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Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com

Why Signal Negative DC Offset at High Volume

Discussion in 'Glowing Bottle Tube Amp Forum' started by robrob, May 21, 2014.

  1. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    While testing my 5e3p Proluxe (fixed bias 6L6) with the Normal Channel cascaded into the Bright I noticed on the oscilloscope the signal (sampled at the Bright Volume input after V1A and B) offsets quite a bit south when the Bright volume is pushed past 9 (of 12). Anyone know why this happens?
     

  2. CoolBlueGlow

    CoolBlueGlow Tele-Afflicted

    Nov 20, 2011
    Arkansas
    Put a VTVM there and look for real DC
     

  3. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    I should have said I'm injecting a 400Hz sine wave into the 'Gain' Hi input. I didn't pay attention to the amount of DC offset when it happened but I'm not finished testing so I'll try to get a good measurement of the offset.
     

  4. CoolBlueGlow

    CoolBlueGlow Tele-Afflicted

    Nov 20, 2011
    Arkansas
    Well, there really shouldn't be any DC there. Are you sure your scope is o.k and set up right?

    Is your bright volume pot inexplicably scratchy when you adjust it WHILE the 400Hz tone is going? That's a clue that DC is actually present and you're not chasing phantoms. DC can make pots get scratchy.

    Maybe you have a slightly leaky coupling cap that only fails when loaded with signal? .1/400 caps there, correct?

    I'd carefully check the ground quality at both vol pots too. Gotta have a good ground there. If they are merely soldered to the back on the pot, you might check the torque and cleanliness of the shafts, nuts, locks and chassis on the pot shafts.

    If DC really is present where you suspect, it will go right to the grid of V2. Practically speaking, this shifts the operating point of V2A in the wrong direction. This will definitely have a negative impact on your tone, which might not be obvious at lower volumes, etc.

    Check for any DC there at V2A grid with your VTVM. If DC appears to be there, then as a final test, temporary in a .2/400 cap in series with your scope probe. If the supposed "DC" disappears, you have a leaky cap. If it remains, you have a measurement problem with your scope.
     

  5. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    Thanks for the help CBG,

    I'll double check my scope coupling. The Bright volume is not scratchy but I'll verify the Bright vol ground to the preamp ground bus. I'll measure for DC at the Bright vol pot and V2A's grid. I'll let you know what I find.
     

  6. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    Looks like it could be classic blocking distortion:

    From aikenamps.com:
    I'm going to focus my efforts on the Gain Channel because I want to leave the Bright Channel alone if possible. My first change will be to reduce the Gain Channel's coupling caps' value. I'm also going to take a close look at how the Gain Lo channel compares on the oscilloscope. If its higher grid stopper value helps reduce the blocking distortion I can increase the Gain Hi and Lo grid stopper values.
     
    Last edited: May 22, 2014

  7. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    I'm looking at the Marshall JCM 800 master volume preamp (lower of two below) for ideas. I'm going to try its first gain stage of the cascade's .022 uF coupling cap value. That first gain stage uses an ECC83 with a cathode bias resistor of 2.7k. Why would it be higher than the 1.5k norm? How would a more positively biased cathode help?

    [​IMG]
     

  8. CoolBlueGlow

    CoolBlueGlow Tele-Afflicted

    Nov 20, 2011
    Arkansas

  9. CoolBlueGlow

    CoolBlueGlow Tele-Afflicted

    Nov 20, 2011
    Arkansas
    Yes, as Mr. Aiken so eloquently explains, Blocking distortion will cause that DC shift, because of the long time constant of the RC network. Effectively, the discharge of the cap appears as DC to your scope.
     

  10. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    I understand common cathode bias and tube load lines but the first preamp stage of the Marshall and Deluxe are almost identical. Same tube, grid stopper resistors and plate load resistor but a much more positive cathode bias. Why? If it helps reduce blocking distortion I may want to change my cathode resistor.
     

  11. Cleeve

    Cleeve Tele-Holic

    Age:
    52
    992
    Feb 19, 2009
    Cincinnati, Ohio
    The higher value cathode resistor allows more voltage swing on the control grid before the grid goes positive relative to the cathode. When the grid goes positive relative to the cathode on signal peaks, the grid begins to conduct current and go into grid blocking land.
     

  12. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    Thanks Clint, I may try a higher value cathode resistor and see if it helps reduce my blocking distortion.
     

  13. CoolBlueGlow

    CoolBlueGlow Tele-Afflicted

    Nov 20, 2011
    Arkansas
    +1 on what Clint said

    ...because Jim Marshall knew that guys were putting fuzz tones and orange squeezers on his amps, and Leo didn't.

    Marshall amps go to eleven.
     

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