Ampeg SVT-CL: Can potentiometer cause crackling/fuzz/buzz when it's not being rotated?

Discussion in 'Amps and Cabs [BG]' started by JuanB, Jun 23, 2021.


  1. JuanB

    JuanB

    Feb 20, 2010
    I have an Ampeg SVT-CL that has been giving me intermittent crackling/fuzzing/buzzing that seems to happen especially when I hit low notes, but can happen with high notes as well. I've gone through the following troubleshooting steps:
    1. Plug in pre-amp out of another amp (Ampeg B2R) into SVT's power amp in. No noise this way. The power amp section is probably good.
    2. Reflow solder in pre-amp board input jacks as one showed a crack. This helped quite a bit as this was contributing to the noise but it wasn't the only source.
    3. Instrument patch cable between pre-amp out and power amp in. Noise still present.
    4. Tapped on the various tubes with power on. Lots of crackling when tapping phase inverter tube. Replaced tube. Seemed to improve but still intermittently present. Could have just been corroded contacts that was contributing to the problem.
    5. Improved ground connection on pre-amp board pad that touches the back left stud (when facing amp). Again, seemed to help a bit but problem is still intermittently present.
    6. Changed pre-amp tubes one at a time. Noise still intermittently present. Put original pre-amp tubes back in.
    I noticed that the gain pot is pretty scratchy if I move it while there's a signal through the amp. I put a little Deoxit in there and rotated it a bunch. This is the last thing that I've done so far and it seems to have eliminated the problem although I haven't had the opportunity to test it long enough to confirm. Is it possible that a potentiometer can cause this issue while it is stationary? Thanks for reading.
     
  2. Wasnex

    Wasnex

    Dec 25, 2011
    Yes. I have amps with worn spots on some of the pots. If the pot is adjusted to one of the worn spots, the signal has a distortion that could be described as a crackle.
     
  3. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    Yes, it is possible, BUT on an amp like that it's just as likely that the underlying problem is residual DC either from leakage or a failure of another part. DeOxit may mask the problem for a while, but if there is DC present it can cause deterioration of the pot's element at the contact points of the wiper on the track.
     
  4. JuanB

    JuanB

    Feb 20, 2010
    I'm assuming this isn't something I can easily check with a multimeter.
     
    Wasnex likes this.
  5. Wasnex

    Wasnex

    Dec 25, 2011

    It not hard, but you need to know how to read schematics, and it is necessary to partially disassemble the amp to get to the test points.
     
    Sailfish and agedhorse like this.
  6. JuanB

    JuanB

    Feb 20, 2010
    I can read schematics. I took some EE courses in college. Schematics typically don't show operating voltages though, do they?
     
  7. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    Sometimes, sometimes not, but there should be 0VDC across all of the pot terminals.
     
    dan1952, Sailfish, FenderB and 2 others like this.
  8. JuanB

    JuanB

    Feb 20, 2010
    I will check that! Thank you!
     
  9. Wasnex

    Wasnex

    Dec 25, 2011

    It's not necessary to know the precise operating voltages for this problem, but some (most) of the Ampeg schematics do show operating voltages. Real world voltages are often higher than what is shown on the schematics, in my experience.

    With exception of the bias controls, the pots should not have any significant voltage on the them. In this particular amp, the gain and volume controls are even capacitor isolated from the small signal tube's control grids. If you look at the schematic for something like a Fender AB763 circuit, the Volume control is connected to the control grid of a 12AX7, so there would be some voltage on the pot, but not the case for an SVT CL.

    The SVT CL schematic shows 7V across the bass and treble circuits and 8V across the resistor network that feeds the Master. I believe these are AC readings since these circuits all appear to be capacitor isolated from sources of DC.

    The preamp schematic can be found here: Ampeg SVT CL-3.pdf (tangible-technology.com)

    Parent directory: Index of /schematics/Ampeg/SVT/Classic/ (tangible-technology.com)
     
    Haroldo and JuanB like this.
  10. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    All of the DC voltages across the pot terminals are relative voltages, the terminals can all be 10V above ground but as long as they are all exactly the same there's no DC current flowing and no issue.
     
    dan1952 and ddnidd1 like this.
  11. Primary

    Primary TB Assistant

    Here are some related products that TB members are talking about. Clicking on a product will take you to TB’s partner, Primary, where you can find links to TB discussions about these products.

     
    Aug 2, 2021

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