Bandmaster Humming?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by inscho, Mar 23, 2006.


  1. inscho

    inscho Senior Member

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    I bought a 67 bandmaster from crusty cabs a month and a half to 2 months ago. When the amp arrived I didn't have a speaker cabinet, so I went to guitar center to plug it into a marshall cab of theirs just to see if everything was fine. Completely quiet. Everything working fine.

    Finally I got my speaker cab yesterday, hooked everything up and turned on the amp. when I switched it off standby there was a fairly loud 60-80 cycle hum. I turned it off unplugged my cable, turned it back on same thing. There was also a cracking/sputtering noise that faded in and out. I tried different outlets in different rooms, different cable, tried all 3 positions of the ground switch, etc. Still no luck.

    When I bought the cab I was told that it had been fully serviced, and he is a very respected cab builder so I wouldn't think it was a cap issue. And it hasn't been played on or moved in about a month.

    any advice would be helpful.
     
  2. brad347

    brad347 Member

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    FILTER CAPS!

    what do you know about them? If they haven't been changed it sure is time (long overdue) and here are the three main symptoms of bad filter caps:

    1. loud 60 cycle hum
    2. crackling sputtering
    3. amp shutting down

    NOW YOU KNOW! :)
     
  3. inscho

    inscho Senior Member

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    well I was told it had been fully serviced when I bought it, and that there was no noise issues whatsoever. are there instances where a filter cap will go bad even after it had been replaced recently?

    and how much does recapping typically cost?
     
  4. brad347

    brad347 Member

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    If you are qualified to do it yourself: about 25 bucks (give or take)

    If you have to pay a tech to do it? who knows. Someone else (one of the pros) will chime in.

    It is also possible your issue is caused by something else. Did you try a different speaker cable? (or is yours brand new?)

    A filter cap can go south after it is replaced if it is defective or of someone installed it wrong, or if it is the wrong rated voltage. I'd inquire about who did the job. If you are careful not to touch anything you can even take the lid of the cap can off and look inside and take a pic of what you see and post it here. I repeat though, be VERY CAREFUL not to touch anything, unless you know how to properly discharge caps.

    Also, "Fully serviced" means different things to different people. To some people, it means "go through the amp and replace what is definitely bad right now." To others it means "Go through the amp and replace every single damn component besides the pots and the transformers." And finally, to some it means "replace filter caps and maybe some other electrolytics if they are old, leave everything else alone of the amp sounds good." There are also various stages in between...
     
  5. Swarty

    Swarty Member

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    A dead or partially dead output tube would be the first thing I would check for.
     
  6. brad347

    brad347 Member

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    oh yeah and there's that :)
     
  7. inscho

    inscho Senior Member

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    meaning power tube? can I just pull a tube when its on to detect if thats the problem?
     
  8. brad347

    brad347 Member

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    Along those lines, also check out the rectifier. A bad rectifier tube can also cause humming and the other symptoms.
     
  9. inscho

    inscho Senior Member

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    well it has a solid state rectifier so that wouldnt be a problem right?
     
  10. brad347

    brad347 Member

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    probably wouldn't be a problem, correct.
     
  11. jpervin

    jpervin Supporting Member

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    This happened to me. I have an Allen Bassman combo that one day, just out of the blue emitted this nasty 60-cycle hum when I flipped the standby switch. Upon David Allen's recommendations, I swapped out the 2-6L6s that were in it for another pair and the hum was much less noticeable. After ordering a matched pair of the exact same 6L6s that were in the amp before, the hum was completely gone.
     
  12. inscho

    inscho Senior Member

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    I hope this is the case, I was planning on replacing the sovteks that are in it anyways. I will have to check when I get off work.
     
  13. jpervin

    jpervin Supporting Member

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    When you pull the tubes that are in your amp now, look for any discerning burn marks or pitting on them. The bad power tube that was causing my amp to hum had a yellow-ish burn mark on top of the center post that the 8 metal pins surround.
     
  14. inscho

    inscho Senior Member

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    Here is what Steve/Crusty said:


    "When I serviced it, I replaced all the electrolytics in the amp. They're usually the only ones you want to replace, because they dry out after a certain amount of time, and they don't affect the tone of the amp. Replacing tone caps is generally done only when one has drifted way out of spec, as to cause abnormal operation, or gone belly up completely.

    The hum I'm not sure about. Could be a tube. The crackling definitely sounds like a tube, or maybe the plate load resistors. I can't remember now whether I changed them out or not. I don't remember what I sent out in that amp.

    I gave it a good workout before I boxed it up, and it performed fine for me."

    Steve
     
  15. brad347

    brad347 Member

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    All signs point to your power tubes.
     
  16. inscho

    inscho Senior Member

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    so I can pull one power tube at a time to check this? it won't damage the amp?
     
  17. brad347

    brad347 Member

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    it might not be too good for your output transformer. It would probably be OK but DON'T TAKE MY WORD FOR IT as i'm not an expert and probably know barely more than you. Wait for the experts to weigh in.
     
  18. jpervin

    jpervin Supporting Member

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    As long as you replace it with a different tube when your doing your check. Do a tube swap, one tube at a time and don't leave any tube sockets empty.
     
  19. inscho

    inscho Senior Member

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    alright thanks!

    also when I pull out a tube to look at it, what are the signs that it has gone bad? I don't have an extra 6l6gc laying around, but I am going to order a jj retube kit fairly soon.
     
  20. brad347

    brad347 Member

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    before you buy a complete retube kit look at the other tubes in your amp. Are there any american or british made tubes or all they all newer production (GT sovtek, EH, etc.)? If so, then those are likely to perform better than the new JJs even though they are old. I'd say replace your preamp tubes only if something is wrong or if you want to experiment with how each preamp tube affects the sound, but otherwise leave them in.

    Especially, your tremolo 12AX7 has NO EFFECT on the sound as it's not in your signal path. It is a good place for your cheapest/weakest 12AX7. Just FYI....
     

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