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Weird noise that may or may not be a tube going bad...

Discussion in 'Amps and Cabs [BG]' started by Lonnybass, Mar 12, 2009.


  1. Lonnybass

    Lonnybass

    Jul 19, 2000
    San Diego
    Endorsing Artist: Pedulla Basses
    Ok, I noticed a weird hissing sound when boosting the volume knob on my #1 Pedulla...I thought it was a bad pot at first, but then, I started noticing the same problem from a couple different instruments. Turn up the volume on the bass and the hiss builds exponentially, then mysteriously drops out once the knob on the bass is turned all the way up.

    Tried a few different cables, same thing, same weird noise. Also tried hooking up the amp to a different power circuit to see if it was line noise, no change.

    I'm fairly convinced it's something in my preamp (Alembic F-1x) or somewhere else in the amp rack, more than likely a tube...but if there is anything else that it might be, I'd like to find out as soon as possible so I can get things into the shop tomorrow and back in time for my gigs beginning early next week. Last time the tube needed replacing, the amp was noisy on its own, but it didn't seem to have anything to do with where the volume knob on the bass was set.

    Troubleshooting suggestions welcome!

    Lonnybass
     
  2. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    I have no idea, but considering a 12AX7 is only $10-15, it's worth a shot. Worst that happens is you have a spare tube.
     
  3. Lonnybass

    Lonnybass

    Jul 19, 2000
    San Diego
    Endorsing Artist: Pedulla Basses
    The strangest part is that the hiss becomes more present when the volume knob is boosted - even when there is no signal coming off the bass. Again, all signs would suggest a bad pot, but this volume knob hiss is present coming off of like 4 basses into my rig. Ideas?

    Lonnybass
     
  4. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    That is reasonably normal for noise...... could be a tube issue, probably is.

    It's a "source resistance" issue. When the tube has leakage or other internal cause of noise, it usually is reduced if you short the grid to ground. The higher the grid resistance, the more the noise.*

    if you "look back into" the pot from the grid of the tube, it is typical for the resistance to be highest "from that direction", with the pot at mid position. That means the largest resistance for the noise to be developed across.

    * Noise can be inside the tube, in which case it is shorted by low resistance to ground, and made more obvious by higher resistance. Noise can also be "thermal noise" which is actually developed in the resistance. You almost certainly have the former.

    So move tubes around and see if it changes. Then you can get a new tube if it does.
     
  5. Lonnybass

    Lonnybass

    Jul 19, 2000
    San Diego
    Endorsing Artist: Pedulla Basses
    This is some great info. I really appreciate it. I actually picked up a replacement 12AX7 tonight on the way home but sadly it is slightly too wide for the sleeve ring inside the Alembic, so I'll have to try again tomorrow morning on the way to rehearsal. Fingers crossed this solves el problemo!

    Lonnybass
     

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