Mojo 6G15 on the way

Discussion in 'Shock Brother's DIY Amps' started by RottenTheCat, Sep 1, 2019.

  1. RottenTheCat

    RottenTheCat Tele-Meister

    Posts:
    336
    Joined:
    May 29, 2019
    Location:
    Drambuie, Africa
    Thought I'd take advantage of the Labor Day sale, and pick up a 6G15 kit from Mojo.

    I've built two of these before and am thinking of doing the "boost only" mod - basically grounding the 2nd triode, and taking its former input to the output through the dwell control. No reverb then, but you get crapload of gain if you want it. Dunno yet. That's easily doable later (I have done it with a relay controlled by switch).

    The other mods are to go a bit heavier on the tone cap, just a touch heavier, trial and error thing, and to reduce the gain on the 6V6 a bit (comes with 6V6 not 6K6...)

    Anyway, waitin on Dorian, bored to crap, and need something to look forward to, do a bit more research on.
     
    Paul-T, dan40 and Robear like this.
  2. muscmp

    muscmp Tele-Afflicted

    Posts:
    1,286
    Joined:
    Jul 1, 2008
    Location:
    california
    sounds like a great plan!

    you have a long wait for dorian to reach africa!

    play music!
     
  3. RottenTheCat

    RottenTheCat Tele-Meister

    Posts:
    336
    Joined:
    May 29, 2019
    Location:
    Drambuie, Africa
    Got email from Mojo .... Everything shipped today. That's the quickest they've ever shipped, only 9 days wait. I suspect we'll see it on Tuesday, because FedEx doesn't deliver on Monday in this locale.
     
  4. RottenTheCat

    RottenTheCat Tele-Meister

    Posts:
    336
    Joined:
    May 29, 2019
    Location:
    Drambuie, Africa
    Otay... for anyone who's following this....

    The MojoTone kit came in very quickly. I expected it Tuesday, but it showed up on the previous Thursday. Good job FedEx!

    I've had a nice good look at the MojoTone layout, which differs slightly from the Fender layout (eliminating the diode board and putting the diodes on the main board, mostly...). There are some minor under board wire routing changes, and one or two over the board/off board changes.

    One change in particular that I do not like..... its potentially dangerous. There is a spot where two 2.2m resistors come together, and one of those 2.2m resistor is grounded to the input jack. The voltage there could be... as high as B+ (about 250-275vdc) depending on the load presented at the grounded end of the 2.2m resistor chain (4.4m total). Very low load (aka high resistance, like a human body), and you get close to the full B+.

    So tell me.... what happens if the nut on the input jack becomes loose and you got your geetar plugged into it? Not a pleasing scenario for your pickup, or your sad sack o' bones you live in. Probably wont kill, as the current is self limiting as the load resistance is lowered, but still... its gonna tingle plenty.

    Better solution is to ground right to the chassis - by whatever means you prefer. I prefer grounding to a copper plate behind the controls and jacks, which is in turn grounded (soldered) to the chassis at the PT end. Lots of ways to do it, but this is my way, and works for me. By the way, what I do is before the plate is installed, I fold over little triangles of metal (corners, or in the middle of the board) and have holes for ground connections pre drilled. Doing that allows soldering to those folded "tabs" with normal low power soldering equipment. Don't have to drag out the 150w iron or gun to make those connections. If you do solder to steel, remember to grind off the chrome plating in that spot first.

    Other than that, as with most Mojo kits, its pretty much inclusive - except for the holes, and means, to mount the boards to the chassis. They sort of leave that up for grabs. Its ok, I got the "skills to pay the bills".
     
    jsnwhite619 likes this.
  5. RottenTheCat

    RottenTheCat Tele-Meister

    Posts:
    336
    Joined:
    May 29, 2019
    Location:
    Drambuie, Africa
    One other thing I wanted to add about the 6G15. I've seen information out on the net that states the dry signal is routed through the cathode follower to keep that signal in phase with the reverb signal. That sounds good on paper if you look at the phase reversals.... Except for two major points.

    First being is the totally disrupted phase relationship of the signal as it goes through the reverb tank itself. The second point is that the reverb send transformer which drives the tank can't be counted on to be in one phase or another just like the output transformer on almost any amplifier. Amp builders know, that it's totally common to have to reverse the output wires, or plate wires, to eliminate squealing on an amp that uses negative feedback.

    So if phase was not the issue, why the cathode follower?

    I was examining the fixed bias scheme using the voltage divider to provide a constant grid voltage and the cathode resistor to provide the offset to the grid in order to bias the tube. And I wondered why they didn't use a standard plate loaded triode amplifier. Okay I guess it's because the cathode follower is such low impedance, and neither does it suffer from the Miller effect. So then I thought well why use it at all? Why not just mix with the guitars raw signal?

    Then it hit me like a ton of bricks. Kind of a goofy thing but if they tried to do that, the output of the reverb would be fed back into the input of the reverb. They would have a hysterical hysteresis going on. AKA feedback of the positive type. The cathode follower acts as a low-impedance "one way" transmitter of the raw signal from the guitar. No matter how high the reverb signal is, it cannot feed back into the input of the amplifier backwards through the cathode follower.

    It's pretty plain to see now, and I've never heard of it explained that way before but I'm sure what I'm thinking is correct. So the remaining question is why use a fixed bias cathode follower?

    I haven't figured that out, but I believe it has to do with the nature of a fixed bias cathode follower being self-limiting and thereby giving a very consistent impedance and output to the raw signal. Put another way, it makes a superior low impedance-to-low impedance buffer.* I think I'm correct on that but I don't know why....yet.

    And lastly.... I'm trying to figure out why Fender did not use the protection diode between the cathode and grit on that same cathode follower. There's a point at start up when the cathode is going to be much higher voltage than the grid, and it's not up to temperature yet. That would make it prone to cathode stripping. RCA documented the protection diode for that circuit which allows current from by the excess voltage on the cathode to pass through to the grid harmlessly. After a few seconds as the voltages stabilized as temperature rose, the diode would just block the reverse voltage which would be present. I got to think about that as well.

    More to come.... it's been well over a decade and a half since I've built one of these, and I found the depth of my own thinking to be different from what it was in that era.... Guess being old has its advantage!

    *Remember, impedance is relative. We think of guitar signals as "high impedance", and they are when compared to low impedance balanced line mics (as an example). But in terms of tube inputs and outputs... the measly guitar pickup is very low impedance. Its all relative.
     
    Last edited: Sep 14, 2019 at 10:11 AM
    Paul-T likes this.
  6. deus56

    deus56 TDPRI Member

    Age:
    47
    Posts:
    46
    Joined:
    May 5, 2019
    Location:
    Texas
    Nice project, look forward to your progress posts.
    Here is my unit, someone went all Johnny Cash on her
    and painted the grill cloth black, but she sounds stellar.
    [​IMG] [​IMG]
     
    RottenTheCat and Paul-T like this.
  7. Paul-T

    Paul-T Tele-Meister

    Posts:
    208
    Joined:
    Nov 23, 2018
    Location:
    London
    I came to this pesky site intending only to build a 6G15 and now seem to be on a long detour. Looking forward to watching this progress.
     
  8. theprofessor

    theprofessor Friend of Leo's

    Posts:
    4,305
    Joined:
    Aug 8, 2016
    Location:
    Chattanooga, TN
    Hey RottenTheCat - I'm going to be watching this build with interest. I've never quite shaken the idea of building a 6g15 myself. I wonder if you might be able to do me a favor and measure the mounting centers on the Mojotone chassis and the cutout on the cabinet? I have an extra 5e3 headshell here that might work, even if i have to dowel the previous chassis-mounting holes and re-drill. Thank you!

    By the way, the reason I'm asking is that I've called Mojotone to get them to do this on a few chassis before, but they've never given me very precise measurements.
     
    Paul-T likes this.
  9. RottenTheCat

    RottenTheCat Tele-Meister

    Posts:
    336
    Joined:
    May 29, 2019
    Location:
    Drambuie, Africa
    Glad to help.

    I think there is lot-to-lot variability to the Mojotone chassis. I have no idea who actually does the metal work for them, but, looking at the process, you have a guy with a punch press who sets up raw sheets and cuts (or shears rather) using the press. Then you set up the press for a hole, using stops as in the shearing process. Intricate presses, used for long production and high volume production, can be set up for multiple holes all "ganged up", but its expensive to do. I doubt the production quantity justifies the sort of expense needed for that. Whats it all end up as? Well.... you might find that holes are smidgen off. I'm sure they use a set-up jig to place the punch and die sets, but... its still a hand operation. My tool and die fabrication days are from the 1980's, and things have changed, but on this particular Mojo chassis, you can see the faint curvature around holes on the finish side, and that tiny lip of raised material on the inside of the chassis, so I'm pretty sure they're punched, not... say... laser cut.

    Hole spacing - 14-1/2 inches give or take 1/32 or so. And chassis cut out is 12-1/2 inches with the same tolerance.

    I've said this before, elsewhere here, but I use copper sheets available from Hobby Lobby as my inside copper plate material. Its easy to cut on a paper cutter, and what I do.... is hold the plate inside the chassis, and mark the holes with a Sharpie. Out of the chassis, predrill each hole to about 3/16 or so - not quite full size. Then back in the chassis and carefully... with one of those really small Unibits, make the first hole full size, using the chassis as a guide. I start with whatever jack is on the end - in this case the output jack. Stick the jack in the hole, snug it up. Then each successive hole, with its jack or pot in the hole snugged up before I move to the next hole. You'll find that the Unibit "self guides" to the chassis openings, which are steel, and cuts the copper "like buttah". The final result is a finished plate that is a 100 percent "perfect fit" for your particular chassis.

    Now if somebody can do ME a favor....

    I used to use PhotoBucket... which has gone wonky. I've got almost no pictures shared, and they still tell me that I've "exceeded my bandwidth". They're on a money grab it looks like. Anyway, I set up a blog type photo share, but its pictures are not well shared on a BBS, and users have to go over to the blog to see full size images (a pain in the tookus). Can someone school me (PM so we don't clog the thread)? Many thanks. ;)

    I hope to get back the 6G15 after my man-jobs are done. Gotta change oil on my old Ranger ('99, about 300k miles), rotate the tires too. Then touch up all the paint where I had plywood on the house and patched the holes. Then do the lawn, which takes me two hours. Sometime... this afternoon, barring rain, I'll be back in the solder fume funhouse.
     
    Last edited: Sep 14, 2019 at 10:03 AM
  10. theprofessor

    theprofessor Friend of Leo's

    Posts:
    4,305
    Joined:
    Aug 8, 2016
    Location:
    Chattanooga, TN
    Thank you very much for providing the measurements for the chassis mount and cutout. It is as I suspected: Center-to-center spacing of 14-1/2", thus 1" longer than the mounting holes on their 5e3 chassis, which is 13-1/2". And the cutout for my 5e3 head also measures 12-1/2". The two chassis have the exact same height as well (2-5/8"). So if I wanted to, I could fill those mounting holes in my poplar 5e3 head and re-drill to fit a 6G15 chassis. I already have tubes, reverb tank, cables, etc. I imagine the transformers and choke ($100 plus shipping) and the board and the chassis ($80 plus shipping) would be the main cost...

    I'm sorry I can't be of help to you regarding the images.
     
    Last edited: Sep 14, 2019 at 10:15 AM
  11. RottenTheCat

    RottenTheCat Tele-Meister

    Posts:
    336
    Joined:
    May 29, 2019
    Location:
    Drambuie, Africa
    Took me most of the day to complete the man jobs, but ....

    I managed to build and mount the cap board, and fully assemble the main board with components and flying leads.

    I noticed that I had stashed away some spares from the 5F4 Super, and there were some longer screws for mounting the boards to the chassis therein. I've got it set up where the cap board is mounted by one sheet metal screw and one machine screw - which does double duty for the main board, plus another machine screw at the far left of the main board.

    Tomorrow, I'll fabricate the copper plate (its already cut to width and length), and drill the chassis for the various mounting screws.

    Barring any fubars... I should have controls and sockets mounted, and the wee beastie wired up by evening - assuming I get the grass mowed in normal time of about 2 hours.
     
    Paul-T and deus56 like this.
  12. deus56

    deus56 TDPRI Member

    Age:
    47
    Posts:
    46
    Joined:
    May 5, 2019
    Location:
    Texas
    ^^^^^^^^Full steam ahead^^^^^^
    Excellent progress Cat!!!
     
  13. Paul-T

    Paul-T Tele-Meister

    Posts:
    208
    Joined:
    Nov 23, 2018
    Location:
    London
    Great work.

    Do take a couple minutes off the mowing and show us some in-progress photos please!
     
    theprofessor and King Fan like this.
  14. RottenTheCat

    RottenTheCat Tele-Meister

    Posts:
    336
    Joined:
    May 29, 2019
    Location:
    Drambuie, Africa
    I hate to blog post... the pictures are minuscule when put here... but.... I'll oblige....
     
    Paul-T likes this.
  15. RottenTheCat

    RottenTheCat Tele-Meister

    Posts:
    336
    Joined:
    May 29, 2019
    Location:
    Drambuie, Africa
    I guess I need to find a new hosting site, again, as use.com is shutting down.

    Got some pictures, copper plate construction, cap board, main board stuffed in there... so... off to find yet another host.
     
  16. Paul-T

    Paul-T Tele-Meister

    Posts:
    208
    Joined:
    Nov 23, 2018
    Location:
    London
    I reckon it's a great advantage of this site that you don't need to host externally. .. life is too short to deal with photobucket.
     
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.