Help please - EL34 Red plate on 1 tube

Discussion in 'The Workbench' started by Deepstone, Aug 12, 2012.

  1. Deepstone

    Deepstone New Member

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    I have a Marshall 1987 Silver Jubilee that i just got and have no idea of the amps history...1 EL34 Red plates and i am not sure why..I ordered new tubes and still same problem. should I change the electrolyte caps? i am not sure how old they are? any tips or help would be great.. also someone put a resistor piggy backed on another on r48 Marshall Amp Forum - Deepstone's Album: Marshall - Picture ...could that be the problem? thanks
     
  2. Wilder Amplification

    Wilder Amplification New Member

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    Does it red plate when you're not playing through it?

    If it red plates all the time, either the grid leak resistor or the grid stopper resistor on that valve is blown open.
     
  3. Deepstone

    Deepstone New Member

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    It seems fine on standby then when i take it off standby it starts to heat up and get kinda red I play it for maybe a minute and it sounds fine then it gets too hot and blows a fuse..I got a new set of tubes then same thing now it goes straight to blown fuse ..so i am thinking it fried another power tube?:( if i send you the schematic could you point me to the resistors you mentioned? I tested the ones around that valve socket and they seem ok but i am not sure the ones you mentioned..thanks for the advice!!!!
     
  4. Deepstone

    Deepstone New Member

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    is the resistor piggy back thing done for tone? does anyone think i should remove it and see if that is the problem? just a thought
     
  5. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Hi Deepstone, hello Jon

    Jon already has the schematic. Could you supply him, the thread, some more pictures of the amplifiers insides at the board and power tubes sockets?

    The piggyback resistor may be a problem but it needs to be seen as to what else may have been done. R48 is a 220k in the bias feed line and the added resistor appears to be a 820k yielding a resistance of ~173k. It does not affect tone but does change the bias voltage which in turn affects operation for the power tubes.

    The resistors Jon was refering to are R21 and R54 on the green wire side to grid pin #5 on tube V4 and R22 and R55 on the orange wire side to grid pin #5 on tube V5. That is how the schematic shows it. Determine which side, V4 or V5, is red plating.

    How long did you red plate the new tube? Keep fuses handy.
     
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  6. plexifier

    plexifier Active Member

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    R48 (and others) set the bias voltage for the power tubes. Someone added a parallel resistor on top of the original 220k to increase the bias voltage. This will reduce the bias current (make the tubes run "colder"). I can't tell for sure what value the added resistor is from your picture but it looks like it was damaged by heat (probably from the bad soldering job there). ;)

    As Jon mentioned, there's a good chance the red-plating tube is not getting negative voltage to pin 5, which sets the bias. Depending on which power tube it is, you need to check R21, R22, R54 and R55 (the resistors Jon mentioned). Make sure both power tube pin 5 wires and connections are solid. Also, check the tube socket pin 5 holders to see if any are loose or broken.

    If the above didn't reveal the problem, turn the amp on BUT leave it in standby and check the negative DC voltage on pin 5 of both power tubes.

    Unfortunately, you may have damaged one of your new tubes. :(

    EDIT: As usual, mickeydg5 and I are posting the same thing at the same time! :dude:
     
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  7. Wilder Amplification

    Wilder Amplification New Member

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    Hi mickey.

    Of course the other possible cause (although unlikely since it rarely ever happens but it's still a possible cause) we missed was the possibility of a leaky phase inverter coupling cap.

    Of course this could also be the valve itself if it draws high grid leakage current. Even if a valve is brand new it can still be bad. So let's not rule that out.

    Have you tried swapping the valves around to see if the red plating issue follows the valve or the socket?
     
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  8. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    I figured you were attacking the bias and grid circuits first and the next step would be to start asking for voltage readings working back towards the PI.

    Deepstone, it seems you are aware but for asking sake, are you familiar with working in amplifiers and taking voltage readings with a meter? There are high voltages in there.

    To double check the power tubes, swapping them around would be a good first step.
     
  9. Deepstone

    Deepstone New Member

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    Wow ...man are you guys smart...I have some experience with some amp building /...I did a marsh amp kit of a princeton reverb and it came out nice..I gig with it all the time and just mic it..in stereo with a blues jr..anyway I will be extremely careful and will discharge caps before poking around:) Thanks!! safety first!!! anyway I am going to get out the amp again tomorrow and see about the things you guys mentioned!! I am dying to play on this Marshall:dude:
     
  10. Deepstone

    Deepstone New Member

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    looks like as follows:
    R21 = .226
    R54 = 5.6

    R22 = 56K ?
    R55 = 5.6

    R23 - BLANK
    R48 (WITH JUMPER) = .458
    TOP RESISTOR GRAY-RED-YELLOW-GOLD 820K
    BOTTOM RESISTOR = R-R-Y-GOLD 220K

    to me it seems like some one did a swap around.

    I am thinking of putting back to schematic specs. as follows

    R21 = 220k
    R54 = 5.6

    R22 = 220K
    R55 = 5.6

    R48 56k

    what do you guys think??
     
  11. Deepstone

    Deepstone New Member

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    thanks !!!!
     
  12. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    DO NOT ARBITRARILY GO AND CHANGE COMPONENTS. (The past post with component numbers must be wrong.) So be careful and make markups and notes on your schematic.

    I forgot about these amplifiers and noticed your component numbering. The schematic numbering does not match your board numbering. I looked around and could not find a schematic to match your component numbering.

    I could only see a few components in your above picture, which all have correct values and circuit placement:
    R22 = 56k (Add: some schematics indicate 68k)
    R47 = 15k
    R48 = 220k
    C15 = 10uF
    C16 = 10uF
    this leads me to believe that:
    R20 = 220k
    R21 = 220k
    R54 = 5.6k
    R55 = 5.6k

    Are you reading .226M (megaohms) and .458M (megaohms)? How are you measuring the resistors?
     
  13. plexifier

    plexifier Active Member

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    Deepstone:
    I (we) were using the wrong schematic for your amp so some of the component numbers mentioned above are different than yours. Wait until we get this figured out! :)

    Please post some better pictures of the power tube sockets and the right-hand end of the PCB (near the bias trim).

    NOTE: This appears to be a "version 2" 2558 so the correct schematic should be here:
    http://www.marshallschematics.com/schematics/2550_revised.zip

    The bias caps are C15 and C16 in Deepstone's picture, which matches the above schem. The Dr. Tube schem that I referred to earlier shows those caps as C16 & C17.

    Grid leak resistors should be R20 and R21 (both are 220k - red-red-yellow). R22 should be 68k but yours looks like a 56k.

    I won't be able to look at this further until tomorrow but hopefully mickeydg5 or Wilder can help in the meantime. :)
     
  14. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Did it again.:lol:

    Notice R22 and RV1 positions are swapped on that different schematic. But they are not on his amplifier board.
     
  15. plexifier

    plexifier Active Member

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    Yep. Sorry! :applause:
     
  16. Deepstone

    Deepstone New Member

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    here are some close ups ...I am not in a hurry...so I will just chill till the dust settles
    :dude:
    Dump Treasure
     
  17. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Deepstone
    Everything seems as I described above. If you have questions, ask. :)

    Do your amplifier a favor, first turn amplifier off and discharge main capacitors, remove tubes and bend those power tube 5.6k grid resistors upward and away from the chassis. I also notice that on some of the 9-pin sockets that some pins are almost touching. Move them apart as much as possible, be careful.

    Are you reading .226M (megaohms) and .458M (megaohms)? How are you measuring the resistors?
     
  18. RickyLee

    RickyLee Well-Known Member VIP Member

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    @deepstone:

    Can you give these guys the revision number on the circuit board? I am wondering if you have an early first version Jubilee?

    What is the serial number? You can put an X for the last digit if you like.
     
  19. Deepstone

    Deepstone New Member

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    I think it ended with C instead of D (JM112D) I will check tomorrow, I remember the serial number being low starting with a 1 I will check and re post more info..Thanks DS
     
  20. Deepstone

    Deepstone New Member

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    I will re check all socket pins...
     

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