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Thread: Crate Blue Voodoo B60 abnormal signal distortion problem

  1. #1
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    Crate Blue Voodoo B60 abnormal signal distortion problem

    In the distortion channel (channel 2), the signal gets distorted on the top half only before it goes to the stage that's actually supposed to distort (clip) it. The signal (sine wave) is fine until about half-way up on the gain (relative to how hard the input is driven) and while the negative half of the wave continues to increase in amplitude, the positive half increases much more slowly and begins to get wider and rounder. I can find nothing wrong in any part of the circuit (swapped out resistors, cap and tubes, etc). Signal chain is as follows: V2-1 -> coupling cap -> grid resistor -> grid-bias resistor -> V2-7. Distortion begins after grid resistor. I have been in contact with Crate technical support and have tried numerous suggestions, but have had no luck as yet. Anybody seen anything like this?

  2. #2
    Senior Member Enzo's Avatar
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    I have to assume you have the BV60H instead of the combo BV60. There is only one channel path through the combo version.

    How large is your test signal?

    Apply the specified test signal to the amp and how do the test points compare? Note the specification also wants the controls set a certain way.

    I wouldn;t be expecting symmetrical clipping here anyway.

    I don;t see any mention of you trying a different tube in V2.
    Education is what you're left with after you have forgotten what you have learned.

  3. #3
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    Hi Enzo,
    Yes, it is the head version. It didn't occur to me that there would be a difference.

    My test signal is 100mv at the input jack. I don't see a spec anywhere on the schematic for a reference signal voltage, just the test points.

    Why wouldn't the signal still be symmetrical? It only changes after the resistor. I don't see anything on the schematic that would change only half the waveform at that point, other than the LDR which might influence it up somehow, but that's not connected to the circuit right now.

    I tried a few different tubes in the V2 socket, even ones with different gain and nothing changed.

    Thanks,
    Ken

  4. #4
    Senior Member Enzo's Avatar
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    Set it up to get the spec'd voltage at TP1 and see if the rest follow.

    You can send tons of signal through a 12AX7, but as with any tube, if you get the grid more positive than the cathode, it starts to conduct current. It attracts the electrons, so it might as well be a small plate competing with the main one.

    I don;t know what is on that cathode there at V2 pin 8, but I bet it is less than 2v, more like 1.5 to 1.8. When your signal on pin 7 goes above that level, the grid conducts and clips off your signal. Going the other way, it meets no such resistance. hence unsymmetrical clipping.

    Or so it seems to me.
    Education is what you're left with after you have forgotten what you have learned.

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