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Mojotone 5F1 Champ Grounding Poll

Discussion in 'Shock Brother's DIY Amps' started by DavesPlanet, Nov 29, 2017.

  1. DavesPlanet

    DavesPlanet TDPRI Member

    35
    Oct 18, 2013
    Bangkok
    In Mojotone's documentation, a photo (A) shows eyelets 1&2 connected together then wired to ground, and eyelets 3&4 are connected together, then to ground.
    However, their layout picture (B), shows all 4 eyelets connected together then wired to ground.
    Please just vote for "A" or "B." I'm a complete beginner. I will do it one of these two ways. Though I've learned there are many other ways to possibly do it, I'm not interested in them at this point. I just want to decide on one way and finish this build in my lifetime. ;-) Thx.

    4 eyelets.png
     

  2. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Hi Daves Planet. Sorry to be "that guy," but I really don't like some of Mojo's grounding stuff. I think it's easiest to do what I said back in your other thread when I said "do it like this." You certainly don't have to, but that's my opinion!

    http://www.tdpri.com/threads/mojotone-5f1-champ-cap-grounding-for-dummies.784621/#post-8008093

    This is @robrob's 5f1 layout that I was referring to.
    5F1_Layout_Robrob.png

    And I know that a 5f2a is not a 5f1, but it's darn close. So if they're helpful, here are some pictures of my own 5f2a build and board. This is hardly the only way to do it, but it works well. There are a few things that would be different from a 5f1, but not many. Here's what I did:

    BACK
    board_FINAL_back.JPG

    I think it's best to see the front in place in the chassis. I'll comment on some things and maybe find a better, detailed picture as well.

    Let's move from left to right.
    (1) See the first filter cap? The positive side of that comes off and goes to the rectifier.
    (2) In the 5f2a, the first and second filter caps are in parallel (that is, both of them = B+1). See the second filter cap? On the positive side, you've got the OT primary attached (it was red according to the schematic, but the schematic was wrong in my case; I had to flip them so that the blue one is going to the second filter cap of B+1 and the red one is going to the 6V6, instead of vice-versa). It's hard to see, but there's a black wire coming off the ground (negative) side of that second filter cap (the eyelet is under the tone control). It goes over to the left, where it joins the red-and-yellow wire (the power transformer's high-tension winding's center tap), where both are grounded together to the chassis. Since the first, second, and third filter caps are jumpered together on the ground side (again, refer to back of board pic above), the third filter cap (= B+2) is grounded by the second one being grounded.
    (3) Now move to the fourth filter cap (= B+3), which is the preamp filter cap. See how I have a bus bar running above it, all the way over to the far input on the right (input 2)? There are a lot of things grounded to this. Skip down to the next picture, where you can see this better.

    IMG_9736.JPG

    You'll see below several things attached to that preamp bus: (1) the Orange Drop from the tone pot ground lug, which is the first black (heat shrink-covered) wire on the very left of the preamp bus. The cap itself is hiding underneath my tone pot (2) then there's the ground lug from the volume pot (3) then there's the eyelet that the last filter capacitor and the carbon composition resistor share (4) then there's the eyelet shared by the Xicon cathode resistor, the Sprague cathode bypass cap, and the carbon comp resistor (5) finally, there's the top eyelet on the right-hand side of the board, which is the ground side of the cathode resistor and cathode bypass cap for 12ax7-triode B. All that goes up to the ground lug of the input 2 jack. Input 1 does not have to be grounded separately, since it is jumpered over to input 2, so it is grounded through input 2.
    IMG_9523.JPG
     
    Last edited: Nov 29, 2017

  3. itsGiusto

    itsGiusto TDPRI Member

    Age:
    29
    54
    Dec 31, 2017
    02141
    I'm starting a 5f1 build now myself, and I'm wondering about having a shared preamp/poweramp ground bus vs a separate one.


    @theprofessor the diagram you posted there, the @robrob layout, seems significantly more complicated than the mojotone or Byoc layouts. I would have to include a ground wire for the poweramp bus from C3 over to where R9 is, because R9, R8 and C6 are a part of the poweramp section. In addition, R6 can no longer be connected in that same ground point as R8 and R9. I can see the logic in this, I guess, but it will make the wiring significantly more messy, and I'd like to know if it really benefits me in any way. Why have separate poweramp and preamp ground busses if they're just going to get grounded together on the chassis anyway? Why not just keep the busses together? Does it help prevent noise to have separate busses?

    I've also seen layouts that have separate preamp and poweramp busses, but ground R8, R9, and C6 on the preamp bus. Is this advisable as a way to reduce messy wiring?
    Thanks!
     

  4. DavesPlanet

    DavesPlanet TDPRI Member

    35
    Oct 18, 2013
    Bangkok
    When I built my mojotone 5f1 a few weeks ago, I posted in a couple forums asking about grounding. This kind of opened a can of worms as all the guys who know this stuff like the back of their hand gave their opinions on how it should be done. They started referring me to whole books on the subject of grounding...making it seem like there was even disagreement among experts as to 'best practices.' It did become clear that my old notion of 'ground is ground' does not apply to tube amps. While their reasoning was undoubtedly sound, as a total newbie, I couldn't really put it into practice.
    I didn't (for example), even know where the 'preamp section' ended and the 'power section' started (I still don't). One message kind of got through, I guess: don't ground everything to one point. Many also thought that, for a low gain amp like the champ, being particular about how it's grounded doesn't make as much difference as in, say, a high gain amp.
    Anyhow, here's how I ended up doing mine. Many of the experienced guys would probably laugh at it. But hey, it works, and it gave me the experience of building my first tube amp. Besides the 50-cycle hum of my building (I live in Thailand), who knows, maybe if I re-did the grounding it would be quieter. It's pretty quiet as is. I actually don't know how much I'll use it, in the end. I have other tube amps and mainly did this for the experience. I may eventually sell it. ;-) my grounds explained.png
     

  5. motor_city_tele

    motor_city_tele Tele-Afflicted

    Apr 25, 2005
    motor city
    1 & 2 to the PT mounting bolt.
    3 & 4 to the Omnibus.
     

  6. jsnwhite619

    jsnwhite619 Tele-Afflicted Ad Free Member

    Sep 10, 2013
    Georgia
    To pick one I'll say A. But I would run it with one more behind the board connection to the V1 bias resistor (the 1.5k on the far right) and ground it all to the input jack.
     

  7. Viejo

    Viejo Tele-Holic

    805
    Feb 12, 2011
    Southern IL
    I would use RobRob's design . Keep the preamp ground isolated from the power tube grounds.
     
    zook and JuneauMike like this.

  8. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    I'm not really qualified to tell you the "whys" or the "best practices" in terms of the differences between grounding schemes. The split grounds (power section | preamp section) make sense to me on a logical level, in terms of keeping two "parts" of the amp separated. Plus, it's what's on @robrob 's 5f1 layout. The narrative of @DavesPlanet immediately above, where he says he was inundated with different opinions and drawn into controversy, is exactly why I said in my first post on his companion thread: "just do it this way." (http://www.tdpri.com/threads/mojotone-5f1-champ-cap-grounding-for-dummies.784621/#post-8008093)

    You can see for yourself, but I don't think the split bus system is too messy:
    IMG_9736.JPG IMG_9523.JPG
     

  9. itsGiusto

    itsGiusto TDPRI Member

    Age:
    29
    54
    Dec 31, 2017
    02141
    Well, I'm not looking to make extra work for myself without cause. If no one can justify a good reason why separate busses make sense, I'm not going to bother.

    I could understand this more if every ground weren't just all connected to the chassis in the end anyway. Like if there were an actual lifted ground. There are some people recommending to do this for the speaker, for example, to not have the ground of the speaker jack connected to the chassis. But I don't really buy the need to have separate grounds if they all get connected to the same point in the end anyway. Maybe I'm wrong, but in order for me to see why, I'd need someone to make a convincing explanation.
     
    Last edited: Dec 31, 2017

  10. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    Whatever you want to do. Ground it however you want. It's just a 5f1, so unless you're trying to do something crazy, nothing's going to be much work anyway. And if you have separate grounds, they don't get connected to the same point in the end, as you seem to suppose (unless you are saying that "somewhere on the chassis" is the ultimately the same point).
     

  11. hackworth1

    hackworth1 Friend of Leo's Vendor Member

    Ground Power Side on the power side and signal side on the signal side. Typically, the third filter cap is dedicated to the signal section.

    The first filter cap is subjected to the greatest ripple current noise. The second filter cap is dedicated to the screen grid of the power tube. Put these and the power transformer HT center tap and the Filament center tap to ground on the chassis side.

    Refer to Rob's diagram.
     

  12. Viejo

    Viejo Tele-Holic

    805
    Feb 12, 2011
    Southern IL
    It was explained to me years ago that there is a potential for the ripple from the main filters to modulate the signal if the grounds aren't isolated. I don't know if that is true but it is simple enough to separate them
     

  13. itsGiusto

    itsGiusto TDPRI Member

    Age:
    29
    54
    Dec 31, 2017
    02141
    It's easy enough to separate them, except the third filter cap.

    But moreover, I just hate the voodoo-esque notions around internet forums that seem to go against what I'm to understand the scientific consensus is about how electricity works. For example, the claim that having two ground busses which are both electrically connected via the chassis is any different from having one ground bus.

    It's very very possible that my understanding of the science of how grounds work is incorrect, and if that's the case, that's totally cool and I'd change my way of thinking about electricity - I like learning the truth of how things actually work and changing my views accordingly. But unless someone actually makes a convincing case that my understanding is flawed, pinpoints why my understanding is flawed, and tells me how I can correct my understanding, I cannot believe the claim.
     

  14. Viejo

    Viejo Tele-Holic

    805
    Feb 12, 2011
    Southern IL
    Well if you have the time try it different ways then and let us all know if there is some effect. Try one star ground for everything. Try it like Fender with the screens grounded with the preamp.
     

  15. jsnwhite619

    jsnwhite619 Tele-Afflicted Ad Free Member

    Sep 10, 2013
    Georgia
    I've figured out that everything I have ever seen or read online about boiled linseed oil is wrong. Everyone either didn't know how to work it and judge how to work it, or was looking for a six-hour finish with something that takes 6 weeks to do. I've had awesome results with linseed oil that the entire internet said couldn't be done.
    Unless you isolate the knobs, the switches, and the jacks, the ground points don't matter that much when you have six knobs that are grounded in part of the circuit plus two output jacks plus four input jacks. I love seeing a ground bus in amp that is grounded from every switch and Jack and knob that is already grounded to the chassis by the nut that holds it down.
     

  16. jsnwhite619

    jsnwhite619 Tele-Afflicted Ad Free Member

    Sep 10, 2013
    Georgia
    The point of the oil story is that this shouldn't be possible. 1514798789811.jpg
     

  17. theprofessor

    theprofessor Friend of Leo's

    Aug 8, 2016
    Chattanooga, TN
    There's a lot of gusto here claiming "voodoo" or that people "can't explain" to you the grounding in some adequate way. Of course it's true that the Internet is a purveyor of many falsehoods and that there is a lot of "group think" going on. And you are more than welcome to do the grounding in any way you please. But it's a bit presumptuous to imagine that just because no one has given you a satisfactory answer to this point means that no one here can or that their views are just "voodoo" and have no part in "science."

    Different folks here have different goals. I was simply offering a system that worked, not one that satisfied my understanding of grounding theory. That's in large part because I don't have a theory! :) But that doesn't mean that everyone here is like me and simply wants the amp to work quietly and safely (I don't need any more than that out of a self-built amp). But if you find the right niche of folks to converse with, I'm sure you'll find more than enough food for thought and be caught in a maelstrom of data re: grounding schemes that will either make you exceedingly happy or possibly even lead you to frustration.
     

  18. zook

    zook Friend of Leo's

    Aug 6, 2003
    Cochise, AZ
    As it has been previously mentioned several times, Ground the power amp filter caps and the cathode resistor for the power tubes together. The preamp filter cap and the rest of the grounds should be grounded together. I put the two grounding points at opposite ends of the chassis ans a rule.
     

  19. kleydejong

    kleydejong Tele-Holic

    591
    Aug 30, 2010
    Orange City, IA
    I really like Rob's grounding method as an initial recommendation. It is pretty simple to use and makes for a quiet amp. I've seen a number of successful builds on this forum use it. To me, that is the primary concern.

    I find it hard to properly isolate variables when it comes to noise. My first build used a single ground bus. It was kinda noisy. But that could be due to other variables like poor lead dress, longer than necessary wires, and mediocre soldering. Subsequent builds used Rob's method, but also show improved workmanship. My most recent build was PTP and used a hybrid star method as shown in figure 15.14:

    http://www.valvewizard.co.uk/Grounding.pdf

    I've used this method in two amps now. One was a PTP Vibrochamp, which is very quiet. The other is an AB763 Bandmaster head with a Trainwreck style second channel. The Trainwreck mod gets a bit noisy, but it is stable. And at those gain and volume levels, with that hot of a circuit, I suspect there are lots of variables in play related to noise. I'd highly recommend Merlin's article for a deeper look.
     
    robrob likes this.

  20. FenderLover

    FenderLover Friend of Leo's

    Jun 11, 2009
    Minnesota
    Better to just get Merlin Blencowe's book and read chapter 15. It's all there, explanations and pictures. The second edition is revised and expanded, and is different than the first printing that you can find on-line. The link above should get you going.
     

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