pick_n_Strum

    Pick_n_Strum TDPRI Member

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    Possibly...I just got the kit from mojo about a week ago. The pots don't seem to be losing their chrome (unless I'm misinterpreting chrome peeling). I unsoldered the ground wire on the pots and am going with the Rob Rob method.

    Thanks! I did intentionally put all the resistors the same way but stupidly enough, didn't even think about it with the caps! I was just double-checking everything as I placed it on the board.

    I'm at a point now where I'm trying to get all my ducks in a row for when it comes time to test (still a ways out yet for me). I have a much better meter on the way; the 2 I currently have aren't as trustworthy as I'd like when getting into some of those large voltage tests.

    On to some questions:

    ----------------------------------------

    I'm going to be making a capacitor discharge probe via these instructions:

    https://www.ifixit.com/Guide/Constructing+a+Capacitor+Discharge+Tool/2177

    I'm going to solder an insulated alligator clip to one end of the probe so I can attach it to the chassis.

    1.) Just want to check - is 12awg and a 20k resistor overkill for discharging capacitors I'll be encountering? I understand that I'll need at least the 600v rating for the wire and 5 watt rating for the resistor no matter the value.

    ----------------------------------------

    In looking at other build threads as suggested by King Fan, I have decided that instead of doing a standby switch I wanted to either do a master volume or 3-way negative feedback switch (both as suggested by RobRob). Instead of deciding, I'm going to do both. They both seem straightforward and doable for me. I'm going to put the 3-way switch in the exact location that is suggested. Big thing is I'll have to be aware of the potential phase issue and remember that I can switch the OT wires.

    For the master volume, I'm going to do the trainwreck type-3 push/pull.

    2.) This is going to be in the standby slot so I plan on using shielded cable since it will be traveling across the chassis. Would this mogami cable (https://tubedepot.com/products/mogami-w2524-instrument-cable) be sufficient? Would I just ground the copper shield portion to any ground location on the chassis or busses?
     
  15. Junior Little

    Junior Little Tele-Meister

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    Just a couple suggestions. In addition to the cap discharge tool, make yourself a current limiter (aka lightbulb limiter). Search the internet (or this forum) for construction details. Super easy and far less expensive than a trip to the ER.
    Personally, I think it's a better idea to build the amp stock, get a feel for it and then do modifications. Of course, that's your call.
    Anyway, you're in good hands here and keep up the good work!
     
    Last edited: Nov 11, 2019
  16. Pick_n_Strum

    Pick_n_Strum TDPRI Member

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    Thank you - will do. Looks like a fun side project.

    Thought I'd do a quick summary of my questions since my post above was so wordy:

    1. Regarding this capacitor discharge probe,
    https://www.ifixit.com/Guide/Constructing+a+Capacitor+Discharge+Tool/2177
    - is 12awg and a 20k resistor overkill for discharging the capacitors I'll be encountering?


    2. I'm going to add the trainwreck type-3 push/pull master volume in the standby slot. Would this mogami cable (https://tubedepot.com/products/mogami-w2524-instrument-cable) be sufficient for running across the chassis?


    EDIT: Just found this written in the rob rob write-up: "Only ground one end of any shielded cable in an amp, preferably the signal input end, to keep from forming a ground loop. If you keep your wire runs short and away from the power transformer end of the chassis you'll probably be fine with non-shielded wire." So that part is answered.
     
    Last edited: Nov 12, 2019
  17. BobSmith

    BobSmith Tele-Meister

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    The place I was wondering about was on bright channel 1 jack front panel. Maybe it’s the reflection in the photo. If it is peeling I would mention to Mojotone.
     
  18. Pick_n_Strum

    Pick_n_Strum TDPRI Member

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    Oh! I see what you're saying. No I actually bought a used Weber chassis off Reverb. Even a couple of the holes for the jacks and switches were drilled larger by a previous owner. The only thing from mojo is the "small parts kit." Thanks for the heads-up though; I appreciate that.
     
  19. Pick_n_Strum

    Pick_n_Strum TDPRI Member

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    So I've been wiring up slowly but surely - repetitively checking continuity as I go.

    Against your guys' warnings and my better judgement, I did decide to take on the mods as I'm building. This had a few logistical challenges (where to put toggle for negative feedback, where to route shielded wire). So on to the photos:

    Master volume pot - I had to add sheet metal because the former owner of the chassis drilled a huge hole in the standby slot:

    [​IMG]




    SPDT from the outside - for the negative feedback switch:

    [​IMG]



    SPDT from the inside and most of the board - I went with shielded wire for both the SPDT and master volume - some of that will be routed back underneath the board for neatness-sake (I definitely need to work on neatness!):

    [​IMG]



    One really stupid thing I did was drill the master volume hole with the transformer in (I know I know, I've been scolding myself) - I ended up nicking the insulation on one of the red high voltage secondary wires of the PT. I'm so mad at myself. I put 2 layers of heat shrink on it, both layers rated for 600v but it would have been much smarter to just pull the transformer out prior to drilling. Lesson learned:

    [​IMG]



    Please go easy on me :cry:
     
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  20. muchxs

    muchxs Doctor of Teleocity

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    If you do this you'll only do it once:

    Imagine you bought brand spankin' new Weber chassis, one of the nice ones when they welded the ends. Doesn't matter if it's a newer stainless chassis or the older chrome chassis. Now imagine flipping your totally naked chassis over and drilling a few pilot holes underneath. You'll only do this once because the chips scratch the crap outta the formerly pristine shiny surface of the faceplate.

    Say you learned your lesson. You stick a layer of masking tape to the show side of the control panel. Still scratch the control panel. You need to mask both sides of the control panel so chips don't sneak through the holes.
     
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