Help diagnose LOUD BUZZ

Discussion in 'Glowing Bottle Tube Amp Forum' started by tacomamc, Jan 23, 2020.

  1. tacomamc

    tacomamc Tele-Afflicted

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    Amp is a Pro Jr, older USA made. Have JJ tubes (pre and power). The other night while playing at home I had been playing for maybe 10 minutes. I then turned it up to about 4/5 and within a minute there was a very loud buzz. More like the buzzer at a sporting event. I bought new power tubes and they arrived today, and the amp did the same thing.... works ok until I turn the volume up to 4/5, then BUZZ! The volume and tone controls have no bearing on the buzz. I’ve ordered some new preamp tubes, but now that doesn’t seem very likely.... they’ll be here in a few days anyway. Does this sound like a tube issue, or something I need a text to look at?
     
  2. Wally

    Wally Telefied Ad Free Member

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    That problem could have any of a number of causes. I would suspect connection problems caused by vibration. What happens when you turn the volume back down? Does the buzz cease? What happens if you slap the side of the cab while the buzz is going on? If you have a way to hook the amp up to a speaker outside of the cab, can you turn the amp up without the buzz occurring?
    what connection problems could one suspect? Dirty tube sockets. Bad connections on the PCB might be more likely. Vibrations can cause a variety of connectivity problems. It is likely that it is tech time. When you get these other tubes...12a-7’s, correct....I suggest installing them one at a time to see if the search for problem can be localized. If you sub in a tube and the problem goes away, you have cleaned a dirty tube socket contact. Simply pulling and reinstalling a tube can be a valid test and/or cure for a problem like this.
     
  3. Wally

    Wally Telefied Ad Free Member

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    That problem could have any of a number of causes. I would suspect connection problems caused by vibration. What happens when you turn the volume back down? Does the buzz cease? What happens if you slap the side of the cab while the buzz is going on? If you have a way to hook the amp up to a speaker outside of the cab, can you turn the amp up without the buzz occurring?
    what connection problems could one suspect? Dirty tube sockets. Bad connections on the PCB might be more likely. Vibrations can cause a variety of connectivity problems. It is likely that it is tech time. When you get these other tubes...12a-7’s, correct....I suggest installing them one at a time to see if the search for problem can be localized. If you sub in a tube and the problem goes away, you have cleaned a dirty tube socket contact. Simply pulling and reinstalling a tube can be a valid test and/or cure for a problem like this.
    The input jacks are problematic in these amps. When the buzz is happening, does lightly jiggling the plug in that jack do anything? For that matter is there any problem with jiggling the plug/jack when the amp is not buzzing.
     
    Last edited: Jan 24, 2020
  4. tacomamc

    tacomamc Tele-Afflicted

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    I will try some of the things you’ve asked. When the buzz is triggered, the volume and tone controls have no impact (increasing them or decreasing them). I will check the input jack, and try wiggling it some too, to see if it changes anything. I’ll also take out and reinstall the current preamp tubes to see if it’s just dirty. I’m hoping it’s something that doesn’t require a tech, as I imagine the cost could quickly add up to replacing the amp. Thank you very much for your suggestions and for taking the time to respond. I’ll let you know what I find later tonight when I can plug back in.
     
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  5. Commodore 64

    Commodore 64 Friend of Leo's

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    Get a non-conductive chopstick. Take the back off the amp and start poking things. Start with the bigger resistors (they dissipate more heat). I've seen this behavior on HRD where the ribbon cable to one of the tubes had bad connection (on the ground pin). So that was an easy fix. Chopstix, FTW.
     
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  6. tacomamc

    tacomamc Tele-Afflicted

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    Ok, so I played some more. Looked for obvious signs on the board of something amiss. While unplugged, I removed and reinserted each preamp tube. I turned it on and played, then increased the volume to 5, and shortly after the BUZZZZZ returned. Still no change by wiggling the input, or the volume, or the tone. The speaker is connected well. When I smacked it on the top of the amp, it stopped and returned to normal operation. I played again and the Buzzzz quickly returned. I repeated this a couple times. While the amp was running normal, I decided to tap the tubes for any sign of “not right”. The right most preamp tube (first in line right to left facing the back of the amp) rings when lightly tapped with a wooden stick. So I’ll start by replacing that tube when the set gets here this coming week. Hopefully that is the culprit.
     
  7. Wally

    Wally Telefied Ad Free Member

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    It sounds as if you have a connection that is breaking during vibration. Imho, the tube would be a long hot....but one can always use spare tubes. And....EL-84s don’t stand up long 8n these amps since they are running a very hot bias for a fixed bias amp....usually around 100% of max plate dissipation. If you have not ordered or can change your order, ask for some tubes that operate on the cool side of tings.
     
  8. tacomamc

    tacomamc Tele-Afflicted

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    Just for fun, I swapped my preamp tubes (one for the other, but the same two tubes). There is a prominent hum at idle, and an occasional “snap/ crackle”, but the buzz does not return even when the volume is increased. I played for a little while and ran with the volume up to 7 or 8 maybe.... couldn’t get the original buzz to return. I don’t understand the function of each tube, but it would appear that the issue is that first preamp tube. I should get my new 12ax7 tubes tomorrow. I’ll let you know.
     
  9. tacomamc

    tacomamc Tele-Afflicted

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    Got new JJ 12ax7’s today in the mail. I replaced the one tube I suspected of being bad and the amp is working just as it did before all the problems. I just ran it about noon (7/8) on the volume with no issues whatsoever. I’m so very glad it was just a single preamp tube. Now I have some spares for the next time. Thank you all for your input and for taking time to help me with this.
     
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  10. MuddyWolf

    MuddyWolf Tele-Meister

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    This is also a symptom of a cracked solder joint. These work for awhile then fail. Then work . Then fail. Even if you find which one is the problem there will be another one that will fail next. Oh the AGONY! But dont worry there is a cure. Find a good tech to reflow ALL the solder joints and whatever maintenance that has been neglected for far too long. Then be happy.
     
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