Longest Metro Plexi 50 Build

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

  1. mickeydg5

    mickeydg5 Well-Known Member

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    Every time I see this thread I am waiting to see a picture of a 12 foot long Plexi head.


    upload_2019-1-18_12-56-34.jpeg
     
  2. Exojam

    Exojam Well-Known Member

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    I will have to find a way to stretch the head to make it longer (that just sounds so bad).
     
  3. mickeydg5

    mickeydg5 Well-Known Member

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    Yes 12 foot long with 8 4x12's under it.
     
  4. Exojam

    Exojam Well-Known Member

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    Well she fired up just fine and the voltage reading are below. The bias is setting around 39mv on V5 and 37mv on V4 with a reading of 450vdc on the HT fuse. This with the bias pot full clockwise for full current. I will wait for everything to settle down before doing the bias again.

    It appears as voltages are moving around a bit as I would expect. I am just going to leave on and connected to the speaker cab (Marshall MG412A).

    The only thing strange to me right know is when the instructions stated to turn up volume one until you hear the normal speaker hiss which I never heard. The head and cab are both 8 ohms.

    Later tonight or tomorrow I will bring the head into the house and change out these JJ tubes with the NOS tubes I have for this head and check out the bias and see if I get any output with a guitar hooked up. This is because I never got my hiss according to the manual.

    450DFC7F-B594-469A-AF61-FDDBFA1E2375.jpeg
     
    Last edited: Jan 20, 2019
  5. Exojam

    Exojam Well-Known Member

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    Well I am going to leave it up in standby over night and check things agains.

    Due to the weather coming in I decided to run a guitar out to the shed to see if I would get any output. Much to my disappointment I am getting sound from all four inputs but the output is really not there. My little Blackstar 3 watt amp sounds louder than this one.

    I did not like the look of V2’s heaters so when my wife brought it in for me I put a 12AX7A GE NOS in that slot and the heaters did come to life but not the sound, this time hooked up to my 8 ohm Mesa.

    I am sure it should be pretty simple so I will break out the schematic tomorrow and grab som more readings, redo the bias and go from there.

    I do believe this amp should be able to reach the neighbors house with the volume on it and the guitar at 11, but it ain’t happening right now.
     
  6. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Hell, my Champ reaches the neighbors. A 50 watt Plexi should be so loud that it would cause pain, at least my 50 watters are that loud.
     
  7. neikeel

    neikeel Well-Known Member

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    Well done that you have sound and all the voltages look correct.

    Classic error on these is that you put a 470k resistor in the PI on the cathode instead of 470R, although looking at your board pics it looks correct.

    Post some pics of the whole amp of inputs, board and sockets.

    Looking over your pics I wonder if you may have a dry joint on one or more of your turrets? Most likely a grid wire. I can't see really good pics of your PI ground wire (from the presence pot) and the ground wire on the output jacks.

    Check the filter caps are empty and reflow all the turrets properly so each one is shiny bright and full of solder.
     
    Last edited: Jan 20, 2019
  8. Exojam

    Exojam Well-Known Member

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    Neikeel,

    I can get some pictures today but unfortunately with the snow and ice we got last night I cannot get my wheelchair through it to reach my tools. I will see if the better half can get them for me tomorrow.

    I suspect something silly I missed or as you put out there a solder joint. To be honest there where times I was nervous about how much solder to flow when putting the components in since I was worried about the wire on the back side coming out, but we shall see.

    Thank you again.
     
  9. Exojam

    Exojam Well-Known Member

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    Neikeel,

    I went through all wires and components and did a light tug test on them and found three wires that need to be resoldered. They are circled below from a page in the manual but I believe you can tell which ones they are.

    I also found an issue with one of inputs 2 wiring where it broke off underneath the board. I also feel I have the inputs wired incorrectly as shown in my terrible painting below.

    Below is a link to 14 pictures I took all around the amp for you t take a look at. If You notice any black on top of the turrets that is from where I use a Sharpie to mark where I should bend the leg of the component that was to be soldered into the turret.

    https://www.dropbox.com/sh/axn4iuo7ab5td07/AAATw8j5EAHUQ18GhJr2it2ea?dl=0

    manual example.JPG

    The painting is how I have them wired.

    inputs.JPG
     
  10. mickeydg5

    mickeydg5 Well-Known Member

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    The 470R show in the pictures.
     
  11. Exojam

    Exojam Well-Known Member

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    Yeah he is in there.

    I am going to probably need to swap out the 47K bias resistor with something lower to help with the biasing though.
     
  12. Exojam

    Exojam Well-Known Member

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    I know it is being discussed in Chris's 2204 build about having the 47K resistor tied to the 8 ohm tap of the impedance selector but I wanted to verify that mine is doing this.

    According to the two below screenshots from the manual it appears this is the normal way to wire the board. My question is how to verify the 47K resistor I circled is the one that is supposed to go to the 8 ohm tap and if in fact it is wired that way. Since according to the schematic I do have the correct 47K resistor circled and it is going to the correct tap.

    I also have to go back and double and triple check to see if I did wire the inputs wrong as they attach to the board.

    47K.JPG 47K1.JPG
     
  13. Guitar-Rocker

    Guitar-Rocker Well-Known Member

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    The 47K resistor, shown as orange, in the photo is the NFB resistor.


    If you happened to omit the green wire (that is non-highlighted between the input jacks), then go back and add that in. And confirm your ground wire on the jacks is as shown, and is tied to ground.
     
  14. Exojam

    Exojam Well-Known Member

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    Both the input jacks are not connected as per the manual but both are tied to ground which I have verified.

    And I do agree with your comment on the 47K resistor, I just want to verify it is tied to the 8 ohm tap as the schematic shows and I was if informed by Neikeel that is the way he really likes it and since he knows wayyyyy more than me I am going for the way he would do it.
     
  15. neikeel

    neikeel Well-Known Member

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    First need to sort the input wiring. On your painting you show the wires in black going from bottom of the high impedance socket (hi input straight to the 68ks. That is not totally correct. There are 2 pairs to the 68ks and these are linked to ensure switched grounding.
    This is a nice clear image. [​IMG]
    The ground wire is taken from the 1M resistor lug nearest the chassis front.
     
    Guitar-Rocker likes this.
  16. Exojam

    Exojam Well-Known Member

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    I changed the input wiring to how it shows in the manual yesterday.

    Going to fix the wiring to the preamp sockets and swap out the 47K bias resistor also.
     
  17. Exojam

    Exojam Well-Known Member

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    Well still a no go. I fixed all of the wiring issues I had found and also replaced the 47K bias resistor to a 33K which did allow for more adjustment. Reflowed all Top turrets. Put in new power tubes and also put new (NOS) tubes in V1 and V3.

    Right now all four inputs work, I am hooked to a 8 ohm cabinet and do get volume from all 4 but it is still not cranking as I know it should.

    I ran through all of the basic reading per the manual again (did not write them down this time) and they were close to last run around.

    Hopefully in the next few days I can troubleshoot some more and also check out the Metro forum to see if anyone has ran into this before. Little bummed but it will be fixed and rock my house!
     
  18. neikeel

    neikeel Well-Known Member

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    Take a look at V1 pin7.
    It is missing this is the grid wire from the junction of two of the 68k resistors on the channel 1.
    I’ll post this and keep scanning the rest of the pics.
     
  19. Exojam

    Exojam Well-Known Member

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    Thank you.
     
  20. FourT6and2

    FourT6and2 Active Member

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    Looks like you have your B+ rail wired wrong. PI node goes to 10K dropper then to one leg of the preamp filter cap. That junction goes to the CF on V2. The other half of the cap (after another 10K dropper) goes to V1 plate. But you have your halves connected wrong. That might be part of it.

    EDIT:
    Maybe you have this right actually. Just looks like you've flipped the two sides around, but still connected ok. But double check.
     
    Exojam likes this.

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