Amp Buzzing?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Rick1114, Sep 19, 2008.

  1. Rick1114

    Rick1114 Member

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    555
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    My 1957 GA5's power transformer recently died - so while i was waiting for a replacement from mercury magnetics I went head and replaced all the electrolytics and tested most of the resisitors (some had drifted but none more than 10-20%). I installed the new PT today (I just wired up the new one exactly like the old one) and the amp plays, but it now buzzes, almost like a blown speaker type sound. The buzzing is more prominent when playing the low E & A strings or while doing bends - when playing with a light touch, it's not so apparent. Does anyone know what this may signify? I'm stumped at this point - all the solder joints look okay to me, but the filter caps were kind of tricky because of the poor layout in this amp....

    Thanks,

    Rick
     
  2. earthmud

    earthmud Supporting Member

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    I'm having the same type of issue with my vibrolux and my tech thinks it's the OT or the coupling caps. The amp sounds great when played at low volumes or with a light pick attack but the buzz really comes out with the low E and A or when using low to medium gain.
     
  3. SatelliteAmps

    SatelliteAmps Member

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    Gotta ask the obvious first. Does it happen when you try it with a different speaker? Does it happen with a different guitar? Having a filter in backwards could be an issue, or even how the filters are grounded. Did you add a three prong plug? Does this amp have a floating ground? I think it did, which means the filters should be floating as well. Are the jacks rusty? Sometimes jacks get rusty and can be intermittent when being played. And of course, go back over all of your solder connections, just to be sure. Don't just assume a visual inspection will be ok. All things to look at.

    Earthmud: Note specific buzzing is probably not the OT.
     
  4. Rick1114

    Rick1114 Member

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    555
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    Jun 20, 2007
    Before the PT blew, the jack, speaker etc... were all fine. I supect a failing component although hooking up another speaker is a good idea. The caps are all positioned correctly and set up with the floating ground. Basically I just swapped in he new components to the old layout and left the two prong for now.

    I just did he chopstick test and he only noisy area was by PIN 2 of the preamp tube thats conncted to the volume pot/coupling cap. I'll ry swapping that cap out and see what happens..

    I hope your right on that one!
     
  5. mattwasserman

    mattwasserman Member

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    I am having a similar problem with my amp (circuit based on a deluxe reverb). I tried a few different speakers and put some rubber around the chassis in case it was vibrating and I still hear it. Also tried different guitars with the same results, could this be a power tube issue?
     
  6. earthmud

    earthmud Supporting Member

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    I've swapped speakers, guitars, cables, all power and preamp tubes, power supplies, and the buzz, fizz, is still there. It's not overly pronounced but still very distracting. My tech is going to install a test OT, and if it's not that then the coupling caps. The guy is super qualified ( he worked for fender when my amp was built) and seems to know his stuff but I'm hoping for coupling caps for the price point sake. This issue has been driving me nuts for months. The problem is more gain specific than note specific. If I run it into a proanalog dual drive at a very low gain settings it does it. If I run the amp clean, no pedals but at high vloume it also does it. It starts when a chord or note is played but fades before the note dies. More like harmonic distortion overtones or a fuzz with a weak battery. The sound the OP is talking about sounds dead on with my own so I'm eager to figure out the issue and get back to playing. Whats the best way to test a OT?
     
    Last edited: Sep 20, 2008
  7. chaz

    chaz Member

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    Saw a similar buzz problem here when a transformer was replaced in an old Twin. Isolating the transformer body from ground cured the problem. Some kind of weird ground loop thing was going on.
     
  8. SatelliteAmps

    SatelliteAmps Member

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    Best way to test a transformer is with a multimeter. Do you have a good one to use? If so, check a few things (tubes out), such as the outside primary to the centertap primary. Check both sides and see how close they are. Be careful, the centertap is directly connected to a cap that has high voltage on it. Enough to cause serious pain or death if the amp is plugged in.

    Sometimes transformers can vibrate, and cause a weird noise. Rare, but possible. You would be able to feel it vibrate when it was on.

    Ground loops are also possible, but don't really sound like the right solution to these symptoms.

    Chopstick test on pin 2, with the tubes in. Did it sound microphonic, or gravely dirty, or what? Could just be a loose socket pin that is intermittent.

    And yes, these issues can be tube related.
     
  9. Rick1114

    Rick1114 Member

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    555
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    Jun 20, 2007
    I just swapped out the coupling cap and and it's not that. I'm going to resolder the joints to the PT wires and then check some voltages with a multimeter. I have a gerald weber video that shows how to do this - I guess i should have done that in the first place. It's a real bees nest in here - looks like a valco inside!
     
  10. SatelliteAmps

    SatelliteAmps Member

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    Try cleaning the tube socket that had noise on pin 2. Then take a toothpick or dental pick and retension the socket there. Might be all the problem.
     
  11. skipm45

    skipm45 Member

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    Buzzing like 60 or 120Hz noise?
    If yes, try decoupling it to ground by connecting a .047u /600v cap across the first filter cap.


    Skip
    www.skipzcircuits.com
     
  12. Rick1114

    Rick1114 Member

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    555
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    I think I got it fellas...(I hope)

    After some research I read that plate resisitors can be very suspect with note crackling. So I tested them and the one going between the coupling cap & the output tube is dead - doesn't even register anything on the meter. The other two register 220 & 320, so I'll be ordering some parts pronto. I didn't check this resisitor before because It required complete removal to test because of the way the amp was laid out. Hope that does it!

    Rick
     
  13. Rick1114

    Rick1114 Member

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    555
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    Jun 20, 2007
    I replaced the blown plated resistor last night and the amp is still
    crackling :BITCH.

    Does this sound like a sympton of a blown output tranny? (crackling/buzz along with note when playing).

    Rick
     
  14. SatelliteAmps

    SatelliteAmps Member

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    Not really, blown output trannies usually sound weak or dead. Crackling and buzzing can be two different symptoms. If you've gone through everything above, and it still doesn't cover it, then there are a few options (but try everything else listed above first). One would be to check every part in the amp, each resistor and cap, and make sure they are all within spec (in particular plate and screen resistors, all of them). Another option would be to take it to a tech to have them check it out.
     
  15. Rick1114

    Rick1114 Member

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    555
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    You are the man! I finally took your advice and checked the 12AX7's tube socket and the noise disappeared. The other two tubes had very loose sockets that I re-tensioned but the 12AX7 had a cover and seemed securely in place so I overlooked it. When I took the spring loaded cover off and adjusted the tube some, the noise finally went away.

    Thanks again for all your help.

    Now i just have to tweak he amp to taste and install a 3 prong plug!

    Rick
     
  16. SatelliteAmps

    SatelliteAmps Member

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    Glad to help. These kinds of things can be maddening.
     
  17. Danohss

    Danohss Member

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    Just finished bringing a beat up TWin Reverb back to life, and I have a similar issue. One of the speakers makes that same weird buzz when hitting the low E and A strings. Disconnected the buzzing one, the other is fine. The buss is weird in that it's not apparent when the string is first picked, but as the note decays the buzz comes on. Thinking since its in only one speaker it's time for a new one, but I've never encountered this odd effect. Normally a blown speaker is buzzy full range, full time. Any thoughts?
     
  18. scrimpus

    scrimpus Member

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    Sounds to me like the speaker is not totally blown but you have a damaged voice coil. Either way its time for a new speaker.
     

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