Longest Metro Plexi 50 Build

Discussion in 'Building the Classics' started by Exojam, Aug 4, 2018.

  1. Exojam

    Exojam Well-Known Member

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    Neikeel,

    That link in your post is not working.
     
  2. neikeel

    neikeel Well-Known Member

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    Try now.
    I tend to use Pihers in my builds but got these recently to do a turret board build of a 1987 reissue.

    Oh and don't give up. I had a ground loop hum on my Matamp build that nearly drove me mad (it uses wire shields everywhere for grounding), but I found the culprit and it is quiet as a mouse now!

    So message is keep going!
     
  3. Exojam

    Exojam Well-Known Member

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    Neikeel,

    Thanks for working on the link.

    Can you verify the the one pins and components that should have any effect on Volume 2 are highlighted in the below screenshot. Could you also verify my post 85 questions.

    And one last one for now. Since there is a .68uf caps coming into play with vol 2 and I do not have anymore Zoso's, would you stick with them or maybe change to the type we went with earlier where we changed the original value one of 220uf to a different value?

    Thanks again, James

    amp.JPG
     
  4. neikeel

    neikeel Well-Known Member

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    The first 0.68 is channel 1 cathode cap and is just channel 1. The corresponding channel 2 is the 250uF/820R in end of board.
    The second 0.68 is on V2a cathode (with the 820R) and is common to both inputs.
    Start with channel 2 jack’s going to the 68ks and via the green wire to v1a socket. Signal comes via the blue wire on v1a through the v1a 0.022uF cap to the first lug on vol2 pot. Then the wiper goes to the first 470k resistor. After that everything is common to both channels. Do you need to check the cathode RK connections, the grid and plate wire connections and the correct coupling cap and wires to the pot (check pot value and it’s ground connection) and the connections to the non-bypassed 470k resistor.
     
  5. Exojam

    Exojam Well-Known Member

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  6. neikeel

    neikeel Well-Known Member

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    That type of pic is very useful as they are clear, you can zoom in and move the cursor to trace the wires.
    From the picture I am still not convinced your input jacks are wired correctly (afraid can't tell from your pics)
    This pic is ok but it is confusing as the two green wires from the switching jack lugs are shown coming together.
    [​IMG]
    The top lug on the top jack goes to one 68k and the bottom lug on the top jack goes to the other 68k.
    The black earth wire goes from the lug on the bottom jack lug closest to the chassis wall (where the 1M resistor and the link wire join).
     
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  7. Exojam

    Exojam Well-Known Member

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    I will unscrew them from the chassis and get side shots of them.
     
  8. Exojam

    Exojam Well-Known Member

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    I have placed some close up shots of the iput jacks. They are V1, V1A, V2, V2A and V2B.

    Let me know what you think. Thanks.
     
  9. neikeel

    neikeel Well-Known Member

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    The jacks look to be wired correctly and the hook ups look correct too.

    If you are sure that there are no dry joints on those grid wires (before and after the 68ks and yoy have tried different ECC83s in V1 then you need to double check the components (Channel 2 pot values, the bypass cap, a 220uF you said? and the AB 820R cathode resistor are all in spec.)
    If all of those things (and the jack grounding is correct there is nothing else, particularly if channel 1 is working normally.
     
  10. Exojam

    Exojam Well-Known Member

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    I can measure them out and swap them if I see something but I do not have any fuses for it right now. I thought I had them in my shed but they are the other fuses the amp needs so I am about to order some.
     
  11. Exojam

    Exojam Well-Known Member

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    While I am waiting for my fuses, I am reading how tube amps work from the below page.

    Very interesting as I had no idea how the signal flowed and how the first tube (and others) were split into V1A-V1B etc. And with the questions Neikeel is asking me I figured I better understand this stuff a little better.

    https://robrobinette.com/How_Amps_Work.htm
     
    Last edited: Feb 3, 2019
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  12. Exojam

    Exojam Well-Known Member

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    Neikeel,

    I did the testing as you had suggested and everything comes out fine. I downloaded some more pics to the link below.

    i guess at this time it is either a bad component or a bad solder joint. My plan is to pull all of the components we discussed and resolder them back in. I am also going to take off the 820R and put some jumpers on the the cap so I can test another 820. If all that fails than I will replace one component at a t time to try and get it working.

    https://www.dropbox.com/sh/jv2qz4vsweuy5up/AADNAKMB_ywy64GakNpz8GXwa?dl=0
     
  13. Exojam

    Exojam Well-Known Member

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  14. Exojam

    Exojam Well-Known Member

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    Well still a blanking no go.

    I replaced the 50uf cap that has the 820 ohm resistor one it, the 50 went in place of the schematics .68 cap.

    Also replaced the the zoso .68 just for fitting purposes, this is the one closet to the end of the board.

    Losing hope of this build as of now. Just pretty fucking disappointed!
     
  15. neikeel

    neikeel Well-Known Member

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    So channel 1 is still working?
    Are you still getting heater voltage on both halves of V1?
     
  16. Adieu

    Adieu Well-Known Member

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    Wow it's THAT simple a curcuit?
     
  17. Exojam

    Exojam Well-Known Member

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    Neikeel,

    I did not check heaters earlier.

    It seems like since I was switching tubes I am getting lower measurements.

    Tomorrow I will try and remember what tubes were wheat and document them and post them.
     
  18. Exojam

    Exojam Well-Known Member

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    Neikeel,

    I apologize but I could not get heater readings as one of the wires going to input 1 broke from under the board:mad: so I did not want to power it up.

    I just really think the issue is with the input jacks wiring based of what Robert from valvestrom sent me (see below).

    “Guessing that you have the jacks wired wrong. When in the top input, the 68ks are wired in parallel; when in the bottom input, one 68k is switched to ground and the input signal goes to the other one. I suspect somehow you are switching the input signal to ground.”

    Input 2 never seems to switch to ground when I am plugged into Input 1’s bottom which I believe should be happening the way I read his message.

    It just makes no sense since I have these wired as the manual states they should be. I have reached out to him for a bit more detailed information.
     
  19. Chris-in-LA

    Chris-in-LA Well-Known Member

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    This is the way that Mojotone wires the inputs on their JTM45. What about using this type of arrangement?

    A14D149C-541C-4BF7-A454-179DBD353B82.jpeg
    @Exojam
     
  20. RickyLee

    RickyLee Well-Known Member VIP Member

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    First off I just want to say that your build looks great on your craftsmanship. I always admire people with much more patience than myself on this stuff.

    And I hope your health gets better as well. And take care of your ears as well when it comes time to crank that bad boy up when you get it sorted. I have been going through a 3 week scare with my hearing, constant bad ringing, and started on antibiotics after I finally went to the hospital Wednesday night. Hoping it is a simple infection and this crap clears up. Anyone reading this, please take caution with your hearing!

    Regarding the issue you are having on input 2, I wanted to ask if this is a 2203 or 2204 type build? On your input 2 jack problem, I got confused as I thought I read some info on a Plexi/four holer type values/branches in some past posts. Just saying in case there is a simple mix up or overlook there between others helping.

    I have some questions for your expertise and experience. On my 2555 Jubilee Clone build, I used Classic Tone iron. At the time I installed the transformers back in 2011, I never trimmed down any of the working leads as well as all the extra aux wires my power transformer has (40-18069). I basically sealed, twisted, tucked them all together into the corner of the chassis away from everything best I could all those years ago. This was due to possibly wanting to swap all the iron out later to period correct 100W Dagnalls I have. I am not having any noise issues that I can tell. Just curious as we are all looking for every edge we can get on the noise issues.

    So basically I am wondering if this is an issue with hum or noise in an amp having all that excess transformer wiring inside?

    Also, regarding the heater filament wiring runs and twist, is there a preferred twist direction starting at the power transformer going down the line that should be implemented? lol Obviously this is AC, but then it seems like I have noticed many many pics of builds using the counter-clockwise twist going out from the power xfmr.
     
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