strange problem.filter caps?

Discussion in 'The Workbench' started by dakrow, Dec 22, 2010.

  1. dakrow

    dakrow New Member

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    Ok so I have a 1989 marshall 2205. I just bought it.I played around with it for a few minutes and noticed it just had no balls...so I look at the tubes and there old and have dust on them.so I unplug it. And order new jj tubes all around.so now I retube it and do a bios adjustment.well now I notice a volume decrease with a slight gain increase that comes and goes....is this a sign of bad filter caps?
     
  2. FourT6and2

    FourT6and2 Active Member

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    Are you sure you know how to bias correctly?
     
  3. dakrow

    dakrow New Member

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    Well as a general rule is 36 to 40 milliamps is what I use.my voltage is 465.so if you do the math it comes to 37.something something.I just made it an even 37.00. I really don't think that made a difference.but then again I just assume this lol.I'm really hoping it's a easy fix.the volume decrease only happens for a second and goes right back to normal.then a few minutes later repeats.
     
  4. chuckharmonjr

    chuckharmonjr Well-Known Member

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    Dak that actually sounds to me more like a solder joint or some other conmnection. First off, let me tell you I am not an amp tech, there are several great ones on here, but I am an Engineer. What you are describing, if it repeats at the same rate almost every time, sounds very much heat related. A bad connection isn't passing current efficiently, hence has resistance in it that shouldnt be there. This builds up heat so it starts expanding, which builds up more resistance and heat till the connection finally breaks open, breaking the circuit stopping current flow so the heat goes away. This allows it to cool and contract back, closing the circuit again, starting the cycle all over. Just my $.02.
     
  5. RickyLee

    RickyLee Well-Known Member VIP Member

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    You replaced the five 12AX7's as well?

    Something to consider is re-tensioning the tube socket terminals. I use a very small jewelers flat screwdriver to do this on the 9 pin sockets, and the end of a paper clip that has been sharpened to a point for the 8 pin sockets. It does not take much to get them tight so don't over do it. Even more important, keep in mind that there could be some voltage stored up in the filter caps that would present itself on a couple of those terminals on the 9 pin sockets, and more of a concern on a few terminals of the octal power tube sockets. I would advise you to drain those filter caps, and you can insulate your tools with electrical tape if it makes you feel safer as well.

    Did you try wiggling the tubes when the amp was powered on and acting up with this issue?
     
  6. dakrow

    dakrow New Member

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    Yeah I replaced all the preamp tubes as well.the volume drop is a hit and miss thing.it does it kinda out of the blue.not really any kinda pattern to it.and sometimes it doesn't do it at all.
     
  7. jcmjmp

    jcmjmp Well-Known Member

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    Its probably a bad connection and/or solder joint.
     
  8. dakrow

    dakrow New Member

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    Any ideas on where to start.
     
  9. FourT6and2

    FourT6and2 Active Member

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    Start with your cables, guitar jack, input jacks and output jacks. Wiggle them around and see if that causes the problem.
     

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