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NAD bummer. Chasing a buzz - Airline/Valco GIM 9171A

Discussion in 'Glowing Bottle Tube Amp Forum' started by ArcticWhite, Apr 15, 2017.

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  1. ArcticWhite

    ArcticWhite Tele-Meister

    142
    Apr 14, 2009
    Portland Oregon
    Bzzzzzzzz....

    That's the sound of this cool old Airline/Valco GIM 9171A I picked up last week (for a $ong). Otherwise the amp works fine. Tone is awesome. Reverb and tremolo work great...

    But there's way too much buzzing and hum.

    Symptoms and facts:
    • The amp has a significant buzz at idle, with all pots turned to zero, and no guitar plugged in. But it really starts to hummmm as I turn the reverb up. With the reverb turned all the way up, the hum is at conversation volume, with no instrument plugged in.

    • Only the reverb pot has any effect on it. The humming does not get louder if the volume is turned up on either channel.

    • The hummm is still present even if I unplug the reverb tank (Hammond by the way).

    • If I bang on V1 with a chopstick, it sounds microphonic. I've isolated this to the 6 and 7 terminals on the tube. 7 is a shielded cable. If I tap either of these terminals, I get a microphonic report.
    • The amp sat in a garage in Florida for years, and the chassis and transformers are rusty. The inside of the chassis is clean.
    • PT is rusty but tightly bolted to the chassis.
    • My power outlet is good. Other tube amps work fine plugged into the same outlet.
    • All electrolytic caps were replaced with Nichicons by a superbly qualified tech/electrical engineer a few years ago.

    • All tubes are verified good. I swapped in different vintage 12ax7s (I have a bunch of 60's RCAs and the like) and Sovtek power tubes. All tubes are tight.

    • There's a new three prong cord and the death cap is gone. Pots are clean.
    Ideas? I've attached photos and a schematic for your viewing pleasure.

    [My qualifications: I have rebuilt and modded several amps. I know how to discharge big caps without killing myself, and I can solder. I'm not trained in electronics theory, but I get the basic ideas.]


    20170414_225355.jpg 20170414_225417.jpg 20170414_225521.jpg 20170414_225536.jpg 20170414_225549.jpg
     

    Attached Files:

  2. ArcticWhite

    ArcticWhite Tele-Meister

    142
    Apr 14, 2009
    Portland Oregon
    El bumpero. Anybody?
     
  3. aerhed

    aerhed Tele-Meister

    Age:
    58
    223
    Dec 24, 2016
    Boulder, WY
    All I know is those amps did that when I was a kid fifty years ago. So I guess you're good.
     
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  5. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    Since the buzz is only controlled by the reverb control it's most likely coming from the reverb circuit.

    Since the buzz is still there when the pan is pulled the problem should be between the pan return jack and the reverb control--the reverb recovery circuit.

    Take a very close look at the pan return jack ground. Is it isolated? Grounded directly to the chassis? A bad ground here is a prime suspect.

    Pull the tube that serves as reverb recovery and verify the buzz goes away.

    Try another recovery tube.

    Clean and retension the reverb recovery tube socket. Pin corrosion can cause your symptoms.

    Chopstick the entire reverb recovery circuit.

    Inspect the circuit with a magnifier, measure component values, re-flow solder joints, etc.
     
    ArcticWhite likes this.
  6. BobbyZ

    BobbyZ Poster Extraordinaire

    Jan 12, 2011
    Snellman MN
    Haven't messed with it in years but my GIM 9111 buzzes too, seemed like more of a transformer buzz than anything.
    They're cool but not exactly built to Fender standards of the time. Lead dress could be an issue as well.

    The reverb pan is really close to the chassis. There was probably a thick piece of paper between them with thin aluminum foil or paint on it at one time. Maybe a new better Faraday shield would help here.
    If you have florescent lights above the bench shut them off. (don't ask how I figured that out)
     
    ArcticWhite likes this.
  7. Bill Moore

    Bill Moore Tele-Holic

    I bought an old Airline in 1968, (used), and it had a hum. While in the Navy, I had some of the techs look at it, they changed a few tubes, and it was better, but still hummed. Fast forward to a few years ago when I rediscovered it, and got it working again, (some caps, resistors, a power tube). It still had a hum, I started moving wires around with a drumstick, and moving one 1/4" stopped the hum! It has been quiet to this day!
     
  8. Greenmachine

    Greenmachine Tele-Holic

    631
    Nov 28, 2011
    New Jersey
    Well I'm no pro, but I had hum in a silver tone amp and did this with some success:

    Redo heater wires and elevate them.
    Star grounding with isolation washers (a hotly contested topic)

    If that were my amp I'd try those things, but get ready it's a big of work.
     
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