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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    Thank you for your response. I am not sure if you saw the link a few post up that has quite a few I believe good close-up shots of the guts of the amp.

    Also I followed the steps pretty good I believe from the instructions so hopefully I did not wire to many things wrong:erk:

    And on a side note, now I half to look most of the things you were talkings about since I am new to this.
     
    Last edited: Jan 24, 2019
  2. Exojam

    Exojam Well-Known Member

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    Neikeel,

    What I found was:

    I have continuity from V1 pin 7 to turret (measured off of resistor legs going into the that turret.

    Anytime I have an input cable plugged into either bottom input on Vol1 or Vol2 I get around 68K going back to pins 2 or 7.

    The top inputs never change from having around 34K on them no matter where an input cable is plugged in or no input cable being used at all.

    https://www.dropbox.com/sh/ix5tkkoveim7quf/AACRGeAGPAmAV_w6OWnXrW5ma?dl=0
     
  3. Exojam

    Exojam Well-Known Member

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    NICE, half way there. I was doing voltage checks with a schematic from a 2204 master model that was very similar to my layout except for a few things. As I was doing this I found pins 2 and 3 on V2 were incorrect and needed to be swapped.

    I did this and on input one it is rocking hard.

    Not so much luck on input 2. Had some humming so I swapped all the preamp tubes and nothing. Pulled V1 and still had it. As it sits no hiss or hum from either of input 2 connections.

    I know I have to go under the damn board again for the V2 pin 2 wire as I can move it and get fluctuations in noise.

    I will keep plugging away as input one sounded mighty nice.
     
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  4. neikeel

    neikeel Well-Known Member

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    Nearly there!
     
  5. Exojam

    Exojam Well-Known Member

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    Just to make sure I am reading the schematic correctly. In the screenshot below I would like to verify:
    Master volume 1 is circled in blue
    Master volume 2 is circled in red
    MV's.JPG
     
  6. Exojam

    Exojam Well-Known Member

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    Can someone please verify I have the pinout correct on this ECC83? Thanks.
    ecc83.JPG
     
  7. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Looks about right.

    B252A768-975A-4D87-95EB-4C92893EEF16.jpeg
     
  8. Exojam

    Exojam Well-Known Member

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    Thank you.
     
  9. Exojam

    Exojam Well-Known Member

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    :mad::mad::mad::mad::mad::mad::mad:E

    Since I had to go under the board to fix one of my wir s I replaced a few I did not like and finished up under there.

    As I was cleaning up I dropped one of my matching NOS Mullard matching EL34’s and in slow motion I just watched it schatter everywhere. I was soooooo pissed at myself since I purchased those tubes specifically for this head.

    I may try some Siemens NOS EL34’s for it now but have to do some looking around.

    Neikeel, if this was your amp what power tubes would you be looking for?
     
  10. neikeel

    neikeel Well-Known Member

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    Your first choice usually. I have one quad left for my Ormat, then I am out
    I quite like the RFTs/Siemens as these are probably best value NOS tubes out there now.
    Military Teslas are good (37 code) too but getting harder to find at decent prices.
    I use basic JJs on repairs for people and in fact have JJ KT77s in my ORST build and they sound pretty decent.
    You can send your tube to a big. Name supplier to match for you. Which is not cheap but better than a new pair. I had a nos GEC KT66 heater die on me but managed to buy a couple and get a good match. Sold the one that did not match.
     
  11. Exojam

    Exojam Well-Known Member

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    Thank you very much.

    On a side note I am just stumped with channel 2. First time I could test it after going under the board.

    Lower volume hiss than compared to channel one by a long shot. It will get pretty damn high if I turn up the presence control.

    Voltages appear to be around the board when I compare it to the 2204 master schematic but.......

    I will keep trying, but now it is starting to bother me as I believe it will be something somewhat easy.
     
  12. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Just had a similar problem with my build, rolling tubes fixed it. See if that helps.
     
  13. Exojam

    Exojam Well-Known Member

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    All ready rolled them and no change. Pulled V1 as described in the manual and nothing.

    Just measured all resistors and they are all of the correct values. I did not need to pull any of their legs except the 47K that sits next to the 10K and 8.2K ones. With both legs in, it was around 4K but when I pulled one leg it was the correct 47K value.
     
  14. Chris-in-LA

    Chris-in-LA Well-Known Member

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    I assumed that you might have done that. I’d probably go through everything with a chopstick at this point. Poke everything, tap on stuff, move wires, etc. See if that reveals any problems or odd noises. Mine’s pretty quiet but it hisses pretty good with everything on 10. Are you sure that the hiss is not normal?
     
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  15. Exojam

    Exojam Well-Known Member

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    It just cannot be in my opinion.
     
  16. Chris-in-LA

    Chris-in-LA Well-Known Member

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    So it’s louder than all your other amps?
     
  17. neikeel

    neikeel Well-Known Member

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    Things that cause hiss:
    High gain (component values or settings).
    Component type (carbon comp resistors early in signal path some ceramic caps).
    First thing is to get channel 2 working then I suggest swapping out the carbon comps at least on your v1 grids
     
  18. Exojam

    Exojam Well-Known Member

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    Well I was in communication with valvestrom and he thought the wiring of the input may be wrong.

    I redid the wiring and the inputs read correctly and switch to ground correctly.

    In a few minutes I am going to do the pencil check and see what happens.

    Neikeel, the resistors tied into V1’s grid are Allen Bradley ones, are those carbon comps?

    Thanks to both you and LA.
     
  19. neikeel

    neikeel Well-Known Member

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    Yes, the Allen Bradleys are carbon comps. They can add what some people call a nice vintage vibe, what others might call hiss and noise. If you find the hiss unpleasant then it may well be worth replacing them, in particular the 68k grids, the 470k grids, the 1M input jack resistors, ie all those very early in the chain. The other thing that often causes hiss is when people use big bypass caps on the volume 1 pot (aka bright cap) as it selects out the high frequencies and amplifies them above all other frequencies.
    Silly things to check will be that you have not swapped the two brown wires to vol pot on channel 2.
    Your pics, whilst nice and clear, do not give an over view of the whole board and the front and back of the chassis so you have to jump back and forth trying to trace things out, looking for the likely simple wiring error or dry joint.

    For MF resistors I have used these:[​IMG]
    https://www.ebay.co.uk/itm/5-X-1M-o...he rest of the amp, they sell them in 68k, 1M,
     
    Last edited: Feb 1, 2019
  20. Exojam

    Exojam Well-Known Member

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    Thank you Neikeel.

    Right now I am at a standstill until I can get out into my shed as I blew a few earlier.

    I also want this back out in the shed since over my workbench I have a very good 2*4 led panel which gives great light and I can roll my wheelchair under the bench so I can reach everything better.

    I have not lost faith on this yet. I am just disappointed in myself since I try to take my time and do everything nice and clean and expect the project I am working on to work when fired up.
     

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