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Problem with my amp is bugging me at work and looking for help.

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  • Problem with my amp is bugging me at work and looking for help.

    Hey everyone, first let me start by introducing myself. I'm Tyler and this is going to be my first post.

    I have come to you all because the amp I play through has developed a very unpleasant issue. It's a B-52 AT-100 head. up until a few days ago it played fine. The only thing that I noticed about it was that the tone was harsh and shrill, not uncommon on these amps seeing how it still has the stock sovteks in it.

    before the "go buy a new amp" "b52 sucks" "components are lame, give up" comments start, understand that I have owned one of these before and after borrowing my friends tung sols and trying them out in the pre amp these amps sound amazing. Sovteks really do sound THAT terrible in this amp, believe me.

    On with the issue, the amp has a class A, class A/B, and solid state mode switcher on the back. Running in class A, the amp sounds alright. It only lacks the room shaking volume it used to have, but it still gets loud. It also lacks gain in this mode, only gets about half as much as before.

    Running in solid state, again it sounds alright. Solid state is the loudest of all three modes and has the most tonal characteristics (bass depth etc)

    Running in class a/b, the amp has almost no volume, the master and channel volume will be at half to 75% and the amp will sound thin, farty, and overall crappy as if it was a blown speaker. Not even one bit useable on stage. (does this while in distortion channel, i forget if it does it in the clean. can check today.) *it reminds me of this old peavy Solid state amp i used to have, when i would forget to put my foot switch in it would get "stuck" in this mode where it was neither distortion or clean, just complete and utter garbled treblish fizz, i would put the F/S in and it would go away.*

    I can't afford a retube for another couple weeks but I have my questions as to whether it's a tube issue, the tubes all light up just fine in the preamp and power section, the recto tube lights up fine too, no microphonics coming from tubes (did the tap test). There WAS a couple of white powderish specs inside my reciftifer tube but very minimal, looks like charring from typical heat TBH.

    What should I be looking for? I love this amp and quite honestly it has the sound i want, and i just don't want to give up. It really does give you that thunderous loose distortion that a mesa does only with the tonal imperfections that make you different from the mesa players (cause there's a real lot of them, and i like to be different.)

    Thank you everyone, I'll be at work until 8:00 AM Eastern time, and awake at home troubleshooting my amp until 9AM to noonish.

    SCHEMATICS can be found here: http://music-electronics-forum.com/t9765/ (Sorry, I don't know how to embed or upload an image!)
    Last edited by tpawlowskis; 10-15-2012, 08:30 AM.

  • #2
    Lack of volume, poor tone... My first suspect would probably be the tubes. Maybe the power tubes, maybe the rectifier, maybe even one of the preamp tubes. They can still light up, look fine, and yet be completely shot.

    Can you borrow some known good tubes to swap in?
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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    • #3
      Originally posted by Steve Conner View Post
      Lack of volume, poor tone... My first suspect would probably be the tubes. Maybe the power tubes, maybe the rectifier, maybe even one of the preamp tubes. They can still light up, look fine, and yet be completely shot.

      Can you borrow some known good tubes to swap in?
      Thank you for your response. I don't have tubes right now and can't afford them for another week from this thursday and then I need to wait for shipping. I also apologize for just getting back to here now, I work an overnight shift so I have to sleep in the day time. The update as of now is that I took the amp out of the chassis, noticed one of the internal fuses was blown. Happy to find the problem, i changed the fuse, started the amp back up and prepared to rock. Took the amp off standby and this time the whole amp died and lost power. Noticed a small but very bright spark out of the corner of my eye right before it happened too. it blew the same fuse again and i put a new one in, started the amp up and this time i can't even get power to go through it. no tubes heating up nothing. checked the pilot light for continuity and its still good. is there other needed?

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      • #4
        Okay, everyone, I have some great news. I was voltage testing the +120 volt lead after the ac power cord and then i just touched the main ground and guess what.. 55V. So i thought this was wierd, i decided to take the power outlet off of the amp (didn't need to do that). but upon studying it very closely there's actually this black cartridge that pops out with another fuse! sure enough the fuse was blown and I changed it and took all the tubes out and we have power and standby working. This points me to the problem, not being a circuitry malfunction but instead probably a shorting tube. Anyone agree?

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        • #5
          Yes, it is probably a power tube or the rectifier tube. If the internal fuse that is blowing is F4, then the rectifier tube is probably ok and it is most likely a power tube.
          Originally posted by Enzo
          I have a sign in my shop that says, "Never think up reasons not to check something."


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          • #6
            Originally posted by g-one View Post
            Yes, it is probably a power tube or the rectifier tube. If the internal fuse that is blowing is F4, then the rectifier tube is probably ok and it is most likely a power tube.
            I feel the same. Now that the amp powers on just fine with all the tubes taken out I think it's safe to say this problem is fixed until I put new tubes in it. Since there are many B-52 troubleshooters out there that could benefit from this thread, I will follow up with everyone when I retube it with some sweet sounding JJ's and gold pins from eurotubes. Those guys are so good to me.

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            • #7
              Just an update for everybody, The full set of JJ tubes were put in the B-52 head today, and it sounds epic. I will be posting sound clips on youtube this afternoon. If I measured 445v across the powertube fuse terminals, increased the bias to 455, does anyone know if that is an efficient setting to run the amp at? Eurotubes specifies 460, so I figured I'd play it safe.

              Thanks for the help everyone.

              Comment


              • #8
                The clip is done, sounds kinda crappy and I don't know why, i used a pod xt live and a conenser mic and it sounded great till i put it in movie maker. I really have to get sleep for work though, try get past the wierd clipping problem.. its not too bad but i can notice it. b52 test - YouTube

                Im not the best player, but i sure try to practice alot. tell me what you guys think of the tone!

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