Yet another "my BFDR makes a funny noise" problem

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by mbratch, Aug 20, 2006.

  1. mbratch

    mbratch Member

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    [FONT=Verdana, Arial]My vintage BFDR, when idle, makes a noise that is I suppose sort of like an occasional wind blowing past an unfiltered microphone. It's not very loud and is not noticeable while playing especially during a gig. But sitting at home it's there and it doesn't seem normal, so I thought I'd ask what could cause this. With volume on say "3" there's a soft white noise background with occasional "soft, muffled crackle" sound.

    [/FONT][FONT=Verdana, Arial]When I try removing any of the pre-amp tubes, the problem seems to be a little worse. More specifically, with everything in place, the noise is slightly there with all volumes set at 1 and gets a little louder (in less proportion) when either volume is turned up. If I pull a pre-amp tube, the noise is a little louder when the volumes are both at 1.[/FONT]

    [FONT=Verdana, Arial] What causes this?[/FONT]
     
  2. Blue Strat

    Blue Strat Member

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    Man, could you have any more amp problems?

    First, does the reverb control affect the noise level? If so, try replacing V3 and V4.
     
  3. mbratch

    mbratch Member

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    Reverb does not affect it.
     
  4. BudLite

    BudLite Supporting Member

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    You want to take a chop stick and poke around the circuit. The resistors get noisy ect....tap them lightly with the amp on and listen for noise...just a thought...
     
  5. mbratch

    mbratch Member

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    BudLight, thanks for the tip. The resistors in the amp are all original, so maybe that's something to check. And I happen to have chop sticks. :)
     
  6. John Phillips

    John Phillips Member

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    Do you mean if you pull either V1 or V2 (but not both) the noise increases? If so, you've almost certainly got noisy plate resistors. Pulling one tube slightly increases the gain on the other channel because they share a common cathode resistor.

    Noisy plate resistors is a very common problem with old amps that use carbon-comp ones - and it sounds like what you're describing. You will probably get differing opinions on this, but if you're going to change them I would stick with carbon-comp if you want to keep the most possible 'vintage' tone. Personally, I wouldn't do it at all unless the noise is really annoying :).
     
  7. mbratch

    mbratch Member

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    Thanks John. Yes, the noise is there when I pull V1 or V2. I didn't try pulling both concurrently.

    I may just leave it. In a gig or practice scenario, I really don't hear it. Only when I'm practicing at home. It may make a good sound man poke around to find out what the noise is when things are supposed to be quiet. ;)
     
  8. slider313

    slider313 Silver Supporting Member

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    Check for a slightly sprung input. This will cause the noise your describing also. If it's the input,retension it and clean it also.
     
  9. Blue Strat

    Blue Strat Member

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    If pulling both V1 and V2 doesn't cure it, try V4. If that does it, it's either the 2nd plate resistor for V2, the coupling cap, or resistor or cap leading to the grid of V4.

    In any case, replacing all plate load resistors for V1 and v2, plus the treble cap, would help eliminate some contenders.
     
  10. mbratch

    mbratch Member

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    Thanks for all the suggestions guys. I hope to check these out the upcoming weekend.
     
  11. jpervin

    jpervin Member

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    Don't know if this would be a possibility or not, but I just recently built a 5E3 clone, and at home I usually have it plugged into a power strip that I also have an overhead fluorescent light plugged into. There's definitely an audible noise, which gets louder when the volume or tone control is turned up. However, I played this amp at church this past Sunday and it was dead quiet, and that was with the volume up to about 2.5 and the amp was miced.
     
  12. donnyjaguar

    donnyjaguar Member

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    I agree with JP on this one. What I have found though is that if the amplifier is making this noise, even at low levels, its a plate resistor that is already way out of spec. Usually you can nail it down with a voltmeter because the plate voltage will be unexpectedly low.
     
  13. mbratch

    mbratch Member

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    I finally replaced the 100k plate resistors with new carbon comps. The problem is gone for now. Time will tell if it stays gone (it's disappeared briefly before, prior to the resistor changes), but I'm optimistic based upon the expertise provided on this thread. If my ears are telling me true, the amp overall just sounds better, too.
     
  14. lchyi

    lchyi Member

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    I have the same problem on two of my amps. An AC30 (new) and a TRRI. They make the same "wind through the microphone" type noise. At first someone told me it was the power, I've been to about 5 different places with the TRRI, it's not the power. Someone told me it was the tubes, so I tried replacing the tubes. Nope not that either.

    Now I will try to go replace the plate resistors. Great, I hope this is it! It seems like this problem is more common that I thought but the solution doesn't seem to be.
     
  15. TopJimmy5150

    TopJimmy5150 Member

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    I had the same problem. I cleaned the tube sockets and all is now well. Get some tuner cleaner from Radio Schmack, spray a little on the tube pins and push them into their sockets, in and out 6 times. Worked for me. It sounded a little scary the first time I fired it up...I think the tuner cleaner hadn't dissapated completely. But since then, it's been a slice of heaven.
     
  16. mbratch

    mbratch Member

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    Well... after updating all the plate resistors and after a couple of practices and a gig, the noise seems to be back. It comes and goes. :(
     
  17. go7

    go7 Member

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    I cured my ab763 circuit of noise .I put a 5751 in v3.If this doesn`t work put another one in v4. The 5751 is an amazing tube.Soon to be "the next dicovered valve" according to Watford Valve Review.Thankfully I already have my stash.
     

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