Bassman Noise

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by stebnera, Feb 16, 2008.


  1. stebnera

    stebnera Member

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    Dec 19, 2007
    Location:
    Ohio
    After much banging of the head into a wall and replacement of parts, I'm down to the last noise in the amp, noise that only comes from turning the "Normal" Volume Pot. AB165 w/ bias circuit & PI plate resistor mod. I've tried 5 different 12AX7's in the tube slot, including the one that's noiseless in the "Bass" position. I've replaced all of the plate resistors, all of the bypass caps, tone caps, checked all grounds 10 times over... I'm not really sure what else to try.

    Last night the noise was amplified by turning the treble pot of the normal channel, which then was further amplified when turning the volume, but the volume didn't generate noise on it's own like it's doing today. I removed all of the leads from the tube socket, cleaned the pins, fresh, nice solder joints, and ended up with constant noise that would have caused the power tubes to red-plate had I not killed the power right away. Replaced the bypass caps today and that got rid of that noise, but I'm now stuck with the "volume" pot noise. Seems like it's just moving around on me, with no apparent rhyme or reason.

    Any ideas greatly appreciated.

    Thanks,
    Aaron
     
  2. Wakarusa

    Wakarusa Member

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    May 7, 2004
    Location:
    Indianapolis, IN
    A wandering noise that risks red plating the power tubes sounds like the amp is oscillating to me. Have an oscilloscope handy?
     
  3. stebnera

    stebnera Member

    Messages:
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    Joined:
    Dec 19, 2007
    Location:
    Ohio
    Yes, I have a scope, function generator, and other handy lab equipment. Just to clarify though, while I've dealt with the red-plating noise a couple of times while working on this amp, presently it is gone. I get more-or-less correct, non-fluxuating voltages at every section of the B+ and at every plate & screen (they're all a little high from the schematic numbers, but within reason). I am more than glad though to check for oscillations however you suggest, as the times the constant, red-plate inducing noise has existed, it has appeared for no apparent rhyme or reason based on my knowledge of electronics and circuits, although I am still very much learning about the specifics of amplifier circuits.

    The only noise presently existing sounds like a spring breeze blowing into a microphone and the only action that makes it occur is adjusting the "normal" volume pot of the preamp (ie if this pot is turned down all the way, the amp is silent). I can turn everything in the "bass" section to 10 and no noise, and adjusting the bass & treble knobs in the "normal" section does not increase/decrease the noise.

    About the only thing I haven't tried replacing in this section of the preamp is the tube socket, but inspection of it gives me no reason to believe it is bad. I've also used the chopstick to wiggle all of the terminals and it doesn't add to or decrease the noise... all the leads in this section of the preamp check out... I've cleaned both sides (receptacles and terminals) countless times, it has great continuity from receptacle to terminal, no signs of arcing, free of flux...


    Open to any and all suggestions!
     
  4. Wakarusa

    Wakarusa Member

    Messages:
    1,477
    Joined:
    May 7, 2004
    Location:
    Indianapolis, IN
    Odd. For the "other" noise, I don't get how replacing bypass caps (I assume you mean cathode bypasses on preamp tubes) would have solved that. If the bypass cap was open (or showing high ESR) you'd just get a hi freq rolloff. If one was shorted I could see the preamp tube redplating, but not the power section (unless maybe you had hi freq oscillation, but it's still damned odd).

    One trick that might help localize the problem - use a jumper with clips to short the normal channel triode grids to ground one at a time. If that makes the noise go away it will at least get you closer to the culprit. Should at least tell you if you have a bad ground or bad shunt on the input jacks.

    What's the "PI plate resistor mod"? Probably not the problem since the bass channel is well behaved, but still curious.

    If you can't further isolate the problem, and I'm reaching a bit here, any chance the amp has been stored in very humid conditions? Once in a while the fishpaper board on these will accumulate moisture and present weird problems like you're seeing.
     
  5. stebnera

    stebnera Member

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    Dec 19, 2007
    Location:
    Ohio
    The PI plate resistor mod just means that when I converted the bias circuit, I also removed the 220K resistors that couple the PI plates with the power tube plates ala AA864 circuit.

    I'm not sure about humid conditions, but this has certainly been mind-boggling. The board doesn't seem particularly damp, but it would make more sense than anything I've been able to surmise. I've actually had the amp noise at more-than-acceptable levels 3 times, but my obsession to get the normal channel as clear as the bass channel has led to various other conditions I can't explain. Replacing resistors with new ones in the preamp shouldn't lead to problems in the power section... Anyway, going to try carbon film plate resistors instead of carbon comp and see what happens.

    I'll try the shorting trick too, thanks!
     
  6. stebnera

    stebnera Member

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    Location:
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    Hallelujah! It was moisture in the board. After the carbon film resistor made things worse, I took a heat gun and started blowing over the board. Puddles came to the surface! 20 minutes of alternating hot & cool air, turn it on, silence! Turn things up to 10, silence! Thanks so much!
     
  7. Wakarusa

    Wakarusa Member

    Messages:
    1,477
    Joined:
    May 7, 2004
    Location:
    Indianapolis, IN
    Great to hear! (puddles? sheesh)

    Something you may want to try is baking the amp in the oven. Low heat for a few hours usually does the trick.
     
  8. stebnera

    stebnera Member

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    Location:
    Ohio
    I think I did close to the equivalent, 2hrs with a heat gun. I will say this, it's one way to clean the boards, although definitely not preferred. Yeah, during this time I remembered he lived in an apartment 5 years ago or so that got flooded. Now the blown fuses and short-lived power tubes he complained about make sense, as well as all the trouble I had trying to eliminate the noise for him.
     

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