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Classic 30 died. Any help?

Discussion in 'Shock Brother's DIY Amps' started by RodeoTex, May 13, 2009.

  1. RodeoTex

    RodeoTex Doctor of Teleocity

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    Let me forst say that this is not my amp so I don't have immediate access to it. It belongs to a friend whom I talked into buying it. Anyway...

    Today he called me to tell me that while he was playing it the sound just started fading to zero. The three 'little' tubes were glowing but the 4 EL84s were not.

    Sounds like a power transformer to me (I'm not an amp guy at all). This is assuming that the preamp stage runs on a different voltage than the secondary. I could be way off here. Please help out as the closest amp tech is about 100 miles away.

    If anyone knows of a direct replacent I'd appreciate it or if that's not the problem based on the facts I know please also let me know.

    All help appreciated guys. thanks.
     
  2. jaimed

    jaimed Poster Extraordinaire

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  3. e-merlin

    e-merlin Doctor of Teleocity

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    Probably a bad tube. Also, it probably took out the main fuse inside when it went. Peaveys are pretty robust. I doubt it's a transformer.
     
  4. RodeoTex

    RodeoTex Doctor of Teleocity

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    OK, so I had him sway in a known good EL84 into each socket, one at a time and it did not work.
    He read on TGP or somewhere that it had to be the power tubes so he bought a new set of 4 Sovtecs. Still the same: little tubes glowing, big ones not, no sound.

    I did check the 3 fuses that I found and checked the output resistance of the power xfmr, all good. Any other ideas?

    I'm sure (well almost) that it is something simple if I just knew where to look. Thanks for any help anyone can offer.
     
  5. e-merlin

    e-merlin Doctor of Teleocity

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    Did you pull the fuses and check for continuity? Oftentimes they may look good or test good on the board but won't be good when you pull them.

    The symptoms you describe are exactly the symptoms of a Classic 30 in which a tube or tubes are going bad.

    One thing to remember is that the tubes are wired in series so if one is bad none of them work. Put in a complete set of good power tubes and replace the fuses with those of the same type and rating. I bet it starts on the first pull...;)
     
  6. Cleeve

    Cleeve Tele-Holic

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    Look at the schematic-
    http://www.schematicheaven.com/newamps/peavey_classic30.pdf

    check R71 and R72. They are two large resistors near one end of the narrow PCB.
    The layout showing the location of those resistors on the PCB is included below the image of the schematic on the link, it's just really small so you'll have to zoom in on your computer, but it's readable. Those wirewound cement resistors sometimes die not from buring up, but they are just big and break the internal connections and/or the solder connections at the board.
    Also check all the solder connections in that series heater circuit, any one crack will take down all four heaters.
    The symptom you describe leads straight to those resistors.

    Oh and as you noticed, all four output tubes must be in place in order for the heater circuit to work.
     
  7. e-merlin

    e-merlin Doctor of Teleocity

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    Any success, RodeoTex?
     
  8. RodeoTex

    RodeoTex Doctor of Teleocity

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    This amp really does belong to a friend so I haven't gotten to see it again yet. I think I'll get him to bring it over and I'll do some checks of the resistors and solder joints myself.

    I appreciate all the input from you guys. The nearest amp tech is 90 miles away, and it would probably mean 2 trips to drop it off and pick it up.

    I'm an electrician with a VERY basic understanding of electronics so maybe I can do something for him.

    I've offered the owner of the amp my acoustic guitar on loan. He's a metal head (stuck in the 80s) and dislikes acoustics. If his amp is down long enough maybe something good will come of it.
     
  9. RodeoTex

    RodeoTex Doctor of Teleocity

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    Well, the owner of the amp called yesterday and told me he found the problem. Seems that one of the jumper wires from one PCB to the other had 'broken'. He soldered it back and everything now works.
    I can imagine one of those wires just breaking, but oh well, whatever.
    Thanks guys.
     
  10. e-merlin

    e-merlin Doctor of Teleocity

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    Wow, that's strange, I've heard of them breaking when people are modding 'em but never heard of one just breaking.
     
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