Longest Metro Plexi 50 Build

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

  1. Chris-in-LA

    Chris-in-LA Well-Known Member

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    You will get a lot of opinions about twisted heater wiring. I don’t believe that there is a wrong direction. Many will tell you it has to be tight but here’s a pic of my stock 2204, not especially tight and it works fine. You can duplicate the direction of the twist and you can make it tighter if you want.

    41869920-3C63-4038-AC9A-B6C832D971D7.jpeg
     
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  2. Exojam

    Exojam Well-Known Member

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    LA,

    I am going to see if Robert from valvestrom replies back to me since it makes absolutely no sense that this is taken place.

    I went back through old photos and verified again, that I wired the board correctly and also the inputs. I ran out of green wire so I ordered some more and both inputs are coming out and being rewired. Add in the fact I double and triple checked the wiring with continuity checks and all is correct. Just some in two damn inputs that are kicking my but, how sad.
     
  3. Chris-in-LA

    Chris-in-LA Well-Known Member

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    It’s too bad that this thing is causing you so much trouble. You’ll get to the bottom of this I’m sure. I spent some time going over your pics and posts but I can’t figure it out. The pics are no substitute for actually having physical access to that amp.
     
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  4. RickyLee

    RickyLee Well-Known Member VIP Member

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    I was just curious to know the input scheme. Is it a 2204 type High/Low?

    Reason was I thought I read some posts regarding a Plexi gain stage layout which translates to the four input design. Just checking. Title mentions "Plexi".
     
  5. ampmadscientist

    ampmadscientist Well-Known Member

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    NOS tubes can have all types of problems.
    For one thing many of these tubes are not NOS. They are just old used tubes.
    I consider these type tubes a huge waste of money, since they are usually unreliable.

    I would definitely test an amp with new tubes.

    There is no advantage to using old tubes....it will just cause a lot of headaches.
     
    Last edited: Feb 11, 2019
  6. ampmadscientist

    ampmadscientist Well-Known Member

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    The majority of hiss is only caused by one thing: the 68K input resistor.
    This 68K contributes about 70% of the total hiss in a tube guitar amplifier.

    (we have tested this extensively, with real test equipment)

    The carbon resistor is the noisiest possible resistor.
    Wire wound or bulk metal foil is 40 db quieter than a carbon resistor.

    Your second choice is metal film resistors.
     
    Last edited: Feb 11, 2019
  7. Exojam

    Exojam Well-Known Member

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    RickyLee,

    It is a JMP 50 or early 2204 as I have been told.

    Thank you very much for your comments concerning my health. I will just leave it as it is still ongoing and since Dec 16, 2016 I have had to be admitted over 70 times. You just want it to be over so I can continue my little life.
     
  8. Exojam

    Exojam Well-Known Member

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    That is correct, high low on both inputs. One on top of the other.
     
  9. RickyLee

    RickyLee Well-Known Member VIP Member

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    Got it. So then your Low input or input 2 you are hving the issue with should be entering the circuit after the 1st gain stage and before gain pot and 2nd stage.

    Your High input has to travel through Low input when contacts are closed.
     
  10. neikeel

    neikeel Well-Known Member

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    Trace the wires out and you will find it is the same;)
     
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  11. Exojam

    Exojam Well-Known Member

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    Well at this I am not sure were to go. Did a lot of work on it yesterday and today and still no input two.

    I completely replaced all the wiring on both inputs, in selected turrets that seem to keep breaking wires, I ran the wire up the back side of the turret and put a few wraps on the leg of the component that goes into that turret slot.

    I have already replaced the 50uf cap that has the 820 ohm resistor one it, the 50 went in place of the schematics .68 cap. I could get to readings on 50uf cap and that is why it went.

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

    All my pictures show I have this thing wired correctly. Just tested the .022uf cap that goes between the sets of 68k resistors and can watch it charge and discharge very fast.

    I will link in all my readings and photos from today sometime tomorrow but again at this time something on the board is failing and it is not my dang wiring.

    Just do not have any idea where to go from here since I have checked this thing out a million ways to Sunday.
     
  12. Exojam

    Exojam Well-Known Member

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  13. Exojam

    Exojam Well-Known Member

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    Well it just goes to show, do not depend on the “hiss” from the speakers as described in the manual to validate your amp is working, plug a freaking guitar into all into inputs.

    Than you will (hopefully) find out your amp IS WORKING!!!!!!!!!!

    Damn, I thought I was never going to get this girl running, feels damn good.

    Now I just have some cleanup work to do to fix some components that I do not like the look of, solder on the 820 and 2.7k ohm resistors I had clipped onto their respective caps.

    Neikeel,

    Based on our conversation where had stated the below:

    “channge the 0.1uF PI output couplers to anything between 0.022uF to 0.047uF (0.022s are expensive but you can get 0.027uF 0.033uF or 0.039uF or 0.047uF much cheaper in 400v rating)”.

    Should I go ahead and change out the .01uf caps while the amp is still open? I know I have one .022uf Zoso left but would need to search my parts to see if I have another cap in that value. If I only have the one I was thinking of putting it on the presence pot. Than get another cap for the .01uf cap coming off of V3 pin 7 (I believe that it were it goes).

    I cannot thank you enough for all of your help you provided me in getting this amp together, but again THANK YOU!
     
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  14. neikeel

    neikeel Well-Known Member

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    Thank goodness
    I would change no components, if the last pics are correct you already have 0.022uF in the PI output slots which will be fine
    Just play it and bed it in.
    Tweak the bias to the sweet spot and get some hours onto it, it will get sweeter and nicer to play.
    Mod it later.
     
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  15. Exojam

    Exojam Well-Known Member

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    Sounds good.

    Thanks, James
     
  16. Exojam

    Exojam Well-Known Member

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    Well I tried the below combination of tubes and am not impressed. The tubes in V4-5 had to go as they were blowing the 3 amp fuse so I just dropped some JJ’s in there.

    V1 - I63 Ice (Mullard)
    V2 - 12AX7 Raytheon JAN
    V3 - GE 5751
    V4/V5 - Amperex XF2 6CA7

    On either input on Input 1, the low E,A and D sounded like they had no definition at all. They were just sounded like the volume was way passed it limits and made everything rattle. Not like it was a clipping sound.

    On the G, B and high E it just sounded like an ice pick.

    On Input 2 either top or bottom, it made it sound like the amp was covered in a blanket and no string had any definition. I am not sure if the Input 2 selection is supposed to a few dB lower than Input 1 or not.

    Not even sure if I want to keep testing with those tubes with the sound I am getting or change them up.

    May need to change that cap on volume 1 channel pretty quick.

    Also, not very impressed with the overall volume levels on the amp as I feel my modded DSL40C is much louder.

    I will keep on trying.
     
  17. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Your description of channel 2 sounds about right, that’s why i rarely use that channel.
     
  18. neikeel

    neikeel Well-Known Member

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    either a 5000pf cap on vol 1 or 100pf.

    - nothing in between!

    Are those JJs old an are they biased correctl?
     
  19. Exojam

    Exojam Well-Known Member

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    Yes the JJ’s were biased correctly. I am taking the plate voltage and putting it into a bias calculator to get my target bias value.

    I decided to put two non matching EL34’s in there since I cannot find any technical documentation that states this cannot be done. I was actually looking for multiple tech docs on non matching power tubes and could not find any. Right now they are biasing just a few millamps apart high I see even with matching tubes in this amp.

    I will see if I have either one of those caps.

    I also may have been to quick on the evaluation of the amp last night. After my first session with it, I just left it on for a few hours so everything could start to break in - settle in and things sound more satisfying.
     
  20. Exojam

    Exojam Well-Known Member

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    Damn and the only one I have is in between (capacitor) for volume 1.
     

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